Flame detectors require an unobstructed view to function properly. Which condition would prevent proper sensing?

• A. Dirty lens or blocked line-of-sight
• B. Bright sunlight
• C. Low ambient temperature
• D. High humidity

Answer: (A)

Flame detectors rely on a clear optical path to see the flame. When the lens is dirty or something physically blocks the line of sight, the detector cannot receive enough flame radiation to sense it reliably. A dirty lens scatters or absorbs radiation, reducing sensitivity, while a blockage—dust, smoke, debris, or a cover—completely prevents visibility. Other factors like bright sunlight can cause nuisance alarms or interfere with readings but don’t inherently block the view, and normal operating ranges usually keep temperature and humidity from preventing sensing as long as the optical path remains unobstructed. The condition that prevents proper sensing is a dirty lens or blocked line-of-sight.