We are discussing how the brain manipulates images before you “see” them in your head.  Or amends them.  Adds or subtracts from them.  Or just plain tries to fool you.

And it does some wonderful tricks with colour.

Colour casts
No doubt over the years you will have seen photos where people have been indoors and the photo has a greenish cast over it.  This is due to an effect known as colour temperature.

Without getting too technical, light is simply radiation emitted from a hot surface.  At certain temperatures you can’t see this light.  e.g. a lump of coal can be too hot to touch without having to glow red.   To start with, the coal glows black.  Infra-red, invisible to the human eye.  Other animals can see in infra red and hence they have “night vision” as they are seeing dull heat.

As the surface gets hotter, the colour of the radiation changes, from a dull red, to red, through orange, and yellow, then green, blue, ending up with violet coloured light.   And then the light gets hotter still and passes out of the eye’s visible range again.  Ultra-violet.

Now, I hear you saying, “But I can see Ultra Violet, it makes my white’s glow blue in discos.”

No, you can’t see Ultra Violet.  What you see is the ultra violet short waves of light ‘exciting’ the special white coloured particles in your white clothing and they start to give off very bright purple light, almost white light, which your eye can see.

Anyway I digress.  What is important to know that different chemical elements give off different coloured light, and this light is measured in Degrees Kelvins (or ‘K).  And light sources have different chemical elements in them.

When you get REALLY technical, you will start to look at the colour temperature settings in your camera.  But for now that’s beyond us.

Let me give you some examples which might make better sense.

The sun, mainly hydrogen, gives off what we consider to be White Light – Or Daylight. But actually white light is a mixture of all the different colours of light that we can see.  As the light passes through the atmosphere, particles in the air scatter light rays in all directions – hence why the sky appears blue as more blue light is scattered than red.  But late in the evening or early in the morning, due to atmospheric changes and the position of the sun, more red light can be scattered than blue (or the blue has been scattered out of view) giving you the beautiful orange sunrises and sunsets:

An old fashioned street lamp glows deep orange, because Sodium in the light glows gives off orange light.   Sodium lights works so well because they have a very long “wave” of light, and thus travel great distances. letting them cut through fog etc.  And the eye can see this colour quite well.

But take a picture at night illuminated just by street lamps, and you will see that characteristic orange cast:

And yet if you look outside now, your street won’t look bright orange – more of a black and whitish colour, with a slight orange tinge perhaps.

Tungsten lighting, which accounts for many old fashioned filament light bulbs, give off a definite orange glow, but more yellow than the sodium lamps:
Again, you’ve probably seen a few photos with this orangy glow across people’s faces,
but like Sodium lights, it never quite appears THAT obvious to your own eyes?

You might ask about LED’s, and you will no doubt be aware that they come in different colours – like christmas lights for example – warm white, cool white.   LED’s can be made to change colour depending on how hot they are made to glow, and what materials they are made of, and also what colour the outer packaging shell is.  Hence it won’t surprise you to find you can buy photographic video LED lights which are perfectly matched to daylight temperatures, and your camera should need no adjustment for these.

My final example is fluorescent lighting, which work by electrons in the lamp making the inner coating of the glass glow.    Nowadays you can get different coatings which give off different temperature light – known for instance as “Daylight Tubes” because the light is closer to the temperature of natural daylight.   But the original older tubes glowed slightly green, and this is picked up by photographs as a green tinge to everything:

But yet again, our eyes don’t notice it when we are under fluorescent lights.   Or do they?

Here is a perfect example of how the brain is processing the image just like you might in photoshop.    The brain has past experience of lighting conditions, and knows what colours should look like.  So, it subtracts the appropriate colour casts from the image to correct the colour back to daylight as best it can.

Think about a red rose.  Isn’t it more or less the same red colour no matter what type of lighting you view it in?  Your brain knows what is SHOULD look like and will do it’s best to adjust the image colour back to normal.

This is vitally important to know, because the image you get on your camera may bear no resemblance to what you are seeing with your eyes.

In another post, I’ll be explaining more about colour temperature, white balance and how to ensure you set you camera correctly for the conditions.

To be continued.