With both CFF and stroboscopic motion, our brains are constantly using effects to give ourselves a perception of stable vision. When we perceive an image that is first briefly seen in one location and quickly is seen in another location, then we perceive the illusion of motion. ![]() With the movies, there is also motion that complicates the scene (pun intended). Under fluorescent light, it’s just an on/off perception which is more apt to be explained solely with CFF. In fact, we spend half of the movie in darkness - just like under fluorescent light we spend half the time in darkness - but our eyes don’t perceive it. This rate is above most of our critical flicker fusion, the point at which the flickering disappears. So even though the film is moving through the aperture at 24 fps, it’s projected to the audience at 72 fps. So, what gives? Why don’t we see movies in the jerky motion of old Charlie Chaplin movies? It’s because Hollywood came up with a solution: a shutter opens and closes three times over a single frame until the frame passes on. ![]() Then when I went to optometry school, they told us that humans can perceive flicker when it’s under 60 fps. The speed seems even faster when something goes wrong and the film starts to pile up on you. I used to work as a projectionist, and I was always told that the film goes in front of the bulb at 24 frames per second (fps). But if (most) humans have a CFF of 60 Hz, then do we notice everything that’s below that frequency? Not necessarily. But in a hawk, a high CFF might allow it to better notice the slight movement in its prey while circling way overhead. In humans, it may drive you crazy because you are bothered by a flickering monitor or light bulb that your friend doesn’t notice. Let’s break it down further than that: The hertz unit represents essentially a cycle (frame) per second measurement, and the higher a CFF, the easier you can detect the average flicker.Ī high CFF is a double-edged sword. This is the critical fusion flicker frequency (CFF) - that is, the number in which we cease being able to detect a flicker in a light and just see that light as constantly on. I did an informal poll of all the people in my eye clinic, and most of us think that in school we read somewhere that the retina can perceive flicker at a rate less than 60 Hz. ![]() That made me wonder lots of things: What’s wrong with her monitor? Is this some kind of cone dystrophy? Can she really detect flicker? Why can’t I detect flicker? What’s critical flicker fusion frequency again? So, I went down a rabbit hole and learned a lot about flicker, and I thought we would tackle it for this month’s article. But recently a patient asked me if her flickering screen could have caused her asthenopic symptoms. Or at least not that we can recall.Īs we know, a lot of these complaints can be attributed to accommodation or convergence or dry eyes - the proverbial computer vision syndrome (CVS). ![]() They come to us for relief, but also with a lot of questions, questions that might not have been on our National Boards. All of these light projections aren’t steadily on - they flicker.Ī lot of our patients will come to us stating that all their computer use has put a strain on their eyes. The screen on which you’re reading this, the fluorescent light over your head, the TV on in the other room. If you continue to have this issue please contact to HealioĮverything is flickering at us.
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