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The photocell degrades with time, you can have one made for about 20UKP (Megatron do them) but as it is only a comparison there are two ways round this.

1) Re-mark the Standard Brightness point on the meter.

2) Reduce the series resistor in the meter housing.

I have done 2), replacing the fixed resistor with a pot.

seipot
And here it is.

The only problem with this is finding a standard light source to calibrate against.  Calibration using test negatives is possible and would be fine for photographic applications of the SEI, but as it has a Foot Lambert scale it is nice to calibrate to that.

Standard Light Source.

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