Archived posting to the Leica Users Group, 1998/03/25

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Subject: RE: [Leica] Lens Glass
From: Jim Brick <jimb@ubet.com>
Date: Wed, 25 Mar 1998 09:54:50 -0800

If you look through an optical supply catalog, you will find many many
different types of glass. Glass that absorbs or transmits various
frequencies of light. Since most of us old farts know that, back in the
40's and 50's, UV filtration at high altitude did indeed make a
difference on scenics in color. Yes... there was color film back then!
And when Leica states that all Leica lenses manufactured after 1965
contain UV inhibitor, mainly in the glue, would logically lead one to
the conclusion that the optical glass used in lenses, for many many
years, had very little, if any, UV inhibiting properties, ergo, the
special glue.

It has been said that you cannot get a sunburn through window glass. I
cannot answer that, but I have a skylight in my library (glass) and a
lot of faded books on the shelves, and a faded carpet. I had to put a 3M
UV inhibiting film on the glass.

Jim

>-----Original Message-----
>Subject:	Fwd: [Leica] UV filtration
>
>>
>>I have read with varying degrees of interest, the UV filter debate
>>(again!). I have heard about multicoating, of special glue in the lenses
>>which blocks UV etc. I must confess to being at a bit of a loss in regard
>>to this. I far as I am aware, glass blocks UV, from greater than 300nM
>>down, which is why, when you use a UV/visible spectrometer, you need to
>>change from glass to quartz cells when you get to around 300 nM. Does
>>anyone at what wavelength a photographic filter will "cut in"?
>>
>>Presumably, a UV filter (esp. one of glass) must filter at greater than 300
>>nM, otherwise there would be no point. Is that the point?  :-)
>>
>>Or maybe leica optical glass blocks somewhere lower than 300 nM
>>
>>*If* I had a UV filter, I could put it in the light path of the UV spec.
>>and find out! The lens caps that I use block everything.
>>
>>Neil.
>>
>>p.s I have just noted that in the 51st ed. of the Chemical Rubber Co.
>>Handbook of Chem & Phys, there are several Corning glass filters which are
>>described as clear UV transmitting, but transmission below 300 nM ( eg for
>>glass no. 0160) is negligable. So UV filters must filter above 300 nM.
>>
>>pps
>>Furthermore, the Kodak Wrattan UV filter No.1 blocks below 360.
>>Borosilicate glass cuts off at 330, Heavy flint glass (whatever that is)
>>transmits only 58% at 384, so we have overlap, depending on the glass.
>>
>>Did anyone want to know this? probably not. I wonder if I could fit a lens
>>into the UV spec. chamber?
>>