The graph here shows the human eye sensitivity as compared to a silicon based camera or sensor chip. The sensitivity of the eye at 810, 850, and 940 nm is way smaller than the highest sensitivity for green-yellow light, but there is still some residual visibility of very strong 810 nm and 850 nm light. The 940 nm is invisible even at high levels, since it is 100 times smaller than that of the 850 nm for humans. Therefore, 940 nm is used for covert observation or when even a faint red glow would be disturbing. The sensitivity of the camera is around 40% of that of 850 nm, but still usable. Some camera manufacturers are improving their sensitivity at 940 nm and this might change in the future.