A method of determining a property of a phase of a heterogeneous medium in motion comprising at least two phases, the heterogeneous medium comprising a solid phase made of a plurality of dispersed particles or fibres in a liquid, the method comprising the following steps: - irradiating the medium with a light beam having at least one predetermined wavelength, - obtaining a measurement signal representative for detected light leaving the medium, wherein the light beam is small in relation to a part of the medium for which the property is to be determined, and wherein the bandwidth of the measurement signal is sufficiently high for distinguishing time periods for the measurement signal which are related to the part of the medium for which the property is to be determined, the method further comprising the following steps: - for time periods for the measurement signal which are related to the part of the medium for which the property is to be determined, using the measurement signal for determining the property of the part of the medium for which a property is to be determined, and - wherein the property to be determined is any of the following: a chemical composition, brightness, and colour, and - wherein the property to be determined is determined by a mathematical calculation using the measurement signal corresponding to the part of the medium for which a property is to be determined. - irradiating the medium with a light beam having at least one predetermined wavelength, - obtaining a measurement signal representative for detected light leaving the medium, wherein the light beam is small in relation to a part of the medium for which the property is to be determined, and wherein the bandwidth of the measurement signal is sufficiently high for distinguishing time periods for the measurement signal which are related to the part of the medium for which the property is to be determined, the method further comprising the following steps: - for time periods for the measurement signal which are related to the part of the medium for which the property is to be determined, using the measurement signal for determining the property of the part of the medium for which a property is to be determined, and - wherein the property to be determined is any of the following: a chemical composition, brightness, and colour, and - wherein the property to be determined is determined by a mathematical calculation using the measurement signal corresponding to the part of the medium for which a property is to be determined. wherein the light beam is small in relation to a part of the medium for which the property is to be determined, and wherein the bandwidth of the measurement signal is sufficiently high for distinguishing time periods for the measurement signal which are related to the part of the medium for which the property is to be determined, the method further comprising the following steps: - for time periods for the measurement signal which are related to the part of the medium for which the property is to be determined, using the measurement signal for determining the property of the part of the medium for which a property is to be determined, and - wherein the property to be determined is any of the following: a chemical composition, brightness, and colour, and - wherein the property to be determined is determined by a mathematical calculation using the measurement signal corresponding to the part of the medium for which a property is to be determined. - for time periods for the measurement signal which are related to the part of the medium for which the property is to be determined, using the measurement signal for determining the property of the part of the medium for which a property is to be determined, and - wherein the property to be determined is any of the following: a chemical composition, brightness, and colour, and - wherein the property to be determined is determined by a mathematical calculation using the measurement signal corresponding to the part of the medium for which a property is to be determined.