A charge-coupled device (CCD) infrared imaging sys- tem (see Problem 12.85) operates in a manner similar to a digital video camera. Instead of being sensitive to irradiation in the visible part of the spectrum, how- ever, each small sensor in the infrared system's CCD array is sensitive in the spectral region 9-12 µ,m. Note that the system is designed to only view radiation coming from directly in front of it.

An experimenter wishes to use the infrared imaging system to map the surface temperature distribution of a heated object in a wind tunnel experiment. The air temperature in the wind tunnel, as well as the surroundings temperature in the laboratory, is 23°C.

(a) In a preliminary test of the concept, the experi- menter views a small aluminum billet located in the wind tunnel that is at a billet temperature of 50°C. The aluminum is coated with a high- emissivity paint, s = 0.96. If the infrared imaging system is calibrated to indicate the temperature of a blackbody, what temperature will be indicated by the infrared imaging system as it is used to view the aluminum billet through a 6-mm-thick fused quartz window?

(b) In a subsequent experiment, the experimenter replaces the quartz window with a thin (130-µ,m- thick) household polyethylene film with T = 0.78 within the spectral range of the imaging system. What temperature will be indicated by the infrared imaging system when it is used to view the alu- minum billet through the polyethylene film?