While rigging this afternoon, I heard what sounded like something falling. Being a bit gloomy (!) I couldn't see much, but I noticed that the beam coming from one of the profile spots (Strand Cantata 18/32) had gone strange with a projected image of the lamp now on the stage. After looking into the lantern, I saw that the rear lens had cracked in half and one half was now in the bottom of the lens tube.

This is a new one on me - I have used that lantern from new for around 12 years. It hasn't been bashed or toured, just been hanging in the rig being regularly used.
Anyone had this happen to them before? I guess (hope) that I can get a replacement lens ...

We've got several Selecon 1200w zoomspots with cracked lenses - although not to the point where half the lens has fallen out of the holder! Our units are probably of a similar age to yours and have had similar usage - we're not sure why they cracked though. Good luck getting a replacement....

Just try Used Lighting John , or any of the usual suspects like WL and SLX etc. Cant see a prob finding a replacement as lantern is still in common use!

The last replacement lenses I had to buy came from a small company called RB Lighting, who appear to specialise in spares and lenses for older lanterns. - give them a ring on 0208 977 9665. I'm not sure ifthey have a web site or not as I just phoned up and get what I wanted, they probably aren't the cheapest around, I think that a pair of Cantata fresnel lenses were about £70 including the postage) but if they have what you need...

A quick look on Used Lighting shows they have a Cantata 18/32 Axial on sale for £50 and several Cantata 18/32 for £65 (+VAT + carriage of course) which might be worth bearing in mind if spares cost more (plus I would email/call Ian Dennis anyway in case they have some spare lenses kicking around).
Peter Vincent (nothing to do with UL!)

I have have come across this before where an inner lens that is close to the gate cracks. It is ususally caused by the lens becoming jammed in its holder and is then unable to expand evenly and/or the metalwork of the fixture stresses it as it expands with the heat from the lamp. Some lenses are held with a silicone spacer others are just held by clips but make sure the silcone is in good shape where used and the lens is correctly installed when fitting a replacement.

For older fixtures if you know the diameter and focal length of the lens you can frequently find another fixture from the same manufacturer or a fixture from another manufacturer use a lens of the same diameter and focal length. This gives you more options in finding new or used replacements. Note on the latest generation of fixtures i.e. Source 4 because of their more precise grinding and specialised coatings these lenses should not be interchanged with anyone elses.

Many thanks for the advice on possible replacements. I shall see how far I get - it would certainly be a shame to loose a good lantern!

Recently I've come across two Fresnels with cracked lenses, one profile with a badly cracked inner lens and a follow spot where the inner lens was in three pieces.

As has been mentioned already the repeated heating and cooling must play a major part in this. It could be the expansion of the glass causing pressure due to poor mounting. I also suspect that the fact that one side of the lens is adjacent to the very hot lamp means uneven expansion occurs creating stresses in the lens. The follow spot lens that broke was about 4cm thick at the centre, I would guess it takes some time for both sides to reach a similar temperature.