# How is the flicker fusion rate interpreted as an on-off rate? [closed]

In our experiment we use a fly to determine the flicker fusion frequency, i.e., that frequency where the fly starts perceiving a pulsating light as a constant light source. The human eye also has a flicker fusion frequency. For example, most people cannot see the flickering of a fluorescent tube, which equals the main frequency (60 Hz in Europe & USA).

Our experimental light source needs two input durations, namely how long the light is ON and how long it is OFF for. The input is provided in ms. From what I found out so far is that the unit Hz equals the frame rate, namely pictures per second. So a frame rate of 50 Hz is 50 pictures during 1 second.

But in my experiment it is about flashes of light, namely a light phase and a dark phase. Does the dark phase then count as a picture? So what I mean is whether 50 Hz represents 50 times lights ON + 50 lights OFF, or does it mean 25 times lights ON and 25 OFF?

Also, how long is then each light pulse?

My assumption is:
50 Hz are 25 lights on and 25 lights off within 1 second.
50 pictures in 1000 ms
1 picture per 20 ms
20 ms / 2 = 10 ms lights ON, 10 ms lights OFF

• Besides the fly, I don't think the main question has anything to do with biology. I'm voting to close as off topic – theforestecologist Mar 6 '17 at 19:09
• @theforestecologist - I think questions like this are not Bio per se, but frequencies and cycle times are so fundamentally important in the visual and acoustical sciences that I strongly argue to keep this post alive. Biologists often don't receive training in signal processing, although it's an essential part of some pretty big disciplines. I have attempted to make the question biologically more interesting too, adding the key term 'flicker fusion' in there + a link – AliceD Mar 6 '17 at 20:00
• @AliceD I agree that they are important, and I can see why the OP asked this question here. However, unless the OP edits in a way to make the question more biologically relevant, I think it's off topic. This is more for their sake b/c they could likely get good answers if asked elsewhere. However, I just saw that you provided a perfectly viable and useful answer (+1), so that seems moot at this point :p. But even your answer does not mention anything biological. This whole Q&A would still probably be more appropriate on Physics.SE in my opinion. – theforestecologist Mar 6 '17 at 20:37
• If questions about chromatography get closed, I can't imagine how this one would be on-topic. – canadianer Mar 6 '17 at 21:12
• Also, mains frequency in North America is 60 Hz. – canadianer Mar 7 '17 at 8:00

A light flashing at 50 Hz means there are 50 flashes of light per second. The unit hertz means cycles per second. 1 cycle is the time it takes for the light to be ON and OFF. 50 Hz cycle time means a cycle time of 20 ms. If light ON and OFF times are equal, then the ON and OFF times are both 10 ms each.

Background
The unit hertz (Hz) is a frequency unit in the International System of Units (SI) and is defined as

one cycle per second

A textbook example where the unit Hz finds its use is a sine wave (Fig. 1). 1 cycle per second would be 1 Hz. A cycle, as can be seen in the figure, is a complete wave consisting of a positive phase followed by a negative phase.

In the right panel of figure 1, a square wave is shown. Again, a cycle time is the time it takes for one complete square wave to develop, i.e. a positive phase and a negative one.

Fig. 1. Sine wave (left) and square wave (right) sources: Simon Fraser University, Integrated Publishing

Now, in your example, you can depict the lights ON as the positive block in the square wave in Fig. 1 above, and the lights OFF as the negative phase. Together (light ON + OFF phases) they form one cycle. So a light pulsing at 50 Hz means it flashes ON and OFF 50 times.

The cycle time then is 1 second/50 = 20 ms. If we assume that the ON and OFF times are equal, which they obviously do not have to be, then light ON is 10 ms, lights OFF is 10 ms.

On the wikipedia page on the hertz unit, a flashing light example is actually illustrated and explained.