There are generally speaking and as far as I can tell 3 options that would work - but only the actual log amp will result in a continuous, analog signal:
1: A log amp. Instead of making one from scratch, I'd opt for one of the integrated types available that use a minimum of external components and offer inherently better temperature compensation than whatever you can cook up at home. Several manufacturers still have a product offering to suit, sometimes even quite recently introduced types.
2: The crude digital approach would be to take an A/D converter of sufficiently high resolution to get the range you need. In practice this means 16 bit+ with overall excellent linearity, low noise etc.
3: A programmable gain linear amplifier will also work, but you need to have some kind of 'auto ranging' logic that determines which gain to set. Then feed the output into an ADC and do the math.
Evidently, 2 & 3 are only useful in systems where you need to have a digital signal. Although arguably, you could do the log & linearization part in the digital domain and then output the linear signal through a DAC (or PWM through an RC filter) to whatever circuitry that wants an analog input.
I'm not aware of any true analog alternative to #1. Log amp IC's are relatively expensive; a quick search suggests that something like TL441 is a reasonable compromise between cost, performance and availability.
Anyway, back to the main question - what kind of output do you need over what kind of range?
With ideal voltage measurement (very high impedance) the voltage goes like log of illumination. Helps achieving dynamic range. Several vintage lightmeters (Selenium) I've seen have a bespoke galvanometer with a variable gap between the pole pieces to achieve [deviation]\propto[\log(lux)]. That is not easy to replicate in the tinkering approach.Also if I measure voltage instead of current I found that the cell is very sensitive to the load impedance. I can have significant lower reading if the load is 50kohms vs 200kohms.
With ideal voltage measurement (very high impedance) the voltage goes like log of illumination. Helps achieving dynamic range. Several vintage lightmeters (Selenium) I've seen have a bespoke galvanometer with a variable gap between the pole pieces to achieve [deviation]\propto[\log(lux)]. That is not easy to replicate in the tinkering approach.
I will try a test with voltage measurement and see.
I bet, at a quite low illumination level. Was that within your design target?Did a quick check. The cell is extremely sensitive to loading by the meter. I can see a different voltage reading when I switched between a 10 MegaOhms input impedance vs 4GigaOhms input impedance on the meter.
from about LV15 down to something relatively dark like LV5 or something
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