المرشحات الضوئية .. كتاب التصوير الملون ، الكاتب عبد الفتاح رياض

Chapter two

optical filters

The shape of the optical filters:

Optically flat glass.

The optical filters used in photography are made of colored glass or dyed celluloid, or from a thin layer of gelatin colored with dyes, or by a combination of two of the aforementioned methods, such as placing, for example, a layer of dyed gelatin between two layers of glass. Optical filters are used in photography, either by placing them in front of the light source, behind the lens, or directly in front of it. In the latter case, the following is required:

completely

To be of flat optical glass. And be free of any bubbles. aerial.

2 Do not put any pressure on the lens. The position of the filter should be secure in front of the lens, so that the rays are not allowed to pass between them.

It should not be inclined to the surface of the lens, but should be parallel to its surface. There are filters that are prepared for special purposes in scientific research, consisting of a container of glass or quartz that is sometimes filled with liquids or gases. It is placed in front of the lens or light (1) source

(1) We have previously talked about this type of filters in detail in the author's book, Invisible X-rays, p. 67 of the first edition. Publisher Anglo Egyptian Bookshop 165 Mohamed Farid Street.

Optical filters function:

D calls the light filter that absorbs a side of the components of the light rays that fall on it, and these rays that it absorbs are those that are considered complementary to its color, and it allows the rest, which is what makes up its color, to pass through it. In other words, it controls the rays that reach the film and affect it. It has a real effect: , which has two consequences: (1) a decrease in the amount of light that reaches the film, which leads to a decrease in the amount of light that reaches the film. To increase the exposure by a certain amount, which is termed the filter factor, and we will come back to explaining this topic in this section. (b) A change in the ratio of the colors of the rays that pass through it from those that fall on it: If we mention, for example, that the white rays consist of three equal parts, blue, green, and red, and that the yellow optical filter is the blue rays that are included in the composition of the white rays, then this means that the rays that will pass through The optical filter and reaching the sensitive layer will be free of blue rays, and this results in a change in the colors of the images of the images of the objects, which does not happen if we do not use this yellow filter. How colorfast the filter is: This one sucks

Since different dyes are used in the manufacture of optical filters, and since the color of these dyes changes due to exposure to light, their properties must change accordingly, which necessitates careful care of optical filters. Therefore, the following should be taken into account:

(1) Preventing the filters from being exposed to strong light, unless it is necessary to use them, and it is better to keep them away as much as possible. from direct sunlight
<-1 The results of the passage of white rays through optical filters: the yellow optical filter

(b) Filters are kept away from high temperatures or extreme humidity. In addition to the effect of these two factors directly affecting the color stability of the filter, they are capable of damaging the thin gelatin layer that lies between the two surfaces of the glass in the filters made in this way.

If a white light ray passes through a yellow filter, the filter, according to the general rule of filters, must allow the rays that make up its color to pass through and absorb. In addition to it, we conclude that the yellow filter will allow the red and green parts that are part of the white rays to pass through, while the third part, which is the blue part, will be absorbed, because blue is the complementary color to yellow (1) (Fig. 7).

If we use a yellow filter, for example, to photograph a red or green surface, the positive image appears in light gray (on black and white film). But if the color of the object is pure yellow (ie, completely in the color of the filter), then its image appears completely white.

Magenta cochineal photofilter

If a white light ray passes through a crimson filter, it lets in the rays that make up its color (which are red and blue), and absorbs the green rays that are complementary to its color (figure). passing by

(1) In application of this, blue colors are seen existing if we look at them through a light filter of a pious yellow color. If we use a panchromatic film (black and white) with a high sensitivity to colors, super panchromatic, imaging a surface or a blue object, this color will appear black in the positive image.
Optical filter «greenish blue» Cyan

If a white light ray passes through a cyan filter, it will allow the rays that make up its color, namely the blue and green rays, to pass through it, and it will not allow the rays that complement its color, which are red, to pass through it (Fig. 9). rays

4- The red optical filter: When the white rays pass through a red optical filter, only the red part will permeate it, which is included in the components of the white rays. The filter absorbs the green and blue parts. The effect of the red optical filter is similar to the effect of combining two filters, one yellow and the other scarlet. The yellow filter penetrates both green and red rays. As for the scarlet filter, it absorbs the green part, and thus only red rays are not transmitted (Fig. 10). Only allowed

110 - green photofilter:

This filter absorbs both red and blue rays, and only green rays are allowed to pass through (which makes up the color of the filter). The effect of this filter is similar to the effect resulting from the combination of two filters, one yellow and the other cyan, as the red and green rays pass through the yellow filter and then only the red rays are absorbed when they pass through the second (cyan) filter, so only the green rays pass through them (Fig. 11)