An optical or infrared spectrometer is suitable for on-line measurements for industrial, agricultural, field, commercial and other applications. Optical spectrometers are very useful for various analytical measurements. On-line operation is needed for obtaining real-time information, which is useful e.g. for process automation and quality control needs. The invention is based on optical design optimized for measuring moving samples at a distance and includes a light guide for signal homogenization, a linear variable filter for defining multiple measurement wavelengths as well as a linear detector array for detecting optical signals relating to the different wavelengths. There is an element for cooling and stabilizing the operating temperature of both the linear detector array and the linear variable filter, while the spectrometer is operating in variable environmental conditions. Thanks to the optical signal chain designed to maximize the radiance at the detector, the proposed spectrometer can provide high signal-to-noise ratio and high speed.