The Dark Art of Light Measurement

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Pro-Lite’s Robert Yeo throws light on the science of measuring light and light sources: radiometry and photometry

When I joined the world of photonics as a newly qualified optical physicist, it didn’t take me long to realise just how much I didn’t know, my education seemingly having missed out on the commercially important area of light and lighting and how to measure and express light outputs. Which is strange when you consider that we encounter light, either naturally occurring or artificial, almost every second we are awake.

So while I was taught how a laser works and that the output ‘power’ of a laser is expressed in Watts (capital ‘W’ as the unit derives from James Watt, the Scottish inventor of the term ‘horse power’, an actual person), I had no idea what a lumen was, nor a candela or why Americans like to define the ‘brightness’ of a TV in nits.

Which introduced me to the science of radiometry and photometry, or as some would describe them, the ‘dark art of light measurement’, presumably because the science is so poorly understood. I should declare an interest. My company, Pro-Lite Group, specialises in the supply of light measurement equipment of all kinds (photometers, radiometers, colorimeters, integrating spheres, laser power meters, goniophotometers and spectrometers) and we have also developed a training programme on ‘Practical Photometry’ which fills a gap in the market for those who like me missed out on the delights of radiometry and photometry in their formal education.

The Language of Light

One of the most confusing aspects to the science of light measurement is the choice of words used to describe the output of a light source and how they overlap with non-technical language. For example, a non-specialist may refer to the ‘brightness’ or ‘intensity’ of light without realising that the two are quite distinct technical terms used to define the amount of light emanating from a light source. Moreover, even trained physicists will often refer to the ‘power’ of a lamp without realising that this has no meaning in either radiometry or photometry, power being the electrical energy consumed rather that the amount of light emitted (which is called radiant or luminous flux, by the way). Moreover, confusing luminance with illuminance when purchasing your photometer would be an expensive mistake. The terminology used in light measurement can be extremely confusing, so allow me to present to you my quick guide to the parameters and units used to express amounts of light in radiometry or photometry.

Before I do that, let me first explain the differences between radiometry and photometry. Radiometry is what you do if you care about the absolute amount of light irrespective of whether a human will see it, an example being a laser used in a materials processing application. Conversely, if the light will be used to illuminate a room, or emanates from an LCD display (in both cases, involving human vision), then you will perform photometry. Photometry is the science of measuring the amount of light as a human being would perceive it. The human vision system has a characteristic spectral sensitivity, meaning that we don’t see all colours (or wavelengths) equally. We can only see visible light (if that wasn’t already obvious!), which is defined as light having wavelengths between 380 and 780nm (from blue to red). A blue lamp would appear less bright than a green lamp having the exact same (radiometric) output. Similarly, a red lamp would appear dimmer than a green lamp having the same (radiometric) output. Put simply, human vision perceives green light (specifically green/yellow light at 555nm) as being brighter than any other colour, a property known as the photopic response of the eye, and mathematically defined in the 1931 CIE standard luminous efficiency function for photopic vision, V(λ). By evolutionary coincidence, the wavelength at which sunlight is strongest on the surface of earth also happens to be at 555nm.

For a light measurement instrument to meaningfully report the performance of a light source or display, it must be designed to match the spectral sensitivity of the human vision system, in other words it must possess a photopic spectral response. Hence the generic term ‘photometer’ for the light measuring device.

So back to my quick guide to radiometry and photometry and the myriad of confusing units used to express the output of various sources of light. First, let’s consider the total amount of light that a lamp emits. This is referred to as either the radiant flux (in radiometry) or the luminous flux (in photometry), with units of Watts (W) or lumens (lm), respectively (lumen with a lower-case ‘l’ because this is a made-up unit and Dr Lumen never existed). Today, EU regulations demand that lamps (not light bulbs please, bulbs are what you plant in the ground!) are sold with lumen specifications. Better that you know the number of lumens produced than electrical Watts consumed is the rationale, although the tabloid newspapers still believe that the lumen is an evil invention of the EU who hate the Watt because it is British.

So if lumens define the total light given out by a lamp, what proportion of that total is received onto a surface per unit area? In photometry, this is called the illuminance and is measured in lux (lx), where one lux equals one lumen per square meter. In radiometry, we refer to irradiance, which is expressed in units of Watts per square meter (W/m2). 500 lux of illuminance is considered adequate in a typical work environment.

What of the proportion of total flux that is shining in a defined direction from the light source? That is called the radiant intensity (in radiometry) or luminous intensity (in photometry), being the flux emitted from the (theoretical point) source per unit solid angle. The unit of solid angle is called the steradian (sr), so radiant intensity is expressed in units of Watts per steradian (W/sr), while luminous intensity is given in units of lumens per steradian (lm/sr), which is reduced to the candela (cd) for convenience. Think of intensity as being the jet of water from a hose pipe. The narrower the jet, the greater the intensity. A concept which can be put to good use in powerful torches. While a torch ‘with the power of a million candles’ may sound amazing, it only requires that you focus the beam from the lamp into a small cone angle and, voilà, you have a torch that achieves a peak luminous intensity of 106cd.

The fourth (and final!) parameter defines the amount of light that is emitted (or reflected/transmitted) from an extended surface per unit area per unit solid angle. This is the case of a display device (e.g. flat panel display) and is commonly referred to as ‘brightness’, although more properly we use the term radiance (in radiometry) or luminance (in photometry). The units used are Watts per steradian per square meter (W/sr.m2) and lumens per steradian per square meter (lm/sr.m2), respectively. We can reduce luminance to candelas per square meter (cd/m2) as we know from the definition of intensity that a lumen per steradian is a candela.

So there you have it, your quick guide to radiometry and photometry. And the nits that I mentioned earlier? The nit is a unit of luminance, the name deriving from the Latin word ‘niteo’ meaning ‘to shine’. The conversion between nits and cd/m2 being very complex: 1cd/m2 = 1 nit.

Robert Yeo is a co-founder and director of the Pro-Lite Group of companies which specialises in the supply of light measurement equipment and which runs training in photometry and light metrology.

Related links

Pro-Lite website

Pro-Lite light measurement equipment 

Pro-Lite training

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Robert Yeo