Abstract: A device for measuring the mass of solids contained in a fluid with light. The device includes at least two pairs of light transmitters arranged crosswise. The light transmitters in each pair are able to, at the same time, transmit pulses of light with a certain wavelength, where the wavelengths of the pairs of transmitters are different from each other. The device further includes at least one detector that is able to detect light pulses reflected from the solids as a result of the transmitted light pulses. The detected light pulses are processed to obtain a measurement of the mass of the solids contained in the fluid.