[LargeFormat] RE:IR filters

[Michael Briggs]

On 09-Jun-02 Clive Warren wrote:
> The R72 and 89B filters are one and the same. 89B is the Wratten 
> designation for filters. The CoCam resin filters are manufactured 
> strictly in accordance with the Wratten specifications.

Some Japanese companies label some of their filters, in addition to a letter
for the color, with the cutoff wavelength in units of nm (nanometers) divided by
10.  Here 72 means 720 nm.  I guess this is the wavelength of 50% transmission.
US and European practice is to use arbitrary numbering schemes of which the
Kodak Wratten scheme is the most common.  The graph in Kodak's filter data book
(publication B-3, Kodak Filters for Scientific and Technical Uses) shows the
89B to have a 50% transmission wavelength of about 720 nm.  (If one wants to be
excessively precise, a table shows the transmission at 720 nm to be 57.6%).

--Michael