When computer screens appear on television why do we see scrolling lines rather than what s actually displayed on the monitor

A.� The answer to this lies in the difference in which our eyes and video cameras perceive images.

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Q.� How is a TV image created

A.� When we see an image on a television screen that image is created by a beam that that scans the inside of the picture tube several times a second, lighting up lines of phosphor pixels across the screen. But in fact the phosphor pixel doesn't stay illuminated for the complete duration of a scan so in fact for a split second there is darkness on the screen.

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Q.� I've never noticed that before, why not

A.� That's because our brains reassemble the broken images into a continuous one. This visual phenomenon is known as 'persistence', it produces a steady image that is an accurate representation of the real world.

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Q.� And how does a video camera 'see' things

A.� In a different way to us: basically it doesn't have this handy trick of persistent vision, it sees things much more literally.

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Q.� So it picks up the split second of darkness that our brain doesn't register

A.� Correct. As the camera rolls, it takes a series of still images. In each of these images the split second when the phosphor pixels are unlit shows up as a black stripe and this is what we see when a computer or television screen is seen on TV.

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Q.� But why does the black line scroll

A.� Because your television and the video camera that recorded the scene have different scanning frequencies. This lack of synchronicity means that the camera sees the black line at a different point each time and it relays this, making the black line move up and down.

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by Lisa Cardy