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Old 02-19-2012, 02:33 PM #1
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Default focusing

Hey. I have some questions that I couldn't find answers which were really satisfying .
I still dont exactly know how focusing a laser really works. I have a small red diode and a collimator both from a dvd burner. what is the best way to find out how to focus it? I mean yeah i can point it on a piece of paper and move the lens back and forth until the dot is as small as possible. but this way I only find out how to focus to the exact point where the papte is. How does focusing to infinity work? and how can I calculate the distance of the focal point from the distance of lens and source?


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Old 02-19-2012, 03:00 PM #2
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Default Re: focusing

From everything I've heard from you, it sounds like you're using the bare diode and a bare lens. It's best to house the diode in an Aixiz module. It not only protects the diode from dust and moisture, but also allows for really great focusing.
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Old 02-19-2012, 03:14 PM #3
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Default Re: focusing

but does the module anything more than just housing diode and lens?
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Old 02-19-2012, 03:35 PM #4
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Default Re: focusing

It acts as a small heatsink for your diode. Keeps the lens in line with the diode and keeps dust, moisture and debris out.

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Old 02-19-2012, 05:40 PM #5
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Default Re: focusing

Quote:
Originally Posted by nikola tesla View Post
Hey. I have some questions that I couldn't find answers which were really satisfying .
I still dont exactly know how focusing a laser really works. I have a small red diode and a collimator both from a dvd burner. what is the best way to find out how to focus it? I mean yeah i can point it on a piece of paper and move the lens back and forth until the dot is as small as possible. but this way I only find out how to focus to the exact point where the papte is. How does focusing to infinity work? and how can I calculate the distance of the focal point from the distance of lens and source?
For a single positive lens the place for infinite collimation least divergence is close to the lenses focal length
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Old 02-20-2012, 02:16 AM #6
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Default Re: focusing

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Originally Posted by steve001 View Post
For a single positive lens the place for infinite collimation least divergence is close to the lenses focal length
I am sorry I still dont know too much about lenses. how do I know the focal length of a lens?
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Old 02-20-2012, 06:44 AM #7
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Default Re: focusing

The focal point of a convex lens is the point where light rays parallel to the axis are brought to a point. The distance from the lens to this point is called the focal length of the lens. If an object is very far away, the angle formed between adjacent light rays is very small. Depending on the focal length of the specific lens, this distance might be anywhere from a few meters to a kilometer. If the object is very far, say 93,000,000 miles (1.5 x 1011 m) like the Sun, the distance is sufficiently far that light rays are essentially parallel. So sunlight is a convenient source of parallel light rays




Use this focal point to create laserdot in the infinity.
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Old 02-20-2012, 01:35 PM #8
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Default Re: focusing

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Originally Posted by nikola tesla View Post
I am sorry I still dont know too much about lenses. how do I know the focal length of a lens?
It's very easy. I could tell you how but that's too easy also and you learn little. If you Google for an answer you can find an answer in 0.31 seconds with about 1,570,000 results.

You'll have to measure it with a metric ruler.
All lenses are in metric units, but you can measure in fractions of or inches and convert to metric

Last edited by steve001; 02-20-2012 at 01:36 PM.
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Old 02-21-2012, 04:50 PM #9
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Default Re: focusing

Focusing to infinity is kinda hard because we can't see to infinity. We can approximate, though, and if we use a large enough difference, the error becomes, for our purposes, negligible.

What I'm about to suggest should be done in a controlled environment where all people are aware not to enter and people already in the environment will have protective goggles.

This is what I do: I set up a mirror at the end of the hallway and mount the laser in a stable setting so that it always points at the mirror and I can adjust the focus. Now, you should be able to position yourself so that your back is to the mirror, and the laser is next to you pointing backwards. You should be able to see the reflected dot in front of you.

Screw the lens all the way in, until the dot starts getting bigger again. Now, slowly start unscrewing the lens, moving it farther away from the diode, until the dot shrinks down to the same diameter as the diameter of the beam right as it exits the aperture. There will be two places where the dot will be the right size, you want to get the first one. If you see the dot get smaller and then expand back to the correct size as you unscrew the lens, you have defocused too much.

Again, to reiterate, safety first. Best to do this with goggles, no pets around, no people around. Lasers are very hard to control when pointing over long distances, hence the recommendation for a stable mount that will still allow you to focus it.
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