In terms of the perception of flickering by CRT monitors, This answer suggests that peripheral vision has faster response and is thus more sensitive to flicker due to being provided by rod cells.

But in another answer it's said that rods are saturated at daylight and even at twilight. These two answers are in contradiction, since the light from CRT mentioned in the question pointed to by the first link above is quite above the photopic threshold.

So, how to reconcile these two answers?

  • $\begingroup$ It's not entirely clear what you mean by "why". Do you mean the mechanism by which the sensitivity is accomplished, or the evolutionary reasons why such sensitivity might increase survival. (That is, something moving fast at the edge of your visual field might want to eat you :-)) $\endgroup$
    – jamesqf
    Feb 12, 2020 at 4:13
  • $\begingroup$ @jamesqf I'm interested in the mechanism. $\endgroup$
    – Ruslan
    Feb 12, 2020 at 6:08

1 Answer 1


I think the most likely explanation is that ambient lighting conditions in the room where OP was sitting, as described in the original question, might not have been bright enough to saturate the rods. Under mesopic conditions, both rods and cones are active. Hence your statement that

...since the light from CRT mentioned in the question pointed to by the first link above is quite above the photopic threshold.

may not hold. Since rods indeed have quicker responses, the periphery may be more sensitive to the flickering of a CRT or light bulb.

Note, however, that cones have flicker fusion frequencies of up to 90 Hz (reviewed by Perz, 2010), and CRT screens and light bulbs flicker at frequencies following the mains (50 or 60 Hz). Hence, cones may still be able to pick up the flickering. Since OP reports on an anecdote, experimental conditions may have been suboptimal and their report might have been unreliable. So we cannot exclude OP was using photopic vision, regardless their report.

- Perz, Master's thesis, Eindhoven University

  • $\begingroup$ "Hence your statement <...> may not hold" — how can this happen? Typical monitors have brightness on the order of hundreds of cd/m² (sRGB standard mandates 80 cd/m²). This is far above the photopic threshold of 5 cd/m² given in BIPM Rapport (Principles Governing Photometry. 2nd Edition. 2019, section 4.5). $\endgroup$
    – Ruslan
    Feb 11, 2020 at 14:03
  • $\begingroup$ It depends on the distance you are from it and the angle you are looking at it. $\endgroup$
    – AliceD
    Feb 11, 2020 at 14:04
  • $\begingroup$ Brightness is distance-invariant. But OK, I don't know what angle of view typical CRTs have... $\endgroup$
    – Ruslan
    Feb 11, 2020 at 14:05
  • 1
    $\begingroup$ @AliceD In units of luminance, it shouldn't depend on distance. FWIW, I much more clearly perceive 120 Hz flickering in LED lamps through my peripheral vision, even for very intense lighting thousands of times more than the photopic threshold, so I suspect that the answer is more complex. Cones in the perhery are sparse and physically larger, so their sensitivity to flicker/saccades may be different. That would make some sense, since peripheral vision is there to alert you to something you need to look at with foveal vision. $\endgroup$ Feb 11, 2020 at 18:48
  • $\begingroup$ @user1850479, that is interesting. Thanks for that $\endgroup$
    – AliceD
    Feb 12, 2020 at 12:20

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