>> > I mentioned this to someone who said that photocells are unreliable
>> > and won't last more than a year or two. He said they all fail due to
>> > light exposure within a few years
>>
>> Sound pretty pessimistic. If you're really worried about UV then just
>> about any plastic will block it. Cut up a sunglass lens, everyone's got
>> a busted pair of sunnies in the drawer
>
>Thanks, that's good to hear. As for the guy I heard it from... Well,
>suffice to say I've had my suspicions that he can't stand being anything
>but the smartest guy in the room, even when the subject is electrial
>engineering and he's a Java programmer... But, wanted to see what you
>lot thought first.

Cds cells used to be used in camera light meters. They suffered form 2 
problems -

1. they have a long time constant going from bright to dark, to get the last 
bit of sensitivity out of them at the dark end required a quite respectable 
time out of bright light (several tens of seconds typically).

2. they suffered from failure to reach a high enough resistance at the dark 
end after a time. This was a specific failure mode for some, and the only 
solution was to replace the cell. The reason for failure was not known by us 
(my father used to repair cameras) but could have been some form of 
contamination during manufacture, or just plain bad manufacturing process. I 
just remember my father did replace a fair number of them over time. More 
recently photodiodes have proven more popular due to better linearity with 
light levels, smaller devices for fitting into ever more cramped camera 
bodies and better overall sensitivity.

I don't know that there is any inherent failure mode in them, but from 
experience as given in point 2, there may be some form of chemical migration 
or similar problem if exposed to high enough light levels. 

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