Method for training a classifier, the method comprising: generating, by an image sensor (106, 108) registering visible light and infra-red light, first image data by registering visible light, the first image data including data representing an object or an event in a monitored environment, generating, by the image sensor (106, 108), second image data by registering infra-red light, the second image data representing a corresponding time period as a time period represented by the first image data, detecting at an object detector an object appearing in the first image data, cropping at a first cropping module from the first image data a subset of data representing the detected object, a cropping area thereof being determined by the position and size of the detected object, determining at said first cropping module features of said cropping area, said features comprising a position of the detected object, an indication of the area occupied by the object, a width of the cropping area and a height of the cropping area, inputting to a tutor classifier the cropped data of the first image data, generating a classification of the object or event in the tutor classifier, receiving the second image data at an apprentice classifier training process, translating at a crop translator said features of the cropping area into a corresponding position represented in the second image data, determining at a second cropping module cropping parameters at least partially based on the translated features, wherein the second image data are then cropped using said cropping parameters, receiving the classification generated in the tutor classifier at the apprentice classifier training process, training (266, 268, 270) the apprentice classifier in the apprentice classifier training process using the cropped data of the second image data as input and using the classification received from the tutor classifier as a ground-truth for the classification of the second image data. generating, by an image sensor (106, 108) registering visible light and infra-red light, first image data by registering visible light, the first image data including data representing an object or an event in a monitored environment, generating, by the image sensor (106, 108), second image data by registering infra-red light, the second image data representing a corresponding time period as a time period represented by the first image data, detecting at an object detector an object appearing in the first image data, cropping at a first cropping module from the first image data a subset of data representing the detected object, a cropping area thereof being determined by the position and size of the detected object, determining at said first cropping module features of said cropping area, said features comprising a position of the detected object, an indication of the area occupied by the object, a width of the cropping area and a height of the cropping area, inputting to a tutor classifier the cropped data of the first image data, generating a classification of the object or event in the tutor classifier, receiving the second image data at an apprentice classifier training process, translating at a crop translator said features of the cropping area into a corresponding position represented in the second image data, determining at a second cropping module cropping parameters at least partially based on the translated features, wherein the second image data are then cropped using said cropping parameters, receiving the classification generated in the tutor classifier at the apprentice classifier training process, training (266, 268, 270) the apprentice classifier in the apprentice classifier training process using the cropped data of the second image data as input and using the classification received from the tutor classifier as a ground-truth for the classification of the second image data.