on Wed, 30 Jun 2004
04:47:46 PDT7
> > Nominally, the eye will not perceive changes above a 30Hz rate,
> > although there will always be variations among individuals.
>
> 30 Hz causes flickering for most people.
It depends on with what. Computer images and moving video images are two
different things. Most people find TV refresh rates acceptable despite the 30
Hz image rate. It's similarly unacceptable for computer and text image
displays. This can be seen in many commercials in which the moving images are
followed by text of the name of the company and maybe a slogan. The images look
fine but the text at the end tends to "sizzle". That's flicker in the
interlacing of images and is why interlacing is wholly unacceptable for computer
displays. Even with frame rates of 60 Hz, many people perceive flicker, usually
at the edges of the screen while they look at the center. The optical nerves
that are most sensitive to motion and flicker are at the edges of periphery.
> Our television system uses a
> 60 Hz rate
Slight correction. TV images are refreshed at the rate of half an image 60
times per second or one full new image 30 times per second. Each half image is
interlaced (every other scan line) with the other half image.
> and our movies are 48 Hz (or 72 Hz with a three-leaf
> shutter). Hewlett Packard's research on eye flicker for illuminating
> with LEDs found a 100 Hz rate acceptable.
Again, for what?
> Fluorescent tubes flicker at
> 120 Hz (in North America).
Most people cannot see flicker at 120 Hz but I believe that there is still a
primary light generation rate of 60 Hz. This is why viewing computer images
refreshed at 60 Hz in ambient fluorescent light to be one of the most harshest
environments. The flourescent light tends to "beat" against the 60 Hz of the
image refresh, also leading to headaches in some, fatigue in others, and
sometimes no effect. This beating can be seen in other circumstances say, when
a TV image containing a computer monitor showing an image. Unless the two (TV
and computer monitor) refreshes are locked against one another, you will see the
rolling horizontal black bar in the TV image of the computer monitor.
> Many people don't realize that television (NTSC) is shot at 60 images
> per second.
Again, see above - this is the half image rate.
> That's why something shot direct-to-videotape looks so much
> cleaner than film even though film has far superior resolution.
There are two things going on here that shouldn't be confused: 1) image
fidelity which is the extent to which each rendered image resemble the source
and 2) motion capture. The first has to do with resolution, color fidelity,
linearity (double the source intensity yields double the brightness of the
image), dynamic range and noise and a variety of other factors that are often
used to judge the quality of a still camera. The motion capture has to do with
image capture rate, shutter speed, lighting and other factors. Videotape looks
cleaner mostly because of its improved characteristics in color fidelity and
linearity relative to film but most don't find it acceptable for movies because
the film going public has been conditioned to expect film and its
characteristics, that is, the softness and relative warmness (opposite of
harshness) relative to videotape. That is to say, factors associated with
category (1). Flickeryness and smoothness of motion are related to category (2)
stuff, namely video capture rate and image capture (shutter) speed. This has
led to the process of filmmakers using video first (for the added convenience as
well) and then transferring to film to cater to the film going public.
> A really simple persistence of vision measurement test is a spinning
> disk with spokes drawn on it -- just speed up the disk until you can't
> tell there's a pattern, then calculate the "flicker" rate. As I recall,
> the human eye's persistence of vision is color-dependent.
There are many interdependencies but there are two different things going on
here as well. Vision characteristics are one thing and image capture is
another. Most people perceive a spinning wagon wheel as moving smoothly while
everyone will also see the "wagon wheel effect" in the captured image of the
wagon wheel spinning at the same rate.
> Note that the eye can "see" extremely short "changes". A photographic
> strobe light might be "on" for just 1/1000th of a second and yet we
> still see it.
Even neurons have a persistence to them. I don't think the question here is
how short of an image change one can see (the eye has an integrating effect) but
how far apart two strobe pulses need to be before you perceive them as two
separate pulses. I guarantee that two 1/1000th of a second pulses separated by
1/1000th of a second will be perceived as one pulse. It's been a long time
since I've looked at this stuff but I think that they have to be separated by at
least low 10s of milliseconds to be perceived separately.
> - Peter
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