Interesting, I did not know there's a wide range of sensitivity of the photosensitive retinal ganglion cells. I guess that might help explain why some people develop SAD (seasonal affective disorder, aka winter depression).
I use light therapy just to combat SAD during the winter months.
What I do is arrange for bright white light at the areas around my computer screen (below my screen, and to the left and right). So while I am working at my computer, I automatically get exposed to the light. I spend around 4 or 5 months each winter using this setup, for the darkest months.
I know when I am not getting enough light, because horrible winter depression will set in within days. When I increase the light level, it takes a few days to disperse the depression,
I use a lux light meter
to check the lux level at my eye position. You can buy such a meter for around £20.
In order to ward off winter depression, it is usually recommended to expose your eyes to 10,000 lux for 30 minutes. If you use lower lux levels, then it requires longer exposure times. For example, 5000 lux requires 1 hour, 2500 lux requires 2 hours, etc.
With my computer screen SAD lamp setup, I get around 1000 lux at my normal eye position when viewing the computer screen, and I expose myself to this for about 5 or 6 hours a day.
By contrast, the room where I work on my computer gets only about 200 lux during the winter, as measured on my light meter, and that's when it's a bright sunny day outside. On a sunny winter day in the UK, I measure about 8000 lux outside, but only about 200 lux in my room.
However, while this SAD lamp setup fixes my SAD, it has never helped my ME/CFS shifted circadian rhythm.
found that light therapy for ME/CFS patients did not help reset circadian rhythm, and did not help with any ME/CFS symptoms.