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405nm vs scotopic vision?

Atomicrox

Atomicrox

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I've been noticing for quite some time that the beam of my 405nm 200mW laser doesn't look the same color as the dot.. The dot is the usual violet and the beam is more of a greyish tone. At first I thought that could be something on the air fluorescing but rules that out because the beam can't be seen with 405nm goggles.

The only other explanation I can think of is that a weak beam of 405nm isn't strong enough to trigger photopic vision, which would explain why it looks considerably brighter than a similar 650nm beam (our scotopic vision is better on the blue end) and the greyish color.

Does everyone see the beams greyish? Can anyone confirm my hypothesis or explain this?
 





Cyparagon

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The beam is brighter than a red's beam because of rayleigh scattering.

The dot is a different color than the beam mostly because almost everything fluoresces under violet light to some extent.
 
Atomicrox

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But shouldn't the eye sensitivity overpower the scattering?
From the CIE data even with rayleigh scattering the red beam should be some 25 times brighter (166 times less sensitive with 6.6 times more scattering). I know that data is known to be off near the blue end but 25 is a lot..

I'm "lucky" enough to have prescription glasses with very high chromatic aberration, all I have to do is look at the dot from a couple meters away at a high horizontal angle and I can see the spectrum decomposed. Usually the violet component really stands out from the rest of the colors, that way I can have a good idea of what's fluorescence and what's diffuse reflection. It sucks but is also pretty cool ;)
 
Cyparagon

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The CIE tables also say 5mW of 650nm light is just as bright as 800mW of 405nm light. Is that true from your experience?
They're off by a lot at blue, but they're off by stupid amounts on violet.
 
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Atomicrox

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Fair enough :p You're probably right about the brightness but I still think fluorescence doesn't explain the beam color. It looks too "unsaturated" to be the "real" color of 405nm. Maybe it's just my eye, who knows.
 
Bionic-Badger

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The beam of the 405nm laser looks like the deep purple outer edge of the laser dot to me, or the dot from a distance on a non-fluorescing surface. I doubt the beam is fluorescing much, but often the laser dot fluoresces the surface it hits as well.

Also, the CIE luminosity curves are usually cited for photopic vision--i.e. daytime lighting conditions. They're not a good estimate for beam-viewing brightness, which is more mesopic. Scotopic vision might apply too, but I think that takes a little longer to achieve than the average beam viewing.
 
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Atomicrox

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Last night I tested something I hadn't tried before - comparing the beam on a lighted room and on total darkness against the sky. I did notice a considerable difference in color, under lights the beam was more like the outer edge of the dot and against the sky it was more greyish. Could be the mesopic vision indeed, the strange thing is that I only perceive this difference with 405nm. Maybe it isn't bright enough to "switch" to photopic vision...
 
Atomicrox

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Up with new info!

I've confirmed that the greyish beam is a case of scotopic or mesopic vision. Now I have a 500mW 405nm laser and the beam always looks violet, while the 80mW laser's looks greyish under the same circunstance (when the higher power laser is off, obviously).
 
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trencheel303

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I can confirm the same. I only have a newwish 405 so it's probably under 100mW, and I see the same thing you do. In a lighter environment, the beam is clearly purple. In a darker environment it trends far more towards grey, exacerbated if I put the dot out of view, in which instance it's almost like my vision switches to monochrome. Really cool effect actually.

What's it like seeing a 500mW beam of 405? I find the colour fascinating and have been considering getting a powerful 405 for light experiments and beam visibility of a colour that to my eyes looks completely unnatural and surreal.
 
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Shakenawake

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outside is when the beam of my 500mW+ 405nm does sometimes seem kinda grey. strange and interesting observation
 
Atomicrox

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To me it looks very ghastly, out of focus and violet (aka awesome)!
Brightness wise it's comparable to a 30mW greenie, maybe a bit dimmer. Clearly visible in the dark.

Never had mine look greyish, but that might be because I live in a very polluted area, which increases scattering and beam visibility.
 
zyxwv99

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I've had the same issue with 405s. Sometimes you can see the color if you look down the beam. Otherwise it looks gray. I assumed from the beginning that it was scotopic.

I also have a very hard time seeing any color in the 490nm dot on my 980. I can see a little shining it on brushed metal, but that's cheating.
 
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trencheel303

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What wizardry did you do to get a 490nm dot out of 980? Some type of frequency doubling I assume, but how?
 
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steve001

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It seems to me some of the problem might be do to the inability to perceive color rather than anything do to other variables. I see the beam as violet and spot on a non fluorescing surface also as violet.
Munsell color test. Take this test and see if there are deficiencies.
Color Test - Online Color Challenge | X-Rite
 
Pi R Squared

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I suspect that the grey effect is caused by the fact that 405nm is at the edge of the visible spectrum. Some people probably can't see the beam at all. The one I just built meters at 590mw and I can only see the beam in dim lighting, or in the dark its much more visible and looks more violet to me, but my damn cameras only see it as blue. For those who don't yet have a powerful 405 if you decide to get one be careful with that thing. It burns anything you point it at, you will accidentally leave some scorch marks before you realize it. Didn't expect that from 590mw.

Alan
 
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