Response to Step wedge use for film testing?

The light source should make absolutely no difference to the result.
However, if you photograph the step wedge, then the flare factor of the camera and lens will affect the results quite noticeably, reducing the contrast range of the step wedge from its specification.
For instance, if you use a light box, then the amount of white light surrounding the step tablet will alter the contrast of the image on film. You'll get a truer representation by masking off all but the area of the step wedge itself.

If the object is to find the transfer function of the camera/lens/film combination, then photograph the step tablet. If you want to only test the film, then you should contact print, using a diffuse light source for exposure.
(Photographic density is always given as diffuse density, unless stated otherwise. Using a collimated source, such as an enlarger, for the exposure may give a misleading result.)

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