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, especially if what you were looking at 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 onscreen. Therefore, even though movies are universally shown at 24 fps, the projector’s shutter may open and close at 48 fps, or perhaps higher.

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.