Lights colours and human night vision?

Can't you just tell her to behave herself?

Bill

Agreed, given humanity has been living off fruit and veg longer than we have meat, and we can't *just* live on meat in any case, the 'fad' is actually our consumption of meat.

But like / understand it or not, not causing animals to suffer and exploited, no matter how much you enjoy doing so, it is what is going to be re-normalised, as we evolve further.

For the rest of you, 'keep banging the rocks together chaps!'.

Cheers, T i m

If you want light to see by, red is the last colour you want. The eye is most sensitive to green. But if you want something as inconspicuous as possible when dark (like a display) you use red.

So given it's probably as easy to produce a torch that emits a green as it is a red light, and if we are talking about a torch and not some sort of safety beacon, why are most torches / headlights that come with white +<another colour>, generally white / red?

Cheers, T i m

So that if you need to stop a train crossing Siberia, you can switch the filter to red. Rather than slashing your arm and soaking your shirt in blood to have the same effect and dying as a hero of the Soviet Union.

What kind of torch/headlight are you talking about? I have a square torch with a white beam light and flashing yellow at the top for emergency use. I think that some countries use yellow headlights for some reason. Never heard of white/red.

For a display, is it better to use a brighter red light (to compensate for the eye's lower sensitivity to it) or a dimmer green light (to compensate for the eye's greater sensitivity)? Which would affect the dark-adaption of the eye more?

Why is it that ambient lighting for reflected-light situations (in submarines, or in an astronomical observatory) is red rather than green, if the eye is most sensitive to green? Is it easier to make a red filter than a green one?

"A recent conversation here about emergency lighting that led (excuse the pun) to head torches and the mention of red lights to help preserve night vision when needed."

I have a circular / magnetic lamp / beacon that sequences LEDs round to look like a 'beacon'.

Head torches, not headlights. ;-)

This is light to see by that doesn't impact your night vision, not lights to attract emergency services.

Cheers, T i m

It's to maintain dark adaptation as the rods in the retina are unaffected by red light.

Ah, I hadn't realised that the rods weren't sensitive to all colours. I'd thought that they were roughly panchromatic, with the standard 20:70:10 (RGB) proportions.

If you swap an LED for an LED I very much doubt that the red LED will protest much. It might get twice the rated current through it in the worst case scenario. I have never had any bother doing this with a bare LED and somewhat spent batteries for astronomy usage. Gets the last little bit of use out of every cell before throwing them away.

Are you somewhere fairly dark where you can get out of direct line of sight of street lamps? You should begin to see the milky way after about

It is new moon right now so you should get nice dark moonless skies.

Not a hope of seeing it when the moon (or sun) is above the horizon.

The ones I hate most are misaligned bluish tinged ones.

It is a tangled tale. Most people have three types of cone cell. People with defective colour vision can have either 2 or 4 types.

The latter are rare and some were much valued in WWII as spotters because they could see the difference between living foliage still attached to a tree and dying foliage on limbs cut down to put onto camoflage netting. To normal colour vision both are just "green".

The normal cone sensors are roughly Blue, Green and Yellow. "Red" is computed by the brain as Yellow-Green. Graph of their respective cone sensitivies labelled S, M & L here on WIki:

Early colour TV's relied on this method to get a decent blue phosphor. Sets without the neodymium glass the colours looked very pastel shades.

Places that work in the dark these days tend to have very well shielded downwards pointing workstation lights that are dim and white. Sometimes with the option of a brighter setting to allow colour discrimination.

Many observatories still have the old red lights though with modern red LED units inside the original red filter glass. It works pretty well.

I once had a deer walk into me in total darkness whilst observing.

If you are emitting light then the longest wavelength red is better.

If you are trying to read charts or wiring diagrams then white light used as dim as you can keep it is preferable for seeing more clearly and in colour. That inevitably means losing some dark adaption.

Neither. Although historically it was easier to manufacture low pass glass filters for any given cutoff wavelength there are plenty of dyes or dichroic filters now that can give you anything at all.

The choice of deep red is because you can retain full dark adapted scotopic vision provided that you don't have any green light at all. The price is that many people find their vision slightly blurred by it.

Increasingly because of the difficulty many people have reading documents and charts in pure red light workstations are now illuminated by very dim white light with a brightness control to allow it to be made just bright enough to distinguish colour (which is already enough to affect dark adaption). Observatory software has a night mode where everything is in shades of red or failing that a red filter to go over the display of something that cannot be customised.

You only realise quite how sensitive the eye is after a couple of hours in total darkness. Cats are even better at night their retina gets two bites at every photon that enters.

You may (or may not) have noticed that at night pillar boxes look black and foliage looks light.

Ok. I believe red were the first / easiest to make and work at the lowest voltage?

And possibly not last as long therefore?

It was with fresh batteries though I was thinking.

Nothing wrong with that, especially if you have easy access to replacements etc.

We can be yes, when walking the dog(s).

Cool.

Our streetlights go off around midnight so that might help.

Noted.

When it was just down to electrical watts and incandescent lamps it was all generally manageable. Even misaligned lamps or being swept by them whilst waiting at a roundabout wasn't 'painful'. With some of these new lamps they are painful even if not straight in front of you. ;-(

Cheers, T i m

Or when you dog runs off in (what seems to be to us) total darkness to chase something.

We have followed ours home in the dark (medium length lead) without using a torch and used him like a guide dog. ;-)

Cheers, T i m

Yes, makes me feel rather inferior when I see how much better dogs' vision is than mine :-)

When I first got a digital camera that allowed manual control of exposure as well as automatic (it had to wait for a digital camera in order that I wasn't paying for every picture I took!) I experimented with very long exposures in conditions of almost total darkness - for example in my bedroom which was lit only by the light from the alarm clock display and maybe by street lights or moonlight coming in through the window. And I first discovered just how much colour there is, if only you can integrate the light over a long enough time, and aren't relying on rods and scotoptic vision. The colour may be very monochromatic - intense green from the backlight of the LCD display, intense orange from low-pressure sodium streetlights - but it is still there.

I must admit that although I had heard of the Purkinje effect, I didn't know what it was, and I'd never really noticed that the relative brightnesses of different colours changed as the light level decreased, just that everything gradually lost its colour at dusk.

As you / one should, and against most of the animal kingdom and the fantastic things they can do *naturally*.

Yeah, and it wasn't cheap till all the mail order photolabs came in ...

Interesting.

I think that is one of those things that just happen and we do (looking), without considering it, like looking up at nights sky and getting further reminder just how insignificant we really are.

Cheers, T i m

Any light more than you actually need is bad, its just that red for some reason seems to have cells in the eye that recover faster than some of the others do, its a chemical process. I know this as its the garbage collecting mechanism that has a faulty gene which has skippered my eyesight.

I also think, defuse red is better than a beam. Brian