appears only in the visible light image and can be neglected for the purposes of the invention the second moving object 27 on the other hand appears only in the thermal image and may be assumed to represent a flow of gas the second moving object 27 being now identified as gas can be presented in any appropriate way to make it easily identifiable to the operator it may be displayed as a field of particular colour shade or intensity in the ir image in the visible light image or in any type of blended image the image 33 denoted c in fig2 shows the resulting image having only the second moving object since this is the part of the image that is of interest in the context of the invention that is the objects that are visible in both images or only in the visible light image have been eliminated of course other objects or the complete screen may be displayed as well but with the second object 27 clearly distinguishable from the rest of the image the detection of any moving objects in each of the images may be performed in any way known in the art for example a method known as u201c optical flow u201d may be used in this method the velocity vector for each pixel is determined along the rows and the columns respectively of the focal plane array 7 13 this may be difficult to achieve in ir images because of noise the optical flow method is described for example in barron j l et al u201c performance of optical flow techniques u201d.