Green or red filter

[Kenny Fowler]

Hi all,

I have been looking at getting a torch for when my eyes are dark adapted.

There was some discussion stating the merits of green light for very low level vision.

Have any of you chaps used green filters instead of red on dark nights?

cheers

Kenny

[ronin]

The technicallity is that you need a red torch that is longer then 620nm in wavelength, up around 650nm is good.

The bits that do the "night vision" are hit by light that is less then this.

Needs a diagram to explain, but it is a case of where the sensitivity (that looks wrong) lies, but green would cause the loss of night vision.

I always wondered why but a few months ago I fell across an explanation, that made sense all round. As in you see red and yet the night vision remains. The Red is bright enough to trigger the colour receptors, but the wavelength is such that the night vision receptors are not desensatised.

Could try one of these: RedTorch

They work OK (ish), they simply cycle through, Bright, Dim, Flash.

They do not start at Bright after being off for a while.

Worth looking through ebay as the same supplier may/does advertise the same at about £3.95 which includes postage.

Actually the same supplier has about 4 different adverts for the same torch, different prices with/without postage.

[laudropb]

I have found that a red torch preserves dark vision better, but you must use it at a dim setting. Just enough light to read charts or change eye pieces.

[Putaendo Patrick]

Red is best - however if you look at something like a map with printed red-brown features (like contour lines), these will be washed out with a red light. Hence some people, especially military, used dim green - maybe it's also slightly less visible at a distance. Neither benefit is particularly useful for astronomy.

[cjdawson]

Red is best for sure.  Also, a dimable red light is better still.  The reason is that there is a chemical in your eye "Rhodopsin" which is responsible for protecting your night vision.  This chemical reacts to light, so when it's hit by a powerful enough light, it reacts and this shuts off your night vision.  The reason it takes 30 mins to dark adapt, is that's how long it takes for your eye to flood with Rhodopsin turning on all your night vision sensors.

The chemical acts light a hair trigger and the moment that any bright light hits it, it will react and shut down all the night vision parts of your eye and cause the pupil to contract.  This protects the sensitive part of the eye from damage.  Bright enough red light can trigger this reaction too, hence dimable torch is best.

Rhodopsin is least sensitive to red light, and this is the reason why we prefer to use red light over other colours.

Your eye is most sensitive to Green light and this is why Green laser pointers are sometimes used when giving the general public a guided tour of the night sky.  As your eye is most sensitive to it, it's easier to see than other colours.

[Kenny Fowler]

Thanks for the info ,so a dimmable torch with a red filter for astronomy and green filter for when I am going out to the hills and will be map reading.

[Moonshane]

I have used a green filter in the past as I found maps easier to read. hat said, it makes the red galaxies in maps turn black which sometimes does not help!

Now I just use a slightly brighter red torch at home where there's lots of light pollution and a dimmer one where it's a darker site.