Abstract: A measuring instrument for reflectometric measurements, comprising a spherical measuring chamber (1) provided with a light source (2) and with a specimen aperture (3); a measuring channel (4) which has been provided with optics (5), with a photometer (6), with an optional stop (7) and with an optional filter (8); a reference channel (14) provided with optics (15), with a photometer (16), with an optional stop (17) and with an optional filter (18); and a signal processing and calculating device (20.sub.s, 20.sub.c) for processing and comparing with each other the light intensity values observed by means of the photometers in the measuring and reference channels. As taught by the invention, the measuring instrument comprises at least one visible light light source (2.sub.s) which emits substantially light having a wavelength mainly over about 380 nm, and a UV light source (2.sub.u) which emits substantially light in the UV range having a wavelength mainly under about 380 nm, advantageously 300 to 380 nm.

Abstract: A reflectometric method of measurement for a 2-channel reflectometer, wherein in connection with the calibration, the standard sample is placed in the measruing channel (4), the dimming of the measuring channel is set to a random position and the light intensities detected both in the measuring channel and the reference channel (14) are adjusted into balance by adjusting the dimming of the reference channel by aid of the measuring diaphragm (17), and the measuring of the sample is carried out by setting the intensities into balance, likewise by aid of the measuring diaphragm of the reference channel. The reflectance value of the sample under measurement is calculated from the reflectance value of the standard, by aid of the set value of the measuring diaphragm obtained in the measurement and on the basis of the set value of the measuring diaphragm obtained in the calibration measurement.