The Wikipedia discussion of luminance includes statement that: The sun has luminance of about 1.6 times ten-to-the-ninth candela per sq meter at noon. ("Luminance". Lighting Design Glossary. Retrieved Apr 13, 2009.)
At the outset here, relate the glow of an LED diode maybe-stared-at, to the glow of this noon-day sun, 1600 candela per sq millimeter. Now convert the Sun's luminance as lumens per sq millimeter. Multiply candela by Pi steradians for 90° practical emitting angle of a diode as offered by the calculator of http://led.linear1.org/lumen.wiz . The Sun's luminance is 5000 lumens per sq millimeter. Low-glare LED lighting will have diode luminance quite a bit less than 5 lumens per sq millimeter, where any toning down by a lens is with waste of lighting power. Whatever our complaint of LED glare, it is nothing like looking at the Sun. Yet, having brightness down two orders of magnitude from that of the Sun surely is not as good an idea, as being down three orders of magnitude.
Here illustrate Array Luminance with a photo of a Nicor DLS4 luminaire, to be mentioned as especially glaring. It will not be correct or fair, to credit diminished glare to the full size of a lens, where the array is much smaller than the lens. Concentrated luminance of an array is quite visible at lens center.
A luminance table for 4" luminaires, in my opinion, should be for about 500 lumens output, like that of the 4" Glimpse, that I have loved. Please see that glare numbers are especially high for this "family." Brightness is pulled from them out of proportion to their size. Look for more-considerate glare in LED downlights of larger size.
Here are photos of luminaires with lens removed. Clockwise from top left: First Version 6" Glimpse , Nicor DLS10 , Utilitech 0752125 , 4" Glimpse , First Version Sylvania 70732 .
Sylvania 75094, 3000°K, 900 lumens, discontinued at Lowe's in deference to 750 lumens Utilitech 0752125.
Nicor DLS56, 3000°K, 1022 lumens.
Home Depot / Commercial Electric T67 and T47, 2700°K, 650 lumens and 600 lumens, improvements of 2016 with 5 mm diodes, no longer losing efficiency by recess in luminaire.
Verbatim 99001, 3000°K, 740 lumens.
Among 750 lumens LED downlights, initial offerings have been by far, easiest to look at. See that diode luminance of as little as 1.28 lumens/ mm2 was achieved. Convert to lux as times 106, 1.28x106 lux. A unity value of diode lumens/ mm2 might be ideal, with luminance at size of the diode array smaller by one order of magnitude.
The evolution of luminance in 1000 lumens luminaires is also in the direction of worsening glare.
Perhaps, the best way to control glare is with good edge lighting, as achieved with
Conturrent™ LED Downlights .
The 250 lumens Conturrent has twenty diodes 3.5 mm square. Diode area is 245 sq mm. Diode luminance is one lumen per sq mm, one million lux. The thick acrylic lens is not obscuring. An etched downward face directs the light output at excellent efficacy of 83 lumens per watt.
The readily-available Cooper SLD4 and SLD6 LED Downlights have much greater diode luminance. Array luminance with edge lighting needs definition. Where etched lens area of Conturrent might correspond to an array fully dispersed on the lens, lens area may not similarly soften the glare with the three clusters of three small diodes in SLD4 and SLD6. Moving patches of brightness on SLD4 and SLD6 may correspond to some phenomenon of extreme energy dispersal.
It is not fair to credit this Cooper SLD6 with even light dispersal to the open diameter of the lens. There is large light concentration at the three diode strips and at random, highly glaring anywhere. Without even dispersal, lens diameter is undefined. I measured 14 watts draw for Cooper SLD6, and 3 watts for Conturrent.
Add a reference found by Google search luminance per unit of diode area .
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A III - IV LED, whatever that means, is credited 1,000,000 to 10,000,00 candela per sq meter. About 1.6 candela per sq meter for Nicor DLS10 is at the low end of this range. Too-glaring Nicor DLS4 is within the range. OK Utilitech 0752125 is way beyond this range. The intervening lens modifies what the eye will tolerate. Definitions get murky for needed conditions beyond a lens, and for innovation in the nature of a lens. I believe lens nature is consistent for luminaires mentioned here. The very thick lens of cheap point-source LED bulbs is of strange other nature, losing half of the emitted light to achieve glow of a sphere.
Google bright light impairing short-term vision
Find Wikipedia, on Flash Blindness :
Flash blindness is visual impairment during and following exposure to a light flash of extremely high intensity. It may last for a few seconds to a few minutes.
For example, in everyday life, the subject of a flash photograph can be temporarily flash blinded. The bright light overwhelms the eye and only gradually fades. A bright spot or spots may be seen for many minutes. This phenomenon is leveraged in non-lethal weapons such as flash grenades and laser dazzlers.