ZnSe - which way round??

[Dave Lock]

Hi all,

Long story short. Out of interest I was looking at a dual focus lens and what they are all about and it struck me looking at the image that the lens appears to be inverted compared to my installation.
My lens is fitted with the 'convex' surface facing down. So I had a browse at some other suppliers websites and they all appear to be used with the 'convex' surface facing up.
I spoke with my supplier and he said that he had seen lenses installed this way but ours should face down and is confirmed by the laser manufacturer.

I can't say I have had any poor results but still felt uneasy. So I decided to do a quick test. I had an image to hand at 300 dpi and rastered it with the lens in it's 'normal' orientation and again with it inverted.

Although the picture below is a phone pic you can see that there is little between the two. There is a pen line under each image to show which way round the lens was fitted for that engraving. Both results are not too bad but the one on the right seems to have burned a bit deeper as if more power had been used.

http://lincs-hft-club.co.uk/forum/uploads/IMG2-1255035953.jpg

Appreciate if anyone can confirm which way the lens 'should' be installed or which way it installed on your machine.

Cheers

Dave.

[Skip Weiser]

Dave, I think we have similar machines and my lens came installed with the convex side down.

I can see the difference in your second image. It looks clearer to me.

[Dan Hintz]

For a plano-convex lens, the flat side should be towards the substrate. If it's a negative meniscus lens (though I don't know why it would be), the convex side will be towards the laser.

[Dean Novakowski]

We install our lens just like the picture on the right. When talking with someone about it.. It really depends on how they are looking at it. If you have the holder in your hand and looking at the lens or ...the installed way which would be the other way around. When installed ours does look like the photo on the right.

[Rodne Gold]

Your focal point distance changed when you flipped the lens cos the lens is not in the same "plane" when you flip it over , hence the different engraving?

[Dave Lock]

Thanks for the comments guys.

I would really love to learn more of the theory on the optics. I was really surprised with the result. When I did the test I was sort of expecting the result to be way off with the lens inverted, like it widening the beam or something.

Thanks again

[Martin Reynolds]

I think that it really does not matter from an physics point of view. To Rodne's point, you changed the focus. It looks like your focusing system is set up for meniscus down. I am thinking that the focus offset is probably about the thickness of the lens.

[Dan Hintz]

I think that it really does not matter from an physics point of view.
Sure it does!