So a flickering actually happens on any CRT, but we can't see it with naked eyes. This is similar to movies which are recorded at a rate of 24 images per second by the camera and are actually projected twice or more this frequency, using a shutter, giving a rate of 48 shorter images per second or more. This apparent frequency of 50 Hz is enough to prevent flickering for humans. The CRT actually displays the two interleaved halves of an image each 1/25 s. The electricity network frequency (50/60 Hz) was used as this provided a stable clocking reference.ĭoes that mean we need to send 50 (60) images to the CRT each second? Actually this would have been too much for the radio transmission associated with TV broadcast, and only 25 (30) images were sent each second, first all odd lines, then all even lines. The refresh rate of the CRT was chosen in order to take advantage of this persistence. For most people, there is no flickering after 40 Hz. This is why we can see a photographic flash which actually lasts less than a millisecond. When our eye sees a bright point, the stimuli is maintained after the point has disappeared. This is also done line by line, and also create areas of different brightness at a given time.Ī human eye also shows a persistence of vision. Passive LCD screens are similar: A passive LCD cell (pixel) is made transparent by storing electrons in a tiny capacitor which is unable to keep it's charge a long time, so this capacitor must be reloaded periodically. This creates a brighter band visible when frozen by photography. As phosphor luminosity cannot be maintained a long time, the screen must be continuously scanned by the beam at some frequency, usually 50 or 60 Hz, but today it's often at a higher frequency.Īt a given time, the most bright lines of the screen are the one currently refreshed, and a few lines which were refreshed immediately before, which number depends on the persistence of the phosphor used. In a CRT screen, the image is built by energizing phosphor material on the screen surface, using a small electronic beam, line by line from left to right and top to bottom. The effect is the combination of the two. In the effect you see there are two reasons: The way a CRT shows images (actually bands on the screen) and the way the camera captures (samples) images. This relates in fine to the Nyquist criteria: If a signal contains frequencies up to f, then it mustn't be sampled at a frequency lower than 2*f. In technical term this means the original object was sub-sampled given it's complexity. Aliasing refers to an artifact created when rebuilding an object from samples that not in adequate amount to get a faithful copy. Stroboscopic effect is one of the many form of aliasing. The sequence of these fixed states may create artifacts, like adding or removing displacements such as a wagon wheel rotating in reverse direction, or a helicopter hovering with stationary blades. This has two effects:Ī fixed state may show things we couldn't see before they were frozen, like a picture which is incomplete at a given time. Instead of viewing the continuously changing real world we see a sequence of fixed states. It appears when the real world is sampled (temporal sampling) and we look at the succession of samples. This is an artifact known as stroboscopic effect. The analogy with CRT is the invisible higher frequency flicker is sampled by the camera at a too low frequency and the result is a low frequency flicker which is visible. When reconstructing a signal from the samples, the result (red) is a different signal. The sampling frequency is obviously lower than twice the signal frequency. In this image the green signal is sampled at black points. It may be surprising, but this effect is the result of not fulfilling the first principle of sampling, known as the Nyquist criterion, sampling must done at a frequency at least twice the largest frequency contained in the signal to sample. Aliasing takes place, artifacts are created, and are visible. Usual CRT flicker by principle, CRT frames are not displayed in their entirety at any time, but this is not visible to the naked eye, due to the frequency this phenomenon occurs.Ī camera is perturbed by any flicker effect, due to the sampling principle it uses.
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