Understanding Flicker and Perceived Frame Rate
Movie screens are not constantly illuminated, although when you watch a movie in the
theater, it appears that this is the case. A film projector’s shutter actually blocks the
light to the screen when each frame advances, but your eyes momentarily retain the
image until the shutter opens again (thanks to persistence of vision).
When you close your eyes, there is a brief moment when you can still see what you
were looking at it, especially if what you were looking it is quite bright compared to the
surrounding environment. This persistence of vision is so brief that you may not be
consciously aware of it, but it is this phenomenon that allows us to believe that rapidly
changing still images are moving continuously.
However, the higher the frame rate, the more film you need, the faster the projector
must operate, or the more electronic bandwidth you need (in the case of video). Early
audience perception tests with movies demonstrated that increasing the rate of flicker
increased the perception of smooth motion, even if the images themselves were not
changing during every single flicker. The perceived frame rate (or flicker rate) can be
increased by opening and closing the projector’s shutter two or three times for each
film frame, creating a less noticeable flicker on screen. Therefore, even though movies
are universally shown at 24 fps, the projector’s shutter may open and close at 48 fps, or
perhaps higher.
One second
60 fps
24 fps
Appendix B
Frame Rate and Timecode
407
V
Early television systems used a different approach for the same result: increased flicker
without increasing the necessary electronic bandwidth. Interlaced scanning fills a
television frame with only half the video lines of a frame (this is known as a field), and
then fills in the remaining lines (the other field). A field effectively fills the television
screen with an image, even though it is only half-resolution, and it does so in half the
time it would take to draw the full frame. The result is a perceived frame rate which is
double the actual frame rate. For NTSC, the frame rate is 29.97 fps, but the perceived
frame rate (the field rate) is 59.94 fps. This causes less flicker. PAL, which has a lower
frame rate of 25 fps (or 50 fields per second) has a slightly more noticeable flicker.