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nubie question on focusing laser

80cj

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i'm brand new to laser pointers and have gotten into them as a result of another project i am working on. i am using simple laser pointers to mark a spot so i can visually see movement of the target (on video). i am trying to get a smaller dot at ~ 50'. the ones that i am using are from a 'laser parking system' that they sell at Lowes. i have taken them apart to better focus them, but the dot is still ~3/8" at that distance. i have been looking at buying ones that have adjustable focus, but i'm not sure what the best that i can hope for would be.

any help would be much appreciated.

thanks
shawn
 

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OK.
Just about all laser pointers and anything else that uses a simple laser to produce a visible dot at a distance, uses just a single lens to collimate (focus) the point source of the laser light, into a beam.

This lens is focused to infinity, so as to give the smallest dot (beam width) for the longest distance.

This is different from the type of focus you would use with a magnifying glass and the Sun, where you move the glass to just the right distance from the objective to create the smallest dot you can. -One is about getting the best beam for any distance and one is about getting the absolute best beam for a particular distance.

Another way to look at it, is like a camera; if you focus it's lens to infinity, everything from two feet to a hundred feet will always be in focus, but at the loss of detail and any magnification. While if you zoom in to magnify and capture detail, you'll have to manually re-focus the lens for every distance, losing the "point and shoot" capability that focusing to infinity gave you.

Basically, the single a spherical lens in the laser pointer is giving you the best "one size fits all" dot (beam width) you can get.

If you want to get a smaller dot, you can try manually moving the internal lens (if it’s adjustable); this will really only be of value if the item you’re focusing the dot on, is less than 10 feet away. Longer distances are just not feasible, due to the short focal length of the internal lens (usually less than an inch from laser point source to the actual lens). Once your beam travels beyond a certain number of feet, the short focal length of the “one size fits all” lens, makes it almost impossible to fine tune any more.

You could try increasing the final focal length by adding more lenses (I've heard using a small telescope or monocular in reverse can work), but you'll have to manually re-focus the beam if the distance to the object you're shining it on changes. –Plus you’ll have to rig something up that will mate all the parts together.

Lastly, if all you want is a smaller dot (thinner beam) and don't mind losing some of the beams energy (brightness), you could just truncate or crop the width of the actual beam as it exits the lens. To do this, look at the aperture where the actual laser beam exits the pointer. This aperture (hole) is usually about 1/8th to 1/16th of an inch across. To crop or truncate the beam, you could take a small bit of aluminum foil and wrap it over the lens aperture, then, taking a sharp pencil, poke a small hole in the foil over the center of the covered aperture. -Depending on the size hole you create, the resulting beam or objective dot will be smaller and correspondingly dimmer, because of the portion of light energy that’s been blocked off. –Such a smaller, dimmer dot may be worthwhile, if using the laser as a signal carrier, signal source or trip sensor.

I hope this helps.
There are guys much smarter than me on this board who can give you better math and theory, but I think this covers the jist of what you're asking.
 

Tw15t3r

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I don't think he wants to know the theory but rather what he should expect and how he should go about doing it.

Since all you require are laser pointers with focusing ability, go get a 5mW one from Hightechdealz.com (the SURE module one). It runs on 3V, so just get 2 AA batteries and it'll last you for hours. Focusing wise, as long as the target is not moving parallel to the laser, you're fine. Focus the laser by turning the ring, and you should get a nice dot, could be as small as a fullstop up close at 1cm (that's my experience. But I haven't tried longer distances. I guess it wouldn't be that small, but definitely smaller than 3/8" at 50' since you are now actively focusing it rather than relying on the "focus to infinity" configuration.
 

rog8811

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Do you have to have just one dot? a diffraction grating will give multiple small dots.

Regards rog8811
 

80cj

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thanks for the replies. i have tried the pinhole approach, but the loss of beam energy is too great.

i have tried using multiple lenses, but there are just too many parts to keep align reliably w/o significant effort to fabricate holders. (i'm figuring that it is already done somewhere and it would be cheaper to buy one than make my own)

the bottom line is that i am trying to show movement perpendicular to the beam. if the spot is 1/2" diameter, a 1/4" movement will be much less noticable than with say an 1/8" diameter spot.

i understand (w/limited understanding) that source quality (size, power, resulting divergence, etc), and lens quality have direct effect on the size dot. i'm trying to figure out if i have just been trying to polish a turd, or if this is about as good as i could get w/o spending significant $$. (or how much i would need to spend to get down to ~1/16 or 1/8")

thanks
shawn
 

Tw15t3r

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All laser diodes provide uncollimated light and require a lens to focus it into a good beam. Unless you venture into other source of laser light like gas lasers, which will mean most of us can't really help, if not, you can't really spend more on a laser to get a tighter beam. You will mostly be spending on the optics.

But anyway, try using a binoculars or telescope. I can't remember clearly which way you are supposed to use it. I believe it's with the objective lens nearer to the source of the laser. It should give a tighter beam. and the optics will all be held in place for u. All u need is something to hold that instrument. I seen a person popping a balloon several tens of feet away by using a 3 feet or so telescope.
 

80cj

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thanks for all the replies. let me try to clarify my questions. i need a very small (~ 1/8" or smaller) dot at ~ 50'. how can do this?

1) can i buy a laser pointer that will have a ~1/8" dot (@ 50') right out of the box (ie w/o making modifications to it)?

2) will i have to assemble components (such as the radio shack microscope) to build my own?

thanks
shawn
 
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1) can i buy a laser pointer that will have a ~1/8" dot (@ 50') right out of the box (ie w/o making modifications to it)?
No.

2) will i have to assemble components (such as the radio shack microscope) to build my own?
Yes.
Unless you pay a lot of money for a lab style laser with an adjustable lens.
 

80cj

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are there any write-ups on using the radio shack microscope? i am struggling with the housing to hold everything. is will be outside and exposed wind, etc so it needs to be pretty robust.

just out of curiousity, what does a lab style laser cost? (ball park?)

thanks again for taking the time to answer my questions.

shawn
 

Tw15t3r

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I'm not sure about write-ups regarding the radio shack microscope for your first question. I'm also not sure what kind of laser you are using, but I'd recommend something like this where all you need are 2 AA batteries in a battery holder connected to the module. It comes with a driver attached to the diode already, so the current is regulated. The power consumption is low, around 10-20 mA, and so will ensure the laser last long, and no cooling problems too. With that setup, u can use it for infinite duty cycle. This module has a focusable lens too, but if you can't achieve the dot you want however, you'll probably have to use an optical instrument like a telescope, binoculars, or what SuicideKing introuced, a pocket microscope.

As for the lab style laser, it really depends on what you consider one a lab style. If you consider a lab laser as one with long or infinite duty cycle at the power you want it at, then the setup that I mentioned is equivalent to a lab style laser minus the mains power supply, and of course, a nice box host. That is, if you only need a small amount of laser power.

Higher laser powers require more cooling, current, and good circuit regulation, hence the laser begins to look pretty big. But the size is mostly due to heat sinks and cooling equipment if any. For your usage, this shouldn't be the case, and your lab style laser would probably fit in a small project box just larger than a matchbox (that is without the external optics of course)
 
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80cj said:
are there any write-ups on using the radio shack microscope?  i am struggling with the housing to hold everything.  is will be outside and exposed wind, etc  so it needs to be pretty robust.
just out of curiousity, what does a lab style laser cost?  (ball park?)
thanks again for taking the time to answer my questions.
shawn
What Tw15t3r said in the previous post covers your last question pretty well, but I'll make one more recommendation.

The simplest, easiest and cheapest way to get what you want (a smaller than normal beam dot), is to modify the apperture (the hole at the end of the laser where the beam emerges). -Now I know that cuts down on the brightness of the beam, so to compensate, just make the beam brighter. Even a dollar store keychain laser pointer can be overdriven, as long as you don't go crazy.

For example, a cheap keychain laser pointer usually runs on 3 button batteries, try using 4 batteries instead; this will increase the brightness of the dot by 50%. You don't even have to monkey with the housing to make room for the extra battery, all you have to do is find another battery type that's a little thinner than the ones that came with the laser, so 4 of them will fit in the housing (just make sure they're 1.5 volts, like the regular ones).

Using the microscope, telescope or any kind of adjustable focal length lens will probably get you that 1/8" dot at 50', but as soon as you move the laser or what its shining the dot on by more than a foot in either direction, that focus will be lost and the dot will be much bigger. You'll have to re-focus EVERY time the distance changes. Whereas, if you just modify the aperture, you'll get the smaller beam and never have to worry about focusing or re-focusing.

You can even find some "el cheapo" keychain laser pointers that already come with multiple, replaceable heads that project different designs. It would be the easiest thing in the world to modify one of them to give you a smaller beam and dot, by just making the aperture (hole) smaller.

I'm including some links to this type of laser at the end of this post, so you can see what they look like, where to get them and how much they cost (average price is $6 bucks).




http://www.amazon.com/Head-Laser-Po...=UTF8&s=office-products&qid=1236104282&sr=8-3

http://store.rebeccas.com/store/merchant.mvc?Screen=PROD&Product_Code=LI1122
http://www.lasermate.com/SPM.HTM
http://www.trueswords.com/golden-bullet-laser-pointer-heads-p-4050.html
 

80cj

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grrrr. i wrote a long explanation (with pics!) to try to better explain this all, but when i tried to submit it, it was rejected b/c i cant post links yet, and i lost all that i wrote..... so i will try again when i get time.


shawn
 

daguin

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80cj said:
grrrr.  i wrote a long explanation (with pics!) to try to better explain this all, but when i tried to submit it, it was rejected b/c i cant post links yet, and i lost all that i wrote.....  so i will try again when i get time.
shawn

When this happens, just hit "back" in your browser.  That should take you back to the copy of the page with the full text.  Then just fix the problem (the link this time) and hit submit again.

If you haven't already closed your browser, you may be able to still get "back" to it, depending on what else you have done in the mean time ;)

Also, when writing long or involved submissions, get in the habit of "copying" the text BEFORE submitting it.

Peace,
dave
 

80cj

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ok. 2nd attempt. (i did try the back button, but message was gone.)

so here is a short version of a long story.

i am in a long battle with insurance b/c my house is moving from high winds. (like i said long story). currently i have lasers mounted outside in the yard, that are aimed at a simple target (grid) that is mounted on the house. i video the target to determine the relative movement of the target as a result of wind. how i got here has been a progression over the last year that started simple and has grown increasingly complex.

it started that i just needed a laser pointer with a switch and A/C power source (didnt want to have to deal with batteries.) so i found the 'Genie Laser parking system' at Lowes and bought that.

as the system has evolved, i have bought more of them and used various parts as needed. (i am currently only using the power and laser.)

i realized that if i wanted to measure small movements (~1/8" or less), that a 1/2" dot (@ ~50') was too big. so i tried using a pinhole, but the dot wasn't that much smaller, and the power loss was too much. so i tried focusing the beam, but the process is hard to do reliably and i still couldnt get it as tight as i would like. (i have pics, but can't post yet)

so that brings me to now. i'm pretty handy and i dont mind fabing some parts if need be, but i really dont want to make the laser the project. (i have a house for that!) my only concern with making it myself is that i need to be able to rigidly mount this so that it does not move from the wind. so how do i get there from here?

thanks again for taking time to answer the new ignorant guy's questions!

shawn
 

80cj

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the focus issue that you mentioned should not be a problem b/c the the target is only moving perpendicular to the beam.

thanks
shawn
 




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