Night Vision. How it works.

[Wayland]

Like many I've known for years that a red light is less disruptive to night vision than anything else but I never thought about why until I stumbled upon it in a book recently.

As you may know, the retina of the eye is made up of two types of cell, rods and cones.

Broadly speaking, cones are sensitive to colour and fine detail but only work well in good light, rods on the other hand don't see in colour but do need less light to see.

The rods can see in lower light because they produce a substance called rhodopsin, sometimes called "visual purple" which is odd because it is actually red in colour.

Rhodopsin is extremely sensitive to light, so much so that white light actually bleaches the pigment from it and it takes about 30 minutes to regenerate fully. Much of it is actually replaced within the first five or ten minutes in the dark but while it is depleted the rods do not function nearly as well in low light.

The useful thing to know is that Rhodopsin is relatively insensitive to the longer, red wavelengths of light so using a low powered red light will preserve your night vision as it will not reduce the retinas Rhodopsin stores in the rods but it will allow the less sensitive cones to become active and give more detail.

Another related fact is that the cones are most sensitive to the green wavelengths of light so in the absence of a red filter for your torch, a green light-stick is useful because the cones can function under very little green light, which because it is so weak, does little to deplete the Rhodopsin.

 

[Ogri the trog]

A little bit of science goes a long way Wayland, thanks!
I remember reading a booklet many years ago about "How to see in the dark" it explained all sorts of ways that you can enhance your night vision. The only two tips I can recall now are the side-to-side movements of your head (like an owl) and deliberately looking to the side of the area that you are interested in. Both these methods help to prevent the part of your eye becoming 'bored' with looking at the same place and your brain ignoring what the eye is seeing, it also allows the part of the retina that is most used to seeing detail & colour to be avoided, favouring a more 'rod rich' parts of the retina. It is quite incredible the amount of detail that can be seen in near pitch darkness.

Cheers

Ogri the trog

 

[Shewie]

So that`s why we now have red lenses on headtorches. Once you`ve finished using the red light, your night vision is not affected and you can still see okay. Whereas it would be if you`d used the normal white light then switched it off.

Fascinating stuff Wayland thanks.


Rich

 

[Mirius]

The reason for not looking directly at something when using night vision is that the part directly at the back of the eye which is used most is largely cones - because we use it all the time for our day vision. So it's an area which is low in rods and so can't see well in the dark. Figure of eight keeps the rods from tiring and avoids that dead spot. About the light of half a moon is all the cones need to start adding some colour vision.

 

[nicodiemus]

Not just headlights. All your military torches generally come with red filters. The whole surefire range has red and blue filters available.

Blue being a good compromise for hunters.. doesn't affect night vis too badly, but does allow greater contrast between blood and earth tones.

 

[Wayland]

Snip> About the light of half a moon is all the cones need to start adding some colour vision.

Another fascinating thing I observed on one occasion is how much info your brain paints in for you.

I was out and about walking by night vision when I heard something coming through the brush ahead of me. I froze and waited.

What emerged was a grey looking dog. As it moved out further I realized it was a fox and all of a sudden I could see that it was reddish brown.

As soon as I knew what it was, my brain corrected my impression of the colour, it was far too dark for me to have actually seen the colour for real.

 

[Mirius]

That reminds me of the time we used to have a black and white TV (this was after having a colour one). After a while I never even noticed that it was monochome - my mind 'edited' what I was seeing into colour.

 

[Peter_t]

is to shut your eyes even if it ia already dark. it makes your eyes adjust quicker and better to the darkness because your eyes cant see any light at all apposed to maby a bit of moonlight so it makes your eyes over react to the dark.
i heard this off an ex para and they do this just befor they jump.

pete

 

[John Fenna]

I once saw an experiment on B&W TV where certain sections of the screen flashed at a certain rate and fooled the eye/brain into seeing colour. Different rates of flash gave the different primary colours (Bengal Rose, yellow, blue). The colours were rather weak but the signal even had complicated designs and moved - rolling spoked wheels, waving flowers etc, but only in a flat plane
As this was only using black and white transmition was it rods or cones that "read" the info?
Was it fooling the eye or the brain?

 

[John Fenna]

Close one eye when using a white light and you can see in the white light better than in red light (especialy colours).
To go back to night vision close that eye and open the one you had kept closed - no loss of night vision. Go back to using both eyes after a few minutes and full binocular night vision is restored!
Paras close their eyes coz they are scared of hights!

 

[crazyclimber]

Don't know if this has something to do with visual purple (urghh that brings back memories of Biol lessons!)... if you haven't seen it before though lean forward so you're close to the screen and scare at the cross. The purple dots should disappear.

Because I love these things I'll include a few more

Moving... or not?

Move your head towards then away from the picture. Do the circles really rotate?

Does it really move?

 

[crazyclimber]

Apologies btw for straying completely off topic.
What we were taught for night vision is dim red light only for 30 mins before flying, then focus about 10 degrees off what you want to see to allow the light to fall onto rods not cones. After 10 seconds they become 'bleached', so you need to keep the eyes jumping around the subject in 10 second chunks to prevent that.
Also for spotting movement at any time of the day and night it's best to focus on a particular point for 10 seconds before moving to another point.
Interestingly one thing I heard was that if the eye has nothing specific to focus on (eg in cloud), rather than focusing to infinity (which is what I would have expected) it actually focuses on a point about a meter in front of you

 

[BushTucker]

Wayland, wear a patch over one eye, sit in a bright light then walk into a dark room after removing the patch, visibillity is 65% improved, do the same test with no patch but make sure you have a first aid kit.

Don

 

[Eric_Methven]

Now I realise why it hacks you off so much at meets when people shine their head torches directly into you eyes.

Eric

 

[Wayland]

Now I realise why it hacks you off so much at meets when people shine their head torches directly into your eyes.

Eric

You got it right there Eric.

It's one of my pet hates which you might remember led to the Bushcraft Etiquette thread some time ago.

The advantages of using a torch rather than a head lamp is that you are aware that you are shining it at someone. I only ever use a head torch when I'm alone and even then it's filtered red most of the time.

 

[Shewie]

You got it right there Eric.

It's one of my pet hates which you might remember led to the Bushcraft Etiquette thread some time ago.

The advantages of using a torch rather than a head lamp is that you are aware that you are shining it at someone. I only ever use a head torch when I'm alone and even then it's filtered red most of the time.

Thanks for bumping that quality old thread Wayland, that was doing the rounds when I first joined the site. It made me chuckle reading that again.




Rich

 

[Ogri the trog]

It's one of my pet hates which you might remember led to the Bushcraft Etiquette thread some time ago.

I recall reading that a while ago Wayland,
Curiously it was a few weeks before attending the 2006 Christmas meet. One night I was walking through the woods carrying my candle lantern. Approaching were three guys wearing headlamps, "Evening Gents" I called, then the three in unison raised their lamps to illuminate my face AAARRRGGGHHHH - blinding me for several minutes!



Ogri the trog

 

[Wayland]

That's it exactly. The natural thing is to focus a head lamp where you naturally look and that means when you look at somebody you shine the light right in their eyes.

It's not done on purpose but you would never be that thoughtless if you were carrying a torch.

 

[match]

I suffer really badly from having over-responsive cone cells. This means that if there's any light at all around, the less effective cone cells 'turn on' and I try (and fail) to see in colour.

This means that when its dusk, or if I've just had a torch shone in my eyes, I find it impossible to see clearly for a few seconds until the cones settle down and the rods kick back in. One of the reasons I hate driving at night, and generally try to leave my torch with a red filter in it, and turned off unless I have to see something clearly.

Besides, tripping over stuff and falling in the mud is half of the fun of camping at night!

 

[Pablo]

You got it right there Eric.

It's one of my pet hates which you might remember led to the Bushcraft Etiquette thread some time ago.

The advantages of using a torch rather than a head lamp is that you are aware that you are shining it at someone. I only ever use a head torch when I'm alone and even then it's filtered red most of the time.

Unless you're doing something specific with both hands you can wear the head torch round your neck. You know where it is, much less anoying when you look at someone and you can still direct it where you want with one hand.

Consider green light as well. Must be used by pilots for HUD for a reason. It's also great for tracking because it casts good shadows thereby highlighting the peaks and troughs of print.

Pablo.