In 1679 Isaac Newton was sitting under an apple tree in Cambridge when a beautiful young undergraduate fell on him. This led him to discover the law of depravity.
A little later, while recovering from the resulting couch injury, he was contemplating into a glass of gin with light streaming-in from the window, split by a prism into a rainbow on his Principia. It was then that he realised that light is composed of the colours of the rainbow, now remembered by school kids with the mnemonic ROYGBIV.
But how much of each colour do you need to make white light? Is it
this much red, or
this much? Is white light purely (pun detector…ping!) arbitrary: who’s to say what mix of each colour you need to get white?
An apparently simple question reveals a rich topic, and by now you may be thinking why should I care, and what’s this got to do with photography, you rambling nong.
You might care because you’re into photography, which is all about light (surprising, eh). And your camera cares deeply because it’s racking its little electronic brain for you, trying to adjust itself to the “natural” white you’re in.
Under fluorescent light, a white piece of paper will look different than in sunlight, and different again under tungsten bulbs. Somehow the AWB (Auto White Balance) feature sticks a metaphorical wet finger into the air and works it out. (anybody have a simple explanation?)
My camera (Canon 350D) seems to get it right most of the time, it’s only about as smart as a I am before my morning coffee, so it can get it wrong.
That’s why the nice camera people allow us to override the auto setting. Mine has Auto, plus 6 others, and one custom. Have a decko at the images, which show one of each (silly me forgot to write them down, so I can't tell you which is which).
Okay, so what’s white? Apparently there’s a fancy theory that all bodies emit light according to their temperature. At 1,000 degrees, it different to 2,000 degrees – the mix of colours varies as you go.
And just to make it really confusing, they call it “black body radiation”. Well I think that’s way off, so I’m renaming it “white body radiation”.
If you want to know more, have a gander at the
Wiki.
PS – in spite of all this, I rarely touch the white balance on my camera
