# TBone's Introduction to Human Vision at Low Light

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TBone
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TBone's Introduction to Human Vision at Low Light

(Low technobabble, one diagram only, no formulas.)

Human eyes have two types of cells that can detect light: cone cells und rod cells. There are three types of cone cells for different colors but they have a lower sensitivity so they can only detect relatively bright light. Rod cells have a much higher sensitivity (up to 100 times) but they ‘reload’ much slower when ‘used up’ by bright light.

The biological stuff is a bit complex. But we are only interested in the outcome so we can cut several corners here.
The most useful diagram for this is the “Spectral luminous efficacy”. The diagram (taken from Wikipedia) shows the sensitivity of the average human eye for light of different wavelengths.
These curves were generated by experiments and they are standardized by the CIE. There is no way to measure them. Your personal experience may vary depending on the remaining transparency of your eye lenses and other factors.

The diagram shows curves for the sensitivity of the cone cells (lower curve at right) and the rod cells (higher curve at the left) for different wavelengths.

(If you are not familiar with reading y,x-diagrams: Pick a number at the horizontal axis, then go straight up until you hit the curve. From this point go horizontally towards the vertical axis to get the corresponding value. So for 450 nanometer (nm) wavelength on the left curve you get a factor of 800 lm/W. For each value at the horizontal axis there is a value at the vertical axis. You could write this as a large two-column table but the graphical view of a curve gives you a better understanding of how things go together.)

Have a look at the right curve K(λ). It starts at 425 nm indigo blue, almost ultraviolet.
The peak sensitivity is at 555 nm (green). Of all colors green is the one humans can see best.
It ends at 700 nm deep red, almost infrared.

The higher curve K’(λ) on the left shows the sensitivity of the rod system that is used to see at low light conditions.
This curve is much higher because the rods are more sensitive. The curve is also shifted towards lower wavelengths, so more blue, less red.
It starts at 400 nm, almost ultraviolet. The maximum of this curve is at 505 nm (blue-green). It ends at 625 nm, red.
.
How to preserve your low light vision (“night vision”)
If you are using any wavelength of light that the K’(λ) curve covers you will trigger the rod cells. If the light is so bright that it overloads the rod cells they will need a long time to reload. The more sensitive system is OFF until then and you completely lose your ability to see at low light.

There is only one area of wavelength that triggers the cone cells but not the rod cells: Above 625 nm, red light.

(The colors of the rod cell curve K’(λ) are a bit misleading. It shows the color of light but not what you see. There is only on type of rod cells. It is triggered by several different wavelengths but this triggering is only on/off, no information about the color. Humans cannot see colors with rod cells. If you see any color at low light you see it with your cone cells. If it is not red it may be so bright that it already reduces your low light vision.)
.

In theory you would need very little green light to see at low light because the rod cells are so sensitive to it. But this is not how it is usually used.
Bright green light is the best way to temporarily destroy your low light vision.
.

This is how I understand the topic after reading some papers. If I am wrong just tell me.

djozz
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Sounds correct to me

Spartan
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What I’m interested is the effect on vision between a thrower and a flooder.

Does the intense beam help at a distance at the expense of losing some night vision?

Does the flooder help more because even though it wont illuminate a distant object, the overall illumination is balanced with better night vision?

Case in point, I get lazy when checking my plant at night (boiler issues). Yes, I can put the plant lights on, but where is the fun in that? I have an tiny olight key chain flashlight and even though it puts out only 120 lumens, fully spread, I’m amazed from a pinky sized flashlight, I can see everything and walk around a 40,000 ft2 building (it has a white ceiling and mostly gray/white floor and machinery). When i use the big thrower, with an intense center beam, I then need to shine to at things to see them. When I turn it off, night blindness.

This will be even more interesting when I get the q8 tomorrow.

Yes, it’s common sense that the intense beam reduces my night vision, so that is where the interesting point lays. As enthusiasts, of course we want flashlights that will blind those pesky mugglers on the moon, but realistically, for emergencies and walk around, actually, for a general purpose flashlight, does one go with a full flooder? And preserve night vision?

Moses came from the mountaintop carrying a tablet. The Words were....WITH GREAT LUMENS COMES GREAT REPONSIBILITY.

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djozz wrote:
Sounds correct to me

I agree ^

Marc E
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Thanks for the concise explanation Tbone.
This is not my area of expertise so any info i have is just from reading other people’s writings but for accuracy only i believe there are 3 known types of cells in the eye that detect light, the 3rd being photosensitive retinal ganglion cells that are linked to neurological functions as opposed to vision and therefore don’t impact your explanation.

And you are correct that we cannot see colour with rod cells but i believe our brains can fill in missing information for familiar patterns, so it’s possible that under some rare conditions while our eyes would only be seeing in black and white our brain would be seeing in colour!

djozz
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There’s someone (obviously a clever person) who maintains his watercolour painting website, and on that site there is a fairly high level (I was glad that I’m a biologist by education) but correct and comprehensive story about human vision. For who likes to dive into human vision without having to refer to numerous sources for that, here’s the link:
http://www.handprint.com/HP/WCL/color1.html

wle
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so why wouldn’t dim green light be best for seeing in the dark and preserving night vision?

you said bright green light is bad but….

wle

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I love this topic. I hope LED lights of the future will be able to change color as needed for certain task, tactical vs searching for example. How difficult would it be to develop something like that? With ramping for different tints?

wle
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ramping for tints? what kind of ui would you have? a phone app? touch screen on the flashlight?

there are lights with red leds included for night vision but of course it is very primitive, just select red or white, i guess red level is assumed to be low

wle

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wle wrote:
ramping for tints? what kind of ui would you have? a phone app? touch screen on the flashlight?

there are lights with red leds included for night vision but of course it is very primitive, just select red or white, i guess red level is assumed to be low

wle

I’m sure that a light with multiple LEDs could be designed to ramp up certain tints and ramp down others, using multiple switches.

I’m hoping to give Toykeeper a challange

nottawhackjob
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wle wrote:
so why wouldn’t dim green light be best for seeing in the dark and preserving night vision?

you said bright green light is bad but….

wle

The OP is apparently more well versed than most on this subject. Butt here goes with my take on it summarized……….

Bright anything tends to wreck night vision havoc on yer eyes – including oft used green. So if yer gonna use green keep it lit soft and low to optimize its inherent eye-friendly wavelength advantages.

“In many things in order to truly understand the small picture you have to understand the big picture first.”

Marc E
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Deputy Dog wrote:
I’m sure that a light with multiple LEDs could be designed to ramp up certain tints and ramp down others, using multiple switches.

I’m hoping to give Toykeeper challange

Ask and ye shall receive , i believe it may be implementated in the BLF lantern: http://budgetlightforum.com/node/44923

The following quotes are from the first post of that thread which is looooong:

“Compact design based from the BLF Thorfire Q8 mid-section and battery section, with great run times, output, and tint ramping, mode groups, built-in USB charging capability, and versatility.”

“The firmware will be developed specifically for lantern use by Toykeeper of BLF, and will have most of the same mode groups as the Q8 flashlight, but with added modes including Candle-mode, sunset-mode, & possibly a tint ramping mode, (which allows the driver to ramp/fade between 3000K LEDs to 4000K LEDs, which will require a third lead for the LED star to have two channels, (one channel for three 3000K 3535 type LEDs and three 4000K 3535 LEDs, possibly the new Samsung High CRI LH351D-3535 series of LED emitters.”

Deputy Dog
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Quote:
“Compact design based from the BLF Thorfire Q8 mid-section and battery section, with great run times, output, and tint ramping, mode groups, built-in USB charging capability, and versatility.”

Shrinking that down to EDC size can only be a small step away.

TBone
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According to the diagram you could use a very low green to keep your low light vision. But it depends on the brightness. If you use your flashlight to find your way, 2 to 3 meters ahead and swing your hand into the beam the reflection of your hand will temporarily blind your low light vision.
In this case I do not see the advantage over red light which should be inherently safe at any brightness.

nottawhackjob
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TBone wrote:
According to the diagram you could use a very low green to keep your low light vision. But it depends on the brightness. If you use your flashlight to find your way, 2 to 3 meters ahead and swing your hand into the beam the reflection of your hand will temporarily blind your low light vision. In this case I do not see the advantage over red light which should be inherently safe at any brightness.

So no matter if I shone a 1,000 lumen red light right into my eyes it wouldn’t/shouldn’t affect my night vision?

“In many things in order to truly understand the small picture you have to understand the big picture first.”

azj
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Actually red lights are used to preserve night vision/seeing in dark situations. That’s what Astronomy hobbyists like myself use and that’s also why you see them used in photographer “darkrooms” when developing film..for photographers not using digtial.

nottawhackjob
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azj wrote:
Actually red lights are used to preserve night vision/seeing in dark situations. That’s what Astronomy hobbyists like myself use and that’s also why you see them used in photographer “darkrooms” when developing film..for photographers not using digtial.

Ok understand that butt again there’s no affect on night vision no matter how bright the red light is?

“In many things in order to truly understand the small picture you have to understand the big picture first.”

azj
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nottawhackjob wrote:
Ok understand that butt again there’s no affect on night vision no matter how bright the red light is?
Didn’t see your post when I typed mine

No, it definitely matters. For Astronomy, the flashlights are kept very dim. 1/2 lumen is generally recommended. Many of the lights have variable adjustments and you turn the up just enough so white papers appear brown. If you can actually see a red spot, it’s considered too bright. It’s not horrible though.

Consider this: You need to sit in a fully darkened room or area for 20 minutes before your eyes fully adapt. One light ruins that. There’s a mythbusters experiment on this that was pretty darn good, which included the reason that Pirates wore eyepatches. It wasn’t because they lost an eye. It was to see belowdecks. They’d move the patch to their other eye once they were in the dark

TBone
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nottawhackjob wrote:
So no matter if I shone a 1,000 lumen red light right into my eyes it wouldn’t/shouldn’t affect my night vision?
Yes, as I understand it. Even if my gut feeling says that being blinded always feels complete.

Common sense is not always up to date about non-natural stuff. Here we have almost monochromatic (single color) light.
In nature most light comes from glowing stuff with a wide spectrum. So you know when light heats up your skin it can give you sunburn.
Then came UV LEDs…

There may be some effect if red light triggers the closing of the pupil but his will be very temporary.
.

The best way to find out if bright red light is a problem for low light vision is the classic way: Go out in the dark with a red and a white flashlight and test it.

TBone
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A possible reason why astronomers dim their lights as much as possible may be historical. Before LEDs existed they used red filters for incandescend light bulbs. Cheap filters do not block light of lower wavelength good enough so with more brightness you get more badly filtered light that will affect you low light vision.

Marc E
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I think there are two slightly different scenarios being discussed here: rod only vision, and vision with the cones engaged at high sensitivity (at which point the rods may or may not be working within their range of function.) My assumption is that most people’s experience with night vision is with the cones still engaged, which is a different discussion to rod only vision.

nottawhackjob wrote:
So no matter if I shone a 1,000 lumen red light right into my eyes it wouldn’t/shouldn’t affect my night vision?
My guess is that if the red light only contained frequencies that cones can’t detect the cones wouldn’t react directly to the light, but the light might overwhelm the rods effectively blinding them. If you turned off the red light you would then need light of other frequencies bright enough to engage the cones above a level the rods are blinded at, or wait for your rods to regain their sensitivity again before being able to see.

azj wrote:
Actually red lights are used to preserve night vision/seeing in dark situations. That’s what Astronomy hobbyists like myself use and that’s also why you see them used in photographer “darkrooms” when developing film..for photographers not using digtial.
Red light is used in photo darkrooms because it’s the frequency to which the photosensitive film reacts to the least. if you were too turn on a normal light it would make a photographers job a lot easier but ruin all the negatives through overexposure to light.
TBone
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Marc E wrote:
My guess is that if the red light only contained frequencies that cones can’t detect the cones wouldn’t react directly to the light, but the light might overwhelm the rods effectively blinding them. If you turned off the red light you would then need light of other frequencies bright enough to engage the cones above a level the rods are blinded at, or wait for your rods to regain their sensitivity again before being able to see.
We have that red. It is called deep blue.
hank
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Great link to ‘handprint’ page, Djozz. Thanks.
— also trained as a biologist.

I recall the discovery of the fourth opsin receptor — the one that regulates sleep/circadian rhythm, without contributing to vision —- was done by searching for molecules similar to the three opsins that make up color vision.

Quote:
In mammals, light perception occurs only in the retina; three different types of photoreceptors are present within this tissue: cones, rods and the newly discovered intrinsically photosensitive retinal ganglion cells (ipRGCs). Researchers believe that the classical photoreceptors (e.g., the rods and the cones) are responsible for the image-forming vision, whereas the ipRGCs play a key role in the non-image forming vision. This non-image-forming photoreceptive system communicates not only with the master circadian pacemaker located in the suprachiasmatic nuclei of the hypothalamus, but also with many other brain areas that are known to be involved in the regulation of several functions; thus, this non-image forming system may also affect several aspects of mammalian health independently from the circadian system.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2848671/
(that’s eight years old, likely there’s newer science to be found)

And unknowns persist:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3200463/figure/F1/

hank
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Oh, and why LEDs and other blue-pumped fluorescent light sources are troublesome for babies and older people who have easily disrupted sleep:

So the LED makers are releasing LEDs that don’t emit in that critical blue-sensitive wavelength:

https://www.ledsmagazine.com/articles/print/volume-14/issue-5/features/b...

Quote:
… It hinges on replacing standard blue-pump LEDs with violet-pump LEDs, and carefully shaping the spectrum of the light source to create a blue gap. By removing energy in a relatively narrow spectral range, it is possible to retain the color temperature of an incandescent light while virtually removing any blue radiation from the spectrum.

Although the use of blue-pump LEDs has been unquestioned for two decades, the last five years have seen the rise of full-spectrum white LEDs based on violet-pump LEDs….

More at: https://www.ledsmagazine.com/articles/print/volume-11/issue-4/features/t... (interesting mention of how new LEDs fail to activate optical brighteners, which are explained here: http://www.ruethedayblog.com/2011/01/why-do-old-ladies-dye-their-hair-blue/)

maukka
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hank wrote:
Oh, and why LEDs and other blue-pumped fluorescent light sources are troublesome for babies and older people who have easily disrupted sleep

If you compare apples to apples, a warm LED light source comparable with an incandescent 2700K bulb doesn’t emit much blue light either.

Barry Lyndon
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Yeah. This is why I want one of these ^.

A flashlight with dual red/white only makes sense. Sometimes you need to preserve your night vision.

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Just playing.

Barry Lyndon
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Just playing.

Did you make that? What would be the cheapest way to convert one of my SK68 clones to a red LED ( >625nm wavelength) ?

These red SK68 clones on Amazon claim 200 lumens for \$8.99 … so it’s probably about 20 lumens.
https://www.amazon.com/WAYLLSHINE-Scalable-Range-Flashlight-Detecting-Bl...
But still, is that a pretty good deal or could I mod one of my existing \$2 SK68s with a better/brighter red LED for less than \$7?

cetary
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One of the cheapest ways to convert your SK68 lights to a red light would likely be using this from mtnelectronics. Just make sure to double check the size of the emitter’s MCPCB in your light. It should be some straightforward pretty easy soldering.

If you are comfortable with emitter reflowing, you could get one of these . It’s one of the so called far red or deep reds. It makes the typical red lights you see around us look almost orange.

The main drawbacks I see to that SK68 clone on Amazon, is the real lack of useful U/I. A ramping U/I like Crescendo or the 17mm Moonlight Special from MTN would provide allot more flexibility and usefulness in a low light environment/night vision preserving situation.

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I used the deep red^^ for my mod. Guessing red is about 70 lumen driven with one 7135 chip.

hank
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SK68 clone with red emitter is \$4.11 from Fasttech
https://www.fasttech.com/products/0/10014229/4479101-1-led-3-mode-150lm-...

Fasttech staff used to show up here and “BLF” was a discount code that may still work
http://budgetlightforum.com/node/41994