DIY Spectrometer/Colourimeter 430nm and Log Amps, Are all log amps equal in this case?
I'd be surprised if you can avoid a microprocessor in your project, which makes calibration way easier. You can build in a simple signal generator and switch it in automatically before each measurement. That solved the problem of calibrating the amp, but not the sensor. Though you could probably program a compensation curve in the processor too.

There are several solutions for log amps. Look at this info from Maxim. It shows the basic theory on how a DC log amp works. Consider that the commercial amps are expensive because they have to compensate internally - you can do that with software.

A trick is to put a series of amplifiers in series, and sum their output. One by one the amplifier will start to saturate, and provide a piece-wise approximation. This circuit does that, but the amplifiers are expensive because they're supposed to work till video. For DC work, any amp should work.

Another interesting article show that even simple circuits can provide quite good results.

I wonder if modulating the light source could provide for a calibration of the entire system...