Collimator Lens

[bdring]

I got my lens installed on my home built and it is working fine, but I have a few general knowledge questions.

Is there a top and bottom? Some of the sketches I saw on the Internet suggest both sided are convex, but the laser side is more convex (see Wikipedia). The actual lens was hard to tell. I shined a laser pen though and picked the way that looked better. It worked, but I would like to know the science behind this.

The lens is 55mm. Is this from the front, back, middle? I just experimented until I got the thinnest lines.

I can control my table via Mach3 so I backed off a little then draw a 4" long line while running up the height 1/4". I picked the thinest part and interpolated the best focal point from that.

Again...I got the result I needed, but would like to know the true definition of where the focal length is measured from.

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