problems associated with using circular shaped attenuating apertures in the design of the sensor device this is achieved by using for example a non circular e g square rectangular etc shaped radiation transparent opening or aperture in other words the radiation window is of a design such that the area of the radiation source e g elongate lamp u201c seen u201d by the radiation sensor device remains substantially unchanged as the radiation window is swept by the radiation source e g elongate lamp in a dynamic sensor system this can be seen with reference to fig3 specifically when a circular shaped attenuating aperture is used in the sensor device the area of the lamp in the field of view i e within the area of the circular aperture of the sensor is constantly changing from when the lamp first enters the field of view to when the lamp leaves the field of view u2014 i e regardless of whether the lamp is wholly or partially within the field of view of the sensor device in contrast when a square aperture is used once the lamp is wholly within the field of view i e within the area of the square aperture the area of the lamp in the field of view is unchanged until the lamp starts to leave the field of view the intensity of radiation sensed will be in proportion to the area of the lamp in the field of view of the sensor in the result the intensity of the radiation when a circular shaped attenuating aperture is used will be constantly changing whereas that seen for the square shaped attenuating aperture a preferred embodiment of the present invention will result in a substantially constant intensity.