A matter of degree Kelvin

A matter of degree Kelvin

There’s a reason that colour in pictures is measured in degrees, but I wonder if you know what it is? Or who Kelvin was that anything’s named after him… This is by way of a lead-in to a routine sort of blog about colour temperature: and the lead image is from my session with AshleyAshton222 yesterday: green curtains, and a lot of trees outside the window mean that AWB might actually have worked well, for once.

Anyway, Kelvin. William Thomson, 1st Baron Kelvin was born in 1824, and was a physicist, particularly interested in thermodynamics, the Third Law of which states that you can never reach absolute zero, the lowest temperature which is possible in the Universe. It’s around 273 degrees below zsero Centigrade, and a temperature scale starting from absolute zero seemed like a good idea: the degrees are the same as the Celsius scale, but start from the bottom. And they’re named after Kelvin.



The missing step is that colour can be defined in terms of the temperature that a perfectly black body needs to be to glow that colour – in practice a piece of iron will do nicely. So red hot, the sort of colour old-fashioned tungsten lamps give, is around 2,000ᵒ Kelvin, and daylight is 6,500ᵒ. This doesn’t take account of the green-magenta dimension that wrecked my portrait of Ashley, but you’ll see it in every editing suite you come across.

What’s this got to do with AWB, you may be asking… Well, it’s the background against which AWB exists, and the idea of AWB is to get accurate colour in any light, although in practice it usually falls short. It seems like a reasonable idea when, for instance, you’re shooting a white plate and want it to look the same in sunlight, lamplight, or shade.



In reality, you don’t necessarily want that sort of fidelity, I’d suggest. For instance, take that sunset view over the lake, with the rosy red tint to the white clapperboard of the boathouse… Do you want that boathouse to be white in your picture, or a delicate pink? Similarly, on a chilly winter’s day, a slight blue tinge conveys the mood of the moment. My solution is to use Daylight balance all the time, unless I see a good and colourful reason to do otherwise…

I know AWB serves many people well – but if subtlety of colouring matters to you, you probably aren’t one of them… And of course you can always adjust things in editing, especially if you shoot RAW files.