I just determined what I had long suspected. I measured the spectral transmission characteristics of one and two thicknesses of unexposed but developed E6 films and found them to be comparable to that of a Wratten 87 IR filter. In addition I also made some pix on HIE film through two sheets of D max EF sheet film and compared the pix to some taken through a "standard" IR filter, the Wratten 87.

The result of this is that it appears that one thickness of E6 film is roughly the equivalent of an 87 filter but with a broader spectral response and with some 1% transmission valleys at 500 and 600 nm. Its transmission starts to drop from 1% at 700 nm to about 95% at 800 nm. Two thicknesses of D max E6 are basically visually opaque with transmission dropping rapidly starting at 720 nm and dropping quite rapidly to 90% or so at 850 nm.

Maybe they might be closer to what a 88 is. Basically the 2 sheets of E6 simply do not have as steep of a cutoff as the Wratten filters do nor as good a maximum transmittance. But they are serviceable!!! especially for placing over a flashgun where expensive Wratten filters tend to fry and buckle!

Picture-taking wise, the two thicknesses of E6 film did not seem to degrade image sharpness significantly when used with 4x5 format. I have not tested 35mm. They would obviously not matter much when used over a flash for inconspicuous flash photography at parties, etc.! (camera lens with or without additional filter over it).

The spectral transmission curves for the 1 and 2 sheets of E6 film, and that of a Wratten 25 and 87 filter are attached below.

Experimental Grade IR Filter spectrophotometric curves:

In case you did not know and might benefit from this information I had some filters often used for Infrared photography, along with some unexposed but developed Ektachrome sheet film, characterized with a spectrophotometer. The films were obviously visually quite opaque. Especially if you stacked two of these. It turns out that the dyes that make up the color layers in this film (and I suspect all color films) are visually opaque but IR TRANSPARENT! This means they can be used as cheap makeshift IR filters, especially to cover a flash source to make unobtrusive flash photos by IR illumination. This info is not totally new as I had already seen such curves at RMIT in Melbourne, Australia, where I spent my summer vacation a couple of years ago. I also used one and two layers of Ektachrome film as a filter in front of my camera's lens and the images were not totally fuzzy and unusable. They had, in fact, a visual quality all their own which you may (or may not) like if and when you experiment in a similar fashion.

 
   3.0|.........    ..........    ....... -------
      |         \__/         :\__/       \    ;  |       :  Wratten #25 (red)
      |                      :            \   ;   |
      |                      :             \  ;    |     ;  Wratten #87 (IR)
D     |                       :             \  ;   |
E  2.0|      approximate      :              \ ;    |    \  1 layer  Ektachrome
N     | Density vs Wavelength :                \;    |
S     |          for          :                 \    |   |  2 layers Ektachrome
I     | 2 Wratten Filters and :                  ;\   |
Y     | Ektachrome Sheet Film :                  ; \   |
   1.0| unexposed & developed  :                  ;  \  |
      |                        :                   ;  \  |
      |                        :                     ;  \  |
      |                        :                       ; \  '- ________
      |                         '.............>           ;-...........
      -------------|------------|-------------|------------|------------|------
     400          500          600           700          800          900
                            wavelength (nanometers)