Why astronomers use red light to read by

You may recall that our retinas are packed with two types of photon-detectors: rods and cones. Under conditions of sufficient light, we almost only use our cones. Since these come in three flavours, each responding maximally to a different range of light frequencies, we get to see in colour during the day. Furthermore, since they are densely packed right where the images focus (at the fovea), and since they are cleverly wired up, the visual acuity that we get with our cones is really good. This type of vision - under well-lit conditions - is known as photopic vision.

When light levels drop, however, we have to switch to low light, scotopic vision, which uses our rods. There's only one 'flavour' of these cells, and so our vision at night is monochromatic. Also, because these cells are 1000 times more sensitive to light than the cone cells are, they permit us to see in the dark. However, they are distributed largely at the peripheries of the retina, and so the image they collect is rather distorted. Furthermore, many rods all club together to send a message to the brain (via the bipolar cells, if you remember your retinal anatomy) and although this increases the chance of a bipolar cell responding to dim light, it also means that localisation is less precise. Together, these two facts conspire to ensure that our visual acuity at night is rather poor.

Enter the astronomer. She comes with an unreasonable demand: she needs scoptopic vision to make out faint stars by night (her cones wouldn't detect anything at these low levels), but she also occasionally needs her high acuity photopic vision to consult her star charts, or to write notes. For this she needs to add enough light to her page for her cones to kick in.

This may sound like a nice plan, but unfortunately to get her rods to see optimally in the dark requires about 30 minutes. Consider walking off the street at noon into a dark theatre: you are functionally blind for a good few seconds, and your retina only gradually adapts to the dark thereafter. Clearly switching frequently between enough-light-to-read-by and dark-enough-to-see-stars is impractical unless one has many hours to kill. So what to do?

Fortunately, there is a partial solution. Shine most shades of light at levels bright enough to excite your cones and you will knock out your rods for several minutes (e.g. walking into the theatre from a bright street). However, your rods are fortunately rather insensitive to pure red light when they are dark-adapted. In fact, they are 50 to 100-fold less sensitive than the flavour of cones that happens to detect red light. What this means is that if you use red light that isn't too bright, you can excite your cones enough to read by, without blowing your rods. In other words, you can then switch quickly between scotopic and photopic vision without having to wait for your eyes to adjust to the change in light levels.