Saturday, 17 September 2011

Colour rendering index

The colour rendering index is a percentage measure of the quality of a light source in terms of colour reproduction. For example, a sodium lamp has a colour rendering index of 0 because when you shine it onto an object you cannot reproduce its colours. It's just yellow, isn't it? Tungsten bulbs or daylight have a colour rendering index of 100 and serve as a reference to other light sources. More strictly the black body radiation is the reference but tungsten light is a good approximation (a more nerdy definition is here). Note that the colour rendering index is not identical to the colour temperature! The colour temperature defines the temperature of the radiator. So, roughly a light bulb with 3200K colour temperature has that temperature!
So, why don't we just use tungsten light? Because it's terribly inefficient. Most of it's output is in the invisible range which is basically heat (think of the 3200K). Anybody who has blown up fuses with red-heads or burnt him/herself knows what I mean.
What are the alternatives? There are quite lot. The only problem is that most other sources have CRI of <100%. For filming a CRI of 90% or higher is required.
The oldest alternative are the so called HMI lights which have roughly 90% CRI. These are gas discharge lights. In the old days these were really just the HMI lights but now you have a large range of discharge lamps with a CRI of well over 90%. They are often called metal halide lamps, for example, the Osram Powerstar D. See my post about HMI style lights. Lights of this type are often used in fashion shops because they need to reproduce colour faithfully.
Three phosphor fluorescent striplights also have a colour rendering index of over 90%. People associate them usually with Kino flos. However, many companies make them now -- and much cheaper. For example, Fotowerkstatt Mainz or Walimex. All fluorescent lights use basically the Osram Dulux 954 55W tube. The rest is just a box with barn doors.
On the left there are fluorescent lights from the Fotowerkstatt with the Osram Dulux tubes in it, mixed with HMIs to create a sunny daytime atmosphere. Here for the film BIG BOOTS. When you watch the film, the lights are on the right of the window which was covered with ND filters to reduce the incoming light by 2 stops. The room was basically lit by the fluorescent lights and the HMI while the ND reduced the incoming light so that the windows won't burn out.
The colour rendering index of both light sources is above 90%. The colour temperatures are actually slightly different but add a bit of texture to it.

Much worse are LED lights. They have often a colour rendering of less than 80%. This is because they have strong blue component which is not very flattering on faces. I've used LED lights in documentary making but I wouldn't use it in fiction, certainly not on faces. It is well known from theatre that blue light brings out blood vessles and makes faces look ugly. Actors will hate them.

cheap ND filters off ebay

For my GH-1 I bought recently cheap ND filters off ebay.
Then the shoot. It was a typical Scottish day: some times it was really overcast and sometimes pretty sunny so that I had to use sometimes the ND4, ND8 or even both on top of each other. These filters I've used have a lilac kind of colour tint. In order to correct in post I shot a white sheet of paper and then put the different filters on. Below is the result.

The ND4 filter adds especially more blue to the image whereas the ND8 adds both blue and red to the image but in slightly different proportions. Pretty annoying. The other option would have been to have 4 different custon WB settings in the camera which the GH-1 won't allow. Bottomline is that I'll tape a greycard to my clapperboard! Taking every time a white balance just takes too much time.