Photoconductive (photoresistive) transducers — what do they actually sense? A photoelectric device whose resistance varies in direct proportion to incident light is called a photoconductive cell. Photoelectric transducers are therefore used to measure parameters that can be converted into controlled changes in what primary quantity?

Introduction / Context:
Photoelectric transducers encompass photoconductive cells (LDRs), photovoltaic cells, and photodiodes. In photoconductive cells, the conductivity of a semiconductor film changes with incident light, making them ideal for converting optical variations into electrical resistance changes. This question focuses on the primary physical quantity these devices are intended to sense or track.

Given Data / Assumptions:

• Photoconductive cell resistance R decreases as incident light intensity increases.
• External circuits translate resistance change into usable voltage or current signals.
• Operating wavelength and spectral response are appropriate for the source/target.

Concept / Approach:
The is light intensity (optical power per unit area). Electrical variables (voltage, current) are secondary signals produced by biasing the photoconductive element in a circuit. While flux density can refer to magnetic or radiant flux, in basic instrumentation the natural wording is “light intensity.” Hence the primary parameter measured is light intensity, with the transducer’s resistance serving as the intermediate variable.

Step-by-Step Solution:

Verification / Alternative check:
Data sheets for LDRs map lux (illumination) to resistance; calibration curves are given in terms of light intensity, not inherent current or voltage.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing “what is sensed” (light) with “what the circuit outputs” (voltage or current). The transducer measures light intensity; electronics translate it.

Final Answer:
light intensity