A device for radiation absorption measurements may include a radiation source emitting electromagnetic radiation having a wavelength in the interval 0.2 μm-20 μm, a detector detecting the electromagnetic radiation, when in a measurement mode at least a portion of the radiation has passed through a medium and been reflected by a surface at a distance from the radiation source, before reaching the detector. The device may further include a fluid calibration cell, which is adapted to be arranged in the path of the electromagnetic radiation between the radiation source and the detector. A method for calibrating a device for radiation absorption measurements may involve emitting electromagnetic radiation having a wavelength in the interval 0.2 to 20 μm, directing at least a portion of the electromagnetic radiation through a fluid calibration cell, and detecting the electromagnetic radiation.