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?


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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crt television work by rapidly drawing the image line by line from top to bottom, of course it’s fast enough that you never notice with the naked eye

however cameras aren’t eyes, and they will only photograph what happen when the shutter was opened, so it will only show some lines rather than all

if you want to take a good picture you need to make sure the exposure time is at least 1/60 (for NTSC television)

modern screens can hold the image on the screen so it’s no longer a problem, however some projectors still have a similar problem where photos turn out rainbow because the colors are displayed sequentially

Old TVs work sort of like ripples on water. So a special tool sends a ripple or a “beam” through the entire screen and then each dot (pixel) on the screen lights up to a certain colour as directed by this beam.

This beam is the black bar that the camera sees. The reason why you don’t see it with your eyes is because the camera doesn’t yet see the next beam after the first one faded. The see the fade which is the black line whereas our eyes would’ve picked the next one.

This probably has to do with how CRT’s (tube televisions) worked in comparison with newer digital technology.

So, back in the day, there was no such thing as a pixel-based display. The computer technology to control thousands of pixels individually didn’t exist. What tv’s did instead was use a single beam of electrons, and sweep it very quickly over the surface of a phosphorescent screen to create an image. Basically the beam would start in the upper-left corner, and create a level of brightness on that spot, and sweep across to the upper-right-hand corner, changing the brightness as it went, to create a whole line of the image. Then it goes to the next line and creates that one and so on. This sounds like it shouldn’t actually work, but it does, because 1. this is happening incredibly fast (covering the whole screen 30 times per second), and 2. our eyes are actually pretty bad at discerning small changes in images, such that as long as the beam moves fast enough we will perceive the whole image.

What’s happening when you try to take a photo of a CRT is that the shutter of the camera is freeze-framing a moment that your eyes can’t actually perceive. The beam is near the top or the bottom of the screen in that precise moment, so some of the parts of the screen are not actually lit up by the beam.

This is incidentally also why Dogs probably have watched a lot more television in recent years than in the past. They can resolve images up to 75 times a second so to them, CRT television was probably just weird flickery lines.

Older cathode ray tube (CRT) TVs drew each image row by row. The phosphor coating briefly lit up very brightly as it was drawn but then quickly faded. If you could take a very short exposure photograph then only a narrow horizontal bar would be bright and the rest close to black. With the longer exposures common with indoor photography, you can see more of the picture but typically a black bar will remain. Modern LCD and OLED TVs can display a complete image all at once so problems of this kind are less likely.

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.