المرشحات الضوئية water for .. كتاب الإضاءة والفلم

Chapter Nine

optical filters water for

The shape of the optical filters:

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 combining 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:


1- It shall be made of flat optical glass and shall be free of any air bubbles. Optically flat glass

2- No pressure on the lens.

3- The filter must be placed firmly in front of the lens. Rays are allowed to pass between them.

4- 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 source (1).

Optical filters function:

The light filter absorbs part of the components of the rays falling on it, and these rays that it absorbs are those that are considered complementary to the color (1), and it allows the rest, which is what makes up its color, to pass through it.

And in an explanation of what we mean by complementary rays, we say that it has been found possible to form white rays from only three colors of rays: red, green and blue, and these colors are what we call primary colors. If any two colors from the primary rays mix, they will produce three new colors We call it "complementary colors".

Red + green = yellow
Green + Blue - Cyan Blue
And red + blue = magenta cochineal
And if rays of a primary color are added with another of a specific complementary color, then white rays will produce white rays according to the following: - blue + yellow rays = white rays
Red rays + Cyan = white rays
Green rays + magenta scarlet

Therefore, we see that the yellow rays are complementary to the primary blue rays, as when they are added together, white rays are produced. The first complements the second to produce (white). And Cyan ll rays are considered complementary to the primary red rays, as adding them together also produces white rays, so the first complements the second to produce “white.” .

The scarlet rays of Magenta are complementary to the primary green rays. By adding them together, white rays are also produced. The first complements the second to produce (white).

Returning to the function of optical filters, we say that the filter performs a process of favoritism, as it allows the passage of all rays that make up its color to the film, but it prevents the passage of any rays that are contrary to its color, which are the complementary rays of its color. In other words, the filter controls the rays that reach the film and affect it in an effective way Actually. This leads to the following two consequences:

(a) A decrease in the amount of light that reaches the film, which calls for increasing the exposure by a certain amount, which has been termed “the filter factor.” We will come back to explaining this topic in this section.

(b) A change in the proportion of the colors of the rays that pass through it from those that fall on it. If we were told, for example, that the white rays consist of three equal parts of blue, green and red, and that the yellow optical filter is included in the composition of the white rays, then this means that the rays that will pass through The optical filter and it reaches the sensitive layer will be free of it, and this will result in a change in the colors of the images of the objects, which does not happen if we do not use this yellow filter.

In the following table, on page 186, we see a summary of the effect of filters on the white rays that fall on them.

Color stability of the filter:

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

(a) Preventing filters from being exposed to strong light, unless it is necessary to use them, and it is better, as much as possible, to keep them away from direct sunlight.

(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.