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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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Richard Sullivan
(richard.s) - MLife

Locale: Supernatural BC
Re: AAA battery choices on 01/11/2004 18:10:14 MST Print View

Happy New Year to all because...

Energizer Introduces World's First AAA Lithium Battery

ST. LOUIS, Jan. 7 /PRNewswire-FirstCall/ -- Energizer Holdings, Inc. (NYSE: ENR), today unveiled its Energizer(R) e2(R) Lithium AAA battery, adding to its exclusive line of lithium 1.5 volt cells. It also announced performance improvements to its AA lithium line, now lasting up to 7x longer in digital cameras.* These two announcements put Energizer at the forefront of round cell lithium technology. And better yet, they arm consumers with the world's longest-lasting AA* and AAA batteries in high-tech devices at a time when the use of digital cameras, electronic games, PDA's, audio players and other high- tech devices is exploding. (*Versus ordinary alkaline in proposed ANSI testing. Results may vary by camera.)

The new Energizer e2 Lithium AAA batteries are now the world's longest- lasting AAA batteries in high-tech devices providing more memories, more songs and more messaging than any other battery available. For the consumer, for example, that can mean 160 more hours of paging messages or 400 more flashes. The product is built on the lithium technology Energizer pioneered in 1992. Energizer remains the only battery manufacturer to harness the power of lithium in a 1.5 volt cell.

"As today's high-tech handhelds like digital cameras and MP3 players get faster computing chips and higher-speed digital processing, their thirst for battery power goes through the roof. Energizer recognizes this, and has been extremely aggressive in providing high performance batteries to meet these demands," says Corey Greenberg, NBC Today Show's Tech Editor.

According to ACNielsen Panel Data (June 2003), the dollar sales of lithium AA batteries grew 25 percent versus a year ago and is helping to drive growth in the battery category.

In addition to the power advantages, Energizer e2 Lithium batteries are lightweight, 33 percent lighter than an alkaline battery. They operate well in extreme temperatures. In conditions where alkaline batteries would fail, lithium continues to operate in temperatures ranging from -40 degrees F to 140 degrees F. The batteries have a 15-year shelf life. The Energizer e2 Lithium AA and AAA batteries come in recloseable packaging for convenient storage and travel. The new AAA battery will be available later in 2004 just in time for the holidays when picture taking is at its highest and shoppers are selecting high-tech gadgets.

Energizer, headquartered in St. Louis, Missouri, is one of the world's largest manufacturers of primary batteries and flashlights and a global leader in the dynamic business of providing portable power.

SOURCE Energizer Holdings, Inc. 01/07/2004

CONTACT: Jackie Burwitz, +1-314-985-2169, or Harriet Blickenstaff, +1-314-995-3939 x103, both for Energizer Holdings, Inc.

Edited by richard.s on 01/11/2004 19:02:19 MST.

Richard Sullivan
(richard.s) - MLife

Locale: Supernatural BC
Re: Aurora criticisms, etc. on 01/11/2004 18:21:37 MST Print View

Further to my previous post, I was able to speak with the manager of the retail store which I mentioned. He noted that he had returned about 75% of the Auroras they had sold, less than 1% of the Tikkas, and no Tikka Plus models.

Aurora problems included "just about everything":
-switch button falling off
-switch contact failure
-electronics failure
-housing failure (cracking)
-pivot failure
-internal corrosion (due to the sealed construction, corrosive battery off-gases cannot vent)

They are continuing to sell the PT Yukon and Pulsar.

Edited by richard.s on 01/11/2004 18:23:00 MST.

Ryan Jordan
(ryan) - BPL Staff - MLife

Locale: Greater Yellowstone
Re: Re: AAA battery choices on 01/11/2004 18:25:53 MST Print View

FYI: will be carrying the new AAA lithium cells as soon as they are released to us for distribution. Next week, we should also receive our first shipments of other lithium round cell batteries, including the AA lithiums already on the market, and for you Black Diamond Ion fans, the lithium L544 BP.

Richard Sullivan
(richard.s) - MLife

Locale: Supernatural BC
Re: Headlamp Failure Modes on 01/11/2004 20:36:35 MST Print View

Well you've pretty well nailed it!

When Tekna came out with it's revolutionary lights in the early 80's, I thought we might have finally arrived. Their designs featured a sealed! switchless! polycarbonate! housing with halogen! bulb options, and one unit with a lithium! battery. Wow! (These design elements are still common to this day.) Ultimately though I ended up throwing them all away. One reason was the sealed design did not vent the battery off-gases and the contact surfaces degraded. You could even smell the gases when you opened up the lithium-cell unit, the MicroLith. Tekna upgraded the springs from phosphor-bronze to nickel-plated PB, but this was not a total cure. The bulb mounts were low precision which added to the problems. And the whole circuit was dependent on a spring pushing on batteries pushing on a bulb with a lead (Pb) contact point. This whole experience clearly evidenced that low voltage circuits are very susceptible to degradation caused by small increases in circuit resistance due to poor contact.

Now 20 years hence from the first Tekna lights, we have a lot more reliability due to LEDs, nickel-plated contacts, weather-resistant housings which will vent battery gases (BD, Petzl), and independent mounting and contacts for the supply and load. But new failure modes include cables and multi-mode or power management electronics. And our age-old nemesis The Switch is still lurking and looking to cause trouble. I am however highly impressed with quality of the current BD switches. And if electronics modules can attain high-reliability, then the switch might no longer be a point-of-failure since ideally it will not carry any current.

I'd like to see manufacturers eliminate single-points-of-failure as much as possible. For example:
-use two cables from the battery pack to the lamp head
-independent switches for flood and nav modes
-implement a bypass switch around the electronics module and multi-mode switch(es) i.e. a crowbar or switch-of-last-resort

In the meantime, I'll be carrying two lights on all trips. For example if you need a nav light then a Matrix 2 and a Tikka have a combined weight which is approximately equal to the Yukon HL, but with no SPOFs! And if the primary light is a Tikka or an Ion, then the backup can be a Photon or Pulsar.

David Spellman
(dspellman) - F
Headlamps and Green lights on 04/22/2004 18:19:00 MDT Print View

While aircraft and navy ships have, for years, used red as an optimum interior light to prevent night blindness, it seems that green is the new color on the block (as noted in the above article). I'm seeing more pilot chart lights with both red and green LED's, and even some tri-color (white is the third) units with a white lockout to prevent it accidentally being turned on.

One more thing -- I have a problem with headlamps in that they're at once in the best and the worst spot for illuminating trail. While it's handy to have them on your head and pointing the same direction you're looking (and they keep your hands free for other things), their flat lighting perspective does absolutely nothing for giving you a good idea of the "texture" of the upcoming terrain. I've found that I "see" the trail a whole lot better if I take off the headlamp and hold it by my side or even away from my body. That gives upcoming terrain features a certain amount of crosslighting and depth. Anyone else tried that?

Arapiles .
(Arapiles) - M

Locale: Melbourne
Re: Re: Aurora criticisms, etc. on 05/27/2004 07:10:29 MDT Print View

I bought a Princeton Tec Aurora last year and it is probably the worst piece of gear I've ever used. Mine was replaced under warranty but I threw the replacement into a corner and don't use it.

Problems included:

- the pivot ...] coming loose so that the headlamp hung down and shone into my forehead; the only way to fix this is to carry a micro-screwdriver (I wrapped a rubber band around it until I got home). Petzl's ratchet is a much better design;
- the rubber cover on the switch coming off in the pocket of my day pack, I got the cover back on and then realised that there was a tiny spacer (less than a centimetre long) between the actual switch (buried in the body of the light) and the rubber cover - this took ages to find, even though I knew it was somewhere in the back pocket of my day pack; an appalling, cheap and stupid piece of design;
- the light is advertised as water-proof - not water resistant, water-proof: I stuck it under a running tap and it immediately stopped working. I then spent several minutes cleaning and drying this "water-proof" light;
- the brightness dropped off spectacularly quickly: I have a number of torches and head-lamps and this was the worst I've seen.

The local importer replaced the light quickly once I complained but the point is that this light was out there, on sale, with a number of very serious design flaws. Yes, they addressed the flaws eventually - but not before I spent my hard-earned cash on one. Given the above, and the rock-solid reliability of Petzl torches you can imagine my surprise that the Aurora won your award as the best headlight out there.

P.S. you need to reset your "profanity detector" so it doesn't object to normal technical words

Edited by Arapiles on 05/27/2004 07:12:45 MDT.

Kevin Sawchuk
(ksawchuk) - BPL Staff - MLife

Locale: Northern California
Re: Headlamps and Green lights on 09/08/2004 17:35:04 MDT Print View

Please see comments on my 12/1 posting and Gary Lewis'12/16 posting about holding headlamps in hand while hiking/running. Yes it works much better.

Jeremy Best
(xtrem3djer) - F
Why these big lights? on 09/10/2004 22:04:58 MDT Print View

It seems no surprise that most people have overweight packs, because they can't give up the kitchen sink!
I carry two Photon II white led's that combined weigh 2/5 oz., and have never had a problem. Give up the straps, the tiny bit of convenience and save you back! If you do this, You will make it to your next destination before sun down and will not need to walk with a light. If you stick a piece of velcro to the light and then to your beany it becomes hands free!
Shoot, carry 5 Proton II's and you will still be lighter than those 5,6,7 oz. head lamps!

Roger Caffin
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Battery Choices - single AAA on 10/09/2004 02:56:13 MDT Print View

The reason many headlights use 3 * AAA is to get enough volts to power a white LED, which needs between 3.0 and 3.6 volts.
I use a single AA or AAA battery and a micro switch-mode power converter (a few grams). I find about 50 mW of power is enough around the tent. A single AA lasted eight weeks continuous this way.
Roger Caffin

Mike Storesund
(mikes) - F
LED Choices on 01/17/2005 23:23:05 MST Print View

I really like the Petzl TacTikka with 3 ‘AAA’ Eveready Lithium batteries (2.3 oz. total). It has a red lens, so night vision is not impacted, that can be flipped down to offer white light when needed.

I also carry the Princeton Tech Eclipse (.4 oz.) as a backup. This 5-position light comes with both a hat visor clip and small key ring clip which are interchangeable. I use the key ring clip to clip it to the drawstring on top of the backpack for ready use when looking for gear.

Now the Lithium triple ‘A’ batteries are readily available at Wal-Mart, Target, Walgreens, Best Buy, Circuit City and many others. I found Target or Wal-Mart offer a better buy than the specialty stores. They may be here, but I have not seen them here yet.

Kevin Shuster
(drshuster) - F

Locale: Northern Arizona Alpine
LED lights on 10/07/2008 23:50:07 MDT Print View

I used the Princeton TEC that uses 2 lithium CR123, for over a year. I recognize that this is a relatively heavy peice of gear BUT.. It seems all thorns need to be removed after dark, and fun reading happen in the middle of the night.

I have numberous items that use the Li CR123 and though expensive... they are relatively light for the amount of charge they store (charge density). So I carry a couple extra CR123 batteries to replenish one or more items that might get extra use.

I am in the process of re-evaluating all my gear with an eye toward weight saving but this headlight keeps making "the cut" trip after trip. Especially in winter months, having such reliable, adjustable, infalable light... means something to me in the way of problem solving.