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200mW of red is equivalent to how much green?

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vivioo7

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Im considering buying a 200mW 650nm and was wondering how visible the beam would be, I have a ~20mW 532 and a ~50mW 405. Obviously the green is my most visible so I was wondering how red compares to green. Thanks
 





holy cow I was expecting 75ish then I saw another members pic of 6 wavelengths together so I can believe it
 
vivioo7 said:
holy cow I was expecting 75ish then I saw another members pic of 6 wavelengths together so I can believe it
Click to expand...

Remember that a camera is going to capture it more 'equitably' than our eyes can see it.

I will say there is a BIG differene between 632.8 (HeNe) and 660 (Diode) ... 20mW of 632.8, to me, seems at least as bright as 80mW of 660nm. So it's not just "red", its what kind of red you're talking about.
 
I don't know why I didn't think of this earlier.

I just plugged the numbers into chroma.

200mw of 650nm red is equal to 24.185mw of 532nm green.
200mw of 635nm red is equal to 49.053mw of 532nm green.

There, now you know... exactly. :D

And for s&g's.. 1W of 445nm = 47.908mw of 532. 600mw of 405 = 3.1573mw of 532.
 
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qumefox said:
I don't know why I didn't think of this earlier.

I just plugged the numbers into chroma.

200mw of 650nm red is equal to 24.185mw of 532nm green.
200mw of 635nm red is equal to 49.053mw of 532nm green.

There, now you know... exactly. :D

And for s&g's.. 1W of 445nm = 47.908mw of 532. 600mw of 405 = 3.1573mw of 532.
Click to expand...

Chroma doesn't take rayleigh scattering into the equation, so those numbers are off.
 
Grix said:
Chroma doesn't take rayleigh scattering into the equation, so those numbers are off.
Click to expand...

Well, they're valid for the 'dot', but your right, when talking about the beam it will come into effect. Blue will get a boost compared to 532, while red will take a steeper hit when talking about beam visibility. But green will still overpower both, since that's the range the eye sees best.
 
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Grix said:
Chroma doesn't take rayleigh scattering into the equation, so those numbers are off.
Click to expand...

This thread got me to thinking, and then to doing some digging! ;)

I wanted to find-out how much Rayleigh scattering varies by wavelength, so that we could adjust qumefox's #'s to account for the change in actual beam intensity based on a laser's wavelength. :cool:

Turns out a fella by the name of Lord Rayleigh (hence the name, and also why the first word should be capitalized ;)) figured this out way back in the 1800's!

He determined that this amount varies inversely proportional to the wavelength of the light raised to the 4th power.

So we can now calculate the difference in Rayleigh scattering between a 532nm green laser and other wavelengths, in order to determine how much we need to adjust the apparent beam intensity to correct this difference! :cool:

So based on that, here is the "approximate";) difference in Rayleigh scattering between a 532nm green laser and other common laser wavelengths -

635nm - 0.49266490683388707592096813556886

650nm - 0.44873826309162844438219950281853

660nm - 0.42215388175097666707984764583813

445nm - 2.0427089195236108955687445081855

405nm - 2.9773263176444960813623876252967

But that's not the end of the story! One of the articles I came across indicated that modern references to Rayleigh scattering often forget to include an additional factor - one that also varies by wavelength!

A feature of Rayleigh's theory that is often forgotten today is that the intensity of the scattering depends on the refractive index of the scattering medium.
Click to expand...

Atmospheric Extinction

Well, it turns-out that the refractivity of air also varies with wavelength! -

Dispersion

Now normally, when a web page claims other references are wrong, I take it with a grain of salt. But this is coming from an extremely authoritative source. These folks' careers depend on making EXTREMELY accurate calculations regarding such things.

So, unlike crazy Google ATM;), I'm going to assume they are a more accurate reference than Wikipedia!

According to this reference, accounting for this extra factor, the change in Rayleigh scattering actually varies by the inverse of the wavelength raised to approximately the 4.08th power across the visible spectrum.

Using these revised calculations, the approximate difference in Rayleigh scattering between a 532nm green laser and other common laser wavelengths is now -

635nm - 0.48573864900825677601320307593276

650nm - 0.44160396556074654015153702131358

660nm - 0.41493512744940499532617990819131

445nm - 2.0720995430911201003047269051986

405nm - 3.043007095689047987747722875912

So, assuming the quoted Chroma values were correct, the revised beam intensity values (including Rayleigh scattering wavelength adjustments), would be approximately -

200mw 635nm red = 23.827mw green

200mw 650nm red = 10.680mw green

And for the other examples given -

1W 445nm = 99.270mw green

600mw 405nm = 9.6077mw green

:cool:

qumefox, given that the commonly-used LPC LOC is typically spec'd at 660nm, I am curious what equivalent 532 green mw your program gives you for that wavelength?
 
^ Enjoyed that info about Raleigh scattering. It definitely puts things into perspective.
 
I didn't read everything people have posted, so sorry if this has been mentioned already.

There is another factor at play here. Green lasers have a finer beam diameter, so the beam is more concentrated.

But ultimately, in my opinion 200mW reds are quite visible at night and it is a sight worth seeing, pointing it at (as the first time i saw a red beam was) a church steeple from 100m away or so.
 
JaiNobeZ said:
There is another factor at play here. Green lasers have a finer beam diameter, so the beam is more concentrated.
Click to expand...

That's correct, but remember: Lower beam diameter means higher divergence (usually), so it's opposite after about 5 meters, then red/violet beams are thinner than green beams :)
 
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