How would you go about creating a lux meter for very low light (0.0001 to 1 lux)?
Astronomers, including amateur astronomers, are interested in the brightness of the night sky, and there are various inexpensive solutions. 

There's a project to monitor the night sky using EURO 100 light meters based on PV solar cells (only available to participants). 

There's a commercial device that is based on (I think) a Taos sensor. Around the same price. Calibrated in visual magnitudes per square arcsecond (logarithmic).



Even an iPhone app that uses the camera (not sure how well that works).