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Placement of light sources when photographing documents by optical means

Documents may be photocopied by optical means, that is, by means that rely on photographic transfer with lenses, or this may be by contact printing on sensitive papers directly to obtain images of the same size using printing devices. What we mean in this book is to talk about the placement of light sources when photocopying documents with a touch screen. We have already discussed this subject in detail in the book Developing, printing and enlarging (1).

The placement of light sources when photographing documents by optical means varies according to the required purpose. We summarize the following different ways of arranging lighting for documents.

(a) Incident Lighting •

(b) Oblique Lighting.

(C) Transmitted Lighting.

First: the incident lighting:

As it is understood from the meaning of this label, in this case the rays fall on the surface of the papers or documents required to be transferred, and this method is usually followed when a document is desired to be photographed to show it as the eye is accustomed to seeing it (2).

Although most of the incident lighting photography operations are done with the help of artificial light sources, there is nothing to prevent them from being performed in normal daylight if it is available (Fig. 93 p. 303). Here, any difficulties arise if the photographing is in an open place, as the lighting itself will be fairly distributed over the entire area of ​​the document.

However, some difficulties may arise as a result of the natural position of the light sources (such as a window or a door) if the photocopying is done in a closed place (such as a room), especially if the document to be photocopied is fixed in a specific place inside the room (such as a large map on the wall, for example). The photographer overcomes these difficulties if he uses reflective surfaces such as a mirror or reflective surfaces such as stretched white cloth (Fig. Although the use of artificial light sources to assist natural light may be acceptable in cases of black and white photography, it cannot be recommended if the photography is in color due to the different spectral characteristics of the light rays that each of them emits, which is something that results in the parts of the image that are illuminated appearing Daylight has a bluish tinge to it, while the other parts illuminated by artificial light appear to have tended to reddish with yellowness. Therefore, if we wanted to use industrial sources when color photography, the spectral characteristics of the sources of artificial light must agree with the spectral characteristics of daylight. (1) Whether we use artificial or natural sources, the first rule that must always be verified is that the lighting must be evenly distributed on all parts of the document, or the papers to be photocopied, especially if the document is large in area, such as a map, for example. To ensure equal distribution of lighting, the reflected light must be measured from the middle and sides to verify its fairness. Some people sometimes follow a method, even if it is primitive - but it will not harm its use, by placing a pencil facing the middle of the document and noting the strength of the shadows that surround it. If the shadows are equal on both sides, then the lighting is equal (Fig. 94).

And if all light sources are artificial, it is better not to use less than two sources of equal power in terms of the light they emit. If the document or map is large in size so that only two light sources are not sufficient to ensure equal distribution of illumination, then there is nothing wrong with increasing the number of sources to four, six, or eight as circumstances require, and so that the rays fall on the document forming an angle of forty-five degrees with its surface (45). Thus, the angle of reflection is 45 (forty-five degrees) as well, to ensure that the rays are not reflected to the lens whose axis is a right angle with the plane, and to ensure that no side rays fall on the surface of the lens, it is always better to put the lens hood on the lens.

And because the nature of some light reflectors and reflectors is not clean or impeccable in manufacturing, it may increase the concentration of light in the middle rather than in the sides, so it is very beneficial to place a diffuser in front of the reflector, and this diffuser may be a piece of white cloth, gauze or paper. Thin or frosted glass...etc.

When photographing glossy or glass-covered surfaces, it is okay to use polarizing filters to prevent reflections.

And Nazi in (Fig. 96) simplified examples to show how to place artificial light sources when photographing documents with incident lighting.