EE-260X
If you look at a well-lit scene and then close your eyes, you will notice that the image can still be sensed for some time after the eyes close. This is due to the amount of time that the retina retains some of the information it has been stimulated with. This phenomenon, which places limits on how fast our visual system can react to changes, is known as visual persistence. Simply put, our visual system has a slow response to change in stimulus. We can take advantage of this to develop techniques for digital video systems.
Although an image on the retina decays gradually, rather than lasting a specific amount of time, there is a critical period during which the stimulus changes so little that the visual system cannot take in any new information even if the eyes are open. This period, on average, is about 50 milliseconds, or one-twentieth of a second. Thus, the average human visual system can only take in about 20 different images per second before they begin to blur together. If these images are sufficiently similar, then the blurring which takes place appears to the eye to resemble motion, in the same way we discern it when an object moves smoothly in the real world.
One can also ask the question of the number of flashes per second of light
that would appear to be a continuous flicker free illumination. Studies
show that this varies as a function of the intensity of the flashes,
but that almost no flashing is evident at above 50 flashes per second
and perception even for the brightest of lights disappears for
rates above 80 flashes per second
.
In fact, if your eyes had a faster response than this, you might find it quite annoying, since the 60 Hz electrical system used in the United States causes electric lights to flicker at a rate of 120 times per second. Because of human visual latency, we only see continuous light. In Europe, the power fluctuates at 50 Hz and hence illumination pulses at 100 flashes per second; they are living closer to the edge of perception there.
The above phenomenon has been used since the beginning of the 20th century to produce ``moving pictures'', or movies. Thomas Edison, the inventor of the motion picture camera needed to balance the needs of human perception with the need to minimize the amount (hence cost) of film that needed to be taken. He determined experimentally that 10 frames per second sufficed to provide the illusion of continuous motion (just barely). He also determined that viewers were quite annoyed by 10 flashes per second caused by the shutter opening and closing to accommodate motion of the film between looks. This phenomenon was simply addressed by having the shutter open and close three times for each single motion of the film, producing a 60 flashes per second presentation. This higher flash rate was tolerated nicely by humans.
Soon after, the frame rate was increased by the motion picture industry to provide a more pleasing 24 frames per second to provide an improved illusion of continuous motion. Again the phenomenon was addressed by having the shutter open and close, this time, twice for each single motion of the film, producing a 48 flashes per second presentation.
Television, interestingly enough displays 30 new images per second, but suffers from the same flash phenomenon if simply presented. This phenomenon is addressed by also presenting the images in a sense twice per frame. The way this is accomplished is that 60 times per second, every other line or raster is changed. So each new image is painted onto the screen in a two step process-first the odd rows, then the even ones-so that at every point on the screen, things are locally changing at a rate of 60 times per second. In this way we do not discern the choppiness we would see if the image were refreshed all at once 30 times per second.
This same phenomenon can be used to create digitized video-a video signal stored in binary form. We have already discussed how individual images are digitized; digital video simply consists of a sequence of digitized still images, displayed at a rate sufficiently high to appear as continuous motion to the human visual system. The individual images are obtained by a digital camera which acquires a new image at a fast enough rate (say, 60 times per second), to create a time-sampled version of the scene in motion. Because of human visual latency, these samples at certain instants in time are sufficient to capture all of the information that we are capable of taking in! When we discuss digitization of audio signals, we'll go into more depth about the idea of sampling a quantity at different times.