Figure 8-4.–Lighting large originals.
Kelvin Temperature of Illumination
When you are producing color copies, the Kelvin
temperature (color) of the light source should match the
color balance of the color film being used. When a light
source produces an illumination color other than that for
which a color film is balanced, filters must be used to
alter the Kelvin temperature of the illumination to
correspond with the color balance of the film.
Rises and drops in voltage also affect the color
temperature and intensity of illumination. Fluctuation in
voltage can be controlled by using a voltage regulator.
Lighting Large Originals
For most copy work, you should position the lights
at a 45-degree angle, about 36 inches from the
copyboard, and aimed at the center of the original.
However, there are occasions when you may have to
copy a large chart, and the normal lighting setup causes
uneven illumination. Light from an artificial source
must travel farther to reach the center of the original, and
the light reflected from the edges must travel farther to
reach the lens. This causes the light to be less intense
along the edges and may result in underexposure of
these areas. You can correct this condition by adjusting
the lights. Keep the lights at a 45-degree angle, but move
them closer to the lens optical axis until the light beams
from the lamps intersect in front of the original (fig. 8-4).
Balance is generally achieved when the beams cross
each other at a point approximately one third of the
distance from the copyboard to the lens. Check the
lighting on the ground glass or through the viewfinder
to see whether it is even from the center to the outer
edges.
When lighting large copy, the use of portrait lights
with umbrellas is a good source of illumination. The
wide coverage and diffused light, produced from this
light source, allows you to light the original easily and
evenly. To check the evenness of the lighting, use a flash
meter and take readings from the center, corners, and
intermediate points on the original.
Reflection Control
Unwanted reflections often affect copying. Proper
placement of the lights is generally sufficient to
eliminate most normal reflections. The three types of
unwanted reflections in copy work are as follows:
Reflections from the light source
Reflections over the entire surface of the original
or copyboard
Optical flare
Reflections from the light source are caused by light
reflecting from the camera stand, lens board, cable
release, or other shiny objects around the copy setup.
The reflections usually occur when you are copying
glass-covered originals, glossy photographs, or other
smooth-surfaced originals. The best way to eliminate
this type of reflection (when changing the position of the
lights does not help) is to use a black cloth or a sheet of
cardboard (painted dull black) as a shield between the
lens and copyboard. You can do this by cutting a hole
the size of the lens in the center of the cloth or board
then placing the cloth or board over or around the lens.
A lens hood also helps in reducing or eliminating this
type of reflection.
Reflections over the entire surface of an original can
occur with rough, scratched, crumpled prints or
paintings with brush marks, canvas texture, cracks, and
so forth. These reflections are caused by high spots on
the surface of the original and cause small light
reflections of the light source. Such small reflections
cover the surface of the original with a haze of light that
results in a low-contrast copy image. Reflections of this
type are more difficult to avoid than reflections of the
light source. As long as the lights shine directly on the
rough surface, such reflections occur no matter in what
position the lights are placed. There are two lighting
methods by which this type of reflection can be
minimized or eliminated. These methods are bounce
lighting and polarized lighting.
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