A method for classifying a particulate contamination on a surface of a test specimen comprising the steps of illuminating the contamination with electromagnetic radiation comprising a first illumination direction relative to the contamination, capturing a first image of the contamination illuminated by the radiation from the first illumination direction under a first observation direction relative to the contamination, illuminating the contamination with electromagnetic radiation comprising a second illumination direction relative to the contamination, capturing a second image of the contamination illuminated by the radiation from the second illumination direction under a second observation direction relative to the contamination, wherein a) either the first illumination direction and the second illumination direction are different from each other and include an azimuth angle different from 0° and the first observation direction and the second observation direction are the same, or b) the first illumination direction and the second illumination direction are the same and the first observation direction and the second observation direction are different from each other and include an azimuth angle different from 0°, wherein the azimuth angle is the angle enclosed by the first illumination direction and the second illumination direction or the first observation direction and the second observation direction in a projection onto the surface, calculating a measure of similarity between the first image and the second image, wherein an image size or a considered image section of the first image and the second image is selected such that only exactly one contamination in the first and second images is considered in the calculation of the measure of similarity, and classifying the contamination into at least a first and a second class based on the calculated measure of similarity, wherein the first class and the second class each denote a type of a material of the contamination, wherein the contamination comprises the type of material of one of the first and second classes, wherein if the similarity between the first image and the second image is above a predetermined threshold, the contamination is classified in the first class and if the similarity between the first image and the second image is below the threshold, the contamination is classified in the second class. illuminating the contamination with electromagnetic radiation comprising a first illumination direction relative to the contamination, capturing a first image of the contamination illuminated by the radiation from the first illumination direction under a first observation direction relative to the contamination, illuminating the contamination with electromagnetic radiation comprising a second illumination direction relative to the contamination, capturing a second image of the contamination illuminated by the radiation from the second illumination direction under a second observation direction relative to the contamination, wherein a) either the first illumination direction and the second illumination direction are different from each other and include an azimuth angle different from 0° and the first observation direction and the second observation direction are the same, or b) the first illumination direction and the second illumination direction are the same and the first observation direction and the second observation direction are different from each other and include an azimuth angle different from 0°, a) either the first illumination direction and the second illumination direction are different from each other and include an azimuth angle different from 0° and the first observation direction and the second observation direction are the same, or b) the first illumination direction and the second illumination direction are the same and the first observation direction and the second observation direction are different from each other and include an azimuth angle different from 0°, wherein the azimuth angle is the angle enclosed by the first illumination direction and the second illumination direction or the first observation direction and the second observation direction in a projection onto the surface, calculating a measure of similarity between the first image and the second image, wherein an image size or a considered image section of the first image and the second image is selected such that only exactly one contamination in the first and second images is considered in the calculation of the measure of similarity, and classifying the contamination into at least a first and a second class based on the calculated measure of similarity, wherein the first class and the second class each denote a type of a material of the contamination, wherein the contamination comprises the type of material of one of the first and second classes, wherein if the similarity between the first image and the second image is above a predetermined threshold, the contamination is classified in the first class and if the similarity between the first image and the second image is below the threshold, the contamination is classified in the second class. wherein the first class and the second class each denote a type of a material of the contamination, wherein the contamination comprises the type of material of one of the first and second classes, wherein if the similarity between the first image and the second image is above a predetermined threshold, the contamination is classified in the first class and if the similarity between the first image and the second image is below the threshold, the contamination is classified in the second class.