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How does your smartphone know the exact moment you step out into bright sunlight to boost its screen brightness?
It relies on a tiny light sensor that turns the intensity of photons into a readable electrical resistance.

Photoconductivity and the CdS LDR

A Light-Dependent Resistor (LDR) is an analogue transducer whose electrical resistance decreases as incident light increases.
In a cadmium sulphide (CdS) cell, incident photons provide energy to break bound valence electrons free into the conduction band, increasing conductivity.
In 1873, electrical engineer Willoughby Smith discovered photoconductivity accidentally while using selenium bars to test underwater telegraph cables.
So if light increases by a factor of ten, does resistance drop by a factor of ten?

The Logarithmic Light-Resistance Relationship

We quantify the brightness falling on a surface as illuminance (EE), measured in units called lux (lx).
An LDR does not respond linearly; instead, its resistance follows an inverse power-law relationship: R=Aβ‹…Eβˆ’Ξ³R = A \cdot E^{-\gamma} where AA is the resistance at 1Β lx1\text{ lx} and Ξ³\gamma (gamma) is the sensitivity slope, typically between 0.60.6 and 0.90.9.
Taking the base-10 logarithm of both sides transforms this curved relationship into a straight-line equation: log⁑10(R)=βˆ’Ξ³log⁑10(E)+log⁑10(A)\log_{10}(R) = -\gamma \log_{10}(E) + \log_{10}(A)