If you have worked with film cameras where colour accuracy was important then you may well have used colour meters.
Figure 1 Unlike the more common exposure meters, which all modern cameras have built inside them, the colour meter is different as it measures the colour values, not the light intensity.
The most often used, and the more affordable, are those that compare the two ends of the colour spectrum which humans see, namely the Red and Blue prime colours. The eye can adjust to varying colour balance, but film can not Consequently film is supplied in preset colour balance levels, usually for daylight at 5000°K, and indoor tungsten light at 3200°K.
Figure 2 The meter reads the balance between the Red and Blue and gives a coloured filter value that will correct the light to match the type of film being used.
These meters work well with natural light but have trouble with artificial light, such as fluorescent tubes. To take account of this type of light a more expensive colour meter is needed that also measure the Green against the combined Red and Blue, ie. the Magenta.
NOTE: Some colour meters refer to Amber, instead of Red, when reading the red and blue balance. This is because the filters used to warm an image in photography are quite pale and so appear more pink than red. The filters used to cool an image are also quite pale but are still clearly blue in colour.
Natural light spectrum
Figure 3 The graphs shown here are idealised but they illustrate that under any natural lighting conditions there is a full spectrum. There are no gaps, meaning that in any image all colours are present to varying degrees.
Natural light is the visible light emitted from an object that has been heated up to a high enough temperature. This covers anything from candles, light bulbs, studio lamps all the way up to the sun it self. What they all have in common is that light is emitted throughout the entire visible spectrum, although not in equal amounts. Light bulbs emit mainly red light but will still emit all the other colours through to blue, just less of them. Likewise daylight from the sun will emit wavelengths from all the colours but in this case with more blue than red.
Figure 4 This balancing act between how much blue and red light emitted is pivoted around the green part of the spectrum.
It is why in photography much of the filtering used on camera lenses is based around the pink and blue filters to warm or cool the image. It is also why many of the standard colour meters will only measure the blues and reds to calculate colour balance.
The important point for correcting digital images in Photoshop is that if the image was lit with natural light then at any given point in that image there must be at least some measure of red, green and blue light. This has a major implication for correcting the RGB levels.
Artifical light spectrum
Figure 5 Un-natural light breaks at least one of the two key points of natural light, namely that natural light has a continuous spectrum and that the spectrum pivots around green in the centre.
Figure 6 Here we see the variations in un-natural light.
Un-natural light falls into two groups:
• Light with a non continuous spectrum (illustrated in the graphs above) just have spikes of light at certain wavelengths. A typical example is Sodium lamps in street lighting. It is essentially yellow orange light and no matter how you filter the camera virtually no part of the blue spectrum exists.
• Light that does have a full spectrum but has extra spikes at certain wavelengths, especially in the greens (illustrated in the graphs below). A typical example is fluorescent light used in many buildings. The problem here is the excess green which requires extra filtering.