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Subject:Why Screens Flicker From:"Johnson, Mike C. @ SLG" <MCJOHNSO -at- SLC -dot- UNISYSGSG -dot- COM> Date:Thu, 16 Mar 1995 09:58:00 MST
Many computer screens flicker badly. This is because the electron beam
that "paints" the horizontal lines that make up one screen frame shuts off
completely while it returns to the top of the screen in preparation for
"painting" the next frame. This is called retrace time. The picture tube
is dark or nearly dark during retrace. This is tough on the eyes and the
nerves, especially when the frame rate is low. Some folks with low
persistence of vision can see the screen flicker when they're using PCs or
going to the movies. As the eye ages, its sensitivity to light (and
persistence of vision) drops, contributing to the problem. Maybe this
explains why my 80-year-old Dad doesn't like to watch TV any more.
American TV has a 30 Hz frame rate (30 frames per second), which would
ordinarily drive many people nuts. Broadcasters compensate for the obvious
flicker and phosphor fade by using interlaced scanning, which boosts the
refresh rate to 60 Hz while maintaining the 30 Hz frame rate. Magic? Sort
of, I guess. They send two interlaced 262.5-line fields that mesh to make
each 525-line TV picture frame. This makes the TV screen flash on and off
60 times a second instead of 30. I have heard that TV screens also have a
rather high persistence phosphor so they don't go completely dark between
fields. This mitigates but does not solve the problem: you're still
staring directly into strobed light.
When a video camera records a computer or TV screen, flicker is usually very
pronounced in the recording. This is because the electron beam scans in the
camera and electron beam scans in the subject are unsynchronized. So the
camera often records a frame of the subject while the subject is in partial
or full retrace. TV studios get around this by feeding the same sync
signal to all monitors and cameras. But that's another story.
I agree with earlier posts that suggest a large, non-interlaced monitor with
a 70 Hz or better refresh rate. If you get a color monitor with a dot pitch
of .28 or smaller, the resolution should be very good. Use a glare filter
to reduce reflections from room lights. Reduce your exposure to fluorescent
lighting as much as possible, because they also strobe at 60 Hz.
This 60 Hz thing is a conspiracy. It's got to be related to Dilbert
somehow.
Mike Johnson
michaelc -dot- johnson%bbs -at- wasatch -dot- com
