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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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