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Hold Time Timing

TVs and computer monitors take advantage of how our eyes work.

Have you ever taken your hand and shaken it in front of a TV or a computer monitor (not an LCD screen)? If you did, you would have noticed a strobe light effect.

Is Seeing Really Believing?

It turns out that the receptors in our eyes will report a light as being continuously on, even it if is off the majority of the time. The critical flicker frequency or CFF is the minimum frequency that a light must be flashed in order for us to not detect a flicker. The CFF ranges from 5 to 55 Hz depending on the size and brightness of the source. The rest of the time the light can be off and we aren't conscious of it.

Did You Know?

Movies, which show only 24 frames per second are interrupted several times per frame, or held in place and quickly switched to the next frame to avoid flicker. Television, on the other hand, which has 25 to 30 frames per second displays every other line 50 to 60 times per second to avoid creating a detectable flicker.

Interesting Observation

One of the first calculators I had used a seven segment LED display. In order to save battery power, the display would only illuminate one of the seven segments at a time, going through all of the seven segments fast enough that I could see the number clearly.

I discovered this one day when waving my calculator in the air. Rather than all of the seven segments being displayed evenly, the display would break up and show only single elements being lit at a time.

Microscopic Animals Use This Principle

The eye of a Copilia Quadrata, an microscopic aquatic life form, uses the same technique as the old calculator. The Copilia Quadrata only has one light sensitive cell per eye. In order to "see the world" it moves its lens in and out at rates up to about 5 times a second. This process, then, let's the Copilia Quadrata piece together an image of the world.