by Tom Bosschaert

21 mei 2012

Tom Bosschaert

This guide explains the various lighting technologies for indoor artificial lighting, with explanations on light quality, efficiency, longevity and various health and usability factors included. The guide contains detailed information on the most common lamp types such as incandescents, fluorescents and LEDs.


This guide has been compiled because of the increased complexity that new energy saving light types have created. We hope to contribute to more pleasurable homes and working environments, while reducing environmental impacts.

This guide has been made with great care, but if you notice any errors or omissions, feel free to contact us about it. For custom light advice, we can provide lighting consultancy, simulation and design.


  1. Properties of Light
  2. Properties of Artificial Lights
  3. Spectrum
  4. Intensity and Energy Use
  5. Spread
  6. Consistency
  7. Format, Fitting and Type
  8. Longevity and Reliability
  9. Material Use
  10. Cost of Ownership
  11. Real World Examples

Light is luxury

Artificial light has made much of human development possible. Since the discovery of fire, light plays a central role in our lives, extending our hours of life, creating mood and atmosphere in our homes, and increasing our productivity.

Since the rising concerns in the last century about the electricity use of traditional incandescent light bulbs, the mainstay of our post-industrial lighting solutions, there have been many alternatives brought to market. Some of these are excellent replacements for the standard lightbulb in most cases but others are not such great alternatives.

This guide sheds light on these issues, an helps pick the right lamp for the right purpose. Also, since energy use is a concern, it’s important to realize that new technologies do not always outperform older technologies in this department. Sometimes smart new ways of using old techniques have remarkable results. This guide is written to show the basic concerns when choosing lighting fixtures and technologies for indoor use, but they may also be helpful in other applications.

This guide starts with some general observations about light, which introduces some important definitions and terms used elsewhere in the guide. This is followed by a discussion of the seven main properties of artificial light sources, which are Spectrum, Intensity and Energy Use, Spread, Consistency, Format, fitting & Type, Longevity and Reliability, Materials and Cost. It concludes with a range of examples of some of our own experiments and experiences.

1. Properties of Light

To understand some aspects of artificial lights, it’s good to investigate some properties of light in general. There are basically only two properties of light in general that concern us: intensity and spectrum. Together these define the light we care about. Intensity is rather self-explanatory: the amount of light emitted. Light is defined as that part of the electromagnetic spectrum that can be seen by an average human, and is a subset of this energy.

Intensity of light can be measured in Lumens, and Lux, where Lux is Lumen spread over a certain area. Lamps are usually expressed in Lumens. More on that later.


The spectrum of light defines the set of wavelengths present in the light in question. Sunlight contains the 'full spectrum’, but also includes infrared, ultraviolet and other radiation beyond those wavelengths. Even though sunlight contains the full spectrum of visible light, it does not contain an equal amount of all wavelengths. The below diagram shows an approximation of the spectrum and intensity of direct sunlight.

Diagram of the solar spectrum. Note that a large amount of energy (area underneath the curve) is present in infrared.

The curve in this diagram is called a 'spectrum-curve’. This corresponds to the perception of sunlight as 'warm’ light, since it has higher intensities in the red spectrum than in the others. The diagram shows light received by earth at atmospheric level, but not necessarily what reaches your exact location. Light is filtered through various atmospheric layers before it hits the ground. This varies per location and with atmospheric conditions.

On an overcast day, when clouds filter certain wavelengths of light from the sunlight, the spectrum looks different. Generally speaking, we can say that daylight on a cloudy day has a more even spectrum-curve. This means the distribution of light intensity per spectrum section is more even, which increases our ability to perceive colors neutrally. This is important, for instance, if you want to judge the color of a certain set of house paints. Doing it on an overcast day will allow you to see the colors better than in direct sunlight, also because of less glare. The same goes for prints and other color sensitive applications. 'Daylight’ is usually considered to be the direct light of the sun, plus that light which is scattered by the atmosphere (indirect). This indirect light enables us to see in areas where no direct light is present, as well as bouncing of light off of neighboring surfaces.

Intensity and Energy Use

If we understand that electromagnetic waves are a form of energy, we understand that radiation in the infrared and ultraviolet ranges also contain energy, even though we cannot perceive them with our eyes. Infrared is generally perceived as heat, and ultraviolet as invisible radiation that may damage our health (UV radiation) but is also used for certain useful purposes by other animal species, such as by bumblebees for navigation. It should also be said that we don’t fully understand all the effects of the electromagnetic spectrum on biological life. We do know that it’s of vital importance to almost all living things and that life on earth has evolved in interaction with the spectrum of our sun.

Why is this important if we want to learn about artificial light sources? There are various clues to be found in this as to the effects of various light fixtures that may not be easily explained from a purely technical perspective. In addition, understanding that visible light is just a subset of the electromagnetic spectrum is important to understand if you’re looking for light sources that both perform well and are energy efficient.

This is a great spectrum diagram by Inductiveload, NASA.

2. Properties of Artificial Light Sources

In this guide we discern seven main properties of artificial light sources. Together these capture the most important properties of all the artificial light sources available, from the tiny LED in your stereo to large floodlights in a stadium. Some are more relevant than others in the discussion for good home and office lighting and will be expanded on in more detail.

The seven properties are:

  • Spectrum
  • Intensity and Energy Use
  • Spread (light shape)
  • Consistency
  • Format, Fitting & Type
  • Longevity and reliability
  • Material use

Many of these properties are related to the technology used to produced the light. A short introduction of the most common types is in place here, with more details about each type further on in each section.

Type of Lights

The type of light is determined by the technology used to produce the light. There’s dozens of types, with a few common in household applications and others more suitable for industrial uses. The five most common light types in household lighting applications are incandescents, halogens, compact fluorescents, LED’s and a few gas-discharge lamps.

In addition to this there are a few solutions that work on sunlight, which are preferrable over most artificial light sources, if they are applicable. The most common ones are reflectors and light tubes, but there are also materials which can store sunlight and emit it at night, for instance.

Incandescent lamps

A typical incandescent light bulb.

Incandescents are the lamps that have illuminated our world for centuries. There are various inventors of incandescent lamps, but it’s usually credited to Thomas Edison in 1880, because he invented the long-lasting tungsten filament. We know incandescents most commonly as the standard light bulb, but incandescents exists in a very wide range of shapes, forms, voltages, colors and applications. They produce a great spectrum, with very poor energy efficiency and a short life span.

Incandescents produce light by flowing power from the wall socket directly through a very thin filament of Tungsten, which then glows, in a glass chamber that protects the filament from coming into contact with air, which will cause the filament to burn up in seconds otherwise.

Incandescent bulbs are cheap to make and buy, have a very good spectrum, consistency and color rendering, produces a lot of heat, and are therefore very energy inefficient. They have a very short life span, and need to be replaced often.

More on incandescents at wikipedia.


A halogen light bulb. You can see
the secondary gas filled chamber inside.

A common use for halogens: spots.

Halogens are incandescent lights where the gas chamber is filled with a halogen gas type, allowing the light to operate at higher temperatures, last a bit longer, and be more compact.

Halogens tend to be somewhat higher in performance than normal incandescents, with slightly higher efficiency, and a longer life span, but typically cost more. Halogens are also made in more accurate production processes that allow very specific models and light spreads to be made. Therefore, high quality spot lights tend to be halogen lights. Because of their excellent color rendering and spectrum, halogens have been the lamp of choice for demanding light applications in the home, office, laboratories and elsewhere.

Compact Fluorescents (CFLs)

Most typically known as a power-saving light bulb, CFLs work by using electricty to 'excite’ mercury vapor. They consist of a gas filled tube and a 'ballast’ which is currently mostly electronic, that prevents the lamp from getting over-excited and burning up internally. CFL’s save energy by emitting light in just a few peak wavelengths, and almost no infrared, reducing the energy needed to operate. While color rendering and spectrum vary quite a bit between models, but have been historically rather poor (the standard having been around a CRI of 65 for a while). Models with much better color rendering are available, up to A CRI of 95. CFL’s can function quite a long time (typically 10.000 hours), but they will eventually dim and finally break.

CFLs are more laborious and complicated to make and are therefore more expensive than incandescent, but their price has come down dramatically since their introduction, making them a cost effective light solution.

CFL’s have various light consistency, but all CFL’s flicker, with some models flickering quite badly, causing people that are sensitive to flicker to respond poorly to them. To a degree, all people are sensitive to flicker, however, which makes CFL’s a less desirable light source. There are high-frequency models that fix this issue, but these are more expensive, and hard to find for home use.

If you have dimmers in your home, make sure to find out of the CFL supports being dimmed. As a default, they don’t.

Almost all CFL’s contain mercury and they are therefore hazards for the household if the glass tube breaks, and environmentally unfriendly if not properly recycled. Try to never break a CFL. In general, we do not recommend CFL’s for use in households now that good LED alternatives are available, because of their mercury content, poor spectrum and consistency and because they have a reduced life span compared to LED’s. Even though they’re cheaper, they’re not necessarily more economical than LED lights. See the last section on Cost of Ownership.

Above: A standard compact fluorescent. Image by Armin Kubelbeck.

A spiral-type CFL. Image by Piccolo Namek.

Light Emitting Diodes (LEDs)

There are a few different LED types. We all know LEDs from the thousands of uses they perform as tiny signal lights on our stereo, phones, and other electric equipment. They are mostly semiconductor devices that operate on low voltage DC currents mostly, often requiring an adapter. Because they are semiconductors, LEDs are quite sturdy and less likely to break if dropped or due to harsh conditions compared to other lamps. Their life span is very long, some are rated up to 50.000 hours.

LEDs used to only be available in red, green, yellow and white, with the blue type only becoming available much later. These signal LEDs (image below) are not suited for general lighting applications, because they are not very energy efficient and their spectrum tends to be ill suited for general lighting.

It’s been only recently that LEDs have been developed that provide decent general lighting. While the first attempts at producing generic LED lighting solutions proved to be somewhat disappointing -with many LED lights having extremely poor spectrum and lower than expected efficiency ratings- today there are ample high quality LEDs available that outperform most other light types in most areas. They are energy efficient, consistent, sturdy, have the longest life span of all consumer lights and can have very good light quality.

The most common ailment of LEDs, especially cheap ones, are spectrum and color rendering, so keep an eye out on that. The light 'spread’ may also suffer in cheap models. Also, for a long time it was not possible to produce LEDs that emitted enough light to replace a 60Watt incandescent light bulb. This is now also no longer a problem. LEDs tend to be a bit pricey, especially the quality kind. However, once you buy one, and don’t, let’s say, throw it around the room, it should last you a life time, providing great economy and worry free lighting.

As with CFLs, LED lamps by default cannot be dimmed, but there are models available that do.

Besides semi-conductor LEDs there are recent developments in Organic LEDs (OLED) and polymer LED (PLED) technology that are quite promising.

Above: A set of commercially available LED lamps. Image by Geoffrey Landis.

Above: A set of Power LEDs. Image by Gophi.

Below: different types of signal LEDs. Image by Afrank99.

Gas-discharge Lamps

Gas-discharge lamps work by sending a current through an ionized gas. We all know Neon lights from the colorful street signs, which are a type of gas-discharge lamp, but there are a great many other types, some of which have very useful properties.

Some gas-discharge lamps contain mercury (mercury vapor lamps, usually blue in color) so be careful with those, because mercury is a very poisonous material (see material use).

Some interesting gas-discharge lamps for household applications are low pressure sodium lamps, which can be used well in outdoor situations where illumination is wanted, but color is not important, such as lighting of a car-park. These are yellow-amber in color and have very poor color rendering. However, they are very energy efficient, sometimes up to 200 lumens per watt, beating every other light type available far and wide in energy efficiency. For uses where a lot of light is desired, the color irrelevant and high efficiency relevant, these could be a very good choice. Note should be taken that they do have a short warm up time.

Low pressure sodium lamps are widely used as street lighting in various countries. It is also the lamp of choice to prevent outdoor light pollution, as its peak spectrum seems to be least harmful to life and ecosystems. Save for turning lights off, of course, which is still better.

Above: A low sodium pressure lamp with its typical amber light spectrum.

Below: A highway illuminated with low sodium lamps. Image by Maarten Schmeitz

3. Spectrum

While most people in consumer situations don’t pay much notice to the spectrum of the light bulbs they buy for in their homes, and instead choose lights by their intensity, the spectrum of light is the most defining characteristic of light sources.

The spectrum defines the color, color perception, psychological effect and energy use of a light source. The spectrum of a lamp is usually a subset of the entire visible spectrum, which we can see when looking at a rainbow, or sunlight through a prism, for instance, where basic white light from the sun is shown in all its sub-wavelengths, and thus its colors.

Color and Spectrum

The color of a lamp results from the frequency of light the lamp is emitting mostly, or how it appears to an observer. It’s very rare for a lamp to emit only a singular wavelength (color), which means lamps emit a range of colors even if it appears a certain hue (also when the light is filtered).

Usually there’s a set of other wavelengths present in the light emitted, together called the spectrum. Light is what we call 'additive’, which means that each color added will make the result closer to white. This as opposed to reflected light, such as with paint or printing, which is a subtractive process, where each added color will make the whole darker.

White light always contains a range of light colors (with at least some red, green and blue), and pure color lights have a narrow spectrum, or a peak spectrum. Light that looks white doesn’t necessary emit the full spectrum though, as we’ll see later in the guide.

Most incandescent lights, among which the classic tungsten light bulb and halogen lights, have an excellent color spectrum, very close to that of the sun (see diagram below). This is largely due to their operating mechanism a glowing filament which comes close to how the sun emits her light.

Because a large part of the incandescent light bulb’s spectrum lies in the infrared region, it also emits a lot of heat. You can feel this when you hold you hand close to an incandescent bulb as the infrared radiation hits your skin and you can perceive it as heat. If this heat is used, it’s not necessarily wasted (as with incandescent heat lamps). Incandescents are therefore not necessarily inefficient, but they do convert a lot of electricity into heat, which in many cases is considered waste.

An incandescent lamp used to melt plastic. Image by bergeycm.

Beyond Incandescents

If we’re to choose a lamp which converts electricity to visible light efficiently, and doesn’t 'waste’ electricity by producing heat, incandescents are not a good option. This has resulted in a great many other light bulb types being introduced in the consumer market in the last decades, most famously the power-saving bulbs, or CFL’s, and the more recent LED lights.

These new lamp types manage to save electricity by making sure there’s as little energy spent emitting infrared and ultraviolet sections of the spectrum as possible, converting as much energy as possible in visible light. For instance, compare the spectra of an example LED, CFL and Incandescent lamp in the following diagram:

Diagram of the spectrum a LED lamp (blue), a CFL (green) and an Incandescent (purple) superimposed the solar spectrum (yellow). Note that the energy used by each lamp is at least the area underneath its curve.

The downside of this method to reduce energy consumption reduction is that some of these lamps have a very narrow spectrum, resulting in poor light performance for color rendering and negative psychological effects. The example CFl in the image above, for instance, has a terribly peaky spectrum that will make everything look ugly.

Therefore, a very energy-efficient light may not actually be a good light for its purpose at all, especially if it makes us sick when working in it for prolonged periods of time (seeing as the energy impact of medicine production in the western world is rather high). The trade-off between spectrum and energy savings, therefore, includes a consideration of the context in which the light will be used. A lightbulb for your garden shed might warrant a different type than that above your working desk. In some cases, an incandescent is still the right choice (for instance, for that garden shed it might be a good option, if never used long).

Color Rendering

*Our eyes can only perceive the color of objects if light that falls onto the object is reflected into our eyes. This means that if you illuminate a blue object with an orange light, it can be hard to see that the object is actually blue. It’ll look very dark grey or black instead. Therefore, our ability to see color is entirely dependent on the spectrum of the light in the room we are in. *

If not all colors are emitted by the light source in question in an even way, you will perceive a biased color set. This is important for those working with color sensitive applications, such as printing and design, but in general you want to make sure that the lights in your home have a pleasant spectrum. For example, skin can look strange under some compact fluorescents (CFL’s) and some LED lights have 'dirty’ looking light that make everything look old. Psychologically, a poor spectrum of light can influence our mood and our health.

In this visual simulation a perfectly blue blob thing called Edgar is set on a white pedestal, while illuminated with a white light.

Another visual simulation, where both Edgar and the pedestal are unchanged, but the light only emits red light. The result is a black Edgar, because his surface does not reflect any of the red light.

For a more useful example of quality of light spectrum and color rendering, here Edgar and the pedestal have received a color test chart paint job. The perfect white light shows all colors well.

In this simulation, the light is not perfectly white. While the white areas of the test chart still seem to be white, the color rendering of the whole is rather poor, and has dimmer, less vibrant colors.

Psychological Effects

The light spectrum also has psychological effects that can be hard to notice consciously, but have been proven to have long term effects on our wellbeing.

Light with a narrow spectrum, such as many fluorescents, besides having poor color rendering, may end up depressing people’s moods and cause other psychological inbalances. Tests have been done on schoolchildren where standard tube lights have been replaced by full spectrum lights and not only their levels of concentration and moods went up, also their performance and general attentiveness. Flickering, whether noticed or not, also affects people’s response to light. For more info on flicker, see further in the section about light consistency.

How to Determine the Right Spectrum

First, you need to consider in what circumstance the lamp will be used. Will it be used often? Will heat be a waste or an asset? Will people work in the light? And so on. Then you can assess which spectrum and intensity you need, and the lamp’s technology will follow.

There are various ways in which spectrum can be expressed to find out which lamp to buy. The most common way is by listing the light source’s color temperature in degrees Kelvin. Typically, these come in three different temperatures: 2500-2900 (warm-white), 4000-5000 (white), 6000-9000 (cool white). You can refer to the chart below for a more accurate listing.

Light Color Temperature Chart (by Mifsud26)

In general, color temperature listings say very little about the spectrum of the lamp. It says something about how you will perceive the color, but whether the spectrum is any good can’t be said from a temperature listing. A 2800 kelvin CFL, for instance, may just have a peak at a orange color, and a smaller peak in the blue range, and the result is indeed warm white, but with a very narrow spectrum. Some manufacturers list the color spectrum profile on their website or can send it to you on request. That’s the best way to gauge a light’s spectrum, as it’s not through some arcane index that may or may not tell you what you need to know.

The way light is emitted from a source can take many different shape or forms. The basic and most common spreads are the omnidirectional lightbulb, the simple spot and the tube light. These cover the majority of uses. There are others as well, for those unfamiliar with custom lighting, that introduce many new possibilities for quality indoor and outdoor lighting. Now that LED lights are becoming more common, more spreads are being introduced. It’s worth noting that the spread of a good lamp is often achieved in co-ordination with the fixture the lamp is in. The best spreads are mostly achieved by a good lamp in an appropriate fixture.

In time, several lamp configurations have arisen that define the spread of light in various ways. These are mostly based on incandescents, but because other technologies are becoming more common, we see these transferred to CFL and LED more and more.

Simulating Spread

The spread of light is important enough for manufacturers to recognise that designers want to know how their lights behave when designing a space and/or lighting solution. Most manufacturers of high quality lamps and fittings therefore release digital files which describe in three dimensions what the spread of a particular lamp is. These files are often formatted as IES files, and they can be loaded by most 3D visualisation software to simulate the effect of the light on its surroundings. The top image of this section has been made this way. This is an excellent way to test lighting design before large costs are incurred. IES files are more commonly available for fittings than for lamps, though.

Some of the available standard spreads are:

Omnidirectional Bulbs

These are the standard lightbulb that spread light in all directions, used for all kinds of purposes. With incandescent we’re used to a perfect omnidirectional spread. For most CFLs and Gas-discharge lamps this isn’t a problem either, although CFL’s are naturally tubes, not spheres, bunching them up in spirals provides virtually the same effect.

LEDs need to be arranged in a spherical pattern in order to achieve proper omnidirectional properties. Good LED lamps don’t have a problem with it, but with cheaper models we’ve seen some issues with inconsistent spreads.

Spot/PAR Lamps

Spotlights and Parabolic Aluminum Reflector (PAR) lights direct the light in a particular direction in order to create a cone of light. This allows for greater control of the lighting situation and is important to make nice lighting designs for homes, commercial situations and others.

Because incandescent are naturally omni-directional, a lot of spots historically have reflectors that reflect the light of a part of the lamp towards the spot’s intended direction. These are usually still indicated in the lamps' specification with a code like R (standard reflector) or ES (Elliptical Reflector). Shaping the reflector differently will lead to different spreads. The image at the top of this section shows what different reflectors can do for the spread of light using the manufacturer’s IES files.

Halogen spots usually have very well crafted reflectors and PAR lights also often receive a lot of attention in their reflector design. This means there’s a lot of varieties of reflectors and resulting spreads out there. Some of these focus the light more towards the middle, with a soft spread outward, others have a hard stage-spot like appearance, and yet others have fancy striping and other effects. These can bring great quality to lighting solutions and should not be underestimated.

LED lights are quite directional in nature by themselves. Because of their directionality, LED lights lend themselves well to spot lights. A standard signal LED even has a very narrowly focused beam. This means that to make a spotlight using LEDs you do not need a reflector necessarily. In fact, some manufacturers don’t even add a lens to control the LEDs, they just put together a lot of small LEDs in a certain pattern and have peace with the resulting spread. Our experience is that those lights have the worse spreads available, and because they mostly only occur on cheap lights, their spectrum is often very bad as well. Try to steer clear of the multi-LED lamps without lenses or reflectors. Good LED lamps always have a custom lens of some kind, which can be molded in with the LED itself or a separate component.

PAR lights focus about four times the amount of light into the center of the spot, and have become somewhat of a standard for a certain spread and intensity level. The number behind a PAR lamp, such as PAR30 is an indication of the diameter of the bulb in 1/8th of an inch. PAR 30 is therefore 3.75 inch in diameter.

Above a traditional incandescent PAR spot.

A LED PAR lamp.

Multi-LED spots that we don’t recommend look like this:

Crown Bulbs

These lamps, which can be hard to find, are basically omnidirectional lamps with a silver coating on the top. They reflect the light back towards the rear of the lamp. This is usually applied in conjunction with specially designed fittings that then reflect the light forward again in a particular way. Several very high quality lighting solutions use this approach, because the larger reflector allows for more precision of the spread. All light types are available with silver crowns, but they may be hard to find.

Decorative Lights

You might have seen the candle- or flame-shaped lightbulbs in a chandelier in some tacky place. Those are decorative bulbs. They come in all sorts and sizes. These are usually not meant for proper illumination and mostly for effect, so do not expect great lighting qualities from them. As an accent in a space they can be used strategically

4. Intensity and Energy Use

The intensity of a lamp determines the amplitude of the waves in the spectrum. Of course, the wider the spectrum, the more energy it costs to produce that spectrum. Because spectrum is not immediately apparent in most cases, it is possible for two different lamps which use the same technology to have very differing energy use.

The 60 Watt Tungsten Bulb Standard

We used to buy lamps based on their wattage. Most people that grew up in the last century have a feeling for what a 40, 60 or 100 Watt lamp 'does’. This is based on the performance of incandescent light bulbs which, regardless of brand or type, used to perform somewhat uniform in all areas. Their spectrum was great, the energy input was a good measure of light output and they were cheap. They just broke a lot, but hey, they were cheap, right? The downside of expressing the intensity of a lamp with the amount of watts it uses is that wattage says nothing about how much light something produces, only how much energy it consumes. So, for energy efficient lights, a 10 Watt bulb may output as much light as a 60 Watt traditional incandescent.

It is because of this reason that many manufacturers have started listing equivalent wattage for their power saving bulbs. This means a lamp may be sold as giving as much light as a 60 Watt incandescent (about 900 lm), but only using 10 Watts. While that may be true in the general sense, be careful, because it says nothing about the spectrum, which will undoubtedly be compromised.


A better way to express the intensity of a lamp is through 'Lumens’, expressed as 'lm’. Lumens are the unit of the power of light as perceived by the human eye. It is the addition of all wavelengths and their intensities. This means that a light that emits the same amount of radiation in the red spectrum can have a lower lumens output than a light that emits the same absolute amount of radiation in the green spectrum, because our eyes are more sensitive to green light.

Lamp Light intensity (lumen)Sun~3.75*10^28Standard 25 Watt Incandescent210Standard 60 Watt Incandescent89025 Watt Halogen2509 Watt Compact Fluorescent400-90015 Watt Compact Fluorescent900-12009 Watt LED fixture400-900Low Pressure Sodium lamp1.000-5.000+

In a way lumens are the light-equivalent to DbA (A-weighted decibels), the measure for sound pressure as perceived by the human ear. Since 2010 the European Union mandates that all light bulbs sold have to state their lumen output. Above you’ll find a small chart listing some common light types and their typical lumen output.

Luminous Efficacy

So, if lumens are a good way to express light intensity, what is a good way to express how much energy the light uses? Well, for that, watts are actually a good measure. So, watts remain important, but only for measuring the amount of energy used. It therefore makes a lot of sense to express the efficiency of a light as lumens per watt, meaning the amount of lumens a light emits per watt of energy spent.

In the table on the right we’ve also listed the lumens/watt range of the typical lamps. You can see that these can vary quite a bit. This measure is also called 'Luminous Efficacy’ and is expressed as 'lm/W’, and also sometimes referred to as wall-plug efficacy. This indicates that there are other factors at play between the lightbulb and the wall socket, which we’ll get to in a minute.

Lamp Luminous Efficacy (lm/W)Class M star (Betelgeuse), 3000 K30Class G star (Sun), 5800 K80Standard Incandescent5-15Halogen12-20Fluorescent Tube Light60-100Compact Fluorescent45-75Signal LED4.5-150LED fixture50-80Low Pressure Sodium100-200

You might find it interesting to know that there is a theoretical maximum to luminous efficacy, which, if you think about it, makes a lot of sense. Since light is a form of energy, there must be a situation in which all energy put into a lamp is emitted as visible light. This is generally considered to be at 683 lm/W, and emits only green light at a wavelength of exactly 555nm, because our eyes are most sensitive for that wavelength.

Other Aspects Influencing Energy Use

There are some other aspects that may influence the effectiveness of your light source versus the amount of power it uses. Some of these are obvious, such as putting a light bulb in a place which blocks some of its light, such as behind colored glass, inside a lampshade and so on. Others are less obvious.

One of the most important losses of energy associated with lighting that is not generally noticed is the transformation losses that occur when using lights that use another voltage level or current than what comes out of our wall sockets, or for electronic ballasts. Incandescent light bulbs work directly off the domestic electricity network. However, CFL’s and LED’s usually require circuitry or a transformer to change that voltage to 12 volt typically, and also from alternating current (AC) to direct current (DC). This transformation is never achieved without energy loss, and when you have a poor transformer these losses may be substantial. It’s therefore often better to have a single, high quality transformer than to have many little cheap ones. Transformers, especially cheap ones, may also emit a humming noise, and depending on its quality this may actually affect the quality of living.

For many modern lightbulbs that are screwed into traditional sockets (retrofits) the transformer is actually part of the lightbulb itself. In that case, and especially when the bulb is a high quality device, one may expect the transformation losses to be minimal. Still, the transformer does take away some energy, uses materials and in some cases quite toxic ones.

Age also influences electricity use. Not so much in that electricity use of a lamp goes up with age, but for CFLs and LEDs, light output diminishes with age, while electricity use does not

5. Spread (Light Shape)

Shown above is a light simulating showing three different light spreads, based on manufacturer’s IES files.

Very Cheap LED lamps

It’s so attractive: a lamp that will last 50.000 hours, has a great spectrum, does not flicker and is very light efficient. And if you see one that’s cheap in addition to this, it’s hard to resist. It sounds too good to be true, and in our experience it always is. Cheap, often Chinese made, LEDs are better left in the store. Mind you, there are excellent Chinese made LEDs available, but the unbranded cheap ones usually have 'made in China’ on them, and perform poorly. Thankfully, they’re somewhat recognizable, no matter where they’re from.

We’ve had quite a few cheap LED lights that fit right into a standard light socket (retrofits). Some of these perform well, but we’ve found that some of them do not reach their advertised ratings, their energy performance is poor, their spectrum very unattractive and in general do not pose an attractive package, not even for their price.

One type that stands out is the multi-LED type, in which a group of 10+ signal LEDs are combined in a single casing, usually without a dedicated lens.

As with our own experiments, these lamps basically combine a large set of very cheap signal LEDs to produce enough light. And as with our own experiments: it doesn’t work. Because signal LEDs are cheap, as they’ve been around for a long time and are produced in very large masses, the cost of these LEDs can be low. Dedicated general lighting LEDs, a subset of Power LEDs in general, are expensive because they’re harder to make and their development has been only in the last decade. Since these lamps are essentially one big signal LED, and given that signal LEDs are very poor general light sources (except for flash lights and such), so are these lamps.

The light spectrum of these lamps is very narrow and the light tends to be on the dirty side of things (yellowish, cold, etc). Their color rendering is very bad. Their spread, due to the lack of a proper reflector and/or lens is dismal. We retain one of these in our office to show the result from a bad LED. For a little bit more money one could buy spots made from three power LEDs. These have a much better spectrum, energy usage pattern and they look better as a light as well.

Above: Steer clear of these, if you can.

Single Power LED spots

If you were looking for a nice general spotlight and buy one of these, you might be looking funnily at your purchase once you start using it. These spots have a single power LED with a clear, very round lens. While these spots have their uses in specific cirumstances, their hard spread, which is more akin to a stage-spot than a spot for home or office use, casts sharp shadows at any distance and has a rather crude cutoff. This is somewhat inherent to its design.

LEDs are point-light sources that require some sort of mediation to have a nice spread, either though a very good lens, a reflector, multiple leds together and usually a combination of those three. Single power LED spots seems to want to cheat on this, and we have not seen a good result from these yet. Let us know if you find one that does, though.

A single-LED with clear lens spot.

AC-driven LEDs

We’ve encountered some LEDs that can be driven by wall-socket current without using an adapter. We purchased some of them, and while their efficacy was not brilliant (50-60 lm/watt), considering there are no other power losses in a driver or transformer, this is actually rather good. Their spectrum is some of the best we’ve seen next to incandescent and they provide new freedoms we didn’t think possible. Even their spread is rather nice for an on-die LED. They save a lot of material and energy by not requiring an adapter and become more reliable as well.

The best example of these are the Acriche LED range. As far as we’re aware, these are the only AC LEDs available. These single power LEDs can be bought in on-die form and then used in whichever application suits you. In our case, we’ve used them as general lighting in the office, at home and are considering various lamp products with them. Making them work with dimmers is a bit of a challenge, but not impossible.

One of the interesting things we found is that they manage to get this to work by actually including two LEDs in one light. Because AC alternates its current, and LEDs are diodes and only work when the current flows in one direction, the two LEDs take turns pulsing light one each side of the current sine, together producing a steady 100 or 120 Hz, which is more than ample to remove any semblance of flicker even for people sensitive to it.

An Acriche AC driven LED

6. Consistency

The consistency of a light source determines both its consistency in spread and in time. Consistency in spread determines wether the light is spreading its spectrum in all directions evenly. With lights that have special spread shapes it’s possible that some directions receive a different light spectrum than others. Spread in time mostly means flicker in the short term, and how the lamp behaves when it gets older.

Spectrum Consistency

The spectrum of a lamp does not need to be the same in all directions. This is not necessarily unwanted, and can be used as a design tool to result in interesting light 'shapes’, as discussed above. If the color rendering is different depending on the angle, that is usually less wanted. This may result in chromatic aberration, which is where the different colors start detaching and rainbow effects start to appear. Some lights, especially those with multi-color reflectors like most halogen spots, may have various forms of chromatic aberration and/or diffraction.

Chromatic diffraction in a halogen light.

Multi-color LED’s, which produce white light by combining individual red, green and blue LEDs can have some interesting color effects in the fringes of shadows that may or may not be unwanted. This is due to the separate LED’s being shaded at slightly different angles.

Consistency in Time

Consistency in time mostly refers to flicker and aging effects. Flicker is almost always unwanted, although it can save energy, and aging effects are also almost always negative.

Quite a few lamp types flicker by design, most notably fluorescents. Any flicker above 60 hertz (60 times on/off per second) may not be directly noticed by the human eye, it is however perceived subconsciously and it can make eyes tired and cause psychological effects. Some people are more susceptible to it than others. As long as lights flicker at at least 60 Hz or more, they should not pose a problem for most people. Unfortunately fluorescents, especially older types, do flicker quite a bit. Only high frequency fluorescents eliminate this problem for its lamp type, but these are 'special’ and thus more expensive. LEDs generally don’t flicker.

Because of our electricity network’s AC current’s frequency of 50Hz in europe and 60Hz in the US most lights that work directly from the socket will have to deal with flicker one way or another. Incandescents did it by using a glowing filament that kept glowing for a bit after you took the power off. The short intervals between each power pulse was bridged by this glow effect. Other lights will solve this in different ways. Light behave differently over age as well. Some, like incandescents and low sodium vapor lamps do not show any negative visual impact over age, which is a beneficial property. They just break when their time is up, and until then they perform like new. Fluorescents and LEDs tend to dim in time, where Fluorescents will eventually also break down, start to flicker and no longer turn on, whereas LEDs will continue to emit light at increasingly lower outputs for the same input

7. Format, Fitting & Type

The format and fitting of lights are mostly dependent on the application. There’s hundreds of formats a lightbulb can take, from the ones in car headlights to those in stadium flood lighting. For household purposes, it’s been common to use two or three fitting types, and about a dozen formats and stick to that.

In Europe these are mostly the fitting types E27 and E14 screw-fitting type and in North America the B15d and B22d bayonet cap fitting types are common as well. These fitting types are designed to allow easy and quick replacement of the bulb, and with thay show their historical usage for incadendescent bulbs that needed to be replaced often. Because CFL’s typically last 10 times longer and LED’s typically last the lifetime of the building they are in, it’s not such a straightforward choice to stick to the old formats.

While the fitting type does not influence light quality directly, it does force a certain technological format. Because the fittings are supplied with power directly from the net they do not allow the efficient use of LED’s without a built-in transformer. Therefore, when constructing or renovating a space, it’s best to choose different solutions for lights, based on low voltage DC currents that end up saving a lot of electricity and provide high quality of light for the duration of the structure. This can be achieved by building-wide DC networks that have been available for some time. Besides lights, they can also power most household equipment without a transformer, such as laptops, phones and so on.
Click on the image below to zoom.

Common base / fitting types

8. Longevity and Reliability

We’ve mentioned the reliability of most light types above, but it’s good to reflect a bit on the topic in general. Lamps are usually listed to have a life span of a certain amount of hours. However, the conditions and pattern of usage may have a large influence on this life span.

Life Span of CFLs
CFLs have as a standard life span about 10.000 hours. Depending on the make and model it will reduce in light output towards the end of its life and eventually stop functioning altogether. Fluorescents, indlucing CFL’s, tube lights and others, can have a severely diminished life span if they are switched on and off often, for instance, sometimes even lower than incandescents. There are special models thart are used for rapid switching. It is suggested that a CFL is better left on if a room is left for less than 15 minutes. A 5 minute on-off cycle used occasionally may already severely deteriorate its lifespan.

Light type Average life span lamps have not yet been around long enough to say for certain what their life span will be. LEDs don’t really stop emitting light, but slowly degrade over time. Therefore, the lifespan is often related to a certain percentage of light output, like 50.000 hours for 60% light output (15 years at 8 hours/day), which is typical. LEDs are sensitive to being 'over driven’, which means receiving higher voltage than their design parameters, which makes them brighter but also can dramatically decrease their life. Under-volting may increase their life span. Over and undervolting can only be achieved by tweaking with the lamps, so in normal conditions should not occur.

One thing to consider is that with LED’s it’s often the adapter or the circuitry that breaks before the lamp does. Especially on cheap adapters or those not manufactured to drive LED’s, the lifespan of a LED light may be reduced drastically. If you are matching adapters and lights manually, take care to choose a proper adapter. If you’re not, there’s no such thing as free, or even a cheap lunch with LED’s. Low quality adapters may cripple any LED.

Color Rendering Index

Another way to express spectrum is through the Color Rendering Index (CRI). The CRI is an indication of the amount of colors a light can render, and therefore, of its spectrum.

CRI is expressed in a value of zero to 100, where zero is no color rendering and 100 is perfect color rendering. Most incandescents have a rating close to 100. The CRI is actually not a good measure for the lamp’s spectrum consistency. If one lamp has a CRI of 50 and another of 51, one could be showing orange and blue well, but be poor at green and red, while the other is the other way around. It therefore does not tell us if one light source is peaky or not.

The math involved in calculating CRI’s is quite complicated, and therefore it’s not very intuitive. If you can find a spectrum graph, it is still the best way to express and compare spectrum.

Light typeAverage life span Standard Incandescents750-1250 hoursHalogen2000-2500 hours Compact Fluorescents (CFL)10.000 hours Light Emitting Diode (LED) 50.000+ hours Low pressure Sodium Vapor Lamp18.000 hours

Besides LEDs, most light types break reasonably frequently.

Replacement Maintenance

We’ve become very used to light types that break occasionally requiring someone to go up to the light fitting and replacing it by hand. It seems to be the most common thing in the world. However, imagine a concert hall, with lights sitting dozens of feet above the audience, or in large public spaces, where lamps can be far away and hard to reach. Most lamps that are currently used break eventually and while building these buildings and structures it has been taken into account that the lamps might need replacing every now and then.

With the introduction of LEDs this may change. Because LEDs outperform most light types, especially for general lighting, and because LEDs have much longer life spans, they can potentially save a lot of material, energy, money and maintenance effort. We can design buildings where lights are embedded within the building to provide a range of interesting and exciting lighting solutions. The division between luminiere and light source, something we’ve become so used to, can disappear. A well designed lamp can not only hold a light source but be the light source, opening up a world of exctiing lighting possibilities.

9. Material Use

A few words on material use. We’re used to the material use of lights to be rather benign both in amount and nature. Incandescents are really not much more than a bit of glass, some thin metal and a glowing filament made of harmless Tungsten.

Not so with some other light types, however. As the energy saving became more important, the CFL has been introduced in our homes. CFLs, other fluerescents and some gas-discharge lamps contain significant amounts of mercury. Mercury is a neurotoxin that is extremely poisonous to all living things, and is also bio-accumulative, which means it stays within the body.

While it is true that the energy saving that CFL’s represent also reduce the mecury output of some electricity plants, breaking a CFL or mercury vapor lamp will introduce a concentrated amount of mercury into our living environment, which is the most dangerous distribution of the toxin. If you break a CFL, get everyone else far away from the room, open the window and clean up the lamp as quickly as possible. Don’t use your bare hands and don’t use a vacuum cleaner to suck up the pieces, as it will blow out the mercury into the room. Read this to know more about what to do if a fluorescent lamp breaks. If mercury containing lights break down, please bring them to your hazardous waste collection point.

LEDs also contain some toxins, but usually far less and less toxic materials than Mercury, and they don’t generally evaporate. With some LED systems, however, the separate adapter cannot be disregarded. This is virtually the same adapter type as we’re familiar with for Halogen spots, and are quite substantial in their material use. This can be avoided by installing a central power converter in the house. The adapter generally lasts beyind the life span of the lamp, and can be produced quite compact, reducing its material use, as seen in modern LED refit models that can be put in a traditional socket.

If LEDs break down, also bring hem to a chemical waste disposal facility

10. Cost of Ownership

The cost of ownership of a lamp extends beyond the purchase, maintenance and energy consumption costs. Because light is vital for us as biological creatures, the quality of light might affect us in ways which may allow us to perform better, be healthier and in general more happy. Of course, saving energy also helps save the environment, which is currently costing our governments a lot of resources as well.

These important considerations are difficult to capture into numbers, so if we consider high quality lighting as a basic necessity and understand that this has added value in the long run, also financially, it should be understood that sacrificing light quality will cost us in the long run. In addition to this, we can use the purchase, maintenance and consumption costs as an indicator of our financial investment, which comes out in our favor in the long run as well.

Below we’ve made a simple comparison of the cost of ownership of generic Incandescent, CFLs and LED lamps for a 10 year period, ~6 hours a day (22000 hours), at a cost of € 0,20 / kWh, which is accurate for spring 2011 for the Netherlands. These are reasonably generic lamp values taken from off the shelf store bought items.

Lamp typelmlm/WSpectrum (CRI)WattPriceLifespanEnergy cost @ 22000 hrCost of Ownership (5 years)Generic E27 Incandescent87014.510060W€ 2,-1.000€ 264,-€ 308,-Generic E27 CFL57051,88211W€ 7,-10.000€ 48,40€ 69,40Generic E27 LED54067,585-958W€ 30,-25.000+€ 35,20€ 65,20

As we can see, the CFL and the LED are both considerably cheaper to operate than incandescent. The difference between the LED and CFL are minor, depending mostly on whether the second CFL will make the 22.000 hour mark or break before. The LED does give a better spectrum during this life span, and can continue to operate for quite some time after the 10 years use therefore the LED is the cheapest option all together.

Note that neither the CFL or the LED reach the lumen output of the incandescent, while both are officially rated as 60W incandescent equivalents. Because these values may vary quite a bit, it’s best to do this calculation again once a number of eligible light sources are found to see which one comes out best. For equivalent cost, the light with the best spectrum and consistency should win, in addition to considerations about health.

11. Real World Examples

We’ve performed many experiments with light types over the years, predominantly with LEDs. We see LEDs becoming the light source for the next century. Our experiments range from soldering together dozens of signal leds to see what their performance would be (poor), making all of our own lamps for our offices to trying to assemble the most energy efficient light available. We’ve seen quite a few good and bad examples of LEDs for various purposes. Some of these stand out, and we’d like to show you a few of them.

Light sourceCRILow Pressure Sodium (LPS/SOX)~5Clear Mercury-vapor17High Pressure Sodium (HPS/SON)24Coated Mercury-vapor49Halophosphate Warm White Fluorescent51Halophosphate Cool White fluorescent64Tri-phosphor Warm White Fluorescent (CFL)73-80Halophosphate Cool Daylight Fluorescent76"White" SON82Quartz Metal Halide85Phosphor Converted Light Emitting Diode (pcLED)85-95Tri-phosphor Cool White fluorescent89Ceramic Metal Halide96Incandescent/Halogen Light Bulb100

List of common lamp types and their CRI. (mostly from Wikipedia)

If none of these are listed for a lamp you are investigating, there’s unfortunately only one way to find out: buy a lamp and see how it performs. In that case you’ll need specialized equipment to do a proper spectrum analysis, or you can just put it through its paces. To be honest though, if you are buying lamps you cannot find the spectrum information for from the company’s website, you’re asking for a poor spectrum. Cheap lamps from unknown origins will guarantee you poor performance in all areas. High quality lights mostly have spectrum graphs available.

How to Affect the Spectrum

Most of the time, we want a broad spectrum from our light source. But, in some cases, we only want a certain subset of it for particular purposes. This can be for setting a mood or an atmosphere, but also for certain practical uses such as in areas where light sensitive material are being handled (like in a photography dark room). It’s not possible to extend the spectrum of a lamp using passive means, only to narrow it down. This can be done with either filtering or by reflection.

Usually it is done with filters. Filters sit in front of a light and just block out all the unwanted wavelengths, and let the rest pass. This is, of course, not very efficient. It’s better to choose a lamp which is already very close or exactly the desired spectrum rather than throw part of it away. Lampshades also throw light 'away’, but converting it into heat and blocking the actual light. While some light shades are important to keep from staring into the light source directly (which can be harmful for the eyes), too thick or large lampshades defeat the purpose of the light ficture in the first place.

The spectrum of light is affected by the surfaces it bounces from. If I have a blue wall, and direct a white lamp at it, I will get blue light reflected from the wall, filling my room. This is also the case with sunlight down a lightshaft or floor. If you want a room to feel warm, for instance, it helps to color one surface of the room, the one which receives the most direct light, a warm color, such as orange. This will have very strong effects on the perception of the space.

Links and Other Tutorials

We hope you have enjoyed reading this guide, and that it has proven useful to you. We have a bunch of other guides available, some of which concerning sustainability, others about 3D visualization and rendering, design and other aspects related to our work.

Except is an integrated sustainability consulting, research and design firm. We provide thorough, exciting and equitable solutions to help build the foundations of a sustainable future. Read more about us  here.


This guide has been entirely written by Except, but could not have been made without the contribution of various sources. Among which, hundreds of Wikipedia articles. Without Wikipedia, who would be able to do decent research? Please  support Wikipedia  if you have the means to do so.

Many of the images used have been contributed to the public domain by generous individuals, often also through Wikipedia. Wherever possible, sources are indicated underneath the images. If you feel an image has been wrongly attributed, not placed in according to its usage rights, or not correct, please  contact us  and we will correct it as soon as possible.

Tom Bosschaert
by Tom Bosschaert

21 mei 2012