Autor: Dennis Jackstien

Light technology: Basics and characteristic values (2 / 10) - Colourimetry

About the qualitative properties of light.

Each light source is defined not only by its luminous intensity or brightness, but also by its colour and the composition of the light spectrum in general. 

The  use of LED luminaires in film & TV has brought the subject of light spectrum and colour quality into greater focus. Luminaires that formally have the same colour and colour temperature specifications can visibly produce different results.

The light spectrum

The basis for everything we will discuss below is the light spectrum. What do we mean by the light spectrum. In short, light is a kind of radiation, arranged between UV radiation and infrared radiation (simply: heat). Unlike other forms of radiation, we can see the wavelengths of light radiation with our eyes.

Each light source consists of different wavelengths. From short-wave blue to turquoise, green, yellow, orange and long-wave red - just like a rainbow.

Each light source has its own individual composition of spectrum. 2 examples:

Images 1 and 2 by Dennis Jackstien

The spectrum of the incandescent lamp shows significantly more red than blue components. Therefore, the incandescent light is perceived as redder / warmer. The HMI daylight lamp has more blue spectral components and is therefore perceived as cooler.

Of course, white light can be described much more precisely than only with warm, cold or neutral white.

Light colour and colour coordinates

Colour coordinate systems are used for the exact determination of light colours (and all colours in general). The best known is the CIE1931 standard colour system shown below:

Image 3: CIE 1931 standard colour system

Here a very specific x,y value is assigned to each colour and also to the white tones. For example, a typical daylight D65 has the coordinates x=0.31 | y=0.33. The different white tones from warm to cold white are arranged in the graphic on a black curve - so-called Planck's curve. Each white tone is assigned a specific colour temperature.

Colour temperature

Colour temperature - or "CT" for short - is a very simple and widely used method of describing white light types. The unit of colour temperature is °Kelvin or short K. A typical warm incandescent light has a colour temperature of 2,700 - 3,200K, while the bluish daylight is 5,000 - 6,500K. As can be seen in the graph, there are of course much warmer colour temperatures (candlelight with approx. 1,800K) and of course cooler ones (like the blue skylight with 10,000 - 20,000K). The cooler, bluer a white is, the higher is the °Kelvin value.

Tolerances & "similar" colour temperature

An incandescent light (e.g. a halogen lamp) will always be exactly coloured on the Planck curve (black arc in the graphic above), since the curve is based on the technical principles of an incandescent lamp (keyword: black spotlight). This is different for discharge lamps, fluorescent tubes and LEDs. For technical reasons, these light sources often lie more or less clearly next to the Planck curve and therefore have a more or less disturbing green or pink tint. Nevertheless, they are described as "3,000K" on the packaging and in the data sheet.

The manufacturer only specifies the so-called "most similar colour temperature" - Correlated Colourtemperature or CCT for short. Simply check which colour temperature comes closest to the light source (indicated by the long, black straight lines on Planck's curve), then round it up or down a little and the result is "3,000K CCT", which can, however, be very different from an incandescent light of 3,000K.

It is therefore important to note that a colour temperature - almost always indicated as "°C CCT" - is only a rough approximation and never a precise indication of a light colour.

Information on colour distances

When do we ever perceive 2 slightly different light colours as "the same"? This varies slightly from person to person, but in general colour temperature differences of up to 200 °Kelvin are not perceived with cold white - with warm white the limit is already 50K. I.e. at 5,500K to 5,600K most people do not see any difference, but at 2,900K to 3,000K they do.

The CIE1931 colour coordinate system is not equally spaced. Therefore, the x,y values and colour temperature data are unfortunately hardly suitable for evaluating whether 2 lamps have the same colour effect or not. Here there are alternative systems for the evaluation.

The most widespread method is MacAdam (so-called MacAdam ellipses), which is communicated nowadays by the specification SDMC (Standard Deviation of Colour Matching). For example, an LED manufacturer could offer its products with a tolerance of SDMC<1. SDMC<1 means that practically no differences are perceptible. SDMC<2 results in small differences and SDMC<3 is often considered acceptable. Anything above this would at most interpret colour-blind as the same colour.

Measurement of light colours

Normal light meters measure only the amount of light, not the light colour. In the past, simple colour temperature meters such as the MINOLTA 3F were very common for colour measurements. They worked with 3-4 photo sensors that were matched to different colours. From this, x,y values, as well as colour temperature and, if necessary, distances to the Planck curve could be determined and displayed.

Especially with LED light sources, these simple colour temperature measuring instruments are very imprecise, which is why today almost exclusively small, mobile spectrometers are used. One example is the SEKONIC C800:

This measures the entire light spectrum and - unlike simple colour temperature measuring devices - can therefore also provide information on colour rendering and colour quality. Both topics are dealt with in Part 3 of the Fundamentals.

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