Figure 1: A low pass (visible light blocking) filter, that allows infrared to pass thru it unblocked.
Normally, IR is invisible to the human eye. So, how could a person detect infrared radiation with the eye? It can be done indirectly. In figure 1 is the low pass visible light blocking filter known to all IR photographers. It blocks visible light (notice that it is jet black, even when held in front of a bright lamp).
The thing is, the human eye can detect IR already. It can detect even down to the level of upper level “far” IR! But, the brain has been, over the period of the evolution of man, purposed to use only the so-called “visible” spectrum for image processing. So, how is the remaining spectrum, mostly IR (Near, Mid, and Far) used by the human eye? That’s an interesting question.
To convince yourself that your eye can sense IR is very easy. Take a walk in the park on a bright sunny day. As you walk, you’ll pass through areas that are deeply shrouded in foliage, and your pupils will open. Further on, you will burst out into an overpoweringly bright patch of sunlit trail, uncovered by the surrounding trees.
You will automatically squint and your eyelids will close momentarily. This is instinct, and protection for your eyes. But, in addition, your eyes (more accurately, the area of the brain behind your eyes) – will feel a tingle, sometimes accompanied by a sort of “pressure” feeling similar to that which occurs when driving to higher altitudes in the mountains. Additionally, in some cases you will see colored circles.
All of this is the brains reaction to excessive stimulus thru the eye. Some of this stimulus is visible light, some of it is IR. Sunlight is about 50/50 (visible to IR ratio). Now, remember the filter of figure 1?
We take two of those kind of filters, and fashion a little set of goggles out of cardboard. Yes, passes-by will look at you and make gestures with their fingers next to their ears. Ignore.
Now, we take black electrical tape, and tape all around to make sure no light gets behind the goggles from left, right, top, or bottom. We take another walk in the park on another bright sunny day. You can probably guess what happens. You experience the exact same response in your head when you enter into the bright patch of trail in the park. Exactly! Your brain has detected the IR (using the photon sensors that you came equipped with: your eyes).
But, we aren’t done. We’d like to double the evidence. How? By using a different kind of filter. You can buy welder’s glasses for about $10 from an industrial safety equipment store, or online (they’re at Amazon too). The ones I use are called IR 5.0 “Shade 5” OTS with UV block. These glasses fit over the top of my spectacles, and block both IR (infrared) and UV (ultra-violet) rays.
So, we take our new IR blocking spectacles, our visible light blocking filters, and we go back to the park. We don’t wear the welder’s glasses while driving, because a person can’t see red with them on, and at traffic lights that is a hazard.
When we get to the park, we use our electrical tape again.