Radiation pyrometry: select the condition under which accurate temperature measurement performance of a radiation pyrometer is not adversely affected among the listed scenarios.

Introduction / Context:
Radiation pyrometers infer temperature from thermal radiation. Accuracy depends on emissivity knowledge, optical path integrity, and background reflections. Recognizing which conditions degrade accuracy and which conditions are benign helps practitioners set up reliable non-contact measurements.

Given Data / Assumptions:

• Pyrometer aligned to a target with some background in the field of view.
• Sighting path may include gases, dust, or windows.
• Object emissivity may vary with surface state and wavelength.

Concept / Approach:
Reflected background radiation is a source of error when the surroundings differ significantly in temperature from the target. If the object and surroundings are at nearly the same temperature, the reflected component has nearly the same radiance as the object's own emission, so the measurement error due to reflection becomes minimal. By contrast, attenuation or enhancement in the sighting path (e.g., due to absorbing or emitting gases/particles) alters the received radiance; varying emissivity changes the proportionality between temperature and radiance; and transparency means the instrument sees through to the background, all of which can cause significant errors.

Step-by-Step Solution:

Verification / Alternative check:
Radiation thermometry guidance notes that reflected radiation bias is minimized when background and target temperatures match, reducing net error.

Why Other Options Are Wrong:

Common Pitfalls:
Ignoring the field-of-view and reflective surfaces; shielding or emissivity coatings can mitigate many issues.

Final Answer:
Object and surroundings are at almost the same temperature