Cracked lenses

[karl]

Just lately I seem to be seeing a number of lanterns with cracked lenses. They are different makes, models and types (fresnel, profile).

 

I am wondering if this is just due to age and repeated heat cycles or whether there is some other factor involved. Obviously if there is anything that can be done to minimise the incidence of this then I'd love to know.

[swampman]

Make a list of the types of cracks: single across, multiple radiating from one edge, shatter....

That will give you an idea if there may be a common cause: knocks when moving, pinching of the lense: uneven holding or pressure points and expansion problems...

Lighting styles: fast heating and cooling ?

Draughts from ventilation or air conditioning?

Dirt leading to excessive heat build up...

 

(Feedback on results would be interesting.)

[David A]

Are you using 650 and 1200 watt lamps in fittings that were designed for 500 and 1000 watt lamps,I find this often and stick with the lower power lamps, which are also much cheaper.

[karl]

We had a 1.2Kw fresnel with a neat crack right across the diameter and a second with a slightly off centre single crack (with another short starting to radiate out at 90 degrees from the mid point). These are lamped at 1200w and I wonder if we should stick to 1000w in future.

 

There was also a profile where the inner lens had pretty much disintergrated. And a 2Kw follow spot where the inner most lense had several cracks radiating outwards.

 

There is no air-con or excessive draughts. Usage is pretty much your average theatre type usage.

[JohnMac]

I had an old Strand Patt 743 in which the Fresnel lens cracked across its middle and came crashing down off the rig and hit a member of the cast. :)

 

Was incredibly lucking and fortunately no lasting damage was done to actor, ;)

Nowadays always makes sure that if the lantern doesn't a lens guard, a filter frame is fitted with a clear gel to catch the bits if it happened again.

 

At the time I put the cause of the problem as having the glass lens too tightly clamped, with the heat of the lamp the glass lens expands, it's got to be able to move to accommodate the thermal expansion else it fractures.

 

Hope this helps

Regards

John Mac.

[David A]

Given that the T19 is 15% cheaper and lasts 175% longer than the T29[1200watt unit] there is seldom a economic case for the 1200, add in cracked lenses and they are very expensive lights.

[Selecon]

Just bare in mine that the filament height of the T19 is totally different to the T29. If you put a T19 into a Fresnel which is set to take a T29, you will get a strange image - a bit like a figure of 8 - the same is vice versa. Though this does not apply to 300w/500w/650w lamps, all filament heights are the same (give or take a couple of mm).

 

As for other reasons for lens cracking; with Fresnels, try backing the focus off a bit so you create a bit more of a gap between the lamp and the lens. With Profiles, make sure that your lamp is not too 'peeky' or off centre. And as was said before, make sure that the lenses are seated properly.

 

But most of all keep your lens clean and always do a good 'slow' rig warm-up when you do your pre-show checks.