Radim
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Thanks a lot, diachi. It's very appreciated.
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So I was bored today and started playing with my 800mw 980nm laser. I noticed in darkness I can see a dark grayish blue dot on the wall at close range if I move the laser slowly. Other than this the laser is not visible. Am I actually seeing the 980nm light? I have been lead to believe only 808nm is possibly visible and 980nm is far to long of a wl to see at all. But after seeing the dot I'm thinking that at these powers, 980nm might be slightly visible after all. It was really dim, by really dim I mean barely visible at all, your eyes have to adjust to the darkness and be really close to the dot.
"They were able to see the laser light, which was outside of the normal visible range, and we really wanted to figure out how they were able to sense light that was supposed to be invisible," said Frans Vinberg, PhD, one of the study's lead authors and a postdoctoral research associate in the Department of Ophthalmology and Visual Sciences at Washington University.
The visible spectrum includes waves of light that are 400-720 nanometers long," explained Kefalov, an associate professor of ophthalmology and visual sciences. "But if a pigment molecule in the retina is hit in rapid succession by a pair of photons that are 1,000 nanometers long, those light particles will deliver the same amount of energy as a single hit from a 500-nanometer photon, which is well within the visible spectrum. That's how we are able to see it. These findings clearly show that human visual perception of near infrared light occurs by two-photon isomerization of visual pigments" ~ from https://phys.org/news/2014-12-human-eye-invisible-infrared.html
Keep in mind:
Color is not a physical property; it is merely the brain’s interpretation of different wavelengths of light. Human vision spans a visual spectrum of approximately 390-720nm. At the short end (390) is what we perceive as blue; at the long end (720) is red. This is nowhere near all light; in fact, it comprises less than an estimated 1% of 1% of the entire electromagnetic spectrum. The narrow range of light we can see is primarily a result of what wavelengths our photopigments are sensitive to; as an engineer would say, the pigments are the bottleneck. Human vision works via a process called phototransduction. In this process, light enters the eye through the pupil, is focused by the lens, and strikes the retina. On the retina are billions of photoreceptive cells called rods and cones, and in these rods and cones are phototransductive compounds called photopigments. These pigments are the target of our project." ~ from: Human NIR Perception - the first project - Science for the Masses
PS good explaination of visual phototransduction is here: https://en.wikipedia.org/wiki/Visual_phototransduction
Yes the 980nm light is stimulating your retina. Here is one report of people seeing 980nm as diffuse gray.
"At these powers, the 980 nm beacon was invisible in the light-adapted retina but was still somewhat visible to most subjects when dark-adapted and displayed in a dark visual field. For example, the required powers were 0.4-0.5 log units above Subject 4's dark-adapted visual threshold for continuous viewing. The beacon generally appeared as a small red spot when fixated, but sometimes appeared as a more diffuse gray spot when off the visual axis. This diffuse gray appearance may possibly reflect the activity of rods, the two-photon/second harmonic generation effect (where light of one half the beacon wavelength is created and potentially seen, Zaidi & Pokorny, 1988), or both. " ~ from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4013550/