When exactly does a romantic “sparkle” become a bothersome “glare”?

I’ve been considering this question now for some time, particularly in relation to Richard Kelly’s six qualities of light.

To me, the primary factor that tips the balance between a delightful sparkle and a disabling veiling glare is not the contrast ratio, but rather the time dynamic of the occurrence.  For example, brief flittering sparkles are pleasant and refreshing; long periods of enduring excessive contrast is tiring on the eyes, or at worst, disabling of vision.

So a basic consideration of sparkle vs. glare is something like this:

(FOCAL GLOW ÷ AMBIENT LUMINESCENCE) × TIME FACTOR = VISUAL IMPAIRMENT

So thinking of examples (without any particular units):

“Sparkle” might be an intensity of 10x Focal Glow divided by 1x Ambient Luminescence, multiplied by 1t of time = (10)

“Glare” might be an intensity of 10x Focal Glow divided by 1x Ambient Luminescence, multiplied by 10t of time = (100)

However, conversely, I also propose that some sort of visual “relief factor” or “stagnation factor” is just as important as considering “glare” or more broadly “visual impairment” as considered via human well-being.  Other patterns of movement, motion, and qualitative change of lighting features can be just as refreshing or bothersome as simple contrast ratios.  An environment that never changes – that is relentlessly oppressive, unyielding – should be penalized somehow over environments that provide consciously scripted or algorithm-based temporal changes to better match natural human patterns.

So maybe something like this:

(FOCAL GLOW ÷ AMBIENT LUMINESCENCE) × TIME FACTOR × STAGNATION FACTOR = VISUAL IMPAIRMENT

It would be nice to turn this into a high-number-is-better calculation…but I’m not sure how to work the math.  Take it from “visual impairment” to “environmental visual quality”.  Maybe this would be a good concept for a PhD student somewhere to sort out!

The industry is pushing color-temperature variability, which is OK, but I feel all of the six properties described by Kelly should be explored.  There is rich territory here waiting to be exploited…