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The stroboscopic effect in electrical discharge lamps

When the electric discharge lamps operate on an intermittent current (.A.C), it turns off. (Extinguish) Discharging in periodic batches, and the number of these batches is equal to twice the number of frequencies of interrupting the electrical current, as the light is emitted in the form of rapid successive flashes interspersed with successive dark periods as well, and this phenomenon is called the stroboscopic effect. This phenomenon has no dire consequences in photography when the exposure time is long, but it is very likely that its damage will appear clear in the case of a short exposure when the shutter speed in the camera is the time it takes for dark periods, so it happens that the camera shutter is opened contemporaneous with the darkening of the source.

If the frequency of the electric current was 50 cycles per second, for example, the number of flashes would be fifty flashes, followed by fifty periods of darkening per second, and then each period of darkening or each flash would take a time of a second, if the shutter speed was greater than a second (for example ) It would have been possible for the shutter to open during the dark period, so the exposure would not have taken place at all. Or it is very possible that the shutter is opened contemporaneously with part of the flashing time and another part of the darkening period, so the exposure will be underexposure. The lower the shutter speed, i.e. the slower it is, the less the possibility of damage, and the possibility of this damage ends completely if the exposure time is very long (2 seconds, for example.).

This is even though the damage seems very likely in the previous case if the shutter is fast, and of the type called a Diaphragm Shutter, but it takes another form if the camera’s creator is of the type called the Focal Plane Shutter, as the film is exposed by This last shutter is in successive batches (1) and its full exposure requires a time that exceeds the time specified as the shutter speed. The time it takes to expose the entire film depends on several factors, including the time that was set as the shutter speed, and the area of ​​the film. As a result of the nature of how the exposure is done in the case of using the focal plane shutter, we find that the negativity will include dense or properly exposed areas, followed by others that are less dense, that is, the underexposed ones that were facing the focal plane creator’s aperture during the period of darkening of the electric discharge lamp.

We see from the foregoing that it is not appropriate to use electric discharge lamps as light sources when the camera shutter is of the focal plane shutter type.
The following three cases are excluded from this:

(A) If the exposure time is too long, as if setting the shutter to T, B.

(b) That the electrical circuit be set up in a three-phase circuit (1) phase circuit (3) and thus it is absolutely certain that the period of darkening of any lamp corresponds to the period of flashing of the other.

(c) Although fluorescent lamps originally operate on the basis of the theory of electric discharge, the potential for damage previously referred to is greatly reduced when used, as the materials covering their glass or mixed with the glass paste shine for a period of time as a result of their exposure to short-wave rays, and this radiance is Sufficient to continue to emit visible rays during periods of darkness, and this is a feature that overcomes the previous damages to some extent.

Electric discharge lamps differ from filament lamps in that the latter continues to emit light during periods of darkness as a result of the continued glow of the filament and its high temperature despite the intermittent flow of current A, C.
(i.e. alternative alternating current).