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The Weird Science of Night Vision: Lighting Considerations for Lightweight Backpacking

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by Rick Dreher | 2003-11-18 03:00:00-07

Be sure to check our Gear Guide to LED Headlamps.

Turn night into day (use bright light), or preserve night vision (use low light)?

Because of the eye’s physiology, it isn’t possible to both use a bright flashlight and preserve your night vision. Our eyes go through chemical changes in response to light, and these changes take time to reverse. It can take as little as several minutes or as long as several days to recover maximum night vision following exposure to bright light. Note that each person’s night vision potential is different, with age, presbyopia, smoking, medications and selection of parents among the many factors that affect how well anybody can potentially see at night.

In very dim (scotopic) conditions, when the cones aren’t contributing to vision, we do not perceive any color or details. We can see objects in black and white only, like on our grandparents’ TV. This is a common experience on starlit or overcast, moonless nights. As mentioned earlier, the eye responds differently to different parts of the wavelength. But it’s even more complicated than that. The cones, which give us our acute and color vision (photopic), are most sensitive at 555nm (yellow-green). This is the reason, by the way, for the newer fire truck and ambulance colors - it’s the color we see most readily during the day - not “fire engine” red. The rods, on which we rely for our night vision, are most sensitive at 505nm (blue-green). One result of this phenomenon is that blue-green light looks brighter at night than red. Unlike the cones, which can see all of what we humans perceive as the visible spectrum, rods essentially cut off all wavelengths greater than about 640nm, the red portion of the visible spectrum. What this means to us is that for all colors other than red, as they are gradually dimmed there comes a point - called the cone threshold - where color is no longer perceived but light still is. At this point only the rods are contributing to vision. With red, however, the cones and rods lose perception simultaneously, and the red light simply winks out.

How can the lightweight backpacker take advantage of this technical gobbledygook?

Most people think they want a flashlight that gives bright, even, white illumination to emulate daylight conditions as best as possible. This approach keeps us in familiar territory perceiving both detail and color after dark. The bright-‘n-white approach, however, is not the most efficient one, either for power management or for making use of our eyes’ natural capabilities.

For example, if you want to see the most detail possible you need to maximize stimulation of the cones because they provide detail along with color. Using yellow-green LEDs in your flashlight accomplishes this, while simultaneously using less current.

Or perhaps you want to use the least amount of light you can get away with for navigating in the dark while, again, using the least power possible. In this case you might want a blue-green light source that maximizes rod sensitivity..

The third approach is to do everything possible to maximize and maintain your night vision potential. This means either using no light at all, or sparingly using a very dim red light. If you must use a light, even briefly, covering one eye will protect that eye’s night sensitivity while sacrificing the other’s to the temporary “flash blindness.” Flashlights sold to astronomers are universally red, despite some thought that green might be more effective. Rigel Systems makes a continuously variable LED flashlight, powered by a 9-volt battery, that’s available in several color variations. The all-red or red/white light is marketed to astronomers, and green versions are sold to folks who use night vision gear. A simple thumbwheel adjusts the light output.

Red or Green-Yellow? The two theories might be summarized like this:

  • To read a map or star chart you need to use your photopic (cone) vision because your scotopic vision will not allow you to read the details. Because you want to minimize flash blindness, you want to use the least amount of light possible, which means a dimmable yellow-green (555 nm) light source turned just high enough to read by. The downside is that you’re affecting the rods along with the cones at this wavelength.
  • Because you’re going to experience a limited amount of flash blindness no matter what, you can limit the effect on your cones by using a wavelength (deep red) that your rods can’t see and won’t respond to. Again, turn up the light only as much as needed to read your map. The downside is that the light required to see will be much more intense than if you were using yellow-green. (Editor’s Note: On many non-military maps, important lines are often printed in shades of red and will be invisible under a pure red light).

Astronomy groups generally forbid the use of anything but red lights after dark at star parties, so they’ve cast their vote. If you’re interested in experimenting for yourself, Rigel can make a red/green dimmable light on special order. Note: a green light will not allow you to distinguish the wooded (green) areas on a topographical map.

Daytime Strategies. Extensive research with pilots has ascertained that a day spent in bright sunlight can significantly affect night vision. Studies have shown that ten consecutive days of sunlight exposure cut our nighttime visual acuity, visibility range, and contrast discrimination in half. With enough daily exposures to sunlight, normal rod sensitivity may not be reached at all, something to consider if you’re vying for the job as cabana boy at Club Med, Bora-Bora. Fifteen percent transmission sunglasses with full spectrum (gray) lenses are recommended to best protect night vision. An ironic note to summiteers and alpine fans is that high altitude negatively affects night vision, diminishing it a reported 5% at 3,500 feet, 20% at 10,000 feet and 35% at 13,000 feet. Tying up the package are the facts that advancing age and, for all you backpacking smokers, the carbon monoxide in tobacco smoke also degrade night vision. With all these factors working against us, it’s a wonder we can see at all in the mountains after sundown, so you might want to take a light along on those two a.m. bathroom breaks to keep from wandering off that cliff.


"The Weird Science of Night Vision: Lighting Considerations for Lightweight Backpacking," by Rick Dreher. (ISSN 1537-0364)., 2003-11-18 03:00:00-07.


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LED Lights
Discussion of light-emitting diode (L.E.D.)-based flashlights and headlamps. The reader is referred to the following articles as basis for this discussion:
In addition, to provide context for the lightware that is currently available on the market, the reader is also referred to:
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Ryan Jordan
(ryan) - BPL Staff - MLife

Locale: Greater Yellowstone
Headlamp of the Future? on 11/26/2003 00:00:42 MST Print View

So, a question to LED headlamp users:

What initially governed your decision(s) to purchase the LED lights you currently use?

Packed size?
LED flood brightness for closeup work?
Long-beam projection for night navigating?
Battery life?

My ideal headlamp has a low weight and tiny size with no sacrifice in short range flood brightness or long range beam projection.

My favorite light is still the Black Diamond Gemini, because of the excellent range of its xenon beam and the battery life/short range brightness of its LED. New technology means they can certainly do better on the LED side, and the BD Zenix is a step in the right direction, but its main light still not as bright as the Gemini's xenon beam. I used a Gemini on an all-night 'emergency' descent of the Middle Teton (10 hours of rappelling down a rotten rock couloir in a storm), so we have kind of bonded. My highest priorities in my "headlamp of the future" is to retain the functionality of this lamp, while improving LED short range flood brightness and shaving weight and bulk.

Your ideal headlamp?

Edited by ryan on 12/06/2003 06:25:35 MST.

Rick Dreher
(halfturbo) - MLife

Locale: Northernish California
Battery choices on 11/30/2003 18:29:30 MST Print View

I have some unanswered questions from preparing the article series for you E.E. or electronics hobbyist types out there. Here’s the first:

Since super-premium alkaline batteries offer more capacity than lithium cells, and since lithiums cost perhaps five times as much as alkalines, might power management circuitry eventually compensate for alkaline battery voltage roll-off such that alkaline performance might equal or better that of lithiums? (Ignoring low-temperature performance.)

--Rick Dreher

Kevin Sawchuk
(ksawchuk) - BPL Staff - MLife

Locale: Northern California
LED lights on 12/01/2003 16:17:32 MST Print View

I prefer a headlamp for versatility--The Petzel I have has three brightness options which is nice for battery conservation and not having a light to bright for reading. It has acceptable brightness for hiking on trails at night, is poor for running at night (I run Ultras and the 100 mile versions require night trail running). It gives better contrast for hiking if you hold it in your hand instead of keeping it around your head. For dedicated handheld use the Princeton Tec Attitude (or their TD40 with a LED module "pushed" to 4 AA cells--this has no warrenty from Princeton Tec but has worked well for me) work well and are light. AA Lithium batteries are also much lighter and last longer than Alkaline.

Ryan Jordan
(ryan) - BPL Staff - MLife

Locale: Greater Yellowstone
Re: LED lights on 12/01/2003 16:24:24 MST Print View

Kevin, tell us more about your TD40. This sounds like a good concept. Did you do the mods or did PTec? and how?

obx hiker
(obxcola) - MLife

Locale: Outer Banks of North Carolina
led lights on 12/02/2003 19:09:26 MST Print View

Please name some relatively common/accessible sources for AAA lithium batteries. I think I've looked at half the major chain electronics big boxes in NC & VA ( w/ no luck)

obx hiker
(obxcola) - MLife

Locale: Outer Banks of North Carolina
Re: led lights on 12/02/2003 19:23:46 MST Print View

I did find this info from a web search: states on their web-site:

AAA Lithium 1.5V batteries are currently not made. If a lithium battery is rechargeable,
it is 3.6V, not the standard 1.5V. There is a lot of confusion about this. You cannot use a 3.6V battery where a 1.5V is required. If you would like a rechargeable option for a standard AA, or AAA battery check out our rechargeable battery page and select the NiMH batteries for your best option.

Rick Dreher
(halfturbo) - MLife

Locale: Northernish California
Re: AAA battery choices on 12/03/2003 10:09:07 MST Print View

As you've discovered, nobody's making lithium AAAs at present. What I've not been able to find out is whether there's a technical reason (they don't scale down well) or a marketing one (wouldn't sell enough of them).

Note that the common rechargables, Ni-MH, etc. put out somewhat less than 1.5 volts, although this doesn't seem to be a problem with flashlights.

--Rick Dreher

John Coyle

Locale: NorCal
Re: Headlamp of the Future? on 12/06/2003 00:12:38 MST Print View

Speaking of the devil, my favorite light, which I have only had for a month, is the BD Zenix(the correct spelling.) I recently did a night hike with my son, who was using a Petzl Myo 3, near Cool CA, the same area a jogger was killed by a mountain lion 7-8 years ago. We both agreed that my Zenix had brighter beam projection and closeup flood light than the Petzl, which is a pretty good light too. I should mention that both headlamps had new alkaline batteries(Zenix 3 AAA, and Myo 4 AA.) Interestingly enough the BD Zenix is totally LED, while the Petzl is a combination long beam Xenon and closeup LED. The Zenix weighs 4.9 oz with batteries, and I can't think of any other LED headlamp in this weight class with such good long beam characteristics. A fairly strong long beam is helpfull in identifying nocturnal animals, a favorite pastime of mine.

I would have loved to have seen a mountain lion on our hike, but unfortunately this did not happen.

John Coyle

Kevin Sawchuk
(ksawchuk) - BPL Staff - MLife

Locale: Northern California
PTec light on 12/12/2003 22:28:26 MST Print View

Sorry I didn't check back sooner Ryan--The PTec module is available commercially for around $30. It is sold so an individual can convert their older headlamps to LED technology. The module supposed to be used with 2 AA cells and the TD40 is a 4AA light. PTec will not warrenty 4AA use, but when I talked to a rep he said that several people had done this for some time without problems. This configuration has seen me through three Western States 100 races (from 9pm when it's dark till I finish around midnight) and many winter commutes home from work. It is NOT a headlamp, but for night hiking/running I prefer my light closer to the ground for more brightness and better contrast/shadows.

Robert Molen
(molen) - F
Rayovac Sportsman 3-in-1 Head-Lite on 12/13/2003 06:54:51 MST Print View

Great review!

Princeton Tec Aurora - I agree that this is a excellent light for around the camp, but it does not have a long enough beam for night hiking.

Black Diamond Gemini - I also really like this light except for the weight... 7.1 oz w/ batteries. I went so far as to make a 3AAA battery pack to replace the 3AA battery pack getting the weight down to 4.9 oz w/ batteries. Because behind-the-head battery packs are uncomfortable lying on my back reading, I moved the custom 3AAA battery pack to the front.

Rayovac Sportsman 3-in-1 Head-Lite - This is the light I am currently using. Weight is 4.5 oz with 3AAA alkaline batteries. It has an excellent short range white LED light, a krypton spot light for longer range and night hiking, and two red LEDs I use all the time to keep my night vision... amazing how many more stars you can see. It is cheap too at $12.99. If I could only get rechargeable AAA lithium batteries for it...

Red Photon Micro-Light 3 - This is my backup light with its red LED and SOS feature weighing 0.4 oz with batteries. Besides preserving night vision, a red LED also extends battery life 10 times a white LED.

Debra Weisenstein
Aurora criticisms, etc. on 12/14/2003 07:27:37 MST Print View

I have 2 negative comments on the Aurora, neither of which seem to have been taken into account in the reviews.

(1) The swivel mechanism loosens over time. I've had to retighten mine twice. It's not a big deal if you're are at home and have the proper screwdriver to tighten it, but in the field it makes the headlamp unusable, as it refuses to stay where you point it.

(2) The switch is in a very awkward place for us left-handers. A centrally-mounted switch is much better from my point of view.

I also own a Moonlight, which I really like except that it once developed contact problems and had to be returned to REI for replacement. I've heard this from other Moonlight owners, and seems to be a design problem when the battery pack is separate from the light unit.

A Gemini is now my light of choice when night hiking and camp use are both possibilities.

Petzl Duo Belt is the one I use in New England winters for the AA (ie. lithium) capability and separate battery compartment that can be kept inside your clothing.

Ryan Jordan
(ryan) - BPL Staff - MLife

Locale: Greater Yellowstone
Re: Aurora criticisms, etc. on 12/14/2003 07:56:28 MST Print View

RE: Aurora pivot head becoming loosened over time. We were aware of this problem in on the Aurora and paid close attention to it. Because this was an issue with initial shipments of the Aurora to retailers, we took it to an engineering lab and cycled the hinge pivot through 5000 complete (down and up) cycles. There were no issues whatsoever about the hinge become loosened and Princeton Tec has appeared to make good on their claim that the problem has been addressed. If you have an Aurora with that has its hinge become loosened after a short time, consider contacting the company and initiating a warranty exchange. -- Ryan

Richard Sullivan
(richard.s) - MLife

Locale: Supernatural BC
Black Diamond on 12/16/2003 23:05:17 MST Print View

My first generation BD Moonlight has developed the broken cord syndrome. BD reports that this is a known defect with early Moonlight and Gemini models, but are now equipped with beefier cords. Retailers are under instruction to replace all lights with this symptom, or you can ship directly to BD.

Note that the new versions of these models can be identified easily. The new Moonlight has multiple brightness control, and the new Gemini has two LEDs instead of one.

Folks - should I get my defective Moonlight replaced or get it upgraded to the new Zenix? BD has said they will do this if I pay the difference. Seems like a no-brainer for the Zenix, but just wondering if anyone had any opinions.

Ryan and Rick - Thanks for all your great work on this topic!

Richard Sullivan
(richard.s) - MLife

Locale: Supernatural BC
Re: Headlamp of the Future? on 12/16/2003 23:09:46 MST Print View

Ryan - do you have the first generation Gemini with the single LED? Do you have any idea how the short range flood brightness of the single-LED model compares with that of the new dual-LED model?

Gary Lewis
(evertrek) - F
Re: Aurora criticisms, etc. on 12/16/2003 23:42:41 MST Print View

My BD Moonlight also has the contact problems you describe, Debra. My quick fix for it is to use a clunky rubberband and twist-tie arrangement to force the wire into an angle that allows the light to work, but to do this I have to wear the battery pack over to the side a bit instead of at the back of my head. The light is the 2002 model, but I only used it about 100 hours and never abused it. Time for a different light.

Regarding my hoped-for ideals in a light:

* LED(s) that would allow me to run on rocky trail without slowing down much, but that has a lower setting for camp use. Battery use for technical/runnable travel: 15-20 hours. Regulation would be needed, I think.
* A combined high-power spotlight capability, probably xenon, that could illuminate far ahead for occasional navagation puzzles.
* Pivoting head, ideally detachable for hand use. I experimented a couple nights ago and found that indeed, as Kevin Sawchuck wrote, one can see better for technical running and power walking with the light at hand level. Not only because of better "shadowing" but also because varying the left/right angle of the light sometimes illuminates better foot placements.
* Use of 123 lithium photo batteries would allow the above ideals for about 4 oz. (Ref. the tactical lights by Surefire and Arc.) Maybe even a design using 2 or 3 AA lithiums could pull that off.

I'm considering buying a good hand-held LED light as my walking/running light, and carrying a tiny light like the Ion for camp tasks. Or even lighter for camp work, I may try gluing velcro to my cap's bill and to a microlight (I like Inova's brand best). For night walking, a reviewer from sewed small elastic straps onto his bill to mount a Princeton Tec Impact LED handheld. An option for relatively smooth trail even when running, if the hat stays stable enough.

Are there any plans to review handheld LED lights here some time in the future? Ryan, your methodology and analysis are much better for backpacking purposes than those used by the several LED light review sites.

Rick Dreher
(halfturbo) - MLife

Locale: Northernish California
Re: Black Diamond on 12/17/2003 10:31:15 MST Print View

Hi Richard,

Thank you for the information from BD. As a gen-1 Moonlight owner myself, it's good to know about this weakness and BD's replacement policy.

The Zenix's flexibility compared to the Moonlight makes it awfully attractive. If you have even occasional need for the high beam, it's an easy choice. The Moonlight does have a broader, more even beam (compared to the Zenix on low) and is lighter. Other than price, those seem to be the only Moonlight advantages.

Best regards,


Richard Sullivan
(richard.s) - MLife

Locale: Supernatural BC
Re: Aurora criticisms, etc. on 12/22/2003 21:41:26 MST Print View

One of my local retailers reports that he no longer carries the Aurora due to numerous switch failures. Anybody else hear of this?

Ryan Jordan
(ryan) - BPL Staff - MLife

Locale: Greater Yellowstone
Re: Re: Aurora criticisms, etc. on 12/22/2003 21:44:41 MST Print View

Again, I am under the impression that this was an issue with the initial production run - but that the issue has since been addressed.

Richard Sullivan
(richard.s) - MLife

Locale: Supernatural BC
Re: Headlamp of the Future? on 12/22/2003 22:15:08 MST Print View

The ideal headlamp at present seems to be the PT Yukon HL, but it's a bit heavy at 8 oz (~7 oz with lithiums).

I am curious about the performance rating of the BD Zenix in the review. It could probably be improved by wearing the battery pack underneath a hat, and this would be fairly easy since the pack is slim. Maybe the Zenix is the best all-round lamp at present if the wearer can keep the battery temperature up?

The ideal headlamp for the near-future appears to be a Matrix 2 with some additional small LEDs for short range lighting. I am surprised that PT did not incorporate this, but hopefully they will soon.

I'd like to add that reliability is critical to the title of "ideal" headlamp. I've been carrying battery operated lights into the wild for 30 years, yet I am still plagued by failures. Ultimately, the thing has to work when it is needed most, regardless of performance characteristics, weight, or usability features.

Rick Dreher
(halfturbo) - MLife

Locale: Northernish California
Re: Headlamp of the Future? on 12/23/2003 10:24:42 MST Print View


Would you mind detailing some of the failures you've experienced over time? Have you noticed some failure categories falling by the wayside as the technology advances; have any new failure modes appeared?

I can note some failures I no longer experience, now that I've moved to LED lights:

* Shattered lenses (this was becoming less common already)
* Burned out and broken bulbs
* Rapid battery failure (due to high current draw or to the light switching on in my pack)
* Failure to operate in wet conditions

We are still dealing with electrical circuit failure (battery contacts, wiring, switches and now, electronic controls); mechanical failure of the lamphead or battery pack and battery failure due to high current draw (in high performance lights) and/or cold temperatures. Also, not all LED lights are immune to moisture effects.