Wikipedia:Reference desk/Archives/Science/2014 July 9

= July 9 =

Pulmonary embolism and COPD
I can see, even with my layman's brain, that COPD can be implicated in cases of PE, but what about the other way round? Is there research to show that a past history of PE without other lung issues (say a non-smoker who had a PE as the result of a DVT) increases the chances of COPD occurring? --86.12.139.50 (talk) 13:47, 9 July 2014 (UTC)
 * PS Before someone says ask my doctor, I'm not asking about an individual patient, but generalised research/data on patientS. --86.12.139.50 (talk) 13:51, 9 July 2014 (UTC)

Eczema herpeticum
I hear often that herpes is carried by a large majority of the population and I also hear that eczema increases the chances of contracting eczema herpeticum if exposed to the herpes virus. So why don't more people with eczema get eczema herpeticum? — Preceding unsigned comment added by 90.192.127.95 (talk) 14:14, 9 July 2014 (UTC)


 * Define "get" there's a difference between carrying the virus and showing the symptoms.  All the various herpes viruses are quite common, but may only exhibit symptoms once or never.  I was hospitalized for quite some time for major abdominal surgery.  At one point I had an itch on my spine like a mosquito bite, which was odd, since I was in one of the most expensive hospitals in the world, on my back, during the winter.  Turns out I "had" the shingles (the most mild case in history, from what I have read and seen in other people), even though I felt an itch for a few times on one day.  Having an infective agent and manifesting the typical or worst symptoms of that agent are two different things. μηδείς (talk) 19:58, 9 July 2014 (UTC)


 * "Herpes" is a misleading term. The herpes class of viruses includes the one that causes chicken pox, which many people have been exposed to.  The ones which cause the venereal disease are less widespread. StuRat (talk) 23:55, 9 July 2014 (UTC)

Why is everything green through night vision goggles?
I got some answers here but didn't feel there was any clear answer I could add to the appropriate Wikipedia articles. I can't believe the word green isn't even used in those articles.— Vchimpanzee  •  talk  •  contributions  •  19:36, 9 July 2014 (UTC)


 * Night vision goggles detect heat. Heat can be cooler or warmer, i.e., brighter or darker, but it doesn't come in inherently different primary colors of heat.  Primary colors exist because we have theree types of cones, not because nature comes in those colors.  Night vision goggles could come in red or white or whatever.  But it turns out that the single color our eyes react to best is green--tthe brightness component of green is better than that of blue or red, so green is a reasonable choice in their design. — Preceding unsigned comment added by Medeis (talk • contribs)
 * Sorry, but that is, at best, misleading. Infrared light comes in different wavelengths just like optical light. Astrophotography makes good use of that to produce fantastic images. For black-body radiators, infrared, intensity and wavelength distribution are coupled. But that's a bit besides the point. Most modern night vision systems don't use infrared illumination, but rather work by image intensification, using a photo cathode and/or a photomultiplier. In other words, the incoming photons are not observed directly, but trigger the release of an electron that is accelerated towards a fluorescent screen, where it forms a secondary image. Each photon either has the necessary energy to release an electron, or not. Thus, the difference in colour (wavelengths/energies) is lost. The single colour one sees is the fluorescent colour of the screen. --Stephan Schulz (talk) 20:31, 9 July 2014 (UTC)
 * Since this is controversial, some sources would be helpful. My objective here is to improve the appropriate Wikipedia article or articles.— Vchimpanzee  •  talk  •  contributions  •  20:43, 9 July 2014 (UTC)
 * I thought this had been pretty well covered last time. The mechanism converts the invisible light to kind of a "gray-scale" visible light. Instead of gray they use green. ←Baseball Bugs What's up, Doc? carrots→ 21:10, 9 July 2014 (UTC)
 * It was, but I didn't get a clear answer I felt I could use on any Wikipedia article.— Vchimpanzee  •  talk  •  contributions  •  21:18, 9 July 2014 (UTC)
 * Maybe you could craft an attempt at an answer, here, and various editors here could refine it. ←Baseball Bugs What's up, Doc? carrots→ 21:26, 9 July 2014 (UTC)

That's pretty much what I was trying to do, thanks. I'm out of time for today and have to return to the real world.— Vchimpanzee  •  talk  •  contributions  •  21:29, 9 July 2014 (UTC)
 * The only special thing about the green is it is a good cheap phosphor commonly used for oscilloscopes and suchlike. See Phosphor. There shouldn't be anything special about it in the night vision goggles articles unless there is a citation for night vision googles saying it is common or giving a reason for the colour -- i.e. the usual no original research business. Dmcq (talk) 22:44, 9 July 2014 (UTC)
 * You can't say "only" and then say "good cheap". Green is gooder that R&B because it is more intensely perceived at a given brightness (good).  I am sure someone who actually studies this sort of physics can explain that, but green is indeed the brightest of the primary colors. μηδείς (talk) 23:01, 9 July 2014 (UTC)


 * Night vision goggles are green because way back in the 1950s when they were first invented, and worked entirely with simple analog optoelectronics, they used a green phosphorescent image intensifier screen. Now, everyone expects the images to look this way, so that's how they get built.
 * A few months ago, I had the privilege to visit Air Mobility Command (60 AMW) at Travis Air Force Base, and I got to wear a pair of real-life combat night vision goggles that the C-5 Galaxy and C-17 Globemaster pilots wear during certain types of night flight. They're entirely digital.  They have a CMOS image sensor and and LCD screen inside of an eyepiece optic, and the entire package was controlled by an image signal processor - it's basically a tiny binocular pair of digital cameras with a live preview and a fixed-length zoom-lens.  Compared to the electronics inside my iPhone 5S with the gain manually boosted, the image intensity was about the same (although the night vision devices actually had a much larger f-number because they double as binoculars - and there's a lot more to image quality than luminosity - so it's not a fair comparison).  But, I can even render the camera image on my iPhone 5S screen using only green pixels, so that I can make it look cool, too.
 * The reality is, modern night vision devices do not need to appear green. Many will display the final image in black-and-white.  Many people confuse thermal imaging with infrared imaging with near infrared with image intensification.  These are each different types of night vision device.  Now that the electronics and the screens are all digital and computerized, the output image can be made any color and brightness that the designer wishes.  They still make them look green because "it looks cool," and you'll hear a zillion theories about human eye sensitivity, or phosphorescent screens, or handwaving about "infrared," that are essentially incorrect.
 * Nimur (talk) 23:38, 9 July 2014 (UTC)
 * Here are some photos from the Travis AFB media page : Night Vision Devices used by ground crews and ground security, but no photos of (or through) the gear the pilots get to use! 60th Air Mobile "owns the night".  Nimur (talk) 00:25, 10 July 2014 (UTC)


 * I would hope that the U.S. military doesn't choose green for its night-vision equipment because "it looks cool". You haven't provided a source for that claim, and if I understand correctly your expertise consists of having once looked through a pair of night-vision goggles.
 * The article Night vision device doesn't contain the terms CCD or LCD, and leaves me with the impression that modern night-vision devices work the same way that they always have. That makes more sense to me, since a device that worked like your iPhone would have a battery life comparable to your iPhone's. If power consumption were not an issue, they would surely use a false-color display, not a monochrome display of any color. -- BenRG (talk) 17:52, 10 July 2014 (UTC)
 * They don't use green: they use a digital screen. It can be made to render in any color.  While I recognize that my comments above are, at best WP:OR (and at worst, it could be possible that I'm just some loony on the internet - although in actual fact I'd like to hope I'm not!), it isn't hard to find sources to back up my claims.  For example: here's a news story about F-16 fighter pilot helmet displays: 177th Fighter Wing receives force multiplier technology.  "The pilots can choose color palettes and layouts prior to the mission. Once we find out who's flying, we install the modular HMIT to each pilot's physical specifications..."    Nimur (talk) 21:52, 10 July 2014 (UTC)
 * The phosphor used on the screen (or the electronic display) could be in any color they wanted - but our eyes are more sensitive to green than to most other colors - so that's probably the reason for the choice.
 * However, our OP remarked that this information is being requested in order that it can be added into a Wikipedia article - and for that, we need to find solid references that say why this particular color was chosen.  It's not enough to know the answer - we need references. SteveBaker (talk) 20:32, 10 July 2014 (UTC)

This website gives the answer and seems pretty authoritative - they make the things! "A green phosphor is used in these applications because the human eye can differentiate more shades of green than any other color, allowing for greater differentiation of objects in the picture". Strangely, the website is given as an external link in the Night vision device article but nobody has used it for references. Richerman   (talk) 21:27, 10 July 2014 (UTC)


 * Some more reference resources for the interested reader:
 * AN/AVS 6 and related AN/AVS 9 systems are phosphor-based screens first used in 1993. They are designed and distributed by Exelis (formerly ITT).
 * US Navy / US Marine Corps Fixed Wing Night Vision Device Manual. "The AN/AVS-9 uses P43 phosphor (Table 2-1) with a spectral output that exhibits a primary peak output in the green."
 * "Integrated Panoramic Night Vision Goggles" from Air Force Research Laboratory.
 * AN/AVS-9 product information sheet. "First developed by Exelis in 1992, the F4949 night vision system continually has improved and is now designated the AN/AVS-9(V) by the U.S. Government. The (V) indicates there are many versions in service." Among the many designs, there are various color filters available.  The product sheet claims 16 hour battery life, but (WP:OR) anecdotally that is overstated by an order of magnitude, not to mention the discomfort and ergonomic effects of a heavy helmet-mounted battery pack weighing on the user's neck.  (The Air Force takes ergonomics very seriously!)
 * PVS/7 Night Vision System - a more compact model
 * US Army's Night Vision and Electronic Sensors Directorate, including a great history page explaining the origins of phosphors and the transition to the "Digital Battlefield" during the 1980s and 1990s, culminating in today's digital image sensors - particularly uncooled infrared sensors (e.g. CMOS imagers).
 * Point is, there are lots of varieties of these devices out in the wild. There are digital systems; there are even still-extant green phosphor image intensifiers in use - but those are "highly toxic" per the User Manual referenced above.  And, like most obsoleted technology, that older gear generally does not perform as well as the newer models.
 * Ultimately, the choice of displayed color depends on the device, the user preference, the purpose of the night vision device, even things like what color the interior lighting of the cockpit is. But the key is, the device can be designed to output visible light of any color; and nowadays the user can change that setting any time they like.
 * Nimur (talk) 05:39, 11 July 2014 (UTC)


 * The suggestion that the human discernability of color is richest for "green" hues is an oversimplifcation of the involved visual perception processes that is somewhat misleading. While it is true that we have preponderance of photoreceptive cells which see in what is designated as the yellow-green segment of the spectrum of visible light, in reality the colors which arise from the interplay between the three types of cone cells (two of which are typically most highly sensitive to different but overlapping spans of frequencies in the yellow-green spectrum -- the "green"/M and "red"/L cones) actually give rise to the perception of many colors outside of what we would in common parlance call green.   What's more, color perception is highly contextual, with the vividness and quality of different hues being subject to a number of complicating effects of the biomechanics of the eye itself (see purkinje effect,  binocular rivalry and sensitization effects) and the neurophysiology of visual perception and cognition (for example, the principle of color constancy) and the interplay of the two (consider the opponent process and chimerical color).  The relationship between the number of cones known to be sensitive to a given segment of the visible light spectrum and hues associated with the name we give to that segment is not a simple 1-to-1 formula.  In any event, since most night-vision devices represent only differentials in luminosity, often to the pure exclusion of other forms visual information, the hue is not a terribly relevant factor.   S n o w  talk 11:11, 11 July 2014 (UTC)