Archived posting to the Leica Users Group, 2000/12/14
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Martin Howard wrote: > weber_leou@itri.org.tw jotted down the following: > > > Why the CdS meter cell need power? > > I believe that all CdS meters need batteries: the CdS cell regulates a > circuit by varying resistance as light falls upon it. A battery is needed > to drive the circuit and provide the voltage/current to vary, which then > drives the read-out. > > Selenium meters generate voltage/current in response to infalling light and > do not require batteries, but function poorly in low light and -- if I > recall correctly -- are prone to "temporary blindness" by exposure to strong > light sources, causing them to give incorrect readings to weaker light until > they've "recovered" for a little while. > Martin, It is just the other way around. The CDS cells are prone to temporary blindness. By the way, Selenium cells are really more accurate with regards to color sensitivity then CDS cells. The problem with Selenium is the large size (area) needed to have a reasonable sensitivity. This large means large windows and also if possible quite a few small collector lenses in front, which means a very large angle to collect the light. They do not work as spot meters or even as reasonable narrow angle meters. They are also prone to aging. But this is not predictable. I have a few Weston meters .Most of them are still spot on. a couple however have lost sensitivity. Amazingly the oldest Weston Master (I would call it Weston Master1 ,since it is the previous model before the model II) ,retained its full sensitivity. Regards, Horst Schmidt