If you intend to use the EPRI film for qualification of your digitizer you should be aware that stray light will have a big influence on the density evaluation. The theory about that is very well described by the American physicist and Nobel laureate Richard Feynman. In Wiki you will find an abstract.
If you capture the complete EPRI Film you will get an image like this:


Now you could evaluate the density steps - e.g. marked with the red rectangle. You will be disappointed, as the maximum density will only be about D=3.8 (gray value 38 000) and you expected a gray value of 45 000.


If you now cover the density step marked with the red rectangle that only light can pass through the area of this step the result will be as expected:

The density is larger than D=4.5 (it should be D=4.65 printed in the certificate).

If we have a look on the image with the covered density step


we can see that the step with density D=0.5 strays light in all directions, the D=1.0 step also a little bit and the effect goes down with higher densities. Even with a very high quality lens this effect can not be eliminated as the physics sets the limit. The main sources of stray light from the EPRI film are the converging line pairs and the line pair sets - the "black areas" in the first image of this posting.
Covering the area of one density step is allowed in ISO EN14096.