mW power to see Blue Beam at Night

[c0ldshadow]

hey guys

i was wondering what output power is required to see the beam of a 473nm blue laser at night? in terms of visibility, reasonably visible, to point out stars, for example.

 

[SenKat]

No EXACT answer...BUT - I can see the beam from my 5mw pointer (532nm) at night...since it is a lower wavelength - let's be non-scientific and guess - say, 10-15mw ? That is a WILD guesstimate....

 

[c0ldshadow]

thanks greg!

c ya around man

peace

 

[pseudonomen137]

Yep, 10mW should do it. My 17mW RPL blue was surprisingly visible for the wavelength.

 

[SenKat]

Yep, 10mW should do it. My 17mW RPL blue was surprisingly visible for the wavelength.

SWEEET ! Lucky guess on my part ! ;D ;D ;D

 

[Ragnarok]

It depends.

Scotopic (night) vision peak sensitivity is at or near 507nm, which means that a blue laser at this wavelength would be brighter than any other color - CAVEAT: scotopic vision is black and white, and much more sensitive and effective than photopic (colpr) vision at low light levels. Assuming two lasers of equal beam power at 473nm and 532nm, as the laser power goes up and photopic vision begins to swamp scotopic vision eventually the green 532 beam would become dominant in brightness.

Mucho good info about that kind of thing here:

http://hyperphysics.phy-astr.gsu.edu/hbase/vision/bright.html

 

[kenrok1]

Interesting... So based on the Luminous Efficacy table would a 405nm laser more visible in complete darkness, since the 405nm does not cross into the phototopic range (assuming it no competition from phototopic light sources)?

 

[aaron_inc]

scotopic vision is black and white, and much more sensitive and effective than photopic (colpr) vision at low light levels.  

i think that is why the beam on my violet laser seems a dim grey sometimes.

 

[Ragnarok]

So based on the Luminous Efficacy table would a 405nm laser more visible in complete darkness, since the 405nm does not cross into the phototopic range (assuming it no competition from phototopic light sources)?

"More visible" implies a comparison with something else. Yes, by itself a 405nm beam will be visible in complete darkness, especially if the eyes have dark-adapted, but will be less visible than a 473nm beam due to the much increased luminous efficiency at this wavelength. If there is not enough energy to stimulate photopic vision, both beams will appear to be a shade of gray.

However, there is another factor which comes into play; atmospheric scattering can increase the apparent luminous efficiency at short wavelengths. The atmosphere scatters 400nm light over nine times as much as 700nm light, so as you go shorter in wavelength, you get more scattered light, which is what makes the beam visible in the first place (if the luminous efficiency were the same at all wavelengths, beam visibility would still increase at shorter wavelengths).

http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html#c2

So there are several factors that determine just how visible "X color" beam will be how visible under "Y conditions".

The tremendous luminous efficiency of 507nm light (blue-green, turquoise) can be appreciated by lighting up a blue-green LED outside at night after the eyes have had time to fully adapt. It lights up the surroundings for a surprising distance, yet there is no color except close by, where there is enough light to engage photopic vision also. On the other hand, a red LED only lights the immediate vicinity.