Night Vision - Red Light Myth

[bluesman81]

Ahhhhh. Conspiracy theories abound................

Sorry, I have found that even using a red light, (even dim), really messes up the vision for a while. I have also found that after a couple of hours in the dark that i dont need a light at all, (only to read), and i dont read in the dark. What really stuffs thing is if i forget to activate dark adapted on the laptop before I start imaging. But even then, it still reflects of the white walls in the obby...

[Stub Mandrel]

I've gone back and read a fair few of the posts and some links.

The eye's rod cells contain rhodopsin which is much more sensitive to green light than any other colour (being purple and therefore absorbing it's complementary colour the most).

This means that to see by red light, the red light must be much brighter than green light.

I suspect that in truth the colour of the light is irrelevant as in order to see cells must be triggered and to see a particular level of detail the SAME number of cells will be bleached whatever colour of light it is. The problem, I suspect, is that because the eye is many times more sensitive to green light it is rather harder to dim an ordinary light source sufficiently.

A fact is that our rods are enough to read with, and if we have enough light (red, green or dayglo yellow) to perceive it as a colour, then strictly speaking it's too bright.

[Gina]

Yes, the mono sensitive rods are much more sensitive than the cones (colour receptors) and the reason for looking to one side of a faint object to see it is that the central area of the retina has more cones than rods and the reverse applies to the peripheral area.  And the reason moonlit scenes appear monochrome is that only the rods are responding.

[psamathe]

52 minutes ago, Stub Mandrel said:

...I suspect that in truth the colour of the light is irrelevant as in order to see cells must be triggered and to see a particular level of detail the SAME number of cells will be bleached whatever colour of light it is. The problem, I suspect, is that because the eye is many times more sensitive to green light it is rather harder to dim an ordinary light source sufficiently....

Re: Colour I had heard that the colour is not relevant and that the reason red was thought best was more because putting a filter over a torch reduced the amount of light a lot (as well as changing the colour).  But I'd heard it from a source I can't remember from before the days of the internet (from my night sailing days).  But everybody kept using red lights anyway - so maybe it really has become a "social thing".

Re: Intensity I would think that in the days in incandescent torch bulbs getting dim enough in e.g. green could be a problem with the bulb alone.  But adding dense filters should reduce the light (just keep making denser and denser.  And these days with LEDs should be easy to get really dim.  But the weird thing is that the commercial motivations do not provide such products.  I have been surprised that I cannot find any case that provides a red screen cover for my iPhone.  Similarly surprised that Apple themselves have not implemented some option to massively step down the screen brightness and switch to red mode.  It's not just Astromomers who want to preserve dark adapted eyes (add people sailing on boats at night (chart plotters, etc.)).  (I appreciate many apps have "red screen" options but I think them pretty useless as the moment e.g. an e-mail arrives or any other app pops-up you are back to the bright screen again.  I have configured the iPhone triple click to additionally dim but it only helps a bit - not enough).

Ian

[umadog]

Stub Mandrel, you're right. There's nothing particularly magic about red light but it's critical that the light is dim. However, red is still the best choice, as you can see by looking at the photopigment absorption curves:

They're pretty overlapping, but the "red" cones (properly called long-wave sensitive cones) are furthest from the rods (shown in black). So with a very narrow 600 nm light source, you're stimulating your red cones a good deal but the rods hardly at all. Thus, you can read and see colour (red) but not be affecting the rods too badly. Choosing the right LED is important. LEDs don't emit at just one wavelength, but have a range. Some are broader than others:

[Stub Mandrel]

Looks like the ideal light is from narrowband red LEDs.

[Stub Mandrel]

http://cpc.farnell.com/kingbright/l-483hdt/led-flat-top-5mm-red/dp/SC11593

That's a 700nm LED that might be a drop in replacement for many existing torches/headlights. Naturally as a red one it will require much less current than a white light one, even before we look at dimming.

[ringz]

2 hours ago, Stub Mandrel said:

http://cpc.farnell.com/kingbright/l-483hdt/led-flat-top-5mm-red/dp/SC11593

That's a 700nm LED that might be a drop in replacement for many existing torches/headlights. Naturally as a red one it will require much less current than a white light one, even before we look at dimming.

unfortunately that LED only outputs 1mcd so it's going to be hard to see much with it, the good news is that your night vision will definitely be preserved.

[Paul M]

There is an interesting article on this very subject in the current (June) issue of S&T.

I think the writer concluded that a particular shade of amber is best for conserving night vision whilst still being useful.

[Stub Mandrel]

57 minutes ago, ringz said:

unfortunately that LED only outputs 1mcd so it's going to be hard to see much with it, the good news is that your night vision will definitely be preserved.

Oops, looks like 700nm is not a good wavelength for LED efficiency...

This one at 660nm and a whacking 2.8cd is more promising:

http://cpc.farnell.com/marl/110069/led-5mm-20deg-clear-red/dp/SC08583

[Stub Mandrel]

57 minutes ago, ringz said:

unfortunately that LED only outputs 1mcd so it's going to be hard to see much with it, the good news is that your night vision will definitely be preserved.

Oops, looks like 700nm is not a good wavelength for LED efficiency...

This one at 660nm and a whacking 2.8cd is more promising:

http://cpc.farnell.com/marl/110069/led-5mm-20deg-clear-red/dp/SC08583