Red or green light, whilst observing?

[Greymouser]

Having seen a post by @Alan White, in the what the postman brought thread, about a new headtorch he had got, my curiosity was aroused. I have always loved torches, since being a small child, so it did not take too much.

I was expecting an expensive bit of kit, but it was not, which was good, but his torch when I looked it up on Amazon, led me to another from the same manufacturer: Black Diamond Storm This is a little more expensive, but what caught my eye was the fact that:

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Functions: full power for high beam and Close range, dimming function, flashing mode, night vision mode (Red, green and blue) and lock mode

That night vision mode in red; green; blue, brought back a memory of a comment from an ex Royal Marine, who once told me, back in the day, that red was not very good for night vision, nor for being discrete. I am sure he said they used blue light at very low levels for very great discretion. I decided to look further, via Google and came across this article: ( Found via Amazon I think. )  Go For The Green It seems to say that red light being best, is a myth.

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There is widespread astronomical mythology concerning the "correct", "proper", or "best" type of light to use while observing. Conventional wisdom dictates that your chosen, appropriate, source of illumination should be primarily red although spectrally pure red is even better. The conventional wisdom is wrong! Millions of observers are unnecessarily compromising their night vision each time they use a red light to read their charts. Why? Because monochromatic red light must be many times brighter than polychromatic green before we can see with it.

Interesting. Was that Royal Marine right all along? Only one way to find out I guess, try the green light and see!

So what do you all think? Worth a try or is it just me finding an excuse to buy another torch? ( Not that I really need excuses... )

[DaveS]

I'll just note in passing that theatre backstage lighting is traditionally blue (Stage blues) and very dim so as not to be visible to the audience.

[JamesF]

I can set my observatory lighting to green, so I shall give it a try.

James

[Jonk]

Well the human eye is most sensitive to green light so a very dim green light may be better than a brighter red?

[mark81]

I wonder if green or blue lights would bring star charts to life the way a red light does? I don't use the light for any other reason than checking star charts and I've never found them (red lights) to disturb my light adapted eyes...

[Greymouser]

I am not sure, but I might just get the headlight, to give it a try. I will also be interested in how @JamesF gets on in his observatory.

The Marine did say that whichever colour you use, there is a payoff, particularly when looking at a map, because it alters the colour on the map! He said that the best idea was to use an ordinary torch, with gaffer tape over the end and a pin hole ( 5mm? ) in the tape, to make it a very small amount of white light, which made reading maps easy. Of course, even then for him, in his line of work, he said you have to very careful to not make yourself a very large target. ( Not a problem for most amateur astronomers, I suspect. ) He did say that red was the worst, because it could be seen from the furthest away.

I have also been told that they use pale blue on Airplanes, by the cockpits, at least military, but can not verify this.

Either way, I thought it was an interesting article and bears some investigation, which does not have to cost much, because you can get a suitable torch, with filters, for about £4! Not that, that is the best quality...

[JamesF]

27 minutes ago, Greymouser said:

you have to very careful to not make yourself a very large target.

I think I'm resigned to being a very large target these days.

James

[markse68]

Or amber? 😂

 

[Second Time Around]

I may be totally wrong here, but if the human eye is most sensitive to green wouldn't a green light ruin dark adaption more than red?  Yes, the green light could be less powerful, but wouldn't it have the same effect as a brighter red light?

Whatever, Rigel who specialise in torches for astronomy discuss this in the article linked to below, including the use of blue-green light, and present evidence concluding that red is best for astronomy.

http://www.rigelsys.com/why_red.html

[Gina]

The colour receptors in the eyes (cones) are less sensitive than the mono ones (rods) and only the rods are used in very dim light.  That's why moonlit scenes look black and white.  It therefore seems irrelevant which colour of light is used - only the intensity.  For seeing charts either weak white light or green light would seem best.  This is from the scientific POV - since I don't observe I have no practical experience.

[Second Time Around]

Like Gina, I too was taught that we have no colour perception in very dim light.  However recent research seems to show that this isn't completely so.  See for instance https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3829393/

More practically, I doubt that many of us observe in dark enough conditions that we use scotopic vision (i.e. rods only).   I'd be interested to hear the experience of others who've observed under darker skies than my Bortle 4 ones.  For instance, does Mars still appear red?  Or is Mars so bright that looking at it damages dark adaption and brings cones into play?   In which case does scotopic vision kick in only on cloudy nights in remote locations?

I certainly agree with Gina that as dim a light as possible is needed.  But even with my dimmable Black Diamond headlamps set to minimum, my white telescope tube still appears tinged with red, so the light is probably brighter than optimal.
 

[John]

When I'm trying to observe the faintest targets (eg: the Horsehead Nebula) I don't want any light at all near me for a good hour or so before attempting to see my target.

When I've been at a very dark site even the illuminated face of a watch seems glaring when the eyes are really dark adapted.

Coversely, when planetary observing, some exposure to light can actually improve the ability to pick out subtle detail. I've read that experienced planetary observers actually stare at an illuminated white surface for a few moments before putting their eye to the eyepiece.

 

[JamesF]

18 minutes ago, Second Time Around said:

I'd be interested to hear the experience of others who've observed under darker skies than my Bortle 4 ones.  For instance, does Mars still appear red?

Seems like ages since I last had a decent view of Mars, but I would say that it does appear to have an orange tint from here.  Aldebaran definitely appears orange to me.

James

[scarp15]

Quite as John has said, no light source at all when deep sky observing. This does require some self discipline and suitable memory recall for determining where targets are situated. I resent having to use a light source at all when required as is often the case to look at a chart. For this a small dim Petzl elite red setting head torch which has an effective swivel, tilt head combined with a large magnifier where I can use both eyes to scan across pages assists. Dark adaption is just about good for every task when out, even when pouring coffee with the coffee cup resting on your lap (well nearly always).

[Gina]

Last time I saw Mars it was orange.

[bingevader]

I seem to remember this coming up before, I don't know if it was on here.

As various people have commented, as little light as possible.

But I think, in the end, it was down to star colour, if that was your thing.

With there being more red stars to look at than green, I think they decided that green light would be better in that instance.

In school, we give out red torches, to avoid people bringing their own (or using their mobile phone torches!).

It's a compromise but does mean the children get to run around in the dark!