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Wednesday, 2 November 2011

Available Light

This is the artificial light that is around us, indoors and outdoors, and it all affects the ambient light in a scene.

Tungsten Light

Heated filament normally in a bulb e.g. traditional; incandescent domestic household light-bulb, which gives an orange/yellow colour cast (think back to the colour temperature and white balance sections).

Fluorescent Light

Electricity causes reaction to a gas, creating certain elements in the gas to glow. The majority of lighting in public places is with this type of lighting; gives a greenish colour cast to cameras. (Again think back to the colour temperature section.
Notes: a shot in normal daylight with the Fluorescent setting on the camera gave a violet/reddish cast to the photograph. This is, in its most basic explanation, the camera ‘adding’ the opposite light colour on the colour wheel to the image, to counter the ambient cast (in this case fluorescent casts a greenish colour... so you’ve probably worked it out now the opposite colour on the wheel is... red!) Think back to the colour theory section.
So going back to the Tungsten giving off a orange cast… yes you guessed it the camera adds a blue/green cast to ‘balance it… Wait a minute…Balance oh yeah ‘White Balance’! Ok this is a simplistic explanation but It works for me!

Vapour discharge lamps

Sodium lamps. The type used for street lighting give that very strong yellow light that you see at night along roads. There are other types of vapour lamps that require other filters/colour temperature adjustments to balance the colour casts.
In the case of street lighting I have found that AWB setting still struggles to correct the yellow cast. I’m not sure why but there is a note in the text to say that it can’t be corrected, however I have found that manually setting a white balance to temperature of around 2000-2500K you can reduce the yellow cast; although at this extreme end of the scale causes other colours casts to show through; take the example below of a night street setting. That I took a few years ago.
10 sec
ISO 400
Light Photography
This first shot shows the extreme yellow cast from the lights; the AWB setting gave this a temperature of 3500K
Light Photography
The exact same shot with a custom white balance of 2000K,  Notice that their is still a yellow cast and with temperate so far up a lot of blue cast is left, so it’s still not a correct representation of the true colours. The central reservation still looks yellowish even though I know in daylight this to be grey.
Light Photography
In the last version I’ve resorted to removing the saturation of colours in Photoshop; I’ve removed all the yellow from the image and all the blue. In terms of colour this would be the most accurate to how you imaging this too look. The roads are grey, It’s night time so the sky is black/grey the trees are green and the break lights of vehicles are red with headlights of white! Interesting experiment in supplementing the white balance and and colour temperature sections.

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