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Medical Forum / General / Vision / April 2009What is the the longest-wave red'?
Hi: What is the longest wavelength of light that is perceived by the average human eye as 'red'? Thanks > Hi: > > What is the longest wavelength of light that is perceived by the > average human eye as 'red'? > > Thanks It depends on intensity. The response is falling rapidly, but the stronger the power, the more likely you are to see it. I accidently got in the beam of a supposedly invisible IR laser one time- I saw it clearly, but jumped out of the way as soon as I could. Problem is, those supposedly IR wavelengths can still burn you, so it is not a good idea to look at those laser diode beams just to see if you can see them. There is annecdotal evidence of folks seeing something even from 1.06 but these were exposures that DID do retinal damage. Whether they actually saw the beam or whether it was a neurological phenomenon is hard to say. >> Hi: >> [quoted text clipped - 9 lines] > those supposedly IR wavelengths can still burn you, so it is not a good > idea to look at those laser diode beams just to see if you can see them. (snip) Infrared laser illuminators for use with image intensifiers usually use 840 or 850 nm diode lasers near the highest power levels considered "eye safe" but are not noticeable to the unaided eye. 808nm lasers in the same application will appear as a faint red dot for most observers. 880 nm LEDs in the same power range also give a noticeable red dot because they have side skirts extending short of 800 nm. The nature of gen III intensifiers limits the longest useful wavelength for laser illuminators to around 950 nm. "eye safe" laser illuminators may operate at any total power level if they are beam expanded so no more than about a milliwatt can enter an eye's dark adapted iris. That would be looking directly into the unit at short range. (still not recommended). They are not necessarily eye safe when viewed though magnifying optics. Beware of cheap Chinese laser illuminators marked 850nm +/-5%. Those of course are 808 nm. There is no perfect answer. But a wave-length of 7000 A is probably correct. > Hi: > > What is the longest wavelength of light that is perceived by the > average human eye as 'red'? > > Thanks > Hi: > > What is the longest wavelength of light that is perceived by the > average human eye as 'red'? > > Thanks Aw crap. He's back. In article <c02bcc13-4034-434f-88ad-0efb82d84f43@y34g2000prb.googlegroups.com>, > Hi: > > What is the longest wavelength of light that is perceived by the > average human eye as 'red'? > > Thanks The nut is back again under a different name. I think it used to be Radium. It must be that he was found out to be the equivalent of radioactive waste on the internet. Bill  SignatureMost people go to college to get their missing high school education. > In article > <c02bcc13-4034-434f-88ad-0efb82d84...@y34g2000prb.googlegroups.com>, [quoted text clipped - 14 lines] > -- > Most people go to college to get their missing high school education. ---------------------------------------------------------------------------------------------- I think you're right in that Radium and GreenXenon are both the same nut, but he's used this name before. Check his profile in Google Groups. It also shows he's now posting a variety of inane troll bait to a variety of innocent and unsuspecting newgroups, victimizing them. Remember that time last year when he claimed someone hijacked his account and posted a bunch of gibberish all over. Yeah, it was him doing it and trying to blame someone else saying that they hijacked his account. And then there were those stupid IRC Flame posts. Don't believe a thing he says. > Hi: > > What is the longest wavelength of light that is perceived by the > average human eye as 'red'? > > Thanks The answer "it depends on intensity" are good, but traditionally 760-780 nm has been given. I think this assumes intensity in the same context you would consider comfortable if looking at e.g. 560 nm. The answer for violet is perhaps more interesting, because 380 nm is traditionally given under similar assumptions, however: retinal sensitivity goes shorter to 360 nm or maybe less, so people with corneal surgery and older implant lenses that didn't absorb enough UV could see light around 360 nm. The short end is also more sensitive w.r.t. age, with young children able to see the shorter end better (from yellowing of lens, etc.) Note: "violet" properly applies to short radiation. It looks like "purple" (mixture of red and blue), because our red sensors have a double peak in the violet hence making for a similar sensation of color. (If Hilary Putnam or other analytical/ordinary-language naive realists are reading this, I'm sorry if you can't understand the idea of "color sensation" because you think we simply "see colors" themselves, which means the actual light "directly" I suppose. (Well, is it the objects we see, as simpleton Ryle argued, or the light from them? What, no internal representation which constitutes the more immediately given percept?) Then they snow-job it over with bafflegab about "seeming." > > Hi: > > > What is the longest wavelength of light that is perceived by the > > average human eye as 'red'? > > > Thanks > The answer "it depends on intensity" are good, but traditionally 760-780 > nm has been given. So 780 is the longest-wavelength most human would see as "red"? I'm guessing this would appear pinkish red because when an electric heating element gets just hot enough to emit visible light, the red light *does* have a pinkish tinge to it. Right? As the heating element get hotter the color starts to appear reddish- orange. Keep turning up the heat, and the element glows yellow. > I think this assumes intensity in the same context > you would consider comfortable if looking at e.g. 560 nm. Yes. Of course. >> > Hi: >> [quoted text clipped - 12 lines] > heating element gets just hot enough to emit visible light, the red > light *does* have a pinkish tinge to it. Right? No, a pure 780 nm would look "deep red" because it doesn't stimulate our green and blue receptors (roughly correlated to those colors in sensitivity) very much. Hot things appear more pinkish or orangeish red because they are not emitting a pure wavelength. Look up "black body radiation" and see the emission curves for various temperatures. BTW, it is little-known that the radiation rule (emission is proportional to absorption) applies to each wavelength, so: we could in principle make an object that would stay purple when hot (i.e., absorb green and reflect blue and red). That object would glow green when very hot, counter-intuitive as that sounds. > As the heating element get hotter the color starts to appear reddish- > orange. Keep turning up the heat, and the element glows yellow. Right, because the curve shifts blue-ward. It's always a range, not a single wavelength (but there is a peak wavelength). Note that not all bodies are BB or even grey bodies, so their emission is not a perfect BB curve. This subtly affects the colors of molten metal, and experienced metallurgists can see when steel is just the right temperature or has too much coke in it, etc. Stars also have emission and absorption lines, so many stars are redder than a perfect BB for their temperature (Betelgeuse should not quite be as peachy, like Mars, given its temperature. Also our atmosphere yellows things a little even looking straight up, and our sun would be more greenish white if seen with no atmosphere in the way. It's peak is about 500 nm, not really a "yellow" star. BTW sorry you seem to get some flack about your posts, this was a good question but people remember whatever bothered them most. >> I think this assumes intensity in the same context >> you would consider comfortable if looking at e.g. 560 nm. > > Yes. Of course. > BTW sorry you seem to get some flack about your posts, this was a good > question but people remember whatever bothered them most. You obviously are new this blokes postings. He's been asking "legit" questions for at least the 10+ years I've been on usenet. They always end up leading to some quack fantasy. Why not ask him about the black light UV laser 3D psychedelic video disc with something like 1 billion by 1 billion by 1 billion pixel resolution yet only a 1 nanosecond sampling rate? hmmm??? He went by another elemental name back then, radium. Google is your friend. Brian Signaturehttp://www.skywise711.com - Lasers, Seismology, Astronomy, Skepticism Seismic FAQ: http://www.skywise711.com/SeismicFAQ/SeismicFAQ.html Quake "predictions": http://www.skywise711.com/quakes/EQDB/index.html Sed quis custodiet ipsos Custodes? > >> "GreenXenon" <glucege...@gmail.com> wrote in message > > >>news:c02bcc13-4034-434f-88ad-0efb82d84f43@y34g2000prb.googlegroups.com... > >> > Hi: > [quoted text clipped - 12 lines] > > heating element gets just hot enough to emit visible light, the red > > light *does* have a pinkish tinge to it. Right? > No, a pure 780 nm would look "deep red" because it doesn't stimulate our > green and blue receptors (roughly correlated to those colors in > sensitivity) very much. What wavelength of light stimulates our red-receptors the most without stimulating the blue and green ones? 780 nm is sort of like red on the edge of being infra. AFAIK, green receptors are best stimulated at 555 nm. Not sure what wavelength would best stimulate the blue receptors while leaving other two alone. The receptor-wavelengths probably also vary [though very slighty] from person to person. ... >> >> The answer "it depends on intensity" are good, but traditionally >> >> 760-780 [quoted text clipped - 14 lines] > stimulating the blue and green ones? 780 nm is sort of like red on the > edge of being infra. Well, "the most" but not green and blue also? Maybe 700 nm or so, which is bright enough but very pure red. That's the subjective sign of the conjunction: bright rich red with no hint of orange. > AFAIK, green receptors are best stimulated at 555 nm. > > Not sure what wavelength would best stimulate the blue receptors while > leaving other two alone. Maybe 420 nm, a deep blue with no green input (not leaning aqua) but bright enough. > The receptor-wavelengths probably also vary [though very slighty] from > person to person. Yes, so none of these answers is the same for everyone. And not just "slightly" in some cases. The weirdest, is that a very few people (women, for genetic reasons) are "tetrachromats" that have four fundamental receptors (an extra one in the yellow band.) I met one, who is a color consultant - it is nearly certain from how she can see extra shades in the yellow area, but she has not been formally "recognized" yet AFAIK. > So 780 is the longest-wavelength most human would see as "red"? > > I'm guessing this would appear pinkish red because when an electric > heating element gets just hot enough to emit visible light, the red > light *does* have a pinkish tinge to it. Right? No, it looks like a deep pure red. 852 nm also does. It seems the blue and green detectors in the eye are completely insensitive to these wavelengths. Cheers, Jürgen SignatureGPG key: http://pgp.mit.edu:11371/pks/lookup?search=J%FCrgen+Appel&op=get |
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