Ok it's not rocket science, so unless you are some kind if photard you should be able to get your head around it. Hmm where do we start? OK, if you got a set of photographs back from the lab, and the pictures taken in doors, by artificial light all appear very yellow, or you are shooting digital and they seem a little warm then you need a tutorial on White Balance. Firstly, I guess a few background notes on the way that colour temperature is measured.

Colour temperature, a little knowledge of which is required to understand White Balance and why White Balance is needed in both Film and Digital Photography. Colour Temperature is a characteristic of visible light, and measured in °Kelvin Colour temperature is based upon the principle that a black body radiator (a quick Google for Planck’s Law of Black Body Radiation will get you more information than you need) emits light whose colour depends on the temperature of the radiator. Black bodies with temperatures below about 4000 K appear reddish whereas those above about 7500 K appear bluish. Colour temperature is important in the fields of image projection and photography where a colour temperature of approximately 5500 K is required to match "daylight" film emulsions.

There is not necessarily an actual correlation between a light source’s colour temperature or and it’s actual thermal temperature, because most light sources are not black body radiators. What is actually measured is its perceived colour (or correlated colour temperature). The simplest analogy that I can thing of is; imagine a blacksmith with his forge and anvil, heating a lump of iron ready for shaping with his hammer. As he heats and cools his lump of metal it will pass through a number of colour changes, from Black (it’s starting point through orange to red and finally white hot. Each of these colour changes occurs at different temperatures.

A small list of different light sources is listed below along with the average colour temperature value.

To the naked eye of an inexperienced photographer the difference between different light sources can go completely un-noticed, not because they eye can’t see it, but because the user doesn’t comprehend the difference, and the mind does compensate to a degree.

However, Film and digital imaging sensors are nowhere near as clever as the average humans mind, in setting colour temperatures so that white is always rendered as white, irrespective of the light source. As a result you get colour casts depending on the type of lighting you use. In the good old days, it wasn't un-common for me to travel with rolls of both daylight and tungsten balanced film in my camera bag. Now digital takes some of the pain out of White Balance. The one thing that is important to know, is that you cannot balance different lights of widely differing temperatures without introducing filters of some sort.

For example shooting with flash and tungsten lights, pictures, where the tungsten has an effect will appear yellowish cast. You either need to take the ambient tungsten light out of the equation, by turning up the amount of flash or fit a tungsten filter to the flashgun and balance the exposure between the two. I prefer for the most part to allow a little tungsten light to be present in these situations. Whereas Flash and Daylight can be combined almost seamlessly.

Most digital cameras have a number of White Balance Options from the simplest of all, the Auto White Balance setting, where the camera tries it’s best to work out the colour temperature of the predominant light source, however this is not always perfect. It depends upon your camera and the circumstances for example, some cameras have a white light sensor on the exterior of the camera, however, if the camera is for example under an umbrella, the colour of the light falling on the sensor will probably be different to the light falling on the subject.

Other cameras simply rely on the information recorded by the imaging sensor. Most digital cameras also include a few, if not all of the following manual

White Balance Settings.

Sunny= 5500K

Cloudy = 7500K

Shade = 9000K

Flash = 5500K

Tungsten = 3000K

Fluorescent = 4500K

* Some may also include: Custom:= enabling you to use a White/Grey Card to create your own specific

WB Kelvin = Enabling you to dial in the exact Temp in K (assuming you have a colour meter.

A point to be aware of Fluorescent Bulbs/Tubes is that they have a large chunk of the colour spectrum missing, even the ones alleged to be “daylight”. As you can see from the image, there are several huge gaps in the spectrum emitted by the Fluorescent Tubes.

No filtering can add the missing wavelengths, so, to a degree you will have accept some colour inaccuracies. This will become even more of a problem as the Government remove traditional light bulbs from the market place. The same problem occurs with Sodium Street Lights, they look orange because they are, they nave a very limited spectrum. This unfortunately means that they cannot be corrected, as there are no other colours present.

The issue of White Balance becomes less of a problem if you mainly shoot in RAW format, however, if you shoot in Jpg mode, then it is best to get the colour balance settings right at the time of taking the image, as most post capture alterations reduce the quality of the image. One way to ensure that you get the white balance spot on, or at least as near as possible is to either purchase a colour meter, you are then able to meter the exact colour temp (used in much the same way as an incident light meter, as you which to record the temperature of the light falling on the subject, rather than the light being reflected) or to use the custom white balance option.

The latter has the advantage that it is a lot cheaper, and if you know how to do it, then if you leave your colour meter at home then you can still get neigh perfectly colour balanced results. Depending upon your camera you will need either a white or grey card, although to be honest almost anything vaguely white will do, even someone’s shirt if necessary. Arrange your card so that the ambient light is falling directly on its front, you will probably need to turn the card to face the main light source, and tilt slightly upwards (assuming the light is from above) and then fill the frame with the card, roughly focus the image in the viewfinder and take a photograph. The using the image as the source for your custom white balance select it and the camera will set that appropriate K value for all subsequent scenes.

The important thing is that things change, what may have been right at the time you did your custom white balance, may not be appropriate when you come to take the photograph. All it takes is for a ENG crew to set up another video light and the balance may have shifted, or for a Press Conference Organiser to decide to turn on a few more lights and you may well be back to where you started. The following images were shot under tungsten lights, in particular, GU10 Mains Voltage Halogen Bulbs, just to prove that not all tungsten lighting is the same.

	The first Image was shot on an EOS1D mkIIN on a 50mm F1.4 at 1/2 sec at F11 at 125ISO. With the White Balance Left to Auto.
	This image was shot using the same exposure but with the White Balance set to Tungsten, as can be seen the colour is a little off.
	The above was with the Manual White Balance set to 2800°K
	The above was with the Manual White Balance set to 3300°K.
	The above was with the Manual White Balance set to 4000°K.
	I did explain earlier that it is not really possible to balance Tungsten and Flash to produce a completely neutral image. Here is an example:Using the same exposure settings as previously, but with the flashgun turned on and bounced from a white ceiling, there is just enough tungsten to influence the colour balance in the image, in this case it may be acceptable, depending upon who and what you are photographing. The only way to completely remove any colour cast is to reduce the affect of the ambient lighting
	I did explain earlier that it is not really possible to balance Tungsten and Flash to produce a completely neutral image. Here is an example:Using the same exposure settings as previously, but with the flashgun turned on and bounced from a white ceiling, there is just enough tungsten to influence the colour balance in the image, in this case it may be acceptable, depending upon who and what you are photographing. The only way to completely remove any colour cast is to reduce the affect of the ambient lighting.
	In this case reduce the shutter speed to 1/125 sec and allow the flashgun to provide all of the illumination for the exposure. By attempting to balance Tungsten and Flash by using the Tungsten Balance setting on the camera and using fill-in flash you will not succeed in correcting the colour cast as instead of being pleasantly warm (especially for skin tones) it shifts in the opposite direction and becomes very blue.
	Two further examples Using the Camera in Tungsten WB mode and the flash gun as a fill in, same exposure settings as previous images, leaves the image with a slightly cold colour cast.

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