الإضاءة المركزة .. الإضاءة والفلم

First: concentrated lighting

It is easy to obtain them through sources that have been termed “spot light” (Fig. 22-d) from which concentrated rays come out. If we receive them at a barrier, they appear to us in the form of a small or large round light spot. The rays of concentrated light sources are characterized by extreme contrast, and if an object intercepts their path, strong, specific shadows appear behind them. And the areas facing the light source are bright, while the shadows areas appear black and specific in the image without a gradation between the white areas and the black areas.

The tungsten lamps that are used in the concentrated light source are characterized by the fact that their filament is small in size, as the smaller the diameter of the filament circle, the greater the contrast in lighting, and on the contrary, the wider its diameter, the softer the lighting. The general rule is that if the area from which the rays are emitted increases, the softness of the illumination increases, and vice versa. This rule applies to any light source (whether tungsten lamps, electric arc, discharge tubes, etc.).

In front of the lamp there is a convex lens (Plano.Convex lens), the flat side of which is facing the lamp and the other side is convex towards the outside, and thus the rays passing through the lens are subject to optical laws, so if the light source (light source) is of the transparent type in which the filament appears It is the resistance wire in the electric lamp, and it happens that this filament falls into the focus of the collecting lens, so the image of this filament must appear on the surface on which the rays fall, and this is wrong, so it is better that the glass of the lamp is not of the transparent type, but rather of the opaque white type Opal glass. There is a defect that factories avoid, which is chromatic aberration (1).

It is a defect that results in discoloration of the letters of the light spot as a result of the passage of rays in an uncorrected lens. Factories avoid this error by narrowing the diameter of the aperture from which the rays come out so that no chromatic aberration appears in the light spot.

It is possible to control the area of ​​the light spot by moving the lamp forward or backward (Fig. Some factories produce concentrated light sources that have a diaphragm hole to control the diameter of the optical spot, and by narrowing the diaphragm aperture, the amount of light emitted decreases, and the light is sharp with a very high contrast.

Some sources are equipped with a reflecting concave mirror behind the lamp to increase the intensity of the light. The electric carbon arc may be the source of the rays in the concentrated light device, and it is used instead of the tungsten lamp, and then this combination leads to the highest degrees of contrast in the lighting.

In the past, defects appeared resulting from the use of reflective mirrors known as Parabolic Mirrors (1) in Spot light Sources, as they used to lead to a spot in the middle of the illuminated space when the rays fall on a surface. It also appeared that the condenser lens was not satisfactory. For many reasons, but it was finally possible to overcome the problem of uneven illumination by combining a large mirror with a convex lens on one side and the other surface being flat. Plano-convex lens. On the convex side, Convex has circular grooves etched into the surface of the glass. These lenses are known as Fresnel lenses and read “Fernel” (Fig. 22-h) in relation to their discoverer, the French naturalist Augustine Jean Fresnel.

It is desirable in practice for the light emitted by concentrated light sources, to reach the maximum intensity of the light in the middle of the light beam emitted by the source, and to gradually decrease its intensity towards the ends, so that it is soft in it (1). The wisdom in that is that the effect of this source is not clear and distinct from the effect of other light sources used in photography at the same time illuminating the same subject, unless the photographer enables it to aim for another purpose.

And in the application of the inverse square law when using concentrated light sources, it is noted that calculating the amount of change in the intensity of illumination whenever the source of rays is near or far from the surface on which the rays fall requires taking into account special considerations, which we explain in
(Fig. 24, p. 88).

Some sources of concentrated light may be used to create special effects in the background lighting of the image, and in Figure 2 (89) we see what illustrates this.