The leaves of my Meyer lemon seem to be infected by a fungus or virus. A newer leaf appears normal until it is put up to a light. The affected areas appear light green next to the healthy dark green portion. A thin pattern of brown lines can be seen in those areas on the underside of the leaf; the pigment is inside the leaf as it does not wipe off. Interestingly, on an older leaf nearing the end of its life (i.e. turning yellow), the affected areas remain green while the surrounding area yellows. Any idea what this is?

Sounds very strange, but with only discription, I hardly have any guess.
A picture would be of great help.

Strange indeed. I would post a picture if I had a camera.

From you description it sounds like you have an infestation of citrus leaf miner.

It is definitely not leaf miner damage; there's no deformation in the leaf. This tree is indoors year-round. The only bug it is exposed to possibly at this time are a small number of spider mites and the damage is not consistent with mite damage.