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Units for measuring the amount of light:

When we talk about the units of measurement of the amount of light, we must differentiate between four things:
(a) The light energy emitted by the source in all directions, i.e. the power of the light source in itself, or as it is sometimes called the intensity of the light source.

(b) Luminous energy that flows over the area occupied by the surface and is sometimes called the luminous flux (1).

(c) Intensity of incident light received by the surface

(d) The intensity of the light reflected by the surface is sometimes called (brightness) or by the name of the intensity of luminance.

The unit of measure for the power of a light source:

The international standard for estimating the power of a light source, that is, the light energy that the source emits in all directions, is built on the basis of the amount of light that a candle of certain specifications emits and burns under specific conditions. Hence the name (caliber candle) originated, and the light energy emitted by this standard candle (Candle Power) became the unit of measurement for the power of the light source.

Since 1936, another unit for measuring the strength of a light source has been established in the United States, which is known as: Candela. And this unit is defined as “a part of sixtieth (1 out of 60) of the light energy radiated by (noun) from the surface of a black body whose temperature has been raised to a degree equal to the melting point of platinum.”

Although there is a slight difference between both units, as the light energy emitted by the American unit Candela is less than that emitted by the standard candle by 2%, but they have been considered equal in the entire process. If necessary, he sought extreme precision in expression, so he differentiated between them by naming the first unit, which is the standard candle, as (the old candle), and naming the second unit Candela (the new candle).

The unit of measurement of the luminous flux that a given area receives:

If we assume that there is a standard candle in the center of a sphere with a radius of one foot (Fig. 6), then this candle emits light energy in all directions. The light energy that flows on a surface area inside this ball of square feet is equal to one Lumen. Lumens, therefore, are a unit of measurement x the amount of light that flows over an area of ​​one square foot that is one foot away from a light source of 1 candela.

If we assume that the area of ​​square feet on the inner surface of the sphere is square in shape, for example, then it will have a hierarchical shape with the radiation source, and this hierarchical shape (shown in Figure 6) will include a luminous flux of one day only. Likewise, if we assume that this patch The area of ​​which was a square foot was circular in shape, and if we connected its circumference to the center of the sphere, then a conical shape would be formed inside the sugar, the base of this circle and the top of it being the source of light, and this conical would include a light flux of one day.

And if we apply the law that determines that the area of ​​the sphere = πr, we will find that the sphere with a radius of one foot, its area is as follows: And as we mentioned in the foregoing, that every single square foot on the inner surface of this sphere receives a luminous flux of two days from the light source that One candle power.

It can be said that the entire inner volume of the sphere would be 12057 lei if the light source had the power of a standard candle. If we assume that the light source was emitting 50 lumens, then the luminous flux that a square foot receives is a foot away from the light source by a linear foot.. a lei per foot. It is also possible to estimate the luminous flux emitted by this source in all directions if we multiply by 50 lumens. x 12057 = 6285 lumens.

Units of measurement of incident rays:

If we assume the existence of a light source of the power of a standard candle (A in Figure 7) and casts its light rays on a surface (E in Figure 7) that is a foot from the source by one longitudinal foot, then this surface is as we know. It will receive a total Luminous Flux of 1 lumen on each foot, and it is symbolized as follows:
(1 day/foot).

And if we assume that the surface area was one square meter, and it is also one meter away from the source, then the total of this surface will receive a light flux of LE as well, because despite the difference in the area of ​​this surface from the area of ​​the previous surface, the amount of light energy that occupies the figure The hierarchical in both cases has remained the same. And since the light energy was distributed in the first case on a square foot, and in the second case it was distributed on a square meter, therefore we see that the intensity of light falling on a square meter must be less than that on a square foot. And when it is 10,764 square feet, so the intensity of the light decreases in the case of the square meter: second by a ratio of 1 over 10,764.

The amount of light falling on a square meter surface that is one linear meter away from the previous source is equal to 1 lumen per square meter, and this is an international unit of illumination. It is called unity
Lux.

As for the amount of light falling on a surface of one square foot and one longitudinal foot away from the previous source, it is 1 lumen per square foot, which is the same unit known as (foot / candle) that has begun to be used less and is replaced by its alternative. Lumen/foot unit.

And since we know that a square meter = 10.764 square feet, therefore, we conclude that: (1 lumen / foot), i.e. (1 foot / candle) = 10,764 luxes.

It seems clear from the foregoing that the exposure meters that are used to measure the incident light must be numbered in any of the previous units (candle feet) ie (lumen / foot). Some of them may be numbered in lux units and are known as Lux Meters.