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PART 2: WHITE BALANCE
Lightbulbs, flourecsent tubes and the sun - all of us see these sources of light as white light . But our eyes are deceiving us they all vary considerably in their colour. Our brains are very good at performing a white balance without us really noticing. However the camera has to be told what type of light it is working in - so that it can correctly reproduce what our eyes see.
To understand the need to white balance you first have to understand colour temperature...
| If I put a
poker in a fire it will start to glow "red hot" - if I put it
in a furnace and really heat it up it will look as if it is "white
hot". As the temperature of the poker rises the colour of light it
emits changes. Red hot is pretty hot while white hot is very hot. There
is a definite connection between the temperature of the metal and the colour
of light it emits. This colour temperature connection is a way of scientifically
quantifying the colour of the light source.
Ok now think about a light bulb - inside is a filament (made of tungsten metal). When you switch on the electricity, a large current flows through a small filament, it gets hot and starts to glow. (ie It behaves like our poker). The colour temperature of tunsten light usually lies at around 3200 Kelvin. It is actually a reddish orange light.
The sun is another main source of light. As you'd expect it is hotter than a light bulb! The temperature on the surfaceof the sun is 5800°K, while inside and especially at its core, it reaches millions of degrees. However, the light that it produces must firstly pass through its atmosphere and then through the atmosphere of the earth before it reaches earth's surface.
So daylight consists of a mixture of the light that comes directly from the Sun with the indirect light that comes from diffusion and reflection, caused by earth's atmosphere and clouds. This makes colour temperature on the ground pretty variable from as low as 2000 Kelvin up to 20,000 Kelvin. The table below shows the range of temperature possible for daylight and the conditions that effect it.
So different sources of light are different colours. Why is this important?
Well we have to tell the camera what colour of light it is working in so that the picture it records looks something similar to what our eyes see. To do this we need to do a white balance.
When the camera does a white balance - it analyses the spectrum of colours hitting a white piece of paper. It juggles these until the white looks white.
In areas with a single source of light this is pretty simple. When we have mixed sources of light we can have problems. A camera white balanced in tungsten light (3200 Kelvin) will give my complexion a blue tint if I am then lit by the sun through the window. Alternatively a camera white balanced in daylight on an average day (around 5600 Kelvin) will make me look like a red lobster if you switch on any tungsten lamps.
Well maybe looking a little sun burnt might make me look healthy but wiTh blue skin I'll look deathly. Getting skin tone to look accurate on screen is always important.
To over come the problems of mixed light a colour correction gel is used to alter the Kelvin output of the different light source. It is usual (because it is often quicker) to try and alter tunsten lights to match daylight. A blue gel over a 3200 Kelvin light source will ensure it matches sunlight at 5600 degree Kelvin. But, it is also possible to use an orange gel over windows to alter sunlights to match indoor tunsten light - you just need more time and plenty of gaffa tape.
To make white objects appear white to the camera we do a WHITE BALANCE. To do a manual white balance the camera must be shown something white (usually a piece of paper) lit by the light source you will be working in. Beware of white balancing in the wrong light source e.g. by a window and then shooting the interview in a corner of the room lit by tungsten light. Whatever light falls on your subject should also fall on the white paper you use to perform a white balance.
WHEN SHOULD YOU PERFORM A MANUAL WHITE BALANCE?
Before you do a manual white balance you must select the correct filter. There are four filter wheel positions to chose from…
WHEN SHOULD I DO A BLACK BALANCE?