Clarifying Laser Output Power and how this relates to photography.

I have done a search for this but could not find the answers I wanted. It would be great to get some help with this from anyone who is willing to explain something that I am sure is quite basic to all you experts, but difficult for someone new to the field to get my head around.

I am trying to work out the Output Power of lasers and how they translate to photography. As you probably all know, most photographers work in 'stops' to determine how to get the correct exposure (very broadly speaking). I am not looking for a photography lesson btw, just trying to quickly get to my point.

So what I am trying to understand is if I need a 532nm laser (for example) that is half the power of a 200mW (photographically speaking a stop less powerful), what would the output power be? Photographic/practical logic would suggest 100mW. However, Adam from DL mentioned this in recent correspondence, "Normally when optical power increased 6 times then the brightness doubled". So mathematically speaking would this be a 3.125mW laser? It seems too weak/small a figure to me.

Any help understanding laser physics greatly appreciated. Thanks, Mark

I can't really understand what you're asking.

Is your question "What 532nm laser power is half as bright as 200mW 532?"

That's a really interesting question. I'd like to know the answer to this too.

-cutyall

cutyall said:

I can't really understand what you're asking.

Is your question "What 532nm laser power is half as bright as 200mW 532?"

That's a really interesting question. I'd like to know the answer to this too.

-cutyall

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Hi, sorry if my question wasn't clear, however you got the gist of it. Yes, that is what I am asking but I was hoping for an answer in photographic terms.

For example, if I exposed a 532nm green 200mW for say 30 seconds to give me my optimum laser beam exposure. Technically if I then did the same test with a 532 green laser that was half the output power (in photography terms), I would then need to expose it for 60 seconds to achieve the exact same brightness/exposure. I hope this helps explain it better.

So my question is, what is half/or double? the output power of lasers (in photograhic terms)? Based on a 200mW as our median. Thanks for showing an interest. Mark

try this. Not sure it's what you need but will allow you to get the relative brightness for each power and color. Some of our photographing experts might have some ideas on how to create the desired level of brightness your pictures require.

http://lsrtools.1apps.com/relativebrightness/?nm1=532&mw1=1&nm2=532&mw2=1&useRaleigh=on

I suspect that what's half as bright to the camera may be different than what's half as bright to your eyes. I also think it may depend on the camera. I am no expert, hopefully one of the photography experts here will reply, but I have always had some difficulty taking photos of lasers, my cameras don't like lasers. It takes some experimenting and several tries and I still don't get a very good photo, my lowest quality camera seems to do the best with lasers.

Alan

starlight said:

try this. Not sure it's what you need but will allow you to get the relative brightness for each power and color. Some of our photographing experts might have some ideas on how to create the desired level of brightness your pictures require.

Relative Laser Beam Brightness Calculator: (532nm 1mw) vs. (532nm 1mw)

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Hi, thanks for the link which was very interesting.



I guess its just a matter of working out whether "Relative Perceived Brightness" translates into photographic terms. It does support the theory that at least output power is based on perceived brightness. I am not sure how that fits in with what Adam from DL was trying to say? Thanks again, Mark

I'll probably try this when I get home. I have two 532nm lasers, one is ~5mw and the other is 18mw. I'll experiment with the exposure/ISO/shutter speed... etc.

-cutyall

Edit -
Yes, there is a very clear difference between the two. I'd say that the 18mw is around 2 to 3 times brighter than my ~5mw. I don't know if it would make a difference in the camera.

cutyall said:

I'll probably try this when I get home. I have two 532nm lasers, one is ~5mw and the other is 18mw. I'll experiment with the exposure/ISO/shutter speed... etc.

-cutyall

Edit -
Yes, there is a very clear difference between the two. I'd say that the 18mw is around 2 to 3 times brighter than my ~5mw. I don't know if it would make a difference in the camera.

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hey thanks for taking a look into this. Would be great to know how the camera 'records' it. If your camera has a 'manual mode' you should be able to play with the shutter speeds to work out the difference in stops. Thanks again for having a play. Cheers, Mark

Alright, the results are in.

Three pictures.
ONE LASER TO RULE THEM ALL...

Sorry, that was unnecessary. On to the pictures.


This one is of the ~5mw beam. Note the beam splash. The beam itself is on the right.


Same perspective, only with my 18mw. Note the non-existent beam splash. Thanks Optotronics


18mw beam on the left, ~5mw on the right. sorry for the not-so-clear beams, the shutter speed was about 10 seconds and i'm not exactly the stillest person. haha

And there you go. I hope this answers your question.

-cutyall

Side note: it was a bit humid outside so the beams may be brighter in the photos.

I would help if I had a LPM (still waiting for the new Radiant ) - without one we're just guessing the power of our lasers.

I've always heard that you need 4 times the power to get double the brightness but have never tested that. It's something I will test (with my camera) when I get a LPM but that's weeks/months away (only ARG knows when the LPM will be released).

Will you be using smoke/fog with your lasers or just outside air?

[I've got an idea that might help, hopefully I get some time tonight]

cutyall said:

Alright, the results are in.

Three pictures.
ONE LASER TO RULE THEM ALL...

Sorry, that was unnecessary. On to the pictures.

This one is of the ~5mw beam. Note the beam splash. The beam itself is on the right.

Same perspective, only with my 18mw. Note the non-existent beam splash. Thanks Optotronics

18mw beam on the left, ~5mw on the right. sorry for the not-so-clear beams, the shutter speed was about 10 seconds and i'm not exactly the stillest person. haha

And there you go. I hope this answers your question.

-cutyall

Side note: it was a bit humid outside so the beams may be brighter in the photos.

Click to expand...

Hi, thanks for doing the test. Did you use the same camera settings for all the pictures (f? @ 10seconds, ISO?). If so, and you are interested and don't mind having a play...it would be interesting to see if you could match the 5mW's beam brightness by giving it a 40 second exposure. Technically this would be an additional 2 stops to roughly match the 18mW (which is around twice the physical power of the 5mW). If you could sit your camera on a tripod or flat surface too so that it was still that would def helps things. Sometimes you can use the self-timer on the camera that you would normally use for group shots to avoid you having to press the shutter and cause camera shake. Hope this helps and thanks again for testing this. Cheers, Mark

I did a test, not sure if the idea was scientifically sane.

I used a filter to half my lasers power (tested with a thermometer based "LPM"). So we effectively have a 52mW and 26mW laser.

I then set the camera up so the 52mW beam was just a little over exposed (it flashes on my LCD). I then used the filter to reduce laser power down to 26mW, and opened up the aperture until it too was just over exposed.

Results (will upload images if interested):
52mW: f16, ISO3200, 30 seconds.
26mW: f11, ISO3200, 30 seconds.

[Used a Nikon D90, 18mm, fixed focus, manual mode, everything on tripods and triggered using a remote shutter cable]

So I think that means half the power, increase by one f-stop? And increasing by one f-stop means it lets in twice as much light IIRC; so kind of makes sense!

clansley said:

I did a test, not sure if the idea was scientifically sane.

I used a filter to half my lasers power (tested with a thermometer based "LPM"). So we effectively have a 52mW and 26mW laser.

I then set the camera up so the 52mW beam was just a little over exposed (it flashes on my LCD). I then used the filter to reduce laser power down to 26mW, and opened up the aperture until it too was just over exposed.

Results (will upload images if interested):
52mW: f16, ISO3200, 30 seconds.
26mW: f11, ISO3200, 30 seconds.

[Used a Nikon D90, 18mm, fixed focus, manual mode, everything on tripods and triggered using a remote shutter cable]

So I think that means half the power, increase by one f-stop? And increasing by one f-stop means it lets in twice as much light IIRC; so kind of makes sense!

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This sounds very interesting and I would love to see the results! It looks as though you did everything right. It was a good idea to test the same laser by reducing its power with a 1 stop filter. The only question would have to be what kind of filter did you use? I would think it would have to be what they call a ND (Neutral Density) filter for the test to be accurate. If it had any colour/tint to it then I guess this could affect the wavelength and this may give inaccurate results. Thanks for testing...Mark

kingphoto said:

This sounds very interesting and I would love to see the results! It looks as though you did everything right. It was a good idea to test the same laser by reducing its power with a 1 stop filter. The only question would have to be what kind of filter did you use? I would think it would have to be what they call a ND (Neutral Density) filter for the test to be accurate. If it had any colour/tint to it then I guess this could affect the wavelength and this may give inaccurate results. Thanks for testing...Mark

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I used one of these: Phot-R™ 52mm PRO-D Slim Variable Adjustable Neutral Density ND2-ND400 Filter | eBay

Adjusted it until I had 50% power. I'll upload the images once my son get's off the computer with the required software

clansley said:

I used one of these: Phot-R™ 52mm PRO-D Slim Variable Adjustable Neutral Density ND2-ND400 Filter | eBay

Adjusted it until I had 50% power. I'll upload the images once my son get's off the computer with the required software

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Excellent! That would work a treat then. Good test...cant wait to see the pics. No rush, just when have some time. Thanks, mark

kingphoto said:

Excellent! That would work a treat then. Good test...cant wait to see the pics. No rush, just when have some time. Thanks, mark

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Probably obvious but make sure if you apply any settings ie contrast, brightness etc in processing that the same adjustments are applied to both frames. Thx,M