If the viewing angle to the target is greater than 60 degrees, which statement best describes how emissivity and reflectivity change?

If the viewing angle to the target is greater than 60 degrees, which statement best describes how emissivity and reflectivity change?

• A. Emissivity decreases and reflectivity increases.
• B. Emissivity increases and reflectivity decreases.
• C. Both emissivity and reflectivity remain constant.
• D. Emissivity decreases and reflectivity decreases.

Answer: (A)

When you view a surface more obliquely, the energy entering the camera becomes more dominated by reflection of the surroundings and less by the surface’s own thermal emission. For opaque surfaces, the energy you detect is roughly the sum of what the surface emits and what it reflects. The Fresnel effect makes reflectance increase as the angle of incidence grows, especially beyond about 60 degrees. As a result, more infrared energy is reflected and the portion coming from the surface’s own emission appears smaller, so emissivity looks lower while reflectivity looks higher. This tendency—emissivity decreasing and reflectivity increasing at steeper viewing angles—is the typical behavior you’d expect in infrared measurements.

When you view a surface more obliquely, the energy entering the camera becomes more dominated by reflection of the surroundings and less by the surface’s own thermal emission. For opaque surfaces, the energy you detect is roughly the sum of what the surface emits and what it reflects. The Fresnel effect makes reflectance increase as the angle of incidence grows, especially beyond about 60 degrees. As a result, more infrared energy is reflected and the portion coming from the surface’s own emission appears smaller, so emissivity looks lower while reflectivity looks higher. This tendency—emissivity decreasing and reflectivity increasing at steeper viewing angles—is the typical behavior you’d expect in infrared measurements.