BLOG: What do I do when my patient sees a flicker?
Everything is flickering at us. The screen on which you’re reading this, the fluorescent light over your head, the TV on in the other room. All of these light projections aren’t steadily on — they flicker.
A lot of our patients will come to us stating that all their computer use has put a strain on their eyes. They come to us for relief, but also with a lot of questions, questions that might not have been on our National Boards. Or at least not that we can recall.
As we know, a lot of these complaints can be attributed to accommodation or convergence or dry eyes — the proverbial computer vision syndrome (CVS). But recently a patient asked me if her flickering screen could have caused her asthenopic symptoms. 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.
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. 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. 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.
A high CFF is a double-edged sword. In humans, it may drive you crazy because you are bothered by a flickering monitor or light bulb that your friend doesn’t notice. But in a hawk, a high CFF might allow it to better notice the slight movement in its prey while circling way overhead. But if (most) humans have a CFF of 60 Hz, then do we notice everything that’s below that frequency? Not necessarily.
It’s not unlike the movies. 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). The speed seems even faster when something goes wrong and the film starts to pile up on you. Then when I went to optometry school, they told us that humans can perceive flicker when it’s under 60 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. So even though the film is moving through the aperture at 24 fps, it’s projected to the audience at 72 fps. This rate is above most of our critical flicker fusion, the point at which the flickering disappears. 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. Under fluorescent light, it’s just an on/off perception which is more apt to be explained solely with CFF. With the movies, there is also motion that complicates the scene (pun intended). 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. This is called the stroboscopic motion. With both CFF and stroboscopic motion, our brains are constantly using effects to give ourselves a perception of stable vision.
So what can we do about flicker? Well, the 60 Hz number isn’t absolute. And remember, that’s the upper limit for most of us. There are variables that can affect CFF, and these variables allow us opportunities to improve patient discomfort. For instance, flicker is less noticeable depending on the depth of the change in illumination. This is intuitive: if a light goes from completely on to completely off, it’s more noticeable than if the light just dims slightly and then bounces back to fully on. Incidentally, this is a difference in LED vs. fluorescent lights. Fluorescent lights flicker from 100% to around 35% and back to 100%. LED lights go from 100% to less than 10%. Still, for most people, even flicker from an LED light isn’t detectable. But, theoretically, for those with higher CFF, it would be more often detected than with fluorescent.
Most importantly, the two main factors we can control about CFF are illumination and size. Respectively, these are the Ferry-Porter Law and the Granit-Harper Law, which I’m sure you all remember from National Boards. Drs. Ferry and Porter showed that if you increase the background illumination, you can increase your CFF. It’s thought this, “is most likely related to a general speeding up of retinal processes that occurs at increasing levels of light adaptation” (Schwartz).
The Granit-Harper Law showed the larger the stimulus, the more likely we can perceive flicker. This is most likely because the larger stimuli will fall more on our peripheral retina, where rods can help detect flicker. As we know, rods are better at movement, so a temporal change in stimuli would be better noticed by rods. There are also CFF differences in the wavelength of light shown, whether the patient is dark-adapted and, of course, the presence of ocular disease.
So if we put the everything together, we’ve found a solution we can give to our flicker-fanatical-friends. Try to dim your screen, try to sit farther back from the screen, and try to take a break. Frankly, this is advice we often give for CVS reasons, but it’s helpful to know that it’s advice we can give for flicker complaints too. And it’s good to know the science behind it, if anything, to remember that Ferry-Porter law that we forgot immediately after we learned it.
- Schwartz S. Geometrical and Visual Optics. 3rd ed. McGraw-Hill; 2013.