+1
I’m very interested in learning about this method.
I was at his home in 2005, but here's what I remember. I wish I had taken a photo of it, but all I got were shots of him and his darkroom.
Alan Ross has a large box that has an array of Wratten ND filters mounted on one side of the box, and then he places the camera in the opposite side of the box. The walls of the box are opaque so no light comes through. I think it was made of foam-core - I forgot to take a photo of it, but I think it was black foamcore. The end with the ND filters is say 2x3 feet, and the opposing side is perhaps 1 ft sq. It looks similar to a home-made softbox.
There's 9 or 12 square holes in the large end in which he mounts the ND filters, with one hole that has no filter and then ND filters in the other holes, each one a stop denser than the next to give a good range of exposure values.
He focuses the camera at infinity, places the lens in the small end of the box, and seals it to keep light from getting in (with a darkcloth perhaps, I forget). The box is aimed at his window and he shoots the sky with it. I think he meters through the box at one of the tagets to get his exposure value. And then he shoots the ND filters. The box is big enough and the filters (I think he used 3 in. sq. Wrattens NDs) are big enough that even though they are out of focus and unsharp, they are large enough on the film that one can easily take readings with the densitometer and not have to worry about measuring the blurry edges of the filters on the neg.
This approach helps take the camera system into account - the lens flare, the camera bellows flare, the reflection of light off the surface of the film back into the camera, the shutter speed and aperature. Of course flare varies with every lighting situation, but this seemed like a good allround way to do it.
Alan also pointed out that one can use theatrical lighting filters instead of the Wratten filters and save yourself a LOT of money...