تقدير عامل المرشح Filter Factor إذا كان مجهولا .. كتاب الإضاءة والفلم

Estimating the filter factor if it is unknown:

And you may get an optical filter without knowing anything about the factor, and here we follow the following.

1- A large piece of white matte paper (such as a sheet of blotting paper) is prepared, and light is shone on it from both sides, so that the light is evenly distributed over it. Then the camera is placed facing the blotting paper, and exactly in the middle of the distance between the two light sources.

2- A sensitive film is placed in a camera and several shots are taken on (one film) with the focal number f fixed. no in all snapshots, and the first image is shown for a second, the second image for two seconds, then four seconds for the third image, and so the exposure time doubles each time for the next one, so the fourth image becomes seconds, then 16 seconds, 32 seconds, 64 seconds, 128 seconds, 256 seconds, and this process is completed. without using the filter.

3- The previous process is repeated after placing the test subject filter on the photographic lens.

4- The two films that were photographed are shown, taking into account the accuracy in the composition of the solution, the appearance and the time required for the display process, so that all conditions are equal when showing the two films.

E - After the flags are dry, the density test is carried out using the device prepared for this purpose (1) Densitometer

Suppose that the part of the first film whose intensity was suitable was the one that was exposed to light for two seconds, and that the equivalent part of the second film in intensity was the one that was exposed for eight seconds, then the exposure must be increased when using the optical filter by four. That is, the filter factor = 4. Although it is practically permissible to follow this method by photographing any scene other than white blotting paper, it may be desirable to follow the previous method so that the colors of the subjects do not interfere in the estimation of the density, and therefore do not interfere in the estimation of the filter factor, because the white color is considered a neutral color. Neutral Color.

Permeation curves for optical filters:

It is usual for factories to prepare graphic curves to indicate the characteristics of the different filters they produce, whether they are filters for visible or invisible rays. It is necessary for the photographer to know how to read these graphs, which are known as Transmission Curves, and they help us in extracting the following information:

(a) The wavelengths of light rays that the filter allows it to permeate and those that it absorbs, assuming that it was a filter. It allows light waves of length less than 6,000 ang to pass. This also indicates that these light waves are the lowest wavelengths absorbed by this filter. For this reason, these curves are sometimes called absorption curves. Absorption Curves.

(b) The permissible amount of these light waves to pass through: we can know, for example, that 50% of the light waves of 500 angstrom length pass through, for example, 30% of the light waves of angstrom length. . And so on .

In the horizontal axis of these graphics, we see a statement of the light wavelengths estimated in any of the known units (angstroms, milli-microns or microns), while the vertical axis shows the following two things that express one meaning.

1- Percentage Transmission or Transparency

2- Density or imam Opacity. Below is an explanation of what we mean by permeation percentage, and “density.”

(a) Permeation Percentage:

When light rays fall on a filter, it will absorb one part of it and allow the rest to pass through. If we want to know the amount that permeated it in relation to the rays that fell on it, it is possible to know this if the quantity of each of the following is measured:

The passive rays, and let us assume that x = a unit of light
And the incident rays, and let us assume that it = 10 x light unit

Then the transmission percentage is calculated mathematically as follows:
Permeation = the amount of rays left over the amount of incident rays = x over 10 x = 1 over 10 .

That is, 1 out of 10 of the rays falling on the filter is the one that only permeates it, and the permeation value can be put in the form of a percentage, so it becomes as follows:

Permeation Percentage = Rays permeating the incident rays x 100 on 1%
= 1 over 10 x 100 over 1% = 10%

In fact, what we called the percentage of the passing rays is in itself a value that indicates the extent of the transparency of the filter, so the transparent filter. Which allows it to pass through the largest amount of rays falling on it, and the lower this percentage (1%, for example), this indicates that this filter is dark.
Let us now turn to what we also mean by the word density:

(b) Density:

And just as it is possible to infer the extent of the filter’s transparency from merely knowing each of the values ​​of the incident rays and the passing rays, we can also infer from these two values ​​the extent of the opacity of the filter, as opacity is the opposite value of transparency, so if we say that:
In practice, we find that the ratio between the incident rays: the retarded rays is often a large ratio (1000: 1) or (500: 1), for example, or (100: 1). Therefore, it may be difficult - when we want to make a graph - to record numbers on the vertical axis that start from 1 to 1000, for example, and may exceed that to 1: 10,000, for example).

Hence the idea of ​​dividing the vertical axis in those graphs by logarithmic division arose. Density is sometimes constrained on the vertical axis instead of constraining the percentage of permeation, or instead of constraining opacity, because Density is in itself a logarithmic value, and it is equal to log opacity to the base 10, i.e. (Lu. opacity).