If you want to experiment with photomultiplier tubes, an easy way to get started might be to pick up an old densitometer on an auction site. These were used a lot in darkroom work. I have some Macbeth photographic-oriented devices, mostly tube-based. They have gorgeous big mirrored panel meters, the power supply for the PM tube is already there, there's a log conversion circuit if you'd like to keep that. Assuming you get one that works or only needs a little attention, you can just start right in. I can't speak to the spectral response, but for photographic purposes, the goal was "flat through the visual spectrum," which was accomplished with three filters and some tuning. I have a vague recollection that there was an IR filter in the path, but that could be my imagination.