Lumens, Lux and Candela

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Lamps & Energy Index

• Introduction
• 1 - Lumens
• 2 - Lux
• 3 - Candela
• 4 - Colour Temperature
• 5 - Colour Rendering Index
• Conclusion
• References

There are basically three different ways to describe how much light a given light source provides.  Which one is used depends on how the measurement was taken, and whether it describes the light output or the amount of light available at the 'destination'.  Candelas and Lumens describe the light emitted from a source, and Lux describes the light level at a given distance from the light source - typically at a work or road surface, etc.

In addition, we may also refer to luminance and/or illuminance.  Luminance (L) is a measure of how bright a light source appears, and is typically measured in candelas per square metre (cd/m²).  Illuminance refers to the light that falls on a surface per unit area.  Luminous emittance is the luminous flux per unit area emitted by an illuminated surface.  Luminous emittance is also known as 'luminous exitance'.

The human eye has a huge range of light sensitivity, from starlight (about 1E-6 cd/m²) up to somewhat greater than direct sunlight (1E6 cd/m²), although at any particular time the range is less (around 1,000:1).  It takes time (20-30 minutes, age dependent) for our eyes to adapt to low light levels after being subjected to bright light.  It also takes up to 5 minutes for our eyes to adapt to bright light from darkness.  See the table below for some examples of light levels.

Figure 1 - Vision And Receptor Regimes (Osram Sylvania, 2000)

Photopic vision is the way we see when the area is well lit (daylight for example), with an luminance levels of more than 3cd/m2.  We have full colour vision in this range because the colour receptors in our eyes, the cones, (red, green and blue) are all activated.  At lower light levels, we are in the mesopic region, and vision is from a combination of cones and rods.  The rods are more sensitive than the cones, but only respond to light intensity on the grey scale, and they do not react to colours.  Colour vision is impaired in the mesopic region, so colours appear muted.

Finally, at very low light levels, we have scotopic vision.  This is based on rods alone being activated, so there is no colour differentiation.  Our eye-brain combination tells us that we are seeing in shades of grey, even though the rods respond most strongly to light within the blue-green range (centred on and around 498nm).  Rods are around 100 times more sensitive than cones.  Night vision is exclusively the regime of the rods, but it is possible to have red light for illumination and it won't cause a loss of night vision because the rods don't respond to red light.

The term 'lumens' (abbreviation lm) refers to the total light output from a source.  Up until fairly recently it was mainly used by lighting professionals, but the introduction of 'high efficiency' lighting has pushed the term into more common usage.  The lumen output gives the total luminous flux of a light source by multiplying the intensity (in candela) by the angular span over which the light is emitted.

Especially since LED lighting has become popular, it's common to refer to the luminous efficacy of lamps in lumens per watt (lm/W).  This allows more-or-less direct comparisons between different light sources, as shown in the following abridged table.  However, it does not refer to the quality of light emitted from the various sources listed.  The figures here are representative only, and in some cases significant variations may be seen, depending on the source of the information.

Where the power rating is indicated as 'n/a', this indicates that luminous efficacy is not affected significantly by the power rating.  Many lamps become more efficient as their power rating increases, with incandescent and CFLs being good examples.  While it is easy enough to imagine that this will be so with traditional lamps, it is a little more subtle with a CFL.  Essentially, the electronic circuitry has limited efficiency, and will consume some current just to operate.  For low power lamps, this basic operating current is a higher percentage of the overall current, so the effective efficiency of the assembly is reduced accordingly.

The amount of light that falls on a surface (workbench for example) is measured in lux (abbreviation lx).  When we use a light meter, we are measuring the light at a surface or at the sensor position if the probe is held in mid-air.  The reading is in lux ... or perhaps foot-candles if one absolutely insists on imperial measurements.  One foot-candle is approximately 10.7 lux.

One lux is the light obtained from a source of one lumen over an area of one square metre.  Light reduction from a source follows the inverse-square rule, so if the distance is doubled the amount of light per unit area is reduced by 4 (1/4), and the light that illuminated one square metre now has to illuminate four square metres.  The illuminance of the surface must decrease because the same amount of light covers four times the area.

These are the figures you'd read with a light meter, and are intended as a guide only.  It is quite normal to see very wide variations, even from a known and 'stable' source.  In most cases, just the position of the light meter probe can cause ±10% variation, especially if the measurement is taken close to a wall or other vertical surface.

When work with fine detail is done (precision work such as jewellery, needlework, colour matching) you need more light than for general purpose home lighting, reading, etc.  Any high-power light source should be entirely free from glare.  Any glare distracts from the task at hand and can easily cause things to become hard to see, even though there is plenty of light.  Intense point sources of light may also create deep shadows, and the high contrast between zones causes discomfort and often poor visibility overall.

Just as a double-check, I measured the light level outdoors in full sun in the morning (during spring in Australia), and read 75,000 lux.

Historically, the candela (abbreviation cd, formerly known as a candle) was defined as the light emitted by a plumber's candle of specified size and burn rate.  It now has a much more scientific definition.  One candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10^12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.  [2]   The candela as it is now defined was ratified and accepted as a worldwide standard in 1979.

Now we need to know what a steradian (sr) might be when it's at home.  A steradian is a dimensionless solid angle, and is defined as that solid angle subtended at the centre of a unit sphere by a unit area on its surface.  For a general sphere of radius r, any portion of its surface with area A = r² subtends one steradian.  Given a sphere of 2m diameter (1m radius), one steradian gives an area of 1 square metre at the surface of the sphere.

A sphere has a surface area of 4 π steradians.  Therefore, an omnidirectional light source that provides 1 candela (over 1 steradian) has a total light output of 4 π steradians, measured in lumens.  Such a light source has an output of 12.56 lumens (4 π steradians).

Candelas are used in lighting, but are not generally particularly useful.  Most of the time we want to know the total light output (in lumens) and the light available at the work or other surface (in lux).  If a lamp is fitted with a reflector, the output in candelas is increased by virtue of the 'wasted' light being captured and re-directed where it is needed.  However, the total output in lumens remains unchanged.  Light is simply being reflected, but no more of it is produced just because a reflector is added to the source.

With LED lighting, reflectors are of limited use because the light is already somewhat unidirectional, and LEDs are often used in clusters and/ or fitted with lenses to increase the spread of light.  Because of the unidirectional light emission, a comparatively low-power LED lamp can often appear to be much brighter than an equivalent incandescent or fluorescent lamp.  Some manufacturers take liberties with this, and may rate an LED lamp as 'equivalent' to (for example) a 40W GLS lamp without a reflector.  While the light falling on a surface directly in front of the LED may well be much the same as the incandescent lamp, there is no side or rear radiation that might otherwise be captured and re-directed by a reflector.

If this is done, and the LED compared with the incandescent lamp with both installed in the customer's luminaire (assuming a 'retro-fit' LED lamp), the LED will be found wanting.  When suppliers hoodwink buyers by this method of 'comparison', all they achieve is to give LED lamps a bad name.  All comparisons must be done in a manner that demonstrates the real usable light output when installed in a luminaire that's comparable to those used by typical customers for residential or office/ industrial applications.

A lamp's colour temperature is measured in Kelvin - 0 (zero) K is approximately -273°C - absolute zero.  For example, 3,000K means that the temperature of the emitter is 2,737°C.  Note that there is no such thing as 'degrees' Kelvin - it's just Kelvin (abbreviated to K).  You will often see the colour temperature of LED and CFL lamps referred to as correlated colour temperature (CCT).  True colour temperature refers to a so-called black-body radiator.  This is determined by a 'black' surface that's heated to the desired temperature, approximated quite closely by the filament of an incandescent lamp.

Figure 2 - Colour Temperature Of Various Light Sources

Discharge lamps (including fluorescent) are not black-body radiators, and they use gas or metallic mixtures and/or phosphors to create light of the desired 'colour'.  'White' LEDs use a royal blue light emitting diode and a colour-shift phosphor that absorbs much of the blue light and re-radiates it as green and red.  The CCT is determined by comparison - looking at (and/or electronically analysing) the light and comparing its apparent colour against known black-body radiators.  This is obviously complicated somewhat by the fact that no known metallic black body can ever be made hot enough to allow it to emit light with a colour temperature of more than ~3,300K (3,027°C) without melting or becoming too soft to hold its own weight.  Tungsten melts at 3,422°C - the highest melting temperature of any metallic element.  Of the other elements, only carbon is higher.  It doesn't actually melt though, it transitions from a solid to a gas at around 3,727°C.

The most natural light source of all, the sun, is difficult to define, because it changes due to cloud cover and with the seasons.  The colour of the light we see is determined by how much of the earth's atmosphere it has to pass through.  Pollution, clouds, time of day and many other factors influence the apparent colour because air and particulate matter act as filters that can make the light 'warmer' (closer to red) or 'colder' (closer to blue) and anything in between.

The ability of any given light source to cause colours to be properly shown (rendered) is called the 'colour rendering index' (abbreviated to CRI).  Lighting with a low CRI shows colours very differently from how they would be seen under ideal conditions - natural daylight.  Most incandescent lamps have the highest possible CRI, with a value of 100.  The CRI of fluorescent and LED lighting is variable.  Most manage 80 or more, but it's uncommon for LEDs, fluorescent or other lamps that rely on phosphors to exceed a CRI of 90 or so.  Anything below 85% distorts colours.

Low pressure sodium lights (LPS/SOX) have a CRI of 0 (it's sometimes claimed to be negative), because the light is monochromatic - of one colour.  It is not possible to determine the colour of any object with any accuracy if the only light source is LPS.  High pressure sodium (HPS/SON) is slightly better, but with a CRI of around 25 it's not usable for any purpose where colours need to be reproduced accurately.

Metal halide lamps are one of the best, ranging from 85 to 96, and tri-phosphor fluorescent can get close to 90.  LEDs are improving, and some of the best LED light sources can achieve a CRI of up to 98.  Achieving such high values may require the use of red (and sometimes green) LEDs in the same lamp or housing as the 'white' LED 3].  While having a high CRI is often considered a requirement, there aren't many applications where it's overly important, although 'cool white' light sources are not flattering to skin tones.  One place where a high CRI is essential is in electronics - you can't read a resistor's colour code if the CRI is poor, because the colour bands won't be reproduced properly.

A high CRI is also desirable for colour matching, food presentation, point-of-sale, photography, cinema and video recording.  Modern digital cameras can all (usually automatically) adjust their 'white balance' to account for colour temperature, but if the CRI is poor all colours will be shifted and objects (including people) will not appear as they should.  This is also done deliberately - an excess of red makes butcher's produce look better, excess green (if done properly of course) will make green vegetables look ... green. 

There are many different ways to illuminate a surface or a room ... incandescent lamps, fluorescent tubes, LEDs, tungsten-halogen bulbs, compact fluorescent lamps (CFLs), electroluminescent panels, mantle lamps (gas, kerosene aka paraffin) or even candles.  The choice of which to use may be limited by the amount of energy available, its cost, or even government legislation ('ban the bulb').  In addition, there are personal choices as well, as each light source has characteristics that make it better suited to some tasks than others.

This short article describes the basics only.  There is a vast amount of additional information available on the Net, and some of it is even useful .  This article is intended only as an introduction, but it should satisfy the curiosity of many people, who don't need to make detailed calculations but just want to understand the terms used.  I encourage anyone who is interested to do their own research, as the amount of info available is prodigious (if somewhat daunting at first).

A simplified way to remember the differences between the three terms is ...

Lumens - how much light is produced
Lux - how well illuminated your surface will be
Candela - visible intensity of the light source from one direction

1. Base unit definitions: Candela - Physical Measurement Laboratory
2. "Brilliant Mix" LED lighting - Osram

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