Why do those moving black bars appear when trying to take a photo of an older tv or computer screen? And not on modern ones?
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Imagine you stand on a bridge and look down onto a road. You have your eyes closed (camera shutter is closed) and while the a car travels underneath you open your eyes for a brief moment. You have now seen the car move on a very specific part of the road. While you didn’t on the remaining part of the road.
It’s the same principle with CRTs. CRT stands for cathode ray tube sometimes also called cathode ray television. A beam of electrons is projected and directed onto the screen. The ray is then manipulated to travel the screen in lines from left to right, top to bottom. This is quicker than our vision allows to register. But if you use a camera the shutter speed (the time the camera opens its eye for) can be much much shorter than the time it takes the beam of electrons to reach the end of the screen. So the camera can only capture the lit up part of the screen the electron beam is able to travel within that exposure time frame.
While the beam travels it excites phosphorus particles to glow, this is what actually shows the image. As soon as the electron beam moves on, though, the phosphorus gradually stops emitting light. It’s like those glow in the dark paints. Shine light on it, and it charges up, take the light away and it will glow for some more time, but will go dimmer and dimmer over time. Same principle applies to the phosphorus in a cart when hit by the electron beam.
Now why doesn’t that happen on modern displays?
Modern displays too build up the image from one edge to the next, but it happens much much faster and the displays show the image for the entire duration until the new image is being built.Thus the entire screen is always showing an image, and there are no dim spots that the camera could pick up.
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