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Possible method of accurate divergence measurement
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toyoda
4.99mW Green Laser Toy
Joined: 03 Apr 2007
Posts: 8
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 Posted: 5/06/07, 7:08 PM Post subject: Possible m |
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Visual methods for determining laser beam divergence are very subjective because such methods are affected by a number of factors such as the colour of the target on which the beam impacts, the distance of the target from the laser, the level of ambient illumination on the target, the visual acuity of the person viewing the laser dot, the characteristics of the laser goggles through which the dot is viewed (if goggles are necessary), and the diameter of the laser beam itself. If the test is done over a long distance (which is usually necessary for any semblance of accuracy), the relative humidity and pollution levels in the area become major factors because their presence makes the beam grow “artificially” in diameter (thanks to the reflective and/or refractive characteristics of contaminants in the air).
Repeatable long distance divergence tests can only be done when humidity and pollution levels are similar from one test to another, or compensated for mathematically. There are, unfortunately, no standards specifying the factors just described, thus making it difficult to compare one person's test results with those done by another person, or even one test to another done by the same person.
The purpose of this post is to suggest a possible way of doing highly accurate divergence testing without expensive photometry equipment. Instead, this method uses inexpensive equipment, and tests the beam divergence over a long distance, where divergence is most obvious.
A TEM00 laser beam is most intense near its center, and then gradually loses intensity toward its perimeter. These changes are so subtle that they are often difficult to see, thereby making visual divergence determinations difficult. There seems to be three commonly used "standards" for stating divergence. The first is a measurement of the apparent beam diameter, which is simply the entire width of the beam (or of the dot produced by the beam when it impacts a target). However, because of the faintness of the intensity near the dot’s perimeter, it can be difficult to determine its true width, even in total darkness.
The second "standard" seeks to find the point in the beam at which the intensity is only 20% of that measured at the beam’s center. This method may be the most meaningful of the three.
The third "standard" seeks to find the point where this intensity is 50% of that measured at the center. This method results in a diameter which is artificially small, and can bear little resemblance to the diameter of the beam seen by the eye.
Trying to visually assess these percentage of intensities is virtually impossible, thereby making such judgments a waste of time; especially if done close to the laser, where the beam is small in diameter.
Accurate determinations of intensity change within the beam can be done with a quality photometer designed for laser measurements, but such equipment isn't available at most small labs. It may be possible, however, to obtain accurate measurements by using a camera and computer programs like Photoshop or PaintShop. Here is how this might be done:
Secure the laser to a rigid mount, and aim it at a target at sufficient distance to get a laser dot of 300 millimetres or more in diameter. For lasers with high power outputs and low divergences, this will probably require a distance of at least 300 metres. Lesser power lasers, or those with large divergence, will require lesser distances. The target should preferably be white, albeit this method might work with targets which are black or any colour. Mount a ruler or scale of some kind across the width of the target and align it so that its center is near the center of the laser dot. A few metres from the target, place the camera on a tripod, and move it as close to the laser beam as possible, without blocking the beam. Adjust the camera's zoom or position until the laser dot nearly fills the width of the viewfinder.
Any type of digital or film camera will probably work with this method, so long as the photographed image can be imported into a photo editing program. Make sure that the target has consistent color and density across it's surface, and is fairly perpendicular to the laser beam and parallel to the film plane in the camera. Once the picture of the laser dot is imported into Photoshop (or other editing program), use the intensity tool to find places on the image where the intensity is only 20% (or 50%) of that at the center of the dot. When these points are determined, use the scale in the photograph to determine the width of the dot at the appropriate percentage, and then use that measurement to determine the divergence of the beam.
Needless to say, the distance from the laser to the target must be precisely known in order for this method to produce any level of accuracy. The reason for the long distance to the target is the probability that the intensity of a small dot (on a target near the laser) would overwhelm the CCD in a digital camera or show as total over-exposure on film. Making the dot large in diameter (by placing the target far away) should eliminate any over-exposure problems and reduce the need for enlargement with the photo-editing program, thus providing for optimal accuracy. Because of the distance, however, accuracy can only be expected in environments with low levels of air pollution and low relative humidity, preferably below 40%. The scale placed on the target should be as narrow and nonreflective as possible in order to minimally affect photo quality; and the camera‘s flash must be turned off. This method may also be able to accurately determine the divergence of infrared lasers, if used with cameras capable of infrared photography.
A test scenario might work like this: The target and scale are placed 400 metres from the laser. For this hypothetical test, assume that the ambient irradiance at the target is less than 1 watt per square meter and that the relative humidity is 40%. Assume also that at 400 metres, the apparent beam diameter is approximately 300 millimetres; and the diameter of the laser beam as it exits the laser is about 1.5 millimetres. A photograph of the dot is taken and imported into the photo-editing program. Using the intensity tool, it is determined that the point of 20% intensity is 115 millimetres from the center of the laser dot, thereby indicating that the dot (using the 20% standard) is about 230 millimetres in diameter. Subtracting the width of the beam at the laser from this measured 230 millimetres, we get a figure of 228.5 millimetres. When this 228.5 millimetres is divided by the 400 metre target distance, we get a divergence of 0.57 milliradians.
(Note: In this scenario, the minimum beam width was taken from the laser manufacturer’s specifications of the beam width as it exit’s the laser. When doing divergence calculations, the narrowest portion ("neck” width) of the beam is the proper number to use, but since locating and measuring the “neck” is basically impossible without specialized equipment, the manufacturer’s number is usually sufficient for these calculations.)
Last edited by toyoda on 5/07/07, 3:36 PM; edited 1 time in total |
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pseudonomen137
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| Posted: 5/06/07, 7:48 PM Post subject: |
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An interesting idea - one that makes me a bit ashamed to not have read your post in its entirety  .
Anyway, a few issues though are that you cannot exactly assume that by letting the dot get larger you can ignore overexposure issues. With the high visibility of the greenies, it would probably take either some attenuation, or some manual camera adjustments. If you take a camera and change the exposure setting step by step between pictures, you'll see the dot appear to change in size pretty dramatically (I did this with the iris on my camcorder and saw a faint point change into a glowing sun filling up most of the screen). I have a feeling that because of this, what sounds good in theory would be near impossible in practice.
Also, where does the 20% figure come from? Isn't the spot size where the field intensity gets to 1/e and the power density 1/e^2?
A minor, but still troubling issue is also knowing the relative size of the image - I guess you could probably print a grid out or something, but that only ends up adding a little extra margin of error.
Also, perhaps for more accuracy the intensity method isn't the best. I'm a bit busy now, but maybe when I get time in a few weeks I'll write up a program to target the center and tally up the power outwards in concentric circles. When ~86.5% is included in the center, then you know your spot size (however, this once again brings a chance of error - it means you'd probably want the spot to be small enough to display 3 of its diameters in the image, yet still be large enough to make decent calculations on).
And of course, settings fo background and overexposure will be very important. For accurate results, the picture taken without the laser in it should return a black screen of all 0,0,0 RGB values, and the picture with the laser should meter the center pixel of the spot as close to 0,255,0 without actually achieving that value.
So all in all, I bet there will be too many factors to control to get this to give a measurement with much accuracy.
That said, when I get time I'll definitely be trying it out and at least hoping it beats doing it visually! |
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Steve0000
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| Posted: 5/07/07, 8:20 AM Post subject: |
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| What the poster suggests is a bit overly complicated and in itself an arbitrary standard. What people I think are concerned about is how large the spot looks rather than knowing actual intensities. A simple method that will produce accurate enough results would be to mark off in a X,Y fashion ticks in millimeter increments on a flat plate of glass. Using the plate and a long tape measure place the plate of glass in the beams path at various distances from the laser and note the diameter of the beam. People can use a program like this http://www.laservisuals.com/sglite.php if they want more, it's free. |
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toyoda
4.99mW Green Laser Toy
Joined: 03 Apr 2007
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| Posted: 5/07/07, 4:09 PM Post subject: |
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In answer to the concerns addressing my original post, I think that this simple photographic method may work. Many times, in order to inspect for lens flaws, dirt on lens, TEM characteristics and changes, et cetera, I have increased the divergence of laser beams using only a simple lens. Several times, when the beam diamter has been expanded in this manner, I have photographed the resultant dot using an inexpensive Canon A620 pocket camera. The photographs taken in this manner did well when brought into a photo-editing program for close analysis.
I assume, therefore, that a large laser dot on a target - hundreds of metres away - will photograph just as well. Using a "ring" analysis of the intensities is an interesting idea. I am not an expert with photo-editing programs, so I am not sure how this would be done. I do know, however, that the simple intensity tool in most programs is very accurate for determining intensities. I have edited hundreds of photos using this tool for guidance.
The reason I chose the 20% or 50% "standards" is because that method of determining divergence is the means mainly stated on laser manufacturer's websites. I have no idea what would be the most "perfect" standard for determining divergence.
Steve, you are quite correct in stating that most people are interested only in the visual size of the dot, and are little concerned with knowing exact intensities within the dot. Most laser users wish no more than to flash a laser around and have fun. Unfortunately, some of us who have spent the bulk of our lives in a laboratory wish to know the exact characteristics of all in our environment. We are never satisfied unless technology is always taken to the max. There may be medications to control these obsessions, but I have not used any myself! On a more practical side, it is sometimes necessary to make sure that a laser system is performing within manufacturers' specifications; or to adjust multiple system to perform the same. If my method will work, it will make the ability to do this affordable to many individuals and small labs.
The biggest danger of this method will be idiots setting up the test in such fashion and places that the beams from their lasers will endanger people, animals, aircraft, etc. Fortunately, most people I know who would take the time to set up such a test are intelligent and mature enough to do it in a safe and scientific manner. |
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Aseras
Fusion Laser
Joined: 03 Nov 2006
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| Posted: 5/07/07, 5:26 PM Post subject: |
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| I measure the divergance on my lasers by setting up a rolling target inside our test building and take a walk with a measuring wheel. When I hit 100 feet I measure the "dot" as seen through laser glasses. This eliminates most scatter and I consider it a worst case scenario. The laser has AT LEAST this divergance, but not any worse. |
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pseudonomen137
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| Posted: 5/07/07, 5:46 PM Post subject: |
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I think the OPs main point is that if you look at a dot visually, whether through goggles or not, you will have trouble figuring out where to determine the diameter from - and as well you should. Since the spot is not really one even circle, but a 'gaussian distribution', it'll be bright in the center, and taper off out to infinity... so where do you measure the spot from?
Well, technically the spot is where ~86.5% of the energy is enclosed within, but you can't really figure that out visually. That's why Coherent charges many thousands for devices made for this purpose.
It sounds like the OP's idea is to do something similar, but without the huge costs. I don't think the OPs idea is quite as viable as it sounds at first glance, but hopefully I'll have time to test this out in the next month or two (In some spare time I started mentally planning out a program for the task) |
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Aseras
Fusion Laser
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| Posted: 5/08/07, 8:26 AM Post subject: |
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I'm just saying if you do measure the ultimate diameter in both cases, you will get a figure that is close enough to being an accurate representation of your laser's true divergance. While your laser may have better divergance that what you measure, outside of lab conditions or for very specific requirments that none of us are ever going to use ( and I use lasers daily as my job ) it's not going to mater or be apparent.
I've seen several high end laser actually use a aluminum pinhole type shroud over the emission aperture to shave the beam just a bit and remove any optical defects or speckle.
Here's a good webpage that explains how the $$$$$$$$ machine measure the "dot" and why it's important..
http://www.spiricon.com/techinfo/application_notes/an4.shtml
and you'll see that it really is for minute changes and for SERIOUS power outputs. A hobbyist or even a professional doesn't need this kind of stuff until you get into the really crazy end of the laser spectrum for manufactures and QA, not for tweaking.. and even if you could find things like this out there's not much you can do about it unles you are the manufacturer. |
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Steve0000
Wicked Lasers Master
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| Posted: 5/08/07, 8:54 AM Post subject: |
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| pseudonomen137 wrote: |
I think the OPs main point is that if you look at a dot visually, whether through goggles or not, you will have trouble figuring out where to determine the diameter from - and as well you should. Since the spot is not really one even circle, but a 'gaussian distribution', it'll be bright in the center, and taper off out to infinity... so where do you measure the spot from?
Well, technically the spot is where ~86.5% of the energy is enclosed within, but you can't really figure that out visually. That's why Coherent charges many thousands for devices made for this purpose.
It sounds like the OP's idea is to do something similar, but without the huge costs. I don't think the OPs idea is quite as viable as it sounds at first glance, but hopefully I'll have time to test this out in the next month or two (In some spare time I started mentally planning out a program for the task) |
I've seen spot size defined as 13.5 % of center beam intensity. You can define spot size any way you want. For most of us a visual inspection will work. The OP's idea can be done for the fun of it.
Last edited by Steve0000 on 5/10/07, 9:20 AM; edited 1 time in total |
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pseudonomen137
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| Posted: 5/09/07, 9:18 PM Post subject: |
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| Steve, I was under the impression that for a gassian beam, the spot size radius technically was, by definition, 1/e the field amplitude on axis, and 1/e^2 the power density (information from some various Springer books on lasers). Are those really just arbitrary values from manufacturer to manufacturer? |
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Steve0000
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| Posted: 5/10/07, 9:19 AM Post subject: |
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| pseudonomen137 wrote: |
| Steve, I was under the impression that for a gassian beam, the spot size radius technically was, by definition, 1/e the field amplitude on axis, and 1/e^2 the power density (information from some various Springer books on lasers). Are those really just arbitrary values from manufacturer to manufacturer? |
They are arbitrary in the sense that this standard was standardized by us it wasn't brought down from some mountain top.
What I mean when I say beam diameter can be expressed any way you want is this. If you profile a beam that is M=1 you'll see that the profile is a Bell curve distribution of power from 100% to 0%. You could make your own standard of what ever percentage of max. beam intensity. Furthermore determining M2 can be complicated because beams can look gaussian but will have an M2>1 . For people like us that do not require this exacting standard 1/e2 a visual measurement will be sufficient.
Here's what I'm using as my reference notice the word typically in the quote
| Quote: |
| In Gaussian beam applications, the distance between two points on the cross section of the beam, directly opposite from each other through the centroid, each with a defined fraction of the peak power of the beam, typically 13.5% (or 1/e2). |
http://www.mellesgriot.com/glossary/wordlist/glossarydetails.asp?w ID=103
and RP Photonics http://www.rp-photonics.com/beam_radius.html
When I look at the spot projected on a surface I don't care what the power distribution is across the spot all I care about is how large the spot looks. |
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Laser Ben
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| Posted: 5/10/07, 11:26 AM Post subject: |
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This a a very good idea, I will see if I can pick up some CCD cameras at the flea market. Any idea where to get that beam modeling software?
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Steve0000
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| Posted: 5/10/07, 4:33 PM Post subject: |
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| Laser Ben wrote: |
| This a a very good idea, I will see if I can pick up some CCD cameras at the flea market. Any idea where to get that beam modeling software? |
C'mon you have to ask with just a click you can have an answer rather then waiting for a reply |
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Laser Ben
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| Posted: 5/11/07, 2:58 PM Post subject: |
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I don't understand what you are trying to tell me! 
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BoomDog
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| Posted: 5/16/07, 11:16 AM Post subject: |
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| Moved to Help/Laser Safety thread to prevent buildup of stickies in general section. |
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Laser Ben
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| Posted: 5/16/07, 3:00 PM Post subject: |
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But seriously, how does this help people with laser problems?
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