Optical radiation pyrometer lower limit: What is the typical starting (lower) temperature for reliable measurement by an optical radiation pyrometer in industry?

Introduction / Context:
Optical (radiation) pyrometers measure temperature from emitted thermal radiation. Their usable range is limited at the low end by weak radiation intensity and background/emissivity uncertainties. A common rule-of-thumb lower bound for classic optical pyrometers is around 700–800°C, depending on design and target emissivity.

Given Data / Assumptions:

• Conventional optical pyrometer (disappearing-filament or equivalent) and industrial targets.
• Normal emissivity and line-of-sight conditions.
• “Starting temperature” refers to practical, not absolute theoretical, limit.

Concept / Approach:
Below roughly 800°C, emitted visible radiation is weak and errors become large for traditional optical approaches. Infrared pyrometers expanded usable ranges downward in modern practice, but the classic value taught for an optical radiation pyrometer’s lower bound remains about 800°C, which aligns with standard exam expectations.

Step-by-Step Solution:

Verification / Alternative check:
Typical vendor datasheets for disappearing-filament instruments cite lower limits near 700–800°C depending on optics and target properties.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing modern IR pyrometers (which can measure lower) with traditional optical radiation pyrometers used in foundational instrumentation courses.

Final Answer:
800°C