The Human Eye, the Brain and .. Planck's Law (??)

For various reasons I couldn't take my camera outside during daylight today, so instead have been experimenting a little.

And, as you may have already guessed, in this photographic context we are in the exciting realm of COLOUR TEMPERATURE. The photo shows the effect of different colour temperature settings on the image produced by a camera.

If your camera is shooting on AUTO, it will make its own decision about the correct colour temperature. However, I have a SLR camera which enables me to set the colour temperature.

As I am curious about the true colour of the subjects of my photographs, in the WHITE BALANCE menu I have selected WHITE as the temperature adjustment which I to be applied. This enables me to see what colours my images really are, before my brain tries to persuade me that they are different.

This means that the colour of any photo taken under artificial light, as happened with this photo, will not be adjusted by the camera (this would involve reducing substantially its component red element and similarly increasing the blue) to look as if it were shot in a white environment.

On the left side of the photo you see the unadjusted colours, on the right the adjusted ones.


So, if you had taken this photograph and wanted to use it as a Christmas card, which version would you chose? The one emphasising its white elements, e g of the star - or the cosyness of the left?


For a little bit of fun, set your white balance to "incandescent", i e the light produced by a standard domestic light bulb, but make sure that you know how to change it back to your current setting. Then go outside to take some photos. You will find that the images are in shades of blue - the camera has compensated for the supposed red imbalance by cutting back the red and adding a lot of blue. Some spooky results, you may even like them! (Don't forget to reverse the setting.)


Planck's law and black bodies.

To start with an example from everyday life: you will probably have noticed that there are different colours of flames, depending on their closeness to the source of heat. The hottest area is white, the next hottest is yellow, and then comes red.

In photography, there is a wide variety of colour sources. The colour temperature scale used is at first odd, I will start by mystifying in hopes that all will become clearer. For instance:
- a candle flame is at 1,800 degrees Kelvin
- an incandescent light at 2,800 degrees Kelvin
- a halogen light at 3,000 degrees Kelvin
- white (a combination of equally bright components of the colours red, green and blue) at 5,000 degrees Kelvin

The colour temperatures are those at which a black body [defined below] would glow in the colours mentioned. (If for instance a black log is heated, it will change colour as it burns.)

So, the left section of the photo is at about 3,000 degrees, the right at 5,000 degrees.


The definition of a black body is as follows:

"an idealised physical body that absorbs all incident electromagnetic radiation regardless of frequency or angle of incidence".

Well, you had already worked that one out, hadn't you? If you hadn't, don't worry, Planck's law applies anyway.