wave length and CdS

brucemuir · Jul 15, 2011

This may be a crazy idea but,
do cadmium sulfide meters read all wavelengths equally?
What I'm getting at is...

will a CdS meter read tungsten as correctly as daylight?

I know some films have a reduced EI under tungsten and this got my mind turning.
Tungsten has very little blue so I was wondering if the CdS meters were more reactive to any certain part of the spectrum.

Maybe I'm not fully understanding how they work and if wave length even plays any role at all

michaelbsc · Jul 15, 2011

brucemuir said:
This may be a crazy idea but,
do cadmium sulfide meters read all wavelengths equally?
...
Maybe I'm not fully understanding how they work and if wave length even plays any role at all

I don't think you're off base, but I think most "modern" meters are pretty well corrected for "about the same as film" response.

Hexavalent · Jul 15, 2011

Plain CdS LDRs approximate the "human eye" response to color. In other words, they peak in the green, and fall off in the far red and far blue.
The sensor itself may have an integrated (or external) filter to alter the spectral response.

samcomet · Jul 15, 2011

Bruce - At a quantum level you are correct but in the macro world we live and shoot in, the discrepancies would be all but indiscernible without sophisticated equipment - and film (silver halide) is equally in the same "macro" world as the CdS meter and thankfully, photographers. As wavelength is inversely proportional to frequency (energy) by the constant named for Plank (http://en.wikipedia.org/wiki/Planck_constant) - you have indeed asked a pertinent question. In my humble opinion your question shows great imagination and thought....stay with it!! Cheers, sam

brucemuir · Jul 15, 2011

Okay, so I guess there is little to worry about in terms of compensation.

I did end up on wiki on the photoresistor page
http://en.wikipedia.org/wiki/Photoresistor
where it describes 2 main types of photo electric devices

one type gooses the response to lower frequencies/wavelengths but I think this may allow some to read lower light levels but I'm still digesting it.

Sam, thanks for the encouragement.

Chan Tran · Jul 15, 2011

No Cds cell does not respond to all wavelength equally but its respond is fairly close to that of human eye much better than silicon cell (which are used on most modern meter) without correction filter. The compensation is really for the film respond and not the meter.

Smudger · Jul 16, 2011

The characteristic of CdS that may be of more concern is Lightsoak.
After exposure to bright light ie sunlight, the cell will give inaccurate shadow readings until it has stabilised. This can take up to 30 minutes for some meters.

brucemuir · Jul 16, 2011

Smudger said:
The characteristic of CdS that may be of more concern is Lightsoak.
After exposure to bright light ie sunlight, the cell will give inaccurate shadow readings until it has stabilised. This can take up to 30 minutes for some meters.

There was also a warning about cold weather in the nikon photomic FTN manual telling you to limit meter usage to 3 minutes in cold weather.

It said the meter would need to warm back up to return to correct readings.

another quirk to be aware of...

Chan Tran · Jul 16, 2011

Cds has many bad things and that is why it no longer being used in meter but spectrum response isn't the reason.

Diapositivo · Jul 16, 2011

The book L'ABC della fotografia, Cesco Ciapanna Editore, 1981, which I bought when I was 15 (just before my first reflex) and which is my "Bible" clearly states that CdS cells have a response which is minimal for blue, maximum for red and extends to the near infrared.

wave length and CdS

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