The Persistence of Vision

Mehruba Anwar

How I Became Interested in LEDs and the Persistence of Vision

When we had to put down our choices for the rotations in my WSE187 class, I chose Optics and Lasers as one of my choices. The initial interest was sparked by the little knowledge that I had in the topic already from high school physics and AP physics, however, the little tricks and demonstrations shown to us by Dr. Noe were quite amazing. We saw demonstrations with sound and an oscillator, diffraction gratings, indexes of refraction, crystal balls as microscopes, reflection, and a lot more.

My main interest is neuroscience, and Dr. Noe tried to help me find something that would relate to both neuroscience and optics. He found a woman at McMaster University in Canada who does experiments with optical brain imaging. I found that very interesting but I did not have any direction with that. I didn't really know where to go after finding that.

I remember Dr. Noe saying something about being observant and being curious in order to find something that you just want an answer to. So, one day I was at the mall and I saw a clock in Brookstone, which is sort of like Sharper Image (a store with neat gadgets). The clock had two columns on the side and looked like there was the time in red light floating in the middle of the air. I mentioned this clock to Dr. Noe and he suggested that I do some reading online to find out more about it. I found out that the trick to the floating lights has to do with LEDs (light emitting diodes) that flash enough times to fool your eyes into thinking that the numbers are floating in the air. This phenomenon, I found out later, has to do with the persistence of vision and the Phi Phenomenon.

These are the same concepts used in making a movie seem like a continuous image instead of flickering frames. If there are enough repetitions in one second, the human eyes and brain gets fooled into thinking that it is a continuous image instead of separate ones.

This idea was really interesting to me because it combined my interest in the human brain with optics. I still don't know what I would exactly experiment with, but this topic seems feasible and promising right now.

A Look at how the "Fantazein Clock" Works!

A good explanation of how the clock tricks the mind

Click here if you want one for yourself!

The First Few Days of Playing Around

Alright, so I knew I wanted to do something with LEDs and the persistence of vision and Dr. Noe was nice enough to get a range of LEDs from RadioShack. This is a list of things I had at my disposal to work with:

  1. 5mm Yellow - 2.1V, 20mA, 720mcd.
  2. 5mm Blue - 3.7V, 20mA, 2600mcd.
  3. 5mm White - 3.6V, 20mA, 1100mcd.
  4. 5mm Green - 2.1V, 30mA, 630mcd.
  5. 5mm Red - 1.7V, 20mA, 800mcd.
  6. Jumbo/Super Bright Red - 2.4V, 20mA, 5000mcd.
  7. Bicolor Red/Green - 2.8V, 30mA, 2.5mcd.
  8. Oscillator (max voltage plus/minus 12.75 V)
  9. Digital camera to take pictures when I saw something cool.

Each LED was set up in a series circuit using a 470 ohm resistor.


The oscillator was really useful because it let me examine the lights at different on/off frequencies as well as different applied voltages. I used the square wave waveform on the oscillator, so the light was on half the time and off half the time.

One of the most obvious observations I made was that as the frequency of the lights decrease, we can detect the flickering more and more. The flickering is also more detectable as the lights are moving. When the output is increased, or when the intensity is greater, it is easier to detect flickering than when the light is dim. While looking through the "Light, Color, and Vision" section of Dr. Metcalf's book "Topics in Classical Biophysics," I found that when a flickering light appears to be blended or fused together to seem like steady light, that certain frequency at which it happens is called the Critical Fusion Frequency (CFF). CFF is dependent on brightness, wavelength, and size (all of which I observed to have an effect on when I could detect flickering and when I could not). Under ideal conditions, the CFF for bright light can be 60 Hz and for very dim it can be as small as 5 Hz.

These pictures below are just some cool pictures of the Jumbo Red LED. The effect is done through diffraction glasses put in front of the camera lens. The first picture is a close up of the light (that's why the light source seems so large and blurry) but the second one is taken from a distance, which creates a small, sharp light source. It almost looks like the lights were flying out at Dr. Noe. This is because the light from the LED is not pure red but it is actually some mixture of colors that create the streaks. If it were a red laser, then you would only see dots of red because red lasers are pure red.


Another observation I made was that when the LED was spun around very fast, the light was not continuous even if the light looked continuous when still. When spun at the same speed, the light tracks seemed to have frozen into the same space. As the frequency was increased, the number of breaks in the light was shorter and shorter, however, the breaks were barely noticeable at frequencies of approximately 10,000 Hz. This sort of showed me what was going on in the "Fantazein Clock." We tried to take pictures of this phenomenon, but we were only partly successful because the shutter on the camera does not stay open long enough in order to catch a whole rotation. Here is a part of what we got though.


Using the oscilloscope it was also possible to see the signal put out by the oscillator. It was cool to see that the oscilloscope also works by the principles of the persistence of vision; the higher the frequency the harder it is to detect the electron beam moving. In the oscilloscope we could see that the oscillator voltage goes back and forth between negative and positive and this is just clearer to see using the sine curve because you can actually see the beam moving back and forth. Below is just a comparison of the signal using the sine wave and the square wave. Also, the LEDs are diodes, therefore, the current will flow when the voltage is positive. The bi-color LED however is a little different because it has two LEDs back to back which enables both LEDs to stay on alternately. If a DC voltage was applied to the bi-color LED only one color would light up.


One of the most interesting things which I noticed was that even when the light seemed completely steady while I was looking directly into it, when I looked away and viewed it only through my peripheral vision, I could detect a great deal of flickering. I was really surprised by that and I had no idea why it happens. We can see the same thing at work when we are trying to look directly at a very faint star. When looking directly at it, it is not noticeable at all, however, when we look away a little bit, we can detect the star in the sky. Maybe these two events are related because we have the best visual acuity at the center of our field of vision and when we use our peripheral vision, maybe we can detect flickering because less light enters our eye from the source.

After just playing around with these things for a couple of hours I became curious about a number of things:

  • What is the frequency at which we can begin to detect flickering looking directly into the light vs. through our peripheral vision?

  • How do the threshold of detection compare with different color lights, intensities, or distance?

My Measurements Using the LEDs

Since I discovered difference in detection of flicker looking directly at the light versus through peripheral vision, I was most curious about that, more specifically, I wanted to see the exact difference between detection using different visual methods. Using the materials provided, I set up an experiment where I measured the threshold of flicker detection at maximum brightness and minimum brightness for eight different LEDs. The results were acquired to get more of a qualitative sense of difference rather than strict quantitative. In an absolutely serious experiment, much precaution would have to be taken to make sure each measurement is taken under specific standards such as distance, ambient lighting, and numerous subjects. In this experiment, I was the only subject.

Direct Vision Flicker Detection Threshold (frequency)

Intensity Red Yellow Green Blue White Jumbo Bicolor
max 36 39 41 41 41 40 39
min 25 30 36 37 36 25 25

Peripheral Vision Flicker Detection Threshold (frequency)

Intensity Red Yellow Green Blue White Jumbo Bicolor
max 49 55 54 58 61 52 31
min 30 44 41 53 46 35 23

The results agree with what was already known about blending and the factors involved. However, by doing this experiment I clearly demonstrated the magnitude of the difference between the flicker frequency that direct vision can pick up versus peripheral vision. Our peripheral vision can pick up frequencies much higher than our direct vision can. This shows us that the eyes and brain are amazing technical instruments.

The difference size makes can also be seen through this demonstration. While the threshold of detection for a normal red LED was 36 Hz at maximum intensity, the threshold of detection was 40 Hz for the Jumbo Red LED.

Another curious occurrence that I noticed was that for the bicolor LED where the Red and Green lights blend together, the individual flickering for each color could also be seen at different angles which shows the difference that color makes even though they were at the same intensity and frequency! The bicolor LED was the only LED where the threshold for detection through peripheral vision was lower than through direct vision. This was probably due to the fact that the bicolor LED was in a translucent lens, whereas all the others were in transparent ones.

Of course, no exact conclusions can be drawn from the experiment because we would need a much wider range of individuals being studied to see if the threshold of detection is independent of the observer. Also, results might differ between age groups as well as between individuals with different medical conditions. However, this demonstration can give a good sense of the way the brain analyzes light.

Some Notes and Resources on Perception and the Eyes/Brain

Everything you want to know about Visual Perception Highly Recommended

Tanya's Project: LEDs as Light Detectors

Nicole's weblinks

How our eyes process motion

The Nature of Perception

Fool Your Eyes: Some neat Optical Illusions

A very In-depth look at Visual Physiology