A instrument of any kind, speed, log, deep sounder and even monochrome radar
, shall have dimmable red light if they shall be used during night and you want to preserve your night vision. If you want to use instruments were colour are essential, for example chart plotters, do best to dedicate one person (who will ruin its night vision) to do this works.
There were several heavy texts Grod have links to (#8). They say that there are tree types of cons, and those cones that are sensitive for red is the absolutely most common. If red doesn’t close down the pupil as much as other colours, that would also contribute to the favour of red.
As noelex say, rods can’t see details. Do a test by have the frontpage of a newspaper in front of you. Pot the focus beside the paper (more than 15°) and try to read. For my self, I can’t read even the biggest signs. The rods are they who beside focus and are most sensitive to blue-green and can’t see colour. The only time blue-green is usable are in cabin
to be able to orientate your self. In my boat, which is quite narrow (2.5 m / 8.3 feet), I have seldom problem with the orientation.
A more serious problem is what colour shall I have on the buoy rescue
light (ore similar equipment)? White has all colours and is most used today. But there is limited capacity in the batteries and you want it to light as long as possible. In other words, the light should use as little power as possible. When you have thrown rescue
buoy and whit that the lamp, it takes a period of time before you are back to that place. During that time the distance to the light have been quite big and the light have dimmed a lot. Way use the limited power on all colours when you only see the blue-green? Its good if it flashes because the rods are more sensitive to changes, and it saves energy.