i have an overhead projector, the lens looks identical to the s15 lens sold on Lumanlab.

It has a sticker saying 315mm(i assume focal length ?)

would this lens be suitable for DIY projector.(i am currently using the overhead projector with my LCD screen for a makeshift projector as i collect the parts for my DIY one.

Absolutely. Just to be sure, measure the diameter. It will need to be approx. as large as the S15 to work with a 15" LCD.

The first one I used came from a 3M ohp. Mine is a 342mm fl and yours at 315mm is even closer to the LL one which is 320mm. If you have the original fresnel it's a perfect match and probably covers a 15" lcd the way mine does. In either case it should also work with the LL 330mm field fresnel.

Hmmm I didnt think OHP were big enough to cover a 15" LCD.......... am I wrong????????

The fresnels in an OHP are not large enough, but the triplets in the higher end Dukanes, 3Ms, and others are essentially the same as the S15 sold by Lumenlab.

I have 2 unsplit ohp fresnels. One was scuffed and bought separately for 2 bucks just for testing and the other comes straight out of my 3M ohp. Size wise both are uncut versions of the LL standard fresnels except my 3M one has a slightly different fl to match the 342mm triplet. I also have the LL fresnels (and triplet) so when I put them on top for comparison the LL fresnel are the exact same width but the LL are cut in height.

What may confuse people is that the glass presentation table of the ohp is usually smaller with rounded edges but the fresnel underneath is larger. It may not be the case with all ohp because I recently saw an older smaller model with a doublet lens in a flea market sale and the fresnel was smaller than mine.

btw) On a side note, if I swap the LL triplet and fresnels with the ones from my 3m ohp, the difference in the fl is obvious even though 320mm and 342mm doesn't seem like much. I might also say that the picture looks a bit crisper (sharper) with the ohp matched triplet and fresnel.

Well, then I stand corrected. Neither my 3M 9800 nor my Dukane 670 has fresnels large enough for a 15" LCD, but I guess some do.