Can the Human Eye See 850 nm Light?

The human eye typically cannot see light at 850 nm, as this wavelength falls in the infrared spectrum which is beyond the visible range. The visible spectrum for humans ranges from approximately 380 nm (violet) to about 750 nm (red). Light at 850 nm is not detectable by the human eye, but some specialized devices like night vision equipment can detect infrared light.

Special Conditions for Detection

Yes, under certain conditions, the human eye can detect light at 850 nm. For example, in a dark laboratory, a 10-mW 850 nm laser can be seen on a piece of paper when held in the beam. However, the spot disappears when using laser goggles designed for 850 nm that filter out that wavelength.

Studies have shown that humans can actually see light at wavelengths above 1000 nm under the right conditions. This demonstrates that the ability to see light is not limited to the conventional visible spectrum.

Historical Experiment

Back when I was a young participant in an experiment to measure the amount of CN radical produced in the lab for astrophysical reasons, I was given a high-tech filter that passed the violet emission band of the CN radical, approximately at 388 nm, which is within the 380 nm lower limit of human vision. When peering through the filter at a broadband source and at a CN emission, I could vaguely see some violet light.

While some humans may claim to see light at the edges of nominal sensitivities, such claims require extensive verification. This anecdote highlights the variability in human perception and sensitivity to different wavelengths of light.

Infrared Band

850 nm is in the infrared band, about 100 nm longer than the longest wavelengths of visible light. Despite this, light is never considered "visible"; it's the light that enables us to see things. Once we define "vision" as the eye's ability to detect light, physical objects become invisible. A more accurate question would be whether 850 nm light is detectable, and the answer is "With the right equipment, any wavelength of light is detectable."

Further, can our eyes directly detect 850 nm light? The answer is a clear 'No!'