Purple lasers. Is purple real? - Page 1
Forum section: electronics / general

author: Circlotron
author_rank: Super Contributor
timestamp: August 18, 2024, 05:36:39 am
content: Seen a bit of stuff lately saying that purple is not a real colour, but is produced in your brain when your eye sees red and blue combined and the green receptors are not triggered. Something like that. Got me totally confused. Then I came across purple lasers, and seeing a laser is monochromatic, if it is purple then it must really be purple and not some trick of the eye/brain combination. So, what’s the story then? Is purple a real colour?

author: Kleinstein
author_rank: Super Contributor
timestamp: August 18, 2024, 05:48:52 am
content: The "blue" lasers are in the 400-410 nm range and thus just on the edge of the visible range. It looks a bit purple / violet. Even if LEDs are similar nominal wavelength, the additional longer wavelength tail is more visible and this changes color to a more normal blue.The naming of colors is anyway a bit confusing and may vary between languages (some have more colors than others and thus no simple translation).

author: coppercone2
author_rank: Super Contributor
timestamp: August 18, 2024, 06:05:45 am
content: UV laser looks grey purple unless its flourescent. On wood its bizzare, it really looks grey purple. If you hit anything that has any activity it lights up bluepurple and glows all sorts of color on plastic and etc objects with dyes or whatever.You see I was curious about the same thing about high red. I got a 750nm laser with a 10nm band width. It looks red. When you put it near a red laser (635 or whatever the cheap one is) with the same amount of attenation, they look similar, but I was thinking that the far red laser looks more red and the other one looks orange. After I was looking at it for a while in the dark I thought, well... thats the most red thing I ever seen.I got high power far-red LED first, but people were just saying "you must be seeing the edge of band". So I got a laser, now I am sure I can say, there is color there, and its red.. I am certain all the people I argued with had no idea what their talking about, because the laser is pretty damn pure, and filtered well to boot (scientific source).its a really narrow band laser.The only danger is, tempting to look into the emitter if its bearly visible. You need real darkness, verified power level and a safe target to try to see these colors.I also think that objects that are glowing from heat look very strange too, in the beginning of their glow region. Like a very dull red heat. Like something substantial like a big steel rod. Like the kinda glow that you don't see until you turn all the lights off and wait a while.

author: IanB
author_rank: Super Contributor
timestamp: August 18, 2024, 06:26:32 am
content: Quote from: Circlotron on August 18, 2024, 05:36:39 amIf you look at the visible spectrum (for example, see first link below), there is a colour similar to purple between blue and violet at the blue end of the spectrum. It's a colour that is often called indigo (see second link).https://www.thoughtco.com/the-visible-light-spectrum-2699036https://www.canva.com/colors/color-meanings/indigo/

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 18, 2024, 06:29:12 am
content: Get a diffraction grating and tell me."Indigo" is usually used for the "super blue" that's on the edge of UV, which also means it's on the rainbow (at least, if it were very visible), whereas purple/fuchsia is not.The perception of color is fascinating. With some hinting, you can just begin to... conceptualize, perhaps, the existence of "red-green", or "red-blue". Purple is kind of there, most people would say it's reddish and bluish in nature, but secondary colors really do have the feel of being their own thing. And this is no accident; they're a byproduct of subtraction and encoding done within the retina itself, you physically cannot experience such colors. You can get close, with combinations of flashing, bright bars or eye tracking to kind of abuse the brain into seeing colors that alternate (never being encoded properly) or never overlap (when they should, due to blurring, involuntary eye movements, etc.).Lasers don't have to be monochromatic; there are many kinds. Very short (fs) pulses are inherently broadband, making the coherency fairly spacially limited, but temporally very useful. Gas discharge lasers can have multiple "fertile" (laser-active) states, and between the large wave number (~10^6 wavelengths between mirrors?) and peak-broadening effects like thermal/doppler, it's very likely (or, possible with some tweaking) that multiple modes resonate. The intensity might not match, but it might also be tunable by various parameters (gas, mechanics, optics).Harmonic generators might also be used. Red could be 2nd harmonic and blue 3rd, of a somewhat oddball IR line (1200nm or so). Or completely separate lasers could be mixed with a dichroic mirror, common for display/projection purposes.Tim

author: Someone
author_rank: Super Contributor
timestamp: August 18, 2024, 06:29:37 am
content: Quote from: Circlotron on August 18, 2024, 05:36:39 amYes:https://en.wikipedia.org/wiki/Spectral_colorBut the intensity required for it to be similar in perceived brightness to "normal" colours makes monochromatic purple quite the hazard.

author: magic
author_rank: Super Contributor
timestamp: August 18, 2024, 07:04:13 am
content: Quote from: Circlotron on August 18, 2024, 05:36:39 amAFAIK the thing is that the "red" cones are sensitive not only near 700nm, but also slightly to the second harmonic, in the vicinity of 400nm. Hence deep violet appears the same as a mix of red and blue.Not all eye spectral sensitivity plots that you will find on the 'net show this effect, but some do.

author: Siwastaja
author_rank: Super Contributor
timestamp: August 18, 2024, 09:18:29 am
content: There are two kinds of purple/violet:One which is really at the end of the spectrum (even shorter wavelength than blue). This looks pretty dim to human eye, as it's close to vision limits. You can just barely see this color when you look at the rainbow.Then you can of course just mix blue and red wavelengths, which is what your computer screen always does to show you purple color.Laser by definition emits a single wavelength so it is the first kind of purple.Same story about yellow: you can have yellow wavelengths like a yellow LED or, even better, sodium vapor light. Or you can take red and green LEDs and mix their light, and get the same sensation.

author: Phil1977
author_rank: Frequent Contributor
timestamp: August 18, 2024, 09:42:23 am
content: There is only one colour you really can't get in a laser: Brown(Source:https://de.wikipedia.org/wiki/CIE-Normvalenzsystem#/media/Datei:CIE1931xy_CIERGB.svg)All colors on the curved outline of this diagram are possible with monochromatic light.For colors on or near the straight line at the bottom, you need to mix different wavelengths. But as already mentioned, not all lasers need to be monochromatic. Lasers can also be a mix of wavelengths.

author: Halcyon
author_rank: Unknown
timestamp: August 18, 2024, 11:31:27 am
content: Quote from: Phil1977 on August 18, 2024, 09:42:23 amOr light in general...

author: Circlotron
author_rank: Super Contributor
timestamp: August 18, 2024, 12:10:26 pm
content: Looking a bit further, it appears that violet is a monochromatic colour but purple is not. Purple is a combination of red and blue, so it would not appear on the spectrum.

author: themadhippy
author_rank: Super Contributor
timestamp: August 18, 2024, 12:14:02 pm
content: Quote from: Halcyon on August 18, 2024, 11:31:27 amyou can get pretty close with filters,lee 746 does a pretty convincing job,now grey light-good luck

author: tooki
author_rank: Super Contributor
timestamp: August 18, 2024, 02:23:27 pm
content: I think that, for practical purposes, it depends on our definition of "a laser".If we mean an actual "raw" laser emitter (like a laser diode or tube), then they are (for practical purposes) monochromatic spectral colors.If we mean a laser product, like a laser lightshow, then it's definitely possible to have multi-wavelength laser beams, which are simply made by overlaying RGB laser beams, modulated as needed, from multiple laser emitters. That's how we get full-color laser shows that include white.

author: TimFox
author_rank: Super Contributor
timestamp: August 21, 2024, 05:04:35 pm
content: Monochromatic light of a given (visible) wavelength has a color.In general, light of a given color usually contains a range (possibly disjoint) of wavelengths.Brown is an extreme example of the latter.Yes, different languages have different distinctions between natural colors.Interesting example: a question to a literary publication decades ago from an author writing an historical novel set in medieval England, before oranges were known there, asking what word should be used to describe that color.The first occurrence of the word “orange” in English was in the 16th century.One suggestion was “tenne”, an heraldic term.Note that orange is a spectral color, between 580 and 625 nm wavelength.

author: SteveThackery
author_rank: Super Contributor
timestamp: August 21, 2024, 06:02:55 pm
content: Quote from: Phil1977 on August 18, 2024, 09:42:23 amI've always thought of brown as "dark yellow".

author: ajb
author_rank: Super Contributor
timestamp: August 21, 2024, 07:46:48 pm
content: It's practically meaningless to talk about specific color perception by naming colors, because color names/perception are highly variable between individuals and circumstances. "Purple" is especially problematic, as evidenced in this thread.The shortest visible wavelengths at ~400nm SHOULD appear deepest blue, stimulating only the S cone in the retina, but 405nm light (which is a common laser wavelength, used for blu-ray) appears sort of a pastel purple. This is because, as magic mentioned upthread, the spectral response of the L cone, which is primarily sensitive to reddish wavelengths, has a small bump near 400nm. L-cone sensitivity is still very low at those wavelengths, but the S cone (sensitive to blueish wavelengths) response also tails off towards those wavelengths, so both responses are significant. It's very hard to precisely quantify the spectral response at such low levels, though, which is why that bump isn't always visible on spectral response plots. Even with a precise response spectrum for the light sensitive molecules themselves, there's a ton of processing overlaid on that chemical response that complicates any attempt to quantify the perceptual results. That processing starts in the retinal ganglia (nerve cells) that aggregate the responses from groups of cones even before those signals leave the eye.If you want to see what a true deepest blue looks like, you could sit in a room lit only by red LEDs for several minutes and then look at a 405nm LED/laser source. The red light will deplete the L cones (and the M cones somewhat) while leaving the S cones (mostly) unaffected, so when you switch to the 405nm source the response of the S cones will dominate.On the other hand, most of the time when you see "purple" light you're seeing a combination of red and blue in some ratio, simply because RGB mixing gets you most (not quite all!) of the colors humans can perceive. Since the eye isn't very sensitive to such short wavelengths anyway, using red and blue sources is also much more efficient in terms of perceptual brightness vs radiant power. A CIE Chromaticity diagram (such as attached) is IMHO one of the best ways to visualize perceptual color space as it relates to wavelengths. The X axis represents the ratio of stimulus between L and S cones, and the Y axis the ratio between L and M cones, which cancels out overall brightness from the equation. The colors displayed in the shaded area aren't accurate, since the RGB color space of your monitor can't display all perceivable colors, but they provide a rough idea of how colors map to the space. The curved boundary around the top and sides of the shaded area represents the entire visible spectrum, so anything on that line is a spectral color that can only be produced by a single wavelength. The straight portion of the boundary is the 'line of purples', so anything along that edge can only be produced by a combination of two wavelengths at opposite ends of the visible spectrum. Any color that is inside of those boundaries can only be produced by a combination of two or more wavelengths. There are actually an infinite number of possible spectra that can produce any perceivable color, though, except for the spectral colors which consist of one single pure wavelength (even lasers aren't quite that pure in general, but they can get pretty close for practical purposes), and the "purples" that are combinations of the very longest and very shortest wavelengths we can perceive. Pick any two points in the shaded area (or on its boundary) and a line drawn between them will cross all of the possible colors that can be made from that combination (its gamut). Pick any three points and a triangle drawn between them contains all the colors that combination can produce. Likewise for any number of points. Note that there is no combination of three spectral colors that can enclose all of the perceivable colors, showing that RGB mixing isn't always adequate -- you can expand the gamut by adding more and more spectral colors, but in fact no finite number of component wavelengths can be combined to reproduce the entire perceivable color space. (To work around this, some color spaces use 'imaginary' colors that lie outside of the shaded area on the chromaticity diagram as their primaries.)Quote from: tooki on August 18, 2024, 02:23:27 pmActually, a lot of tube (gas/ion) lasers are capable of lasing at multiple wavelengths. Before DPSS lasers, which came before direct injection diode lasers, there were "white light" gas lasers, typically using argon to produce blues/cyans/greens, and krypton for yellows/reds, and these were commonly used in laser light shows for that reason. As a result, the laser light show industry adopted typical Ar/Kr wavelengths when standardizing color control channels, so you still see optional deep blue/yellow/cyan in addition to r/g/b channels on some control systems, although for the most part they go unused these days. HeNe lasers can also lase at multiple wavelengths in the red-green range, although it's more unusual to see them lasing at multiple wavelengths at the same time. What you actually get out of the laser will depend on how it's set up, and can be controlled to some extent by how the laser cavity is aligned and what additional components are used in the cavity. For some applications you really only want one wavelength, so it's common for lasers to be set up to suppress all but the target wavelength.(According to a friend with decades of experience in the early days of laser light shows, they used to have multiple sets of cavity mirrors for their ion lasers that could be swapped out depending on what colors were most important to a particular show. Very slight differences in the shapes of the mirrors would cause have different effects on the path lengths through the cavity, which could shift the distribution of power between the different longitudinal modes. So if you had a show that needed a lot of red, you would swap in the red-heavy mirror set -- of course then you have to do a from-scratch alignment of the cavity, which is tricky enough on a small single-line HeNe, let alone on a big multi-line, water-cooled 4ft glass tube that pulls a few kilowatts from the wall!)