Lights colours and human night vision?

[T i m]

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.

An example of that was we were walking the dog last night with the Mrs and daughter and because the Mr's night sight seems to be pretty s*1t and so she generally carries a small pocket torch.

Thinking it was helping us (walking in front) she shone her torch forward and because only daughter and I were keeping an eye on the dog, we would often be turning round and ending up with a face full of bright white light, then we could see less when we were looking forward again.

I believe red light is used to help lessen the impact of night vision loss but does it matter how the red light is formed?

Eg, Years ago I made up a sun visor for my Messerschmitt with some green film that was used in stage lighting. The first set of traffic lights I came to I thought were out so had to double take under the visor and seen they were indeed set to red. ;-(

So, if I wanted to say manage her torch to make it less of an issue to us, would *any* red filter (within reason), or a red LED work re that?

Cheers, T i m

[nightjar]

It is more important to keep the light intensity low. Bright red light will kill your night vision just as effectively as bright white light. Red is traditionally used because a dim red light is better for keeping night vision than dim white light. However, for map reading at night, dim green or blue green may be better, as it allows you to see different colours better.

[polygonum_on_google]

Quite a number of medical/health issues can cause impaired vision, specifically including poor night vision and reduced colour perception. Hypothyroidism being one.

[newshound]

I have a number of head torches, one of them has a relatively dim red option (I think it is separate LEDs). It was excellent when the comet was visible because on that setting I could still pick my way across my overgrown garden, set up a camera and tripod, etc. The comet was barely visible to the naked eye from background lighting, but an easy binocular object and not too difficult to frame with a digital camera. So for astro use or wildlife at night I'd strongly recommend one of these.

[Fredxx]

Red light stimulated the cones but not rods in the eye. That means you can still see things during the process of dark adaption.

The eye is sensitive to broadband red, so anything that is in the red part of the spectrum will be perceived as the same red.

Most red filters associated with incandescent lamps will allow some green through. A red LED will be near pure pure red, or very narrowband.

[T i m]

<snip>

Well I thought that of course but wondered if there was some strange phenomenon that made red light, or some specific red light 'better', than say white for the same number of lumens.

Ok. Part of the reason for the question was because I have a BIGLarry2 multipurpose [1] torch / lamp that has both white and red illumination and testing both whilst out walking, the red didn't seem to be much better at minimising the effect on my night vision as I thought it might?

The red at max brightness is supposed to be 40 lumen, versus the white

500 ... but (3 of the 4 modes) can be dimmed by holding the button to a fair bit less bright (it cycles round, flashing briefly at the extremes).

I think I'd need to give the torch to the Mrs on red and see if she can 1) still navigate by it and 2) do so without it impacting my night vision as much as her white LED torch did.

Interesting (and reminds me of the colour changing torch I had as a kid). ;-)

Cheers, T i m

[1] I'd not had the torch long when I'd used it's magnetic base to stick onto the bench saw as I was working in my own light otherwise and stuck on the garage door to provide extra light to the PIR light, again, to minimise the shadows.

It seem to have a very 'snug' fitting battery tube, so much so that a couple of my rechargeables I've tried so far won't fit in it (well, one will as it's fatter at one end but I didn't risk pushing it right in in case I couldn't get it out). ;-)

[Chris Green]

A 'dark green' light was used in dark rooms for working with panchromatic (i.e. sensitive to all colours) film, the logic being that our eyes are most senisitive to green light so it could be dimmer than any other colour used.

Red was used (a *long* time ago) with orthochromatic film that wasn't sensitive to red.

A yellow light is/was used with bromide paper used for printing B&W prints.

[T i m]

<snip>

Ok.

<Googles>

Ah, ok, well she might be suffering some of the (other) symptoms but we are both due for a blood test so I'll see if we can get them (this year)?

FWIW, she is a gold star blood donor so do they routinely test such donations do you know?

Cheers, T i m

[Martin Brown]

Although it is basically true you really don't want to stare at a high power red LED torch either. You will see a green after image from it.

The trick relies on the fact that colour vision on the cones is still sensitive to red whereas the low light scotopic vision is only sensitive to the mid green. You can sacrifice the red cones and preserve most of your night vision by using a pure long wave red light source.

A red LED at the longest wavelength you can find is the most effective way to preserve night vision with reasonable levels of brightness. (you don't want a "bright red" LED)

This Kingbright one is as good as any at 660nm its a pure dark red.

As a filter the Lee filter 789 "Blood red" is about the closest to a cheap night vision preserving filter. But the more common Wratten 25 or

29 would do at a pinch. As will the red glasses/goggles used for making laser levels visible in sunlight. I have a pair of those too.

Red LED would be fine though it might need a small series resistor with it or be in a torch containing spent batteries to limit the current.

Not true. You can get away with a fair amount of pure red light without losing any significant night vision. The snag is that many people have great difficulty reading maps or documents in red light so for military purposed mid green or well shielded dim white light is now preferred.

You can see by the light of a white LED torch with about 10uA of current flowing through it once dark adapted. Few people these days have experienced total darkness any more. Bridging the on/off switch to have an emergency torch you can find in total darkness is a handy trick.

[polygonum_on_google]

No - they don't.

And, unfortunately, the default standard test is TSH (Thyroid Stimulating Hormone) which as many problems. Starting with a ridiculously wide reference interval.

[Martin Brown]

That was actually because the panchromatic films of the day were almost completely insensitive to green wavelengths around 500nm. As a result famous astronomical nebulae slides from pre 1971 were invariably pink and powder blue. Despite the fact that the brightest ones look green!

In the early 70's someone made an emulsion that was sensitive to green

500nm OIII emission line and nebulae were imaged in true colour for the first time. It made the front cover of Scientific American at the time.

These days all CCDs can render true colour remarkably well.

Red and yellow light both are fine with bromide papers. The key thing is that dark adapted vision is very insensitive to red light so you don't lose dark adaption on the rods even though the cones get bleached.

Note the sensitivity curve it is almost at baseline for 625nm.

In stronger light the cones sensitivity curve applies:

[Tim+]

No, but you can be pretty sure that she doesn?t have HIV, syphilis or Hep B or C. ;-)

Tim

[Max Demian]

The rods in the normal retina are sensitive to blue/green light. Exposure to red light doesn't stimulate them at all, so preserving night vision. There's a scene in the film Ice Station Zebra where people sit around in the cabin wearing red goggles. Then the cabin lights are switched from white to red and the people take off their goggles and use the periscope. This is with people with normal night vision.

In your situation I think you will just have to keep the illumination to the minimum and avoid flashing the torch around too much as a brief exposure to bright light cancels night vision which takes about 20 minutes to be restored. I don't think the colour of the torch or any kind of visor will help.

[Fredxx]

One of them is B12 deficiency that damages to the optic nerve.

Veganism has a lot to answer for.

[T i m]

<snip>

I can remember doing some of that when I was processing prints in my darkroom (ok, my Mums downstairs toilet). ;-)

We did it when going though the induction at Kodak as well, really as a general backgrounder for when working on the microfiche / film machines.

Cheers, T i m

[T i m]

;-)

Neat.

The BIGLarry5 torch can produce 40 lumens of red in it's brightest setting and whilst it does dim when you hold the button, I'm not sure by how much. You can see it dimming but it gives a little flash as it reaches the thresholds of each cycle.

Handy, ta.

;-)

Understood. I note some of the older hand held torches with individual LEDS switch between 1, 3 or all four so if I was to replace the one (and suitable load resistor) it cold be dual purpose?

Interesting.

I wonder how much of my 'night vision' (compared with the Mrs) is actually being able to see (say the pavement) in front of me or taking in all the other features around me (like the kerb or walls / fences etc).

It's like when you are driving towards someone with very bright / misaligned headlights and you can't actually see where you are going but you can see the kerb / white line, mostly by their lights and so you work from them?

Cheers, T i m

[T i m]

<snip>

Phew! ;-)

Cheers, T i m

[T i m]

<snip>

I wonder if we evolved that or was it coincidental? I mean, could we have evolved such a thing during the time we had access to bright lights or could the sun have been enough? ;-)

Cool.

Tell me about it. ;-(

We can't have her in front with her torch without steering her about at junctions by voice. ;-)

Even with a (small but bright / COB) torch she doesn't to be very sound of step, certainly compared with good daylight.

I have taken to making sure I hold her hand (if the width of the path allows) because she walked off the edge of the kerb a while back and landed in the road (not good at 70+ and with her arthritis and metal knees). ;-(

The only things that might catch me out at light levels that she wouldn't even try to walk in at any pace in are overhanging branches, potholes and the like.

But then she's not had 'good vision' (unaided) since she was a child so ...

That said, in reasonable light she can read (unaided) finer print than I can with my x2.5 readers so is still useful sometimes. ;-)

Cheers, T i m

[williamwright]

I used a red safelight when processing prints, but used total darkness when processing monochrome film.

Bill

[williamwright]

Coz your knob would have dropped off by now.

Bill