What happens when the viewing angle to the target is greater than 60 degrees?

• A. Emissivity decreases and reflectivity increases.
• B. Emissivity increases and reflectivity decreases.
• C. Emissivity increases and reflectivity increases.
• D. Emissivity decreases and reflectivity decreases.

Answer: (A)

When you image a surface from a steeper angle, more of what you detect comes from reflected ambient radiation rather than the surface’s own emission. The amount of radiance you measure is a combination: emitted radiation scaled by emissivity plus reflected ambient radiation scaled by (1 minus emissivity). As the viewing angle increases, Fresnel reflection makes surfaces more reflective, so the reflected portion grows and the emitted portion becomes relatively smaller. This makes the apparent emissivity decrease and the surface appear more reflective when the angle is greater than about 60 degrees. In practice, this means temperature readings can appear cooler than the true temperature unless you account for the angle by adjusting emissivity settings or avoiding steep viewing angles.