A method of calibrating an optical surface condition monitoring system, the method comprising: - emitting a first light beam at a first wavelength (λ 1) and a second light beam at a second wavelength (λ 2) by the optical surface condition monitoring system to a reference substrate, - receiving first signals scattered from the reference substrate at a first distance (L 1) between the optical surface condition monitoring system and the reference substrate by a receiver of the optical surface condition monitoring system, - varying a distance between the optical surface condition monitoring system and the reference substrate from the first distance (L 1) to a second distance (L 2), - receiving second signals scattered from the reference substrate at the second distance (L 2) between the optical surface condition monitoring system and the reference substrate by the receiver of the optical surface condition monitoring system, - emitting at least the first light beam at the first wavelength (λ 1) and the second light beam at the second wavelength (λ 2) by the optical surface condition monitoring system to a sample substrate which is arranged at a specific distance (L specific) from the optical surface condition monitoring system in the range between the first distance (L 1) and the second distance (L 2), - receiving at least third signals scattered from the sample substrate at the specific distance (L specific) by the receiver of the optical surface condition monitoring system, - calculating ratios between the signals, which have been scattered from the sample substrate, and signals, which have been scattered from the reference substrate at the specific distance (L specific). - providing input data regarding a measurement distance (L measurement) between a target surface and the optical surface condition monitoring system, and - calculating for a target material, which is the material of the sample substrate, dry calibration values as the product of said ratios and signals of the reference substrate at the measurement distance (L measurement). - emitting a first light beam at a first wavelength (λ 1) and a second light beam at a second wavelength (λ 2) by the optical surface condition monitoring system to a reference substrate, - receiving first signals scattered from the reference substrate at a first distance (L 1) between the optical surface condition monitoring system and the reference substrate by a receiver of the optical surface condition monitoring system, - varying a distance between the optical surface condition monitoring system and the reference substrate from the first distance (L 1) to a second distance (L 2), - receiving second signals scattered from the reference substrate at the second distance (L 2) between the optical surface condition monitoring system and the reference substrate by the receiver of the optical surface condition monitoring system, - emitting at least the first light beam at the first wavelength (λ 1) and the second light beam at the second wavelength (λ 2) by the optical surface condition monitoring system to a sample substrate which is arranged at a specific distance (L specific) from the optical surface condition monitoring system in the range between the first distance (L 1) and the second distance (L 2), - receiving at least third signals scattered from the sample substrate at the specific distance (L specific) by the receiver of the optical surface condition monitoring system, - calculating ratios between the signals, which have been scattered from the sample substrate, and signals, which have been scattered from the reference substrate at the specific distance (L specific). - providing input data regarding a measurement distance (L measurement) between a target surface and the optical surface condition monitoring system, and - calculating for a target material, which is the material of the sample substrate, dry calibration values as the product of said ratios and signals of the reference substrate at the measurement distance (L measurement).