If the viewing angle to the target exceeds 60 degrees, which statement is true?

If the viewing angle to the target exceeds 60 degrees, which statement is true?

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

Answer: (B)

When you look at a surface from a steep angle, it starts acting more like a mirror than a heat emitter. The infrared camera picks up two things: what the surface itself emits and what is reflected from the surroundings. As the viewing angle grows beyond around 60 degrees, the amount of radiation coming from the surface’s own emission toward the camera decreases, while more ambient radiation from the surroundings is reflected into the camera. This happens because reflectivity rises with angle due to Fresnel effects, and for opaque surfaces the relationship emissivity + reflectivity ≈ 1 means that a drop in emissivity corresponds to a rise in reflectivity. So the true trend is that emissivity decreases and reflectivity increases.

When you look at a surface from a steep angle, it starts acting more like a mirror than a heat emitter. The infrared camera picks up two things: what the surface itself emits and what is reflected from the surroundings. As the viewing angle grows beyond around 60 degrees, the amount of radiation coming from the surface’s own emission toward the camera decreases, while more ambient radiation from the surroundings is reflected into the camera. This happens because reflectivity rises with angle due to Fresnel effects, and for opaque surfaces the relationship emissivity + reflectivity ≈ 1 means that a drop in emissivity corresponds to a rise in reflectivity. So the true trend is that emissivity decreases and reflectivity increases.