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-   -   Fresnel lense crack (http://www.dvinfo.net/forum/photon-management/87481-fresnel-lense-crack.html)

Michael Best February 24th, 2007 09:25 PM

Fresnel lense crack
 
Does anyone know if extreme cold could cause a lense to crack. I'm very careful with my lights and I've got a 2K with a sudden, very bad crack in the lense. The light has been used a lot lately in a scene where it is outside blowing through a window. We've had some extremely cold temps and it seems the crack just came out of no where. Typically it is cooled before being moved and I realize jarring could do it but I can't think of an instance where it was mishandled. No big deal, just thought I'd check. Thanks

Bill Hamell February 25th, 2007 09:17 AM

Micheal,

If the lens was extremely cold and you struck it, you put 500 some degrees right behind a cold lens. I am thinking the rapid expansion cracked the lens.

Bill

Michael Best February 25th, 2007 09:40 AM

Boy that does make sense to me, I can recall setting the light a few times and not striking it right away. Glad I asked and thanks for that repsonse.

Ralph Keyser February 27th, 2007 12:15 PM

It's also possible that it worked the other way too. The body of the lamp retains heat and stays warm while the face of the lens cools rapidly due to exposure to cold temps. I worked with a gaffer that would drop two double scrims in front of large fresnels about 5 minutes before turning them off if it was very cold outside. His theory was that the scrims would hold some heat and reduce the stress on the lens as the lamps cooled. I have no idea if this actually helped or not.

Bill Davis February 27th, 2007 01:17 PM

Anybody else have a "fun with physics" childhood where they took glass marbles, heated them in the toaster - then dropped them into a glass of cold water?

LOVELY patterns of cracks. Then you'd try to play with them, and inside a minute they'd shatter into a thousand pieces.

I remember that every time I face a hot/cold situation around glass of any kind.

FWIW

Jaron Berman February 28th, 2007 08:10 PM

I think Ralph hit it dead-on. Glass is actually somewhat resilient in expansion. All types of glass can be heated at a fairly quick rate. But NO glass can cool quickly, even Pyrex (borosilicate glass, like used in these lenses). I studied glass in college, and have shattered more than my share of every type of glass by cooling too quickly. The double scrim trick actually does work. While it may seem like a minor amount of heat, ANYTHING which allows the glass to cool more slowly will also relieve some of the internal stresses. Lenses for these lights are expected to undergo certain extremes, but something as small as a gust of wind can cool the front surface of the lens by as much as a couple hundred degrees instantly - and at 500 degrees, that's a HUGE amount. You can get away with a lot at higher temps, but between 400 and 900, glass is most susceptible to cooling stress... and that's where these operate. Also be sure not to touch any cool metal to a hot lens. Metal that hasn't warmed to the same temp as the glass alongside it will act like a lightning-rod for heat, causing a very small, very cold spot on the glass where the contact was made - instant crack. Oh, and any scratches in the surface of the glass turn into fissures when stressed by heat or pressure.

So the scrim trick works by trapping heat as well as blocking wind. Just try not to touch the cold scrim to the lens face. Boro glass is pretty forgiving, but a little caution can save lots of money in replacements down the road.


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