Acme festival light with fresnel lens

[RPDSoundandlight]

I should know the answer to this. But I confess I dont.

I have a disco light fitted with a fresnel lens. The lens has fallen out. I wish to refit it. My question is this. Is the smooth side of the lens facing outward or is it facing inwards? I am under the impression that the lens is a fresnel type because one side of it is rough I think this may be the 'fresnelled' side.

Please don't laugh.

 

Bob

 

[richardash1981]

A Fresnel lens has (circular) steps on it. It can actually have them on both sides, but the common theatre ones have them on one side, as in the drawing near the top of Wikipedia:

https://en.wikipedia.org/wiki/Fresnel_lens

When used in spotlights, these are normally fitted with the ridged side facing out, flat side towards the bulb.

Separately, a lot of fresnesl lenses seem to be made with little bumps or pits all over the flat side of the lens - these are I belive to soften the beam further, not technically part of a fresnel lens. You could find these on either side of the lens, or both ...

[sleah]

It sounds like it could be a PC or Pebble-Convex lens, which could be described as having a 'rough' side. Very similar uses to a fresnel lens.

https://www.fluxitylighting.com/tutorials/lighting/fresnels-and-pcs

In theatre lanterns they tend to be curved side out / flat side in.

Edited July 10, 2022 by sleah

[p.k.roberts]

I think this is a 'dots 'n' spots' disco light with a plastic lens? If so, it may not make a huge difference. Are there any signs of glue on the lens which might give a clue? If not, I'd whip the front off and use a bit of gaffa to temporarily try it both ways and then pick the best.

[RPDSoundandlight]

6 hours ago, p.k.roberts said:

I think this is a 'dots 'n' spots' disco light with a plastic lens? If so, it may not make a huge difference. Are there any signs of glue on the lens which might give a clue? If not, I'd whip the front off and use a bit of gaffa to temporarily try it both ways and then pick the best.

Yes its a dots n spots type disco light you are absolutely right. The previous owner of the light had glued it in place and used a funny yellow coloured glue to hold it in place. Luckily, as you quite rightly say, I can see which side the glue is on and to anybody reading this the 'rough' side goes towards the inside of the light. So the smooth side of the plastic lens is outward facing. As standard, the manufacturer uses 'bend over' very small bits of metal to hold the plastic lens in place. These are spot welded to the front panel of the light. Whoever thought of this idea must have been out for a few too many beers at dinner time. Because in time these bend over tabs snap off! They are crap! Hence the need to glue the plastic lens in place. What a load of rubbish! Not a good item to buy on the second hand market because I bet that 9 out of 10 of them have issues with the lenses falling out. Usually inwards. The lens is bigger than the hole it is laughingly 'attached' to.