silvered central portion 70 the annular portion 68 of the extractor 64 is dimensioned to correspond to the dimension of an annular beam of selected magnification and the central portion 70 thereof is dimensioned to correspond to the dimension of an annular beam of selected different and preferably successive magnification as can be seen in fig4 b by way of example the extractor 64 is shown in solid outline and has fully and partially silvered mirrored portions 68 70 dimensioned to correspond to the dimensions of the annular beams 52 54 having selected different magnifications of 34 nine 34 and 34 twenty seven 34 respectively optics generally designated 72 and a sensor 74 are positioned to receive light reflected thereto off of the light extractor 64 during the interpulse intervals the optics 72 includes a lens 76 operative to image the extracted annular received beams of selected different magnification and spot size on the sensor 74 a butterfly valve 78 of known design is operative to sequentially occlude corresponding ones of the annular received beams of selected different magnification in such a way that the sensor 74 at any given time is responsive to only one of the annular received beams of selected different magnification in operation received energy oscillates between the cavity mirrors 60 62 producing.