purpose of source locator 120 is to determine the location of the x ray source 200 and to record that location information in an appropriate digital storage device the digital storage device is then associated with circuitry affixed to x ray head 115 so that once the source locator is removed or the x ray head itself is moved the location of the x ray source in a particular x ray head is stored and accurately known at all times shown in fig2 a is x ray source 200 the location of which must be determined x ray opaque object 201 and an image 202 of the x ray opaque recorded on film 203 as described below determination of the size differences between object 201 and image 202 along with appropriate computer calculations based on these differences allow a precise determination of the x ray source location when the mobile x ray machine is turned on x ray radiation 204 is generated which passes over object 201 and is recorded on film 203 as image 202 as object 201 is x ray opaque the size of image 202 will vary based on the relative locations of x ray source 200 object 201 and image 202 referring now to fig2 b there is shown the manner in which the location of the x ray source can be calculated more particularly the location coordinates of points a and c are.