Hi--

>>Yes - you can tape a gel or glass filter behind the pinhole
>>quite easily (the cheaper red filter works almost as well, too).
>
>Where can I buy such a filter and do you know how much I should pay for
>one? Are there many types, if so what's the difference?
>I really appreciate your sharing of your knowledge.
>
>Lots of appreciations
>Ske Gasut - Norway

The cheapest solution would probably be a used red glass
filter for a camera lens (it must be perfect and clean near
the center). Next would be a red (25A) Kodak gel filter in
50mm square size (about $10 here [B&H in NYC]) - the number
87 IR filter gel is $30 in a 75mm square...). BTW, everything
between a dark yellow up toward the IR side will work, with the
IR effect only subtly stronger with each stronger filter. An old
dark yellow, orange (not color-correction type...), or red filter
should get you started, and camera shops may have these cheap used...