Which photodetector is noted for excellent linearity?

• A. Barrier Layer Cell
• B. Phototube
• C. Photodiode
• D. Photomultiplier tube

Answer: (C)

Linearity is about the output signal being proportional to the light intensity across a wide range. A photodiode, when operated in reverse bias and read with a transimpedance amplifier, produces a photocurrent that is essentially proportional to the number of incident photons. This direct proportionality holds over a broad dynamic range, giving predictable, quantitative responses with good speed and stability. The main limitation occurs only when the device starts to saturate, at which point the relationship between light input and current begins to bend.

In contrast, photomultiplier tubes rely on a high-gain dynode cascade. While incredibly sensitive, their response can become nonlinear at higher light levels due to space-charge effects and saturation of the dynodes, leading to compression of the signal. Barier layer cells and some photon detectors can have more restricted linear ranges or be more sensitive to operating conditions, which complicates strict quantitative measurement. Therefore, for excellent linearity, the photodiode is the best choice.