A question of night vision sensitivity

I left my last post with the final design of my new fitness watch face not quite decided. It has an OLED display. I favoured green and red hands on a black background, and wondered whether there was a solid optical reason for this instinctive preference.

Indeed there was. I did a little research into the best lit-up colours to use at night, and it was most illuminating.

To recap a bit: I need to be able to see what the time is at a glance, at night, without glasses on. I want the watch face for this to be perfectly usable in daytime too. It's always possible to set up at least two watch faces on the watch, a light one for day and a dark one for night, as I did with my Fitbit Versa 3. But it's less faff if I stick to one face for all occasions. For use at night, an analogue watch face is best, as one can then judge the time simply from the angle the short hour hand makes with the long minute hand, and it doesn't matter if these hands are a little out of focus. It does help, however, if the hour and minute hands look bright in very dim light. This is where choosing the right colour comes in.

My researches took me to astronomical websites, and to discussions on which kind of illumination has the least detrimental effect on one's night vision. No star-gazer wants to be rendered blind to dim stars if a torch or other night-light is switched on. The discussions strayed into military and air force practice. 

It turns out that, for a very long time, low-intensity red light has been used for night-time illumination, as red has the least effect on the receptors in the human eye. In other words, the human eye isn't very sensitive to red light. But that insensitivity means that low-intensity red light is not ideal for seeing details with any clarity. Stepping up the intensity to make those details clear is not a good solution, as it makes the red too bright, and night vision is gone. You can get around this problem by using green light instead. Green isn't as good as red for preserving night vision, but it reveals detail even at low intensity.

The colour to be avoided is blue. Human eyes are very sensitive to blue, and blue illumination will destroy night vision. This is why doing late-evening things on blue-rich phone screens is discouraged, as it over-stimulates one's eyes, and therefore the brain. Also why cars with super-bright blue headlights seem so dazzling.

This is the watch face I've now adopted:

The green hour and minute hands really stand out in all lighting conditions, but especially in darkness. If I wake up at four o'clock with sleepy eyes and no glasses on, I can easily see what the time is, and still fall asleep again with no difficulty. 

As related above, there is some science behind my being drawn to using green and red on this watch face. But it's also striking how often red and green are dominant colours in the meals I make for myself. I must find red and green a vibrant and exciting combination of colours. For instance, in these meals:

It's as if my new watch were a dinner plate!

And in the wider world, red or green (or both) are often used where unmistakable visibility at a distance really matters. Traffic lights, railway signals, warning lights on tall structures, lane-marking studs on motorways, flashing lights on channel-marking buoys at sea, all come to mind.