I'd like to see a discussion of the various exposure and color balancing aids, such as gray card, expodisc, Pringles lids, etc. There are of course at least two aspects, one being exposure determination for incident light and another being white balancing (e.g. taking a photo of the light source with your expodisc on the lens, and using that info in the darkroom to get good light balance.)

To get the discussion rolling, last night I thought about using a mailing tube cap as a poor-man's expodisc. I measured the transmission of a translucent mailing tube cap with my densitometer. The overall transmission was surprisingly useful, i.e. 16% in the green channel, not too far off from the supposed ideal of 18%.

The white balance was a little off, reading about 0.06 absorbance units too transmissive in the red channel and about 0.09 units too dark in the blue channel.