Here: on CdS light meters v0.3.pdf you can find the most up-to-date version of my writeup on CdS meters.
This is work in progress, but I wanted to use this excerpt in a web discussion already.
I will update this post as new versions are available. PLEASE don’t link to the file directly. The link will break as soon as I update the file.
April 7th, 2010: initial release
April 10th, 2010: expanded part on mercury battery based meters
April 27th some updates, mostly diagrams, not much new work done
Enjoy,
Marek
Hi Marek,
Great article on the basics of light meters. This gives the theory behind the deceptively “simple” CDS meter. I have been looking for an article of this detail for some time. Congratulations on producing this so far. I can’t wait to see they rest.
Best Regards
Greg Nixon
nickon51
Hi Marek,
This is a great article. Do you think it is possible to add a CDS meter to a camera that doesn’t have one using parts from the CDS meter of a completely different camera?
Your opinion on this would be greatly appreciated.
Looking forward to updates on this article,
– Jacob
Hi ! I recently wrote this iFixit guide for the Westone Ranger 9 (the one and only CdS based Weston):
https://www.ifixit.com/Guide/Weston+Ranger+9+Lightmeter+CdS+Cell+and+Battery+Replacement/213318
But I’ve been stuck on how to calibrate the thing against modern meters, mainly because the needle readings on the Weston (even though they are 1-21) are not really EV values but rather some proprietary scale which is made to look like EV. So I started looking to CsD theory and I found your paper which is absolutely phenomenal. I will publish the values of the Advanced Photonix PDV‑P8103 cell I used, based on your methods.
The reason I am reaching out is because I think I screwed up the balance counterweights on the galvanometer and I have no no idea what the procedure is to re-balance it before I can actually re-calibrate it electronically. Do you know what the proper method is, or point me to a good guide or video to balance a galvanometer that is supposed to be unsed on the field in many different angles ?
Great to hear from you! What counterweights do you mean? Generally galvanometers may have small counterweights to compensate for the weight of the needle, so that the readings don’t change when you flip the meter around. While it is difficult to fix it if broken, misadjusted counterweights will only mean that the meter won’t be precise in orientations other than the one it was calibrated in. You can check, how bad they are, by allowing a controlled current through the galvanometer and checking if and by how much does the reading change if you e.g. move it from horizontal to vertical in each direction.