I chopped up a solar cell (cost 6p in = 0.06 GBP) into 4 quarters, then glued it onto the back of my callipers where there's a useless reference panel for thread sizes that is too small to read anyway. But luckily the reference panel is plastic- non-conductive. Underneath each panel is a wire trapped by the pane and ground, this is +ve from each panel, and I connected them all up in series, to get 1.3V. The callipers only need 18uA to work so you don't need a lot of light. My next step is probably to replace the shonky one that snapped badly, and then put clear resin over them and tidy up the wiring. I may also add a capacitor to store charge for a bit longer if you need it in a darker place. But - it works well! Can also be used in conjunction with the battery so it never runs out.