I know this overlaps a lot of previously asked questions but:

I understand that 'A photon hits a photosensitive molecule in a photoreceptor in the retina, which causes a chemical change. [...] Vision is slow: the cascade in response to a single photon takes on the order of 100s of milliseconds.' (for a persistent perceptual experience, why is video able to have a lower frame rate than audio?)

1) What is the longest amount of time a bright light can shine without it being visible to the human eye? (assume it goes from dark to bright to dark)

2) Would it make a difference if, at other times before and after the bright light, the light was on but dim?

My guess:

It would depend on the brightness of the light, and that even then it's almost impossible, as it would be to do with the number of photons hitting receptors, rather than how brief a period it happened. (I tried flashing a screen for 1/60th of a second and I could still see it).

I would also guess that the opposite - how long would a period of darkness or dim light have to last in between bright light in order for the brain to not register the darkness - would be much less time, certainly no more than 1/60th of a second. (I tried a 120Hz '3d' screen with glasses producing a 60Hz image).

  • $\begingroup$ My guess is that not only will it depend on the brightness of the light, but also depend on ambient light. $\endgroup$ Jan 30, 2019 at 10:16

1 Answer 1


It depends on ambient light, but in darkness, humans can detect as few as several photons, perhaps even down to a single photon (though all detection at these low levels is probabilistic - see Tinsley et al, 2016).

Therefore, this depends on the ambient light (ideal detection in darkness), and the number of photons. At this level of detection, it doesn't really make much sense to think about the duration of the signal: the duration can be infinitesimally brief as long as the intensity is enough to hit the retina with a few photons in the visible range.

A flash of 'darkness' is much different. Introducing dark flashes to a signal that previously had no dark flashes will be perceived as a change in intensity; you can read about contrast sensitivity to find these detection limits. The other attribute that is studied is the flicker fusion threshold which is the frame rate at which a flicker (i.e., alternation of dark and light flashes) can be perceived. The exact frequency of the flicker fusion threshold varies with condition, so there is no single number I can give to answer that question (see Brindley et al, 1966 for some ranges), except to say that it is typically less than 60 Hz for still images and higher frequency for brighter light (that is, if the light is brighter, you detect flicker at higher frequencies) and higher frequencies for moving objects.

Brindley, G. S., Du Croz, J. J., & Rushton, W. A. H. (1966). The flicker fusion frequency of the blue‐sensitive mechanism of colour vision. The Journal of physiology, 183(2), 497-500.

Tinsley, J. N., Molodtsov, M. I., Prevedel, R., Wartmann, D., Espigulé-Pons, J., Lauwers, M., & Vaziri, A. (2016). Direct detection of a single photon by humans. Nature communications, 7, 12172.

  • $\begingroup$ Thanks Bryan, much appreciated, exactly what I was after. So 1) if a really bright light (eg bright torch, car headlight, or similar) was shone at someone even for an infinitesimal time, they'd still see it. 2) If that light was on and then was turned off and on once for an infinitesimal time, they'd at best perceive it as a slight dimming, and at worst not perceive it at all. $\endgroup$ Feb 1, 2019 at 5:36
  • $\begingroup$ @JamesCarlyle-Clarke Mostly correct, although be a bit careful with infinitesimal, because once you are talking about real light sources they may be very bright, just not infinitely bright. What you would want to do though is calculate the number of photons that would be hitting the retina to make your light seen: for that all you need to know is how many photons are produced, the distance from the viewer, and the size of the pupil: that is, all physics/geometry, not biology. $\endgroup$
    – Bryan Krause
    Feb 1, 2019 at 16:16

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