Why must a photomultiplier tube be shielded from stray light?

• A. To prevent heating
• B. To avoid spurious signals and inaccurate measurements
• C. To increase signal speed
• D. To calibrate the wavelength

Answer: (B)

Stray light must be kept out because the photomultiplier tube is extremely sensitive to even a few photons. The photocathode emits electrons when light hits it, and those electrons are amplified through many dynodes to produce a measurable current. If stray light reaches the photocathode, it generates unwanted photoelectrons that appear as signals even when there is no intended light signal. This adds background noise and can distort measurements, especially at the low-light levels typical in fluorescence or luminescence assays used in clinical chemistry. Shielding reduces these spurious signals by preventing ambient light from entering the tube, thereby improving accuracy and precision. The other options don’t address the fundamental issue: shielding from stray light does not primarily prevent heating, speed up the signal, or calibrate wavelength.