A sensor having a detector array is provided with means for determining whether the normal field of view of the sensor has been obscured. The image output from the detector array is compared by a processor to an image of the normal filed of view, previously acquired and stored in a memory. Where there are significant differences between the images the processor activates an alarm. The location of high spatial frequency detail in the image may be compared and the absence of such detail used to give an indication that the sensor may be masked.