Monday, 28 July 2014
It would be ideal to change the white balance of the camera when there is a change in the colour temperature of the light illuminating a potential shot. There are some cameras that will compensate for colour temperature variation but carrying out a white balance set-up before filming is always a good idea. Sometimes it will be needed to carry out the specified white balance procedure even though the camera switch may be marked “auto white balance” to ensure it is automatically compensated. To successfully white balance a simple routine will need to be followed:
1. Selecting the correct filter for the colour temperature of the light being used. (e.g. tungsten 3200K)
2. Selecting white balance position A or B; it will memorize the setting that will be achieved by the white balance. On either filter position, the two memories can be either A or B but if preset is selected, no setting will be memorized. This is the standard default setting.
3. Filling out the frame with a white matte card that is lit by the same lighting as the intended shot.
The card should not move during the white balance and it should not have any reflections or shading. The card needs to be correctly exposed, you will need to avoid any colour cast from any surrounding light sources and have the white balance the main source of shot illumination. During the white balance process, you may see a progress report appearing in the viewfinder, which will include a number that will indicate the colour temperature that has been assessed. You may need to check the position of the white card and other requirements of white balance if you may find the number higher or lower than the anticipated estimate of the colour temperature. It may indicate the camera is either not lined up correctly or it may be malfunctioning. Whilst the white balance is under way, the auto-iris circuit will adjust exposure to make the output of the green signal correct, then the red and blue channels will be adjusted to equal the green output. The white balance procedure will therefore be dependent on by the fidelity of colour reproduction since the white will influence all other colour combinations.
BLACK BALANCE There are a lot of cameras that may not require a manual black balance meaning that the adjustment will have to be carried out automatically whenever required. The black has no colourcast since the black balance will set the black levels of the red, green and blue channels. This will only normally be only required if: · The camera has not been used in a while · The camera has been moved between different air temperatures · The air temperature has altered · When the gain selector values have been changed If a manual black balance must be done, you must do 2 things: 1. White balance to equalize the gains and then black balance 2. White balance again.
The tungsten light source: The heat effect from the electric current that flows through the tungsten filament which is how light is produced. The visible spectrum from these sources is continuous and produces a smooth transition of light that is outputted between the adjacent wavelengths. The intensity of the outputted light will be varied if the current is changed (dimming) which affects the colour temperature also although this is usually kept within acceptable limits. Light from a discharged light source is produced as a by-product of an electrical discharge through a gas. The particular mixture of gas present in the glass envelope effect the colour of the light, the colour of the light also depends on the phosphor coating of the fluorescent tube. HMI’s; which are discharged light sources designed for film and television lighting, are not as stable as tungsten but do have a greater efficacy, they are compact and they produce light that approximates the daylight.
PULSED LIGHT SOURCES
Fluorescent tubes; for home, office, factory and neon signs, does not produce a constant light output but it does give short pulses of light depending on the mains supply. The light output of a usual daylight-type fluorescent tube for the normal eyesight give of a bluish green cast to a tungsten balanced camera but does not register high intensity blue and green spikes.
The improvement of Phosphors has made the fluorescent tube/cold light acceptable for television and film came after recent years of development. They are able to provide tungsten matching and daylight matching colour temperature. More or less steady light output are a result of high frequency operation; i.e. >40 kHz.
COLOUR RENDITION INDEX (Ra)
The colour rendition index or Ra can be used to indicate the suitability for use in television production (Ra of 70 is regarded as the lower limit of acceptability for colour television). The use of a scale of 0 – 100 is used as a method of comparing colour fidelity and consistency of a light source.