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Colour Temperature & Dimmers
Colour temperature exists quite independently of any light output level; unfortunately the two issues do become adversely connected when trying to control the light output of a filament lamp with a dimmer. Dimmers were widely used in the early days of black and white cinematography, but with the advent of colour their use rapidly diminished for all but certain aspects of specialist work in colour cinematography.
It is a fact of physics that the colour temperature of a filament lamp will vary as the voltage applied to it is changed. This can cause problems if you are trying to maintain a correctly balanced shot which also includes a contribution from other lights. In this situation a dimmer would not be appropriate. It is far better to use a light with a spot-to-flood facility, such as the Paglight, in order to control the light output intensity without affecting colour temperature. For greater range, when using your Paglight, just exchange the plug-in lampholder for one with a more appropriate lamp wattage. Plug-in halogen lampholders for the Paglight accept 20W to 100W halogen bi-pin lamps, and wattage label kits are available to aid identification.
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Dimmers Used to Good Effect
Camera top lights which employ dimmers to control the light output level naturally move their colour temperature towards the red end of the spectrum as the output is reduced. This aspect can be used to good effect by giving a warmer look to an otherwise cold image. An improvement can very often be made to medium close-up facial shots by adding a little warmth, and at the same time reduce the problems of glare or whiteout in contrast to the rest of the scene. PAG offers a lamp dimmer facility for the Paglight called VariLux, incorporated into its PowerMax Control Unit (Model No. 9958). PowerMax contains a voltage control circuit which efficiently regulates the voltage at the lamp base to 12V, regardless of battery voltage. This gives a constant and correct colour temperature whilst extending battery run time by as much as 25%, and dramatically improving lamp filament life. Despite some manufacturer's claims to the contrary, changing the voltage on a lamp filament does affect its colour temperature output. This is the last thing you want when you are trying to balance two light sources in a direct comparison situation. Use the spot-to-flood facility to control the light output, or select a more appropriate lamp wattage.
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What is Polarisation of Light?
Polarised and unpolarised light exists all around us, yet we do not have the ability to detect the difference between these light waves without employing the aid of a polarising filter. There are clues however: sunlight passing through the earths atmosphere is in the main unpolarised until being reflected at 90° from its direction of travel. This has the effect of polarising some of the light by refraction. Further to this, reflective non-metallic surfaces, such as water or glass, also have the effect of polarising daylight when viewed at about 33° to the surface, whereas shiny metallic surfaces along with soft or matt surfaces such as wood, paper, bricks, all reflect unpolarised daylight. A scene that has a polarised light reflecting from a particular surface can very often be improved by using a polarising filter. The unwanted reflections can be partially or completely eliminated by rotating the filter in front of the lens. The full effect is achieved when the polarising plane of the filter is at 90°to the reflected light's plane of polarisation. The unpolarised light from the rest of the scene passes through the filter. Polarising filters possess a structure composed of long parallel molecules that are aligned in one plane only, and it is this that gives the filter its very special quality. The appearance of a polarising filter resembles that of an ND filter, but it can be used to far greater affect in many other applications.
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© PAG Limited 2005
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