AI Tutor
Every automatic camera shutter and solar tracking panel relies on a sensor converting raw photons into electrical signals. But if you take a sensor out of the box, how do you know the exact equation connecting the light level you shine on it to the voltage or current it outputs?
To find out, engineers put sensors through controlled bench tests to record voltage and current data across known light levels and extract their mathematical model.
How do we build a lab setup that gives us trustworthy optical measurements?

The Optical Characterization Bench

To measure optical sensors accurately, we need to control illuminance, which is the total luminous power hitting a unit area, measured in lux (lx\text{lx}). We place an adjustable light source on an optical bench rail directly opposite our test device.
A calibrated lux meter placed side-by-side with the sensor provides our independent variable, while digital multimeters record the sensor's voltage and current under steady DC bias.
What mathematical relationship should we expect when testing a Light-Dependent Resistor (LDR) on this bench?

LDR Transfer Characteristics

A Light-Dependent Resistor (LDR) is made of semiconductor material like Cadmium Sulfide (CdS), where incoming photons excite bound electrons into the conduction band, decreasing the sensor's bulk resistance.
This resistance change is non-linear and follows an empirical power law: R=Aβ‹…Eβˆ’Ξ³R = A \cdot E^{-\gamma} where RR is resistance in ohms (Ξ©\Omega), EE is illuminance in lux, AA is the device resistance at 1Β lx1\text{ lx}, and Ξ³\gamma (gamma) is the sensitivity index (typically between 0.60.6 and 0.90.9).
Taking the base-10 logarithm of both sides converts the curved power law into a straight line: log⁑10(R)=log⁑10(A)βˆ’Ξ³log⁑10(E)\log_{10}(R) = \log_{10}(A) - \gamma \log_{10}(E) Plotting log⁑10(R)\log_{10}(R) on the y-axis against log⁑10(E)\log_{10}(E) on the x-axis produces a linear graph with slope βˆ’Ξ³-\gamma and y-intercept log⁑10(A)\log_{10}(A).