Looking for a particular type of IR Blocking Filter . .

[xpatUSA]

The basis of this post is shooting visible light with a camera that has been converted to "full spectrum" by removal of the internal hot mirror. The principle being to just put an IR block (short-pass) or a band-pass filter on the lens in order to restore normal shooting.

For me, conversion is easy. My Sigma DSLR hot mirror is just under the lens mount, easily removable and replaceable.

The choice of filter is much less easy!

Having already bought a couple of candidate filters, the so-called "cut filters" (dichroic), e.g. B+W 486, are out due to color shifting at wide angles. The B+W 486 shot at 17mm (25.5mm equivalent):



See the color shift towards green at the edges? Unacceptable. My grass just isn't that green, ever.

The Kolari hot mirror:



Better but the filter characteristic itself is lacking in red response, necessitating a lot of correction in post.

(Pardon the lack of focus in the above shots, they were for quick testing of color accuracy.)

The Kolari is an absorptive type of filter and does not suffer the angle problem, I though I would "just" find one with a higher red response, perhaps accepting a little so-called "IR leakage" over 700nm.

The big problem is that any absorptive filters I've found with reasonable visible response have this sudden rise in transmissivity above about 850nm. This means that if I shoot vegetation in sunlight, the sensor will respond to those wavelengths giving me (Foveon sensor) pink leaves.

So I seek an absorptive IR blocking filter that blocks IR up to say 1200nm but with no 850nm-upward passband.

For any suggestion to stack another filter of suitable characteristic, I would be reluctant because the Foveon sensor needs all the light it can get for good shoots, unfortunately.

[xpatUSA]

I found the type of glass:

Schott Colored Glass Filter KG3.

It has this characteristic:



But what I can't find is a company that sells it polished flat, 1mm thick and mounted in a 72 or 77mm filter ring.

[xpatUSA]

After a lot of googling I've found a similar filter - but with a $320 price tag!

Meanwhile, I've ordered a Schott BG40 which might just do and doesn't go anywhere near so high above 700nm.

[Black Pearl]

If the hot mirror is easy to remove and replace why put yourself through so much trouble and expense with filters?

[xpatUSA]

If the hot mirror is easy to remove and replace why put yourself through so much trouble and expense with filters?

It's a bit fiddly to do in the field and the internal hot mirror is quite delicate and easily damaged.

Plus there is the intellectual challenge to be considered . . ;-)