Advanced math/physics oriented: Alternatives for measuring laser beam diameter?

[Elektr0n3Ro9000]

Hi! I hope this thread is considered as laser measurements. Okay, so I'm trying to get intensity of the EMW (laser) with formula I=P/A power/ area.
know knowing this, i have 100mW cylindrical/ circular beam of laser so A=pi*r^2. now how in the H should i know what my radius is?

I've searched for methods to measure laser beam diameter, aperture method and the knife edge technique and also burning plates. I can't do any of these options since i have no good devices for these stuff and my laser 100mW can't burn plates! *made in china, output is not really 100mW* Any ideas how? thanks!

 

[Bionic-Badger]

Well, that's the problem: you do need to know the intensity of your beam in order to gauge the beam diameter. Otherwise, you'll be relying on eyeballing the brightness or width, which isn't entirely accurate.

The knife-edge technique would probably be easiest. You can try spreading out the beam with your lens and then chopping off parts of the beam for measurement using a homemade edge or aperture made of paper, metal, etc. As long as your position and power measurements are accurate you can figure out the diameter.

 

[Cyparagon]

There are quite a few ways to define beam width. The difference between them isn't going to be too different from eyeball-error. Use a caliper and block out most of the light to make the edge more pronounced.

Why do you care, and how accurate does this measurement need to be?

 

[pschlosser]

You could measure your beam divergence and then work backward to calculate the theoretical beam width at any distance from the laser, even distance zero.

Beam divergence is the natural spreading outward of the light from a tight beam to a large spot over distance. In a manner, light emitting from a laser spreads out in a triangular shape. The narrowest part of the triangle is typically the point of emission. The larger is some distance away. Of course, we are not factoring in any optics that may muddle with the beam shape. Some lasers have them, others may not.

To calculate divergence, you measure the beam thickness (diameter) at two known distances from the emitter and do a little trigonometry. The edge of the laser beam is defined at the point where the intensity of the beam is 1/e (approximately 0.37) times the intensity of the light at the center.

FYI, the special value of “e” is discussed:

e (mathematical constant) - Wikipedia, the free encyclopedia

This isn’t as hard as it sounds if you take pictures of the beam at these two points. Get a sheet of white paper and place a ruler next to it (for scale) and take pictures at the two known distances. Then, analyze the images in a graphics program, like Photoshop, and determine where the beam intensity falls to 37% of the brightest point in the center. Draw some points at these locations, and then draw a circle that intersects these points. Lastly, using the ruler in the image, determine the real-world size of the circle.

FYI, beam divergence is discussed at:

Beam divergence - Wikipedia, the free encyclopedia

Once you pull this off, and measure the power of the beam at some known distance, you can also calculate the beam diameter at this same distance, using the data calculated above. Once you got power and diameter values, you should be able to figure out intensity from there.

If you need more help with this, let us know. We’ll be glad to help.

Edit: rather then measure the beam up close. I would take the two images some distance away. Maybe 2 and 20 meters away. Or 1 and 10m. Using two distances a magnitude apart should introduce the least amount of error.

 

[Elektr0n3Ro9000]

pschlosser wow that's awesome advice. I also have read about beam divergence, i have one shop that already has divergence info for the laser but unfortunately it's china made so not that accurate. can you please tell me how to measure the beam thickness? can you please provide a more explicit explanation about this:

To calculate divergence, you measure the beam thickness (diameter) at two known distances from the emitter and do a little trigonometry. The edge of the laser beam is defined at the point where the intensity of the beam is 1/e (approximately 0.37) times the intensity of the light at the center.

i can't imagine what you said here. at two known distances so i was wondering how to do that? sorry for such a stupid question. haven't really worked with lasers and really has a tight schedule to learn more things a bit in a rush tho. :banghead: and thanks so much

 

[pschlosser]

I'll throw something together, tomorrow, and show you how easy this is. once you do it, or see it done, you will understand it is easy.

 

[Blord]

We need an independent measurement tools for the dot diameter. I was thinking about a cheap webcam with a laser filter in front. When the laser is shined upon the camera the CCD can record the intensity of the dot. With this formation and the distance of the camera to the laser we can calculate the divergence of the beam.

 

[pschlosser]

I've completed a tool for calculating divergence. I welcome your feedback. If there is something you don't understand, let me know. I tried to make the text easy to understand. But I can't help being a geek.

LPF Beam Divergence

 

[Elektr0n3Ro9000]

I have here the specs of the laser given the divergence and diameter.
- Beam divergence angle (mrad): < 1.5
- Beam diameter (mm): < 1.0
but i also need to re-measure them. can't I just put a mm ruler and stick it on the wall and point the beam on it and take a photo of it? it sounds stupid but can it be done?
and pschlosser I found it also, the site you gave for divergence of the beam calculator saw it on youtube but it says that I have to have an optical breadboard or something. alternatives? thanks a lot!

 

[pschlosser]

and pschlosser I found it also, the site you gave for divergence of the beam calculator saw it on youtube but it says that I have to have an optical breadboard or something. alternatives? thanks a lot!

You don't need to do the photograph and intensity analysis unless you want super accuracy. And the further you are away from the source, when you take your measurements, the more accurate your results will be.

Measuring a 1mm beam right at the source, with a 0.5mm error is huge. But a 1mm error on a 100mm spot measured some distance away is a very small error.

Just follow the directions on the web page at:
Beam Divergence

I recommend measuring at 10 feet and then again at 40 or 50 feet. I've forgotten, but if your laser is focussable, you want to focus it for the tightest beam possible at your longest distance. Do not refocuss between taking the two measurements, either.

 

[Elektr0n3Ro9000]

I've been in this site here How to Calculate Laser Beam Divergence | eHow.com which is easier to do but as you mentioned i have to do 10 feet then 40 then 50 so the instruction is 3m then do some computations. I'm okay with measuring the beam diameter on the target (white paper) that can be 10 feet away. But what I cannot understand is how could you measure the beam right at the source?

4. Measure the diameter of the laser beam in millimeters where it exits the laser pointing device. The diameter is the distance across a circle.

I can't seem to understand this.
I don't think my laser is focusable because it's china made it's hard to find original laser US made in my country...

 

[pschlosser]

I've been in this site here How to Calculate Laser Beam Divergence | eHow.com which is easier to do but as you mentioned i have to do 10 feet then 40 then 50 so the instruction is 3m then do some computations. I'm okay with measuring the beam diameter on the target (white paper) that can be 10 feet away. But what I cannot understand is how could you measure the beam right at the source? I can't seem to understand this.
I don't think my laser is focusable because it's china made it's hard to find original laser US made in my country...

You can do this how ever you want. But measuring the beam at the source may sound easy, and maybe it IS easy, but it isn't easy to take a measurement there and have enough precision to make it worth it. With my page, you need only three measurements. No math. Just measure, and then put the numbers in the form, and click submit to calculate.

If there is something about my page that is confusing, or sounds hard, I would welcome hearing more about it, so I can make the page better.

Chose any two distances from your laser, as long as they are at least 10 feet apart, and at least 3 feet from the origin.

 

[hakzaw1]

great thread- learned a lot from all-- Ty and +2

 

[Elektr0n3Ro9000]

okay now I get it thanks so much! anyways can you help me with one more thing? given I have already a beam diameter and computed divergence from there how do I know the beam intensity and power output? As I have said laser is china made so not necessarily give me 200mW the person I brought it from said if I have a 100mW (china made) then the output is only around less 75mW only. Assuming I know the divergence and diameter can how can I know the power and/ or intensity? without of course having some fancy devices..

 

[hakzaw1]

okay now I get it thanks so much! anyways can you help me with one more thing? given I have already a beam diameter and computed divergence from there how do I know the beam intensity and power output? As I have said laser is china made so not necessarily give me 200mW the person I brought it from said if I have a 100mW (china made) then the output is only around less 75mW only. Assuming I know the divergence and diameter can how can I know the power and/ or intensity? without of course having some fancy devices..

If you will add your location in your personal profile page you may find a fellow forum member that lives close-by and owns a LPM.

The best help come faster when we know your location- it will show in every post below your username.
I have an extra hobby LPM I may want to sell- PM me if your have interest.

hak

 

[steve001]

pschlosser wow that's awesome advice. I also have read about beam divergence, i have one shop that already has divergence info for the laser but unfortunately it's china made so not that accurate. can you please tell me how to measure the beam thickness? can you please provide a more explicit explanation about this:

i can't imagine what you said here. at two known distances so i was wondering how to do that? sorry for such a stupid question. haven't really worked with lasers and really has a tight schedule to learn more things a bit in a rush tho. :banghead: and thanks so much

First off your browsing of methods should have brought to your attention the first measurement is to be done at 10 meters. The second is to be done at any arbitrarily greater distance beyond 10 meters. But if it didn't that's where you need to start.
As for determining a beams maximum width that is arbitrarily up to you how it's defined or you can follow one of the methods you've read about.
Now to actually measure beam diameter requires a metric ruler for ease of use and something to stop the beam, like a piece of paper to measure spot size [= beam diameter].
A piece of non white paper works well, it reduces blooming. Personally I use a flat piece of clear glass and mark two lines intersecting 90 degrees to each other like the plus sign + and mark both lines in 1 millimeter increments. I place that in the beams path at 10 meters and subsequent measurements beyond 10 meters. Is this clear ?
Look up Rayleigh Length when you have time.