measurements made at three or more knife edge angles second moments cannot be obtained from measurements at only two angles the present invention preferably includes three or more different angles of knife edge 32 numbers up the maximum allowed by the mechanical constraints of designing the drum 31 are contemplated for example edges at 0 u00b0 15 u00b0 30 u00b0 may be employed if slits 32 are used fig3 b instead of windows bordered by knife edges fig3 a then the slits formed by pairs of closely adjacent knife edges should also be variously angled of course if pinholes are used in the present invention this is moot slits 32 are depicted in fig3 b there are several advantages to the mathematical analysis of the present invention over the manual mechanical methods of the prior art the expense additional inaccuracy and fragility of a mechanical bearing and locking device are eliminated while no extra mathematical calculating equipment computer hardware need be provided the various scanned intensity profiles from the corresponding various angled edges are measured in quick succession almost simultaneously so that the measurements are less likely to suffer from drift by way of contrast the prior art manual rotation requires a relatively long time during which the beam profile might drift due to laser instability vibration accident or misalignment caused by the act of rotating the device the drum rotates along the drum axis of rotation 33 a special wide opening 34 is made on the drum for direct power measurements if desired the present invention can eliminate the window 38 of the prior art since the detector is located inside the hub or measuring head drum and there is no need for a beam exit this is an advantage with high powered.