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The effect of changing the electric potential difference on the intensity of the light energy emitted by the lamp, and on the color temperature of the radiation. Color

Electric lamps are prepared to operate on an electric current with a certain voltage difference (220 volts or 110 volts, for example). If it is assumed that the lamp was made to fit a current whose potential difference is: = 220 volts, and it happened that the electrical potential difference decreased from this limit, then the lamp’s efficiency decreases a lot, i.e. the light energy that it emits decreases. In this case, the efficiency of the lamp does not decrease in the same proportion as the electrical potential difference decreases. Rather, its efficiency decreases a lot if the electrical potential difference decreases slightly.

Let us take an example of this with a lamp that consumes 500 watts and emits 10,000 lumens when it operates on a current of its electric potential difference. 200 volts, so if the electric potential difference decreases by 20, i.e. it becomes 160 volts, then the optical power of this lamp will not decrease by 20% either (it becomes 8000 lumens, for example), but rather the percentage increases much more than that and its optical energy decreases by 50%, for example, i.e. no Sent only Liu from .

Based on that, we see that it does not depend at all on what the electric lamp consumes (estimated in watts) to indicate the light it emits. Two lamps from the same industry may be equal in the amount of electrical current they consume (500 watts, for example), but one of them emits more light energy than the other, and that If the electric potential difference on which one operates differs from the other.

The specified amount, and we see from the other side that in increasing the electric potential difference from the lamp when it is manufactured, it will also result in three results, which are:

(a) Luminous Efficiency increases significantly without this increase being commensurate with the increase in the electric potential difference.

(b) The color temperature of the rays emitted by the lamp rises, i.e. the spectral characteristics of the rays change, so the percentage of blue rays increases, while the opposite happens if the electric potential difference falls below the estimated limit of the lamp, so we see the rays tending to reddish.

(c) The life of the lamp, i.e. its working life, is reduced, as the filament material evaporates quickly, causing its thickness to decrease, and the evaporation products accumulate on the inner glass surface.

In Figure 14, page 51, we see a graph that can be used to know the consequences of changing the electric potential difference.

And for the previous reasons, we see that it is necessary in color photography to stabilize the voitage by means of a special Voltage Stabilizer device, as it may happen that the electric potential difference changes by increasing or decreasing during work. Which basically entails a noticeable change in the amount of light energy and its spectral characteristics. An increase or decrease in the electric potential difference by + 4% may result in a change in the color temperature of the rays by + 50 Kelvin.

Likewise, the color temperature of the rays is affected by the age of the lamp. To prove this, we mention that the color temperature of the rays of the lamp used in photography, known as the Photolita 500, decreases by 0170 Kelvin between the start of its use until just before the end of its life. It has been facilitated to take advantage of the effect of increasing or decreasing the electric potential difference
In the manufacture of light sources known as (Color-tran) (1) and these sources are used in cinematography. The idea of ​​its manufacture is that it is possible to control the color temperature and intensity of the rays by controlling the electric potential difference that feeds the usual tungsten lamp, as the increase in the electric potential difference results in the glow of the tungsten lamp filament increasing as a result of the high temperature of the filament, and the intensity of the light increases and tends The color of the rays is bluish. The usual tungsten lamps that are fed by an electric current of 120 volts and emit rays with a color temperature of 2600 Kelvin increase their color temperature to 3200 Kelvin with 160 watts, and 3450 Kelvin with 185 volts, and in this last case the light energy intensity increases to 400% of the first case.

This type of light source has other advantages:

1- Portability: It is light in weight, so that it is easy to carry in the hand when filming, when compared to the large, heavy-weight light sources used in studios that emit similar light energy.

2- It is well known that the color temperature of the rays changes with increasing the duration of the lamp consumption, and by means of the Coloriran light sources this problem can be overcome, so it is easy to control the color temperature of the rays no matter how old it is (i.e. no matter how long the lamp is consumed).