The Colour of Light.

Generally we do not consider light to have a colour, mainly because the human eye is excellent at making allowances for differences in light colour. However, sometimes we do notice these differences. For example, on a very bright sunny day, when we enter a dimly lit building, other than the level of illumination, we notice the yellow/orange cast everything has, but normally by the time our eyes have become accustomed to the gloom, the colour cast has been accepted by our brain and the bright daylight entering through a window then appears almost blue compared to the ambient light.

Colour temperature

The scale used for expressing colour temperature was devised by a British scientist, Lord William Thompson Kelvin 1824-1907, who compared the visible appearance of a light source (candle, sky etc), with the emitted light of a "black body radiator".

Kelvin. Unit of measurement used to express colour temperature. Kelvin = (°C +273). Using absolute zero (-273°C) as the start point.

The colour of light is measured by its' wavelength in nanometres (nm), starting at the ultra violet (not visible) through violet 400nm, to red 700nm from where it becomes invisible once again at the infra red end of the colour spectrum. Daylight is around the blue area of the spectrum (455-492nm). Tungsten is around the yellow/orange area (700nm).

This is all fine, and as long as we are only using our eyes, we can ignore it! However as soon as a camera is involved in recording an image, it becomes very important. Cameras are not as clever as people, and rely on the people operating them to supply them with the correct information and settings so they can produce the wonderful pictures we envisage! Therefore we must ensure that we match any light we insert artificially into the picture with what is already available. Outdoors we are dealing with sunlight, whilst indoors we are invariably dealing with incandescent light, sometimes mixed with daylight entering through a window or door.

So What Lights Do I Need

So you've spent all your money on your camera, lens(es), tripod, and possibly even audio kit, and a suitable vehicle to transport you and your gear around, gone out and found work, and are hopefully starting to repay some of the above debt.The next thing you know, someone turns around and says you need to spend even more money on buying a large heavy box, which takes up even more room in the back of the estate/MPV/van!

What are the Options? Here are the pro's & con's;

Tungsten = Orange 3200K

Black body radiator- Tungsten filament surrounded by Halogen gas encapsulated is a quartz envelope heated until it glows, therefore the light emitted is a by- product of heat.

Advantages: - cheap heads & bulbs, instant light

Disadvantages: - Hot, fragile filaments, inefficient use of power, if balanced to daylight up to 50% of output lost, Loses colour temp as it's dimmed.

Daylight = Blue 5600K

AC current excites a mixture of gases, which produces light with heat as a by-product.

Advantages: - very efficient power to light ratio, daylight balanced, no filament to break, less heat than equivalent wattage tungsten fixture, depending on bulb/ballast can be dimmed by up to 50% with no loss in colour temp (in fact it tends to increase).

Disadvantages: - takes time on cold strike to reach full power output, requires ballast to work, Kits and bulbs expensive to buy.

Uses

Generally, freelance cameramen will buy tungsten kits because of cost; they could buy 2 or 3 three head kits for the cost of a 200w-daylight kit.

However, the News broadcasters crews generally have a daylight kit each, often as well as a tungsten kit.

When it comes to large OB's and location shooting, the advantages become even more apparent with the low power consumption compared to high output, especially with the added cost of generator hire.

It is rare (but not unheard of) to use daylight fixtures in studios.

Normally all studio luminaires will be controlled via dimmer racks from the lighting control desk in order to provide optimum flexibility.

Here are some comparison tables to give an idea of the output of different types of fixture (based on Current Catalogues):

Inverse square law. Light falls off at a rate determined by the reciprocal of the square of the distances, double the distance and you get a quarter of the light.

How much Light Do I Need?

With the ever increasing sensitivity of modern video cameras and indeed the emulsions available to the film cameraman, the amount of light required to get an image has reduced considerably over recent years. Redheads and blondes were de rigeur in a location lighting kit, to give tube cameras enough light to see. Whereas now, 300-650w fresnels are more commonly seen in kit bags. Conversely, one can argue that with a change in television toward wide format, 16:9 etc, then more light/s are required because a bigger area is seen on the screen.

In studios, a "standard" level of light would be around 800 lux, with a lens aperture of f4.5. This should give enough depth of field for most situations. Bear in mind that to reduce the aperture by a stop, would require double the amount of light (heat, power, ventilation, dazzle, etc,) to give the same level of exposure! There has recently been situations where sets lit for digital cameras had had to be re-lit for the talent, as the actors couldn't see props, prompts, and each other!

On location, the lens aperture is generally chosen to suit the conditions prevailing at the time. Any lighting then introduced must colour balanced for these conditions (remember that you will lose half the output of your tungsten fixture if you have to colour balance it with full blue). It should also be powerful enough to combat the stark contrast created by bright sunlight, to enable the camera to record a useful image. Remember that "on location" doesn't just mean outdoors, it's anywhere that you don't have total control of the lighting.

Outdoors

It's probably best to make the most of available light whenever possible. Utilize reflectors to bounce light onto the subject, remember that anything light coloured will reflect light; white will retain the colour temperature of the source, whilst golds and yellows will warm up the colour and silver tends to cool the colour down.

Battery lights or "Sun Guns", are great if you can afford them, but remember that the run time currently is always shorter than the recharge time so you may need two or more batteries for the kit.

Obviously, if you have access to a nearby building you may be able to use mains extension leads to power your lights.

Indoors

Here you're most likely to encounter lighting of mixed colour temperatures with a sometimes annoying addition, fluorescent lighting. This kind of lighting tends to emit a green light which can be neutralized with a magenta filter known as minus green!

One of the worst scenarios is a mixture of all three colours, daylight, tungsten, and fluorescent in one room.

Hints in this situation are:

• Can you get rid of one or more sources?
• Curtains, blinds & light switches should not be overlooked!

If you brought enough CTO with you, it is quite easy to stick it over the window and convert the daylight (wet the window with a sponge, cut the filter to size, and float it on!).

Wrap the fluorescent tubes in minus green, and re-insert them in the fixtures.

HMI.

Hydragyrum Medium arc Iodide. Generally double ended. Can be dimmed by 30% of output on a suitable ballast.

MSR.

Medium Sodium/Source Rare earth, Single ended with outer secondary envelope. Can be dimmed by up to 50% of output with a suitable ballast.

ARRI pioneered Daylight Luminaires as we know them today; the 1972 Munich Olympics were lit with Arrisonne heads using HQI lamps developed by Osram. This paved the way for the development of the Apollo Daylight fresnel range utilising the double-ended HMI bulbs. In 1990 the first of the Compact lampheads were produced, which were designed around the Phillips MSR single ended lamps, also resulting in a revamp of the re-named Arrisun range.

By Andy Barnett, ARRI GB.