The last article in this series looked at filters. This article covers some of the same ground, but concentrates on the light being filtered rather than on the filters themselves.
What is White?
Imagine you are outside on a cold winter's night. The snow is white all about you and, in the distance is a house where you know it will be warm. The light spilling out of the windows is a warm yellow. Now imagine you are inside the building. The room lights are switched on. It doesn't seem to be yellow inside the house but, as you look out through the window, the snow looks blue.
Does this sound confusing? Why did the snow look white when you were outside, but blue when you were inside, and why did the colour of the room light look white when you were inside, but yellow when you were outside? The answer is that, despite what you may think, there is no such thing as white! The human eye, backed by a rather large computer called the brain has a habit of adjusting things so that what we expect to be white looks white. It happens completely automatically. It's only when you stand outside looking into a room with artificial light that the effects become apparent. Artificial room light is yellower than daylight, and when you were outside you could see it as yellow. However, when you came indoors your brain adjusted things it made the room light seem white. Blue and yellow are what are called complementary colours - sort of opposites. You make things less yellow by making them bluer. This is what your brain did. The result was that when you looked through the window at the snow, it seemed blue.
Unfortunately, colour film can't do this. It can't shift the colours to make what you see as white come out as white on the film. You have to make the adjustments yourself.
Colour Temperature
Every 10 year old knows that if you want to describe something as being really hot, you call it "red hot". There is a germ of truth in this. If you took a lump of iron and heated it up, it would eventually glow red. If you carried on heating it it would go white and, at very high temperatures, blue. You could specify a colour in this range, from red through white to blue, by quoting the temperature the lump of iron had to reach to produce that colour. This temperature you could call the "Colour Temperature". Now, when physicists want to talk about colour temperature they don't use a random lump of iron; they need to be much more precise. The principle, however, is the same.
Some colours can't described in this way. Now matter how long you heated a lump of iron it would never go green! It turns out, though, that most light sources consist of things which light up because they are hot. The main exception is fluorescent light. I will talk about that later.
The standard temperature scale, used for colour temperature, is the Kelvin scale (°K).
Now, while colour temperature measured in this way is common, it is not the easiest way of working. There is a alternative scale called the "Mired" scale which turns out to be more useful. I'll explain why when I describe the use of colour correction filters.
The following table gives the colour temperature in degrees Kelvin, of some common light sources.
| °K | Mired | |
| Shadow under a Clear Blue Sky | 12,000 | 83 |
| Overcast Sky | 7,000 | 142 |
| Standard Daylight | 5,500 | 182 |
| Photoflood | 3,400 | 182 |
| Tungsten-Halogen Bulb | 3,200 | 182 |
| 100 watt domestic bulb | 2,900 | 182 |
It is also useful to think of the way the colour changes during the day...
| °K | Mired | |
| Sunrise | 3,100 | 322 |
| 1 hour afterSunrise | 3,500 | 286 |
| 2 hours after Sunrise | 3,900 | 256 |
| Midday (Sun in a Blue Sky) | 5,500 | 182 |
| 2 hours before Sunset | 4,000 | 250 |
| 1 hour before Sunset | 3,600 | 278 |
| Sunset | 3,100 | 322 |
Films with Different Colour Balances
To match these different colours of light there are different types of film. Films, that is, which will reproduce a white subject as white under different lighting conditions. There are three types of film which you may come across:
- daylight (5,500°K)
- type A (3,400°K photoflood)
- type B (3,200°K tungsten)
Daylight film is the normal film which you use most of the time. It is designed to give good colour rendition under average lighting at midday. At dawn and dusk the colours will seem distinctly yellow.
Type A films are designed to work with photoflood studio lights. These are the studio lights most likely to be used by amateur photographers.
Type B films are balanced for tungsten light. The most common source of tungsten light, for the amateur, is a slide projector. All modern slide projectors use tungsten-halogen bulbs which give light with a temperature of 3,200°K. (This means that if you are doing table top photography and don't want to spend a lot of hard earned cash on new lighting equipment, use your slide projector.)
Filters for Colour Balancing
It is not always convenient to change film as the colour of the light changes. Furthermore, you may find that you have light to cope with for which there is no corresponding film. You need some way of adjusting the colour. There is a whole range of filters designed for exactly this purpose. If you used daylight film with tungsten light, the result would be too yellow so, use a blue filter to take out the excess yellow. These filters are known as "Wratten" filters (the name "Wratten" is actually owned by Kodak. Each filter in the range will shift the colour by a fixed number of mireds (that's why mireds are useful). The filters you are most likely to come across are:
| Filter | Mired shift |
Use |
| 85B | 130 | Type B Film with Daylight |
| 80A | -130 | Daylight Film with Tungsten Light |
| 80B | -110 | Daylight Film with Photofloods |
| 81A | 20 | Type B Film with Photofloods |
Incidentally, the 81A is also useful as a kind of "strong skylight" filter for taking the blueness out of an overcast sky when using daylight film. If it is actually raining, try an 81C (35 mireds) or even 81D (45 mireds).
Flash
All electronic flash guns, whether built into expensive studio units or designed to be thrown away with disposable cameras, are balanced for daylight. The flash will overpower any other light sources, so use daylight film.
Studio Lights
Most modern studio lights use flash. There is often a tungsten modelling light to allow you to see the effect you are getting. This modelling light takes no part in the final photograph. You can ignore its colour.
Flourescent Light
This is the big problem!!
Fluorescent light does not fit into the standard idea of colour temperatures. What is more, with some tubes, it may not be possible to get a good filtration!
Fluorescent lamps produce a green cast, so you need a magenta filter to compensate. What is more, the filtration will depend on the colour of the tube; White, Warm White, Cool White etc.
You can buy filters which claim to enable you to use daylight film with fluorescent tubes. These do a good middle-of-the-road job. They are fine for capturing candid, gritty, photo-journalistic pictures. For other subjects, do all you can to find a different light source.
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