A computerized infrared microspectrophotometer has been designed which can measure over the infrared wave-length range 2.5 to 14.5 μm (4000-690cm-1) from areas as small as 20 x 20 /um. The system can routinely measure the thickness of silicon dioxide, silicon nitride, photoresist or multiple layers of these and other non-metallic films from several hundred angstroms to several μm In addition, concentration of phosphorus over a wide range can be determined in phosphosilicate glass. The measurements are nondestructive and can be performed by unskilled operators in less than one minute. Since the instrument scans over the fundamental IR range, some contaminants on wafers can be identified by the interpretation of their spectra. The NanoSpec"/20 IR computerized infrared microspectrophotometer employs an all-reflecting optical system which allows viewing of small sample areas at 150X. Wavelength scanning or its inverse, frequency in waves per centimeter (cm-1), is accomplished by the computer controlled rotation of a circularly variable infrared narrow-band wavelength filter. This can also be positioned very quickly at a specific wavelength to select a narrow band (approximately 1% of wavelength) for specific analysis or monitoring purposes. Chopped infrared energy emitted by a heated ceramic rod is transmitted through the filter, the sample and a variable aperture which allows a specific sample area to be measured. The infrared radiation is then focused on the target of a liquid nitrogen-cooled HgCdTe photodetector. The amplified signal is digitized and the data processed by a microprocessor computer with large memory. CRT display of a large variety of interactive operating modes and convenient instructions is provided. Thickness and concentration measurements can be printed out on paper tape. Spectra are recorded from the computer on an X-Y recorder, linear in wavelength or cm-1, and in transmittance or absorbance. Up to eight complete working spectra can be stored in RAM memory and compared using a variety of arithmetic operations.