One of the most important elements in copying is proper, even illumination of the original. Originals that are not properly and evenly lighted yield negatives with uneven density, which are hard to print. This is true whether the original is illuminated by daylight or artificial light. Uneven illumination can be caused by improper placement of artificial lights in relation to the copyboard or by improper placement of the original in daylight.

Artificial lights are normally placed on two sides of the copyboard. A 45-degree angle is recommended for general use (fig. 8-3). At this angle, a minimum of unwanted reflections from the surface of the original occurs. However, depending on the type of surface of the original, the best angle for the lights may change.

For example, an original artwork may have brush strokes that produce reflections. These reflections may be reduced somewhat by placing the lights at an angle greater or lesser than 45-degrees.

Do not position artificial lights too close to the copyboard. The circles of illumination will not cover the original completely. Movable lights should not be positioned so far from the copyboard that the intensity of the illumination falling on the original is greatly reduced.

The evenness of illumination on an original can be checked with an exposure meter. Do this by placing a gray card on the original and taking a reflected light meter reading from the card. Do not allow the shadow of the meter or your hand to influence the reading. When a gray card is not available, a white card (the back of a sheet of photo paper) can be used, but you must compensate your exposure by two f/stops. Reflected light from the four corners and center area of the original should read the same light values.

Daylight provides two choices of illumination: direct sunlight and diffused daylight, such as a cloudy, bright day or open shade. Evenness of daylight illumination is controlled by ensuring that the original is completely in direct sunlight or in diffused daylight, and no shadows are cast on the original.

Although an original is uniformly illuminated over its entire surface, apparent unevenness in illumination may still appear in the copy if a wide-angle lens is used. This is caused when the light transmitted through the center of the lens is more intense than the light transmitted near the edges of the lens. When the entire angle of view of the wide-angle lens must be used, more illumination to the edges of the original is needed to compensate for the falloff of light at the edges of the lens. This can be achieved by turning the lights slightly toward the edges or by moving the lights close to the edges. The amount of light increase necessary for the edges of the original is best determined by conducting exposure tests with the type of film being used.

Lighting control is more critical when using an extremely high contrast film, compared to a high, moderate, normal, or low contrast film. Uneven lighting is more visible in a copy produced with an extremely high contrast film because of the limited exposure latitude of the emulsion.

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.

Figure 8-5. Using bounce light to control reflections.

BOUNCE LIGHTING. When a white surface is low enough, you should direct the light sources upward so diffused light bounces off the surface onto the original (fig. 8-5). When the surface is too high or other than white, it may be possible to use a white reflector positioned horizontally over the upturned lights. This reflector could be a large sheet of white cardboard.

POLARIZED LIGHTING. The most efficient method of eliminating unwanted reflections in copy work is by using polarized light. In regular photography, a polarizing filter is placed over the camera lens to subdue reflections. This works because the light from the sun is polarized as it passes through the atmosphere and is reflected by the object being photographed. Using a polarizing filter over the lens only does not greatly reduce unwanted reflections in copy work. In copying, polarizing screens must be used over the lights as well as a polarizing filter over the lens.

When polarized light is used in copying, a considerable increase in exposure is required. This exposure increase is from about 10 to 16 times the normal exposure required with the same lights without polarizing screens. The exact increase is best deter-mined through a series of exposure tests.

Reflections caused by flare are common with dirty or poor quality lenses. When available, lenses designed for copy work should be used, and like all lenses, they should always be kept clean.