Wikipedia:Reference desk/Archives/Science/2016 May 8

= May 8 =

Paint color matching.
When I take a chip of paint to a DIY store and have them make a matching paint - what kind of sensor do they use? Seems like a regular RGB camera wouldn't cut it...do they switch light sources? How does it actually work?

SteveBaker (talk) 02:21, 8 May 2016 (UTC)
 * Colorimeter?--178.107.62.251 (talk) 03:34, 8 May 2016 (UTC)
 * One actually can use a regular camera, for example, Sherwin-Williams has an app. DMacks (talk) 04:13, 8 May 2016 (UTC)
 * I'm DEEPLY skeptical that a cellphone camera, with unknown lighting, could possibly, remotely, come up with a useful color match to existing paintwork - and to be fair to Sherwin-Williams, that's not what they're claiming. They suggest using colors captured in photos for "inspiration" and other such things - but they never say it's going to match...and nor would it. SteveBaker (talk) 18:00, 9 May 2016 (UTC)


 * Not sure how good it is, but has some technical discussion. That and other sources like   (second one is about car paint colour matching) says a spectrophotometer. One source I came across called it a Spectroradiometry, but I think photometer is more accurate for the type of machine normally used. Admitedly I wonder if spectrocolorimeter may be even more accurate for what being done, but I didn't read enough to know for sure.  Anyway I found this PR about one machine used  which may help find out PR about this specific machine, if you cut through the PR I would guess you could get some info on what's actually involved. Colour Match Navi CT-X is evidentally one device used for cars although I could find very little about it. The device mentioned in the Gizmodo link is more productive . I also noticed a lot of AliBaba results e.g. , I would guess some stores are using products sold there.  I would also assume/hope, there must be some published science about these commercial computerised paint colour matching machines, I didn't notice any but a targeted search may work better.  Nil Einne (talk) 14:23, 8 May 2016 (UTC)
 * Hmmm - so according to the HowItWorks article - they shine "pure" white light onto the target then capture the reflected intensity across the visible spectrum in 10nm steps using a bunch of interference patterns. That's kinda what I thought.
 * SteveBaker (talk) 18:00, 9 May 2016 (UTC)

Mobile phones in walkie-talkie mode - range?
I understand that a mobile phone connecting directly to another mobile phone instead of via a cell tower would have greatly diminished range because the cell tower provides the power so that the phone doesn't have to but what would that range be? Using the hardware usually used to connect to the nearest cell tower, how far could phones talk to each other in a such a walkie-talkie mode? — Preceding unsigned comment added by 78.148.108.58 (talk) 19:52, 8 May 2016 (UTC)
 * This question really devolves pretty quickly when you start getting into technical details. Cellular networks are specifically designed to be asymmetric, not peer-to-peer; and this design is embodied in every detail of the hardware, from the amplifier design to the antenna design to the frequency choice to the digital and software protocols that are used. A significant limiting factor - perhaps the most significant limiting factor - is the nature of the shared channel, and if we re-architected a mobile phone for direct peer-to-peer operation, this changes everything about the communication signal quality.  Every single detail of your mobile telephone's radio is defined by its mechanism for cooperatively sharing that channel with a tower and with other users.   So, we simply can't answer the original question as posed - not accurately - because cellular telephone electronics don't work in this mode.  If we start changing details or making approximations, we haven't got a cellular telephone anymore.
 * How familiar are you with radio technology? Before we start lobbing specifications at you, it might be good for you to review some basic terminology for telecommunications engineering and radio-frequency engineering.
 * At best, we could show you some specifications for other types of radios whose size and shape are "similar" to mobile cellular telephones. For example, a handheld VHF radiotelephone for use at aviation radio frequencies will reach one or two miles (if you're on the ground); and you might manage to reach as far as ten or twenty miles if you're at altitude - say, two miles above the ground, with a clear line of sight to your remote station.  Aviation VHF is quite a bit lower frequency than the bands used by a modern mobile cellular telephone.  To first-order approximation, that means that your cellular telephone will have a shorter range and will be much more limited by any obstacles (like walls, buildings, and terrain, and even humidity in the atmosphere) that block your line-of-sight.
 * I happened to be reading this article - on natural antennas - earlier this weekend. It's fascinating to see some cold, hard numbers - a 15 watt HF radio (which is much lower frequency and much much much much greater power than your mobile telephone) will work over a few miles through dense forest; but if you use a tree as an antenna, you can get as much as 20 or 30 dB of antenna gain.  If you were in a forest and wanted to use a modern mobile cellular telephone, you'd probably have to be within a mile of the tower, or the signal would drop below usable levels.
 * Nimur (talk) 20:47, 8 May 2016 (UTC)


 * Some cell-phone manufacturers marketed cellphones which were known for walkie-talkie modes (known as Push-to-talk). Nextel was one of the best known.  -- Jayron 32 23:10, 8 May 2016 (UTC)
 * Yes, but that technology was still mediated by the centralized cellular network. There was no direct device-to-device radio link.  For example, iDEN, the Motorola-branded technology that powered Boost Mobile and NexTel, was a trunk radio carrier.  It has the superficial appearance of being a "walkie talkie," but that technology actually has more in common with VoIP than with a real, direct radio link.  Nimur (talk) 02:42, 9 May 2016 (UTC)
 * TETRA (Terrestrial Trunked Radio) Mobile Stations (MS) designed for use by emergency services can communicate in direct-mode operation (DMO) in situations where network coverage is not available. DMO also includes the possibility of using a sequence of one or more TETRA terminals as relays. AllBestFaith (talk) 13:27, 9 May 2016 (UTC)
 * Neat! That's very similar to a lot of research into mesh networks and ad-hoc digital packet radio networks used by advanced HAM operators.  But I'm pretty sure TETRA is not used in the United States by emergency services.  We have a diverse set of local, state, Federal, and special-purpose services, so perhaps some organizations use it... but they are exceptions, not the rule.
 * Here's a pretty cool rundown of the historical and current radio technologies used by the CHP: California Highway Patrol radios, from WB6NVH. That guy has great photos, does a deep technology dive, covers current and historical systems, and even lists sources .... he even links to the authorization-legislation!
 * Such radios are neither "mobile phone" nor "walkie-talkie" - the preferred terminology is "Tactical Network Radio" or some variation therein.
 * It's hard to find good technical documentation on those technologies - what's left of the Motorola radio technology division is struggling to stay afloat ever since our friends at Google decided to hack the company to pieces. Most of the technology offerings are listed on the Motorola Solutions webpage - the real Motorola - and these are pretty much the full spectrum of the radios and radio network technologies that American emergency services use today.
 * Nimur (talk) 15:14, 9 May 2016 (UTC)


 * Presumably you could build a cell phone with other radios for direct communication, right? Actually isn't that already the case, as many phones these days have Wi-Fi and/or Bluetooth? Do those use the same antenna as the cell radio? I'm a programmer, and I'm fairly knowledgeable about software in general, but all that signal processing stuff is voodoo to me. Now certainly, it might not be economical to add radios to a cell phone for peer-to-peer communication. --71.110.8.102 (talk) 00:02, 10 May 2016 (UTC)
 * You could build such a device - but RF electronics are difficult. Like, really difficult.  Like, you don't see nifty "do-it-yourself" RF electronics at the Maker Faire.
 * Test equipment for RF is expensive and difficult to use. Most engineers with a degree in electronics can't tell you the difference between a TDR and a VNA, let alone would they know how to use them.  When enthusiast hobbyists see the price tag for the test equipment - without which it is literally impossible to build a modern radio - they gargle and guffaw and the hobby project fizzles out pretty quick, with the budget diverted to cheaper thrills like Ferraris or fast airplanes.
 * Radio electronics in this decade is a far cry from an amateur pursuit.
 * For the ultra-enthusiast who really really wants to get into RF: don't despair! Find a great research university who can afford the equipment you need, and prove that you're worthy of time-sharing their effort by being very good at math and engineering theory.  An internship wouldn't hurt, either.
 * So, it's unlikely a hobbyist will build such a device; but if a research team or an industrial consortium settles on a standard - say, AX.25 (with all its awesome features and deal-breaking limitations), you might see some mobile handsets that start supporting it.
 * But for now, especially in the United States, it looks like we're going to be using centralized radio networks for the foreseeable future. You can read about our FCC's radio spectrum policy roadmap on the Radio Spectrum Allocation webpage.
 * Nimur (talk) 01:12, 10 May 2016 (UTC)
 * There must be something obvious about your answer I'm missing. I am under the impression that ham radio, shortwave sets, walkie-talkies, pirate radio and so forth are very common consumer hobbies on a shoestring budget.  How is the situation you describe different? Wnt (talk) 02:12, 10 May 2016 (UTC)
 * HAM radio sets don't work with mobile telephone bands or protocols. Nimur (talk) 03:34, 10 May 2016 (UTC)
 * The closest that ham radio gets is probably D-STAR, which uses similar digital protocols for voice communications on the VHF and UHF Ham bands. This technology was adopted by amateurs (hams) quite a few years ago and taken up by one of the big three amateur radio manufacturers, Icom Incorporated, in Japan. It is now a well and truly established technology. Of course, it doesn't interwork with consumers' phones. Users with D-Star capable sets are, just like their non-D-Star capable cousins, able to talk direct (line of sight), or via a compatible repeater to extend the distance, or via a satellite link to communicate with other countries. Akld guy (talk) 06:22, 10 May 2016 (UTC)
 * A gift for the spook who has everything, the Motorola APX Transportable Base Station's “mouse-hole” exit for a power cable allows for covert closed case operations. You did not read this here. AllBestFaith (talk) 12:14, 10 May 2016 (UTC)

Driest place east of the Mississippi river
Which place in the United States east of the Missisippi river receives the lowest average annual rainfall? — Preceding unsigned comment added by 24.207.71.235 (talk) 23:09, 8 May 2016 (UTC)
 * I did some searches, and while I couldn't find any specific locale, This overview of Virginia climate notes that certain valleys of the Appalachian Mountains, such as the Shenandoah Valley and the New River Valley are noted for their lower-than-normal rainfall in the Eastern U.S. That may help you narrow down your searches.  Such places lie in the rain shadow of the peaks of the Appalachian ridges, so that makes some sense.  -- Jayron 32 23:16, 8 May 2016 (UTC)
 * Minnesota. Sagittarian Milky Way (talk) 05:13, 9 May 2016 (UTC)
 * Hard to tell from just that map. There are places east of the Mississippi in other states which are colored the same color as those parts of Minnesota which are also east of the Mississippi (which is only a small part of the state). -- Jayron 32 08:35, 9 May 2016 (UTC)
 * At the river's northernmost point I draw a meridian to Canada. I've seen people draw the line due north from the source. Sagittarian Milky Way (talk) 16:47, 9 May 2016 (UTC)


 * Here's a better mean annual precipitation map of the USA, for the period 1961-1990, using downsampled NOAA data . Looks like Northern MI has the largest area in the 25-30" bin east of the Mississippi. SemanticMantis (talk) 13:49, 9 May 2016 (UTC)
 * Huh. So "Northern Michigan" does mean the northern Lower Peninsula, not the northernmost part of Michigan.  Who knew? --69.159.61.172 (talk) 23:51, 10 May 2016 (UTC)


 * If someone answers this with a source, please do tell. But I exect either somewhere far up north, such a Maine, where the lack of evaporation compensates for the lack of precipitation.  That, or somewhere in Long Island. μηδείς (talk) 04:46, 12 May 2016 (UTC)


 * What's wrong with this:? And what's so special about Long Island? It only has pine barrens cause the soil is crappy. Sagittarian Milky Way (talk) 05:27, 12 May 2016 (UTC)