From:             "Marc Falcone" <marcfalcone@hotmail.com>
To:               infrared@a1.nl
Subject:          Custom made IR filters of wavelength of your choice
Date sent:        Wed, 13 Mar 2002 21:25:29 +0000
Send reply to:    infrared@a1.nl

I have just received an RG-9 filter that I have ordered from schott glass. (cut
in 82 mm diameter to fit a 86 mm filter ring). I asked the company why they make
many glass filters available from UV to near IR (RG 715) about every 15 or 20
nanometer, but there are none available between RG 715 and RG 780 (except RG 9,
but RG 9 is band-pass, not long-pass). They replied me that long-pass filters
are made in an interesting way: they originally are clear as a BK7, then they
are cooked in an oven (and beaten) for a amount of time, the longer the time,
the farest the wavelengh change, so RG 850 is cooked a longer time that RG 830.
One time, they made a "RG 840" by mistake, but the glass was then recooked
another time to make it become RG 850. The interesting think is that the company
can make a filter of the wavelengh of your choice, but only for a minimum amount
that is of several thousands pounds of glass! I don't know at what temperature
they are cooked, nor know how they are beaten, but I will try to change a piece
of RG 665, if I succeed, I can have a filter the wavenlengh I want!