Part Three - Colour Temperature

Two questions for you now.

1. What colour is moon light?

2. In a movie scene in Los Angeles at night why does it always appear to have been raining?

I will tell you later in another post. Hopefully you can work it out from the information here.

This is probably a good point to discuss the inherent colours in lighting in more detail. As previously intimated different types of lamp emit inherently different colours of light when used for photographic reasons, and mixing the two is not a good idea unless you are intentionally doing it for an effect. Each light can be colour corrected to the other spectrum, so daylight balanced lamp (blue) can be covered with specific orange filter that has a precise transmission (this will be a full CTO gel) and it will be extremely close to a tungsten balanced source.

Similarly a tungsten balanced source (warm or orange in nature) can be corrected to replicate daylight by putting a full CTB (blue) gel filter over the front of the lamp. Be careful as CTB and CTO are very precise and specific transmissions to correct one colour temperature to another, and not any orange or blue filter will do this! They also come in 1/2, 1/4 and 1/8 densities incidentally.

This is common practice during any film or video shoot, and sheets of CTO filter have been used over windows so that it can be matched to the tungsten lighting inside. Often nowadays this is left to be a blue window light if it is drama as the viewer becomes more and more conversant with the convention of daylight being bluer.

CTO = Colour Temperature Orange

CTB = Colour Temperature Blue. I hope you can see why?

Measuring colour temperature

Often an experienced cinematographer can walk into a room and instantly see the mixture of colour in the lighting in that room. It may be imperceivable to many others, as the 'Mark One' eyeball kicks in to action again and is fooled into thinking everything as neutral colour, no matter what colour light is actually hitting it!

A cinematographer is more tuned into this however, and can establish the available light and then work out the best way to utilise the light in the room and what colour temperature is the predominant colour. Clearly if it is a massive office or supermarket we would not be expected to turn off all the lights and light the scene with our own lights, so we have to balance the prevailing light with the light we are going to put on a subject. In a supermarket it will normally be lit by fluorescent lighting, often daylight balanced. In a professional kitchen environment it is often much bluer light, as this normally equates to being 'clean' in our perception. Take a look next time you are in a restaurant if you can!

Lighting colours have a numeric value expressed as degrees Kelvin. 

For example tungsten lighting is classed as having a colour temperature value of 3200K or perhaps 3400K, and this is a warm orange colour. A daylight balanced lamp such as an HMI will be given a colour temperature of 5600K. A normal computer monitor screen is balanced as daylight if you didn’t know, and is given a normal value of 6500K. Television screens are also daylight balanced. If you don’t believe me take a photograph of the television screen at night with the normal house lights on, and the television will appear to be quite blue.

The Kelvin scale in photographic terms goes from about 1500K to 12,000K or more. A match or a candle may be 1700K to 1800K, the sun at sunset will be about 2000K. Whilst an overcast day will be between perhaps 6000K to 7500K and a partly cloudy sky could be anything up to 10,000 Kelvin.

This is important to know because your camera has a button or dial on the side of it to tell you what filter to change to. On a professional camera it will have a rotating knurled wheel marked 1-4, and may have another one around it called A, B, C, D. It will normally tell you on the side of the camera what these relate to, as they are different from manufacturer to manufacturer! Superb! Normally ABCD relate to different strengths of neutral density filter, in case the sun is too bright or you wish to reduce the depth of field to isolate something photographically.

The camera will have a small label telling you which one has a value of 3200K and 6500K that relates to one setting on the knob. If you’re shooting in daylight you turn it to 6500K and it will be close to giving faithful colours. Turning the knob to their setting will introduce a small orange (CTO) filter in front of the sensor, If you are shooting interiors and artificial light you need to change to 3200 K as this takes out the filter and gives you slightly more light as well.

A great example of the way a director of photography can use the natural colour of the light as it is without correction for effect is Chris Menges BSC, and a particularly good film to watch to see this is Dirty Pretty Things from 2002. There are sections of this film that are actually vibrantly colourful with the variations in fluorescent lighting, tungsten and daylight and neon. Incidentally Chris Menges also won the Academy Award for his work on the superb film The Killing Fields in 1984.

Some other examples : The Informant!, Contagion, The Conformist and Haywire. In particular, The film The Limey (1999) has a lengthy handheld shot that transitions from daylight to interior fluorescents with no colour correction.

Neutral density filters

These do exactly what they say they do! They are carefully constructed filters that reduce the amount of light that can be passed through them without altering the colour at all. This is important because you may need to 'dial in' some neutral density or ND filters because suddenly the sun has come out and it is too bright! We don’t get that happening a lot in England but when we do we like to get rid of it!

By adding a neutral density in front of the sensor it makes us open the aperture of the lens, and this has the effect of a less depth of focus within the scene you’re looking at. This will be examined more in the lenses section, but ultimately it cuts down the light without changing anything else such as the colour, otherwise we would need to white balance every time we changed the ND filter, which would not be ideal.

Incidentally neutral density gel filters can be bought to reduce the intensity of the light without changing its colour. This is useful if you have a light in the right position that you don’t want to dim because it would change the colour, and you can’t move the light further back because it is in the perfect place. A full ND filter will halve the light transmission from that lamp. You can add another layer of ND filter and it will cut it down even more of course!

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Posted on June 17, 2015 and filed under basics, Part Three.