referring now to the figure , there is seen a system 20 for obtaining radiographic images of a reference subject 22 and a clinical subject 24 by means of a radiographic camera 26 and a display 28 . the camera 26 includes a collimator 30 , a scintillator 32 , an optically transparent spacer plate 34 , a set of scintillation detectors each of which incorporates a photomultiplier 36 and a signal conditioning circuit 38 , and a signal combiner 40 which are supported by a case 42 . each signal conditioning circuit 38 includes integration and pulse shaping while the combiner 40 includes resistor matrices for forming the x , y and z axis signals in the manner such as that taught in the aforementioned stout patent . in accordance with the invention , the system 20 comprises analog - to - digital converters 44 and 46 , a timer 48 , an adder 50 , memories 52 and 54 , a digital - to - analog converter 56 , a multiplier 58 , a timer 60 , an address generator 62 , a counter 64 , a calculator 66 , a source 68 , and switches 70 and 72 . the adder 50 and the memory 52 are utilized for providing the sums of the event signals for z axis signals identified by the letter z in the figure , the sums being stored in the memory 52 in accordance with addresses provided by the x and y signals from the camera 26 . the counter 64 counts the total number of events , the total number being identified by the letter t in the figure . the average number of events per cell of the memory 52 is provided by the ratio t / n where n is the number of cells in the memory 52 , the number n also being the number of resolution elements of the image produced by the camera 26 and presented on the display 28 . the scale factor is identified by the letter k in the figure , a separate value of k being produced for each resolution element of the image by the calculator 66 , the values of k being stored in the memory 54 . since the ratio t / n is constant , the value of k is seen by the formula shown in the figure for the calculator 66 to be inversely proportional to the value of b where b represents the magnitudes of the sums stored in the cells of the memory 52 , there being one value of k for each value of b . the multiplier 58 multiplies the relative energies of the z axis signals , shown in the bottom line of the graph 74 by the factor k to produce either a variation in magnitude of the z axis signal , as shown in the second line of the graph 74 , or a variation in the duration of the z axis signal as is shown in the first line of the graph 74 . the product of the multiplication , identified in the figure by the symbol kz , is applied to the display 28 . the storing of the sums of the event signals in the memory 52 during the calibration of the system 20 is accomplished as follows . first , the reference subject 22 is placed in front of the camera 26 . the reference subject 22 is typically in the form of a flat sheet uniformly inpregnated with a radioactive material for uniformly illuminating the front face of the collimator 30 . the switch 72 is manually set in the calibrate position as shown in the figure . the counter 64 is manually reset to zero by the reset button 76 . in the reset condition of the counter 64 , the counter 64 provides a logic state of zero on line 78 under which condition the switch 70 has the position as shown in the figure . in response to the illumination provided by the reference subject 22 , scintillations appear on the scintillator 32 , the scintillations being converted by the photomultipliers 36 and the signal combiners 40 to pulse signals which are coupled via the lines 80 to the signal combiner 40 which combines the signals on the lines 80 to produce the x , y and z signals which are seen to fan into the line 82 . the x and y signals of line 82 are coupled to the converters 44 and 46 which convert the x and y signals from an analog format to a digital format , the digital formated signals being seen to fan into line 84 . the z , or event , is coupled via the line 82 to the timer 48 , the adder 50 and the counter 64 . in response to each strobing of the timer 48 by the z signal , the timer 48 provides clock signals on line 86 for operating the converters 44 and 46 , the timer 48 also providing clock pulses , identified by the legend c1 , which are seen to fan into the line 84 for operating the memory 52 to store digital numbers at locations corresponding to the address of the x and the y signals . in response to the clock signals c1 , the contents of a cell of the memory 52 is incremented upon the occurences of successive events at the address of the cell . the contents of the cell is coupled along line 88 from the output terminal of the memory 52 via the switch 70 to an input terminal of the adder 50 to be summed with the z signal . the resulting sum from the adder 50 is coupled via the line 84 and the switch 70 back to the same cell of the memory 52 , this cell being addressed by the x and the y signals of the converter 44 and 46 . with each appearance of a z signal in combination with a specific address of the x and the y signals , the memory 52 makes available to the adder 50 the previously stored sum at that address , the adder 50 then incrementing that sum by a count of 1 corresponding to the one event signaled by the z axis signal . in this way , each cell of the memory 52 stores a count of the number of radioactive events from the reference subject 22 which have appeared at the image location corresponding to the address of that cell of the memory 52 . when the counter 64 attains a count of t events , this being a predetermined number to which the counter 64 is preset , the counter 64 provides a logic level of 1 on line 78 which serves as a strobe signal for operating the switch 70 to its alternate position whereupon the line 84 is disconnected from the memory 52 . thereupon , no new data enters the memory 52 . the signal on line 78 also strobes the timer 60 to operate the memory 52 in a read - only mode via clock pulses c2 . in addition , the timer 60 strobes the address generator 62 which sequentially addresses each cell of the memory 52 by means of x and y signals generated by the generator 62 . signals are coupled from the output terminal of the memory via line 88 and the switch 70 to the b input terminal of the calculator 66 . the timer 60 also provides timing signals c3 for operating the calculator 66 to perform the calculation in accordance with the formula shown on the figure in the block identifying the calculator 66 . thus , for each sum b coupled from a cell of the memory 52 to the calculator 66 , the calculator 66 provides electrical signals in a digital format representing a value of the scale factor k , these electrical signals being coupled via the switch 72 to the memory 54 and representing a measure of the activity of radioactive events at a specific location of the reference subject . the clock pulses c2 and the address signals of the generator 62 , which are seen to fan into the line 90 and be coupled via the switch 70 to the memory 52 , are also coupled via the switch 72 to the memory 54 for strobing and addressing the memory 54 in accordance with the strobing and addressing of the memory 52 . thereby , the memory 54 stores a set of scale factors k at locations corresponding to the resolution elements of the image corresponding to the x and y coordinate locations of the resolution elements . during the second phase of the operation of the system 20 , the switch 72 is manually positioned in the run position and the reference subject 22 is replaced with the clinical subject 24 . thereupon , the camera 26 and the display 28 are operated to produce an image on the display 28 in the manner commonly employed in hospitals , as disclosed in the aforementioned stout patent except that , in accordance with the invention , each z axis signal which signals a radioactive event is modified by the multiplier 58 before being applied to the z axis terminal of the display 28 . with respect to the multiplication by the multiplier 58 , it is noted that since the scale factors are stored in a digital format in the memory 54 , the digitally formated scale factors are first converted to scale factors in an analog format by the converter 56 and then applied to the multiplier 58 . the multiplier 58 may be an analog multiplier wherein the amplitude of the voltage provided by the converter 56 modulates the amplitude of the z axis signals to provide the aforementioned scaling of the amplitude portrayed in the second line of the graph 74 . or , alternatively , the multiplier 58 may incorporate a pulse - width modulator circuit wherein the amplitude of the voltage provided by the converter 56 is utilized for varying the duration of the z axis signal on line 92 to provide the aforementioned pulse - width modulated signal on the first line of the graph 74 . thereby , the products of the amplitudes of individual ones of the z axis signals times the duration of the corresponding z axis signals , this being equivalent to the relative energies in the pulses of the z axis signals , are modulated to compensate for variations in the addresses of z axis signals as may result in drifting of the gain characteristic of individual ones of the photomultipliers 36 . in addition , it is noted that if the reference subject 22 be provided with a design , as by masking a portion of the reference subject 22 with a cross or a circle , the outline of the masked portion appears superposed upon an image seen on the display 28 . if desired , a digital multiplier ( not shown ) may be utilized in lieu of the multiplier 58 in which case the converter 56 would be deleted and a pulse - width modulator ( not shown ) would be coupled between the digital multiplier and the display 28 . with reference to the calculations performed by the calculator 66 , it has been found that , in an experimental model of the system 20 wherein the value of the total count t was held constant ( 7 million counts being utilized ) as well as the number of resolution elements n being held constant , the variations in the magnitude of the sums b were sufficiently few in number such that the calculator 66 could be replaced by a read - only memory in which the memory address was supplied by the magnitude of the quantity b and in which the output of the memory was the desired scale factor k . thereby , for each value of b presented to the read - only memory , a corresponding value of k was read out of the memory . it is understood that the above - described embodiment of the invention is illustrative only and that modifications thereof may occur to those skilled in the art . accordingly , it is desired that this invention is not to be limited to the embodiment disclosed herein but is to be limited only as defined by the appended claims .