Abstract: In an optical instrument which provides an analog signal representing the intensity of the reflected light from a sample, the sample is irradiated with narrow band wavelength light with the wavelength being repeatedly swept through a range of values to produce a repeating analog signal. Sampling pulses are produced simultaneously with the repeating analog signal to determine a sampling interval. An analog-to-digital converter is connected to receive the analog signal and converts the analog signal to a binary value several times in response to each sampling pulse. The binary values produced in response to each sampling signal are averaged and fed to a digital computer. In the digital computer, the received values are again averaged with other values produced in response to the sampling pulse produced at the same point in the scan of the wavelength irradiating the sample.