referring now to the drawings and , more particularly , to fig1 there is shown one embodiment of a multicolor laser printer 10 including laser print heads 12 , 14 , 16 , 18 , a black toner cartridge 20 , a magenta toner cartridge 22 , a cyan toner cartridge 24 , a yellow toner cartridge 26 , photoconductive drums 28 , 30 , 32 , 34 , and an intermediate transfer member belt 36 . each of laser print heads 12 , 14 , 16 and 18 scans a respective laser beam 38 , 40 , 42 , 44 in a scan direction , perpendicular to the plane of fig1 across a respective one of photoconductive drums 28 , 30 , 32 and 34 . each of photoconductive drums 28 , 30 , 32 and 34 is negatively charged to approximately − 900 volts and is subsequently discharged to a level of approximately − 200 volts in the areas of its peripheral surface that are impinged by a respective one of laser beams 38 , 40 , 42 and 44 . during each scan of a laser beam across a photoconductive drum , each of photoconductive drums 28 , 30 , 32 and 34 is continuously rotated , clockwise in the embodiment shown , in a process direction indicated by direction arrow 46 . the scanning of laser beams 38 , 40 , 42 and 44 across the peripheral surfaces of the photoconductive drums is cyclically repeated , thereby discharging the areas of the peripheral surfaces on which the laser beams impinge . the toner in each of toner cartridges 20 , 22 , 24 and 26 is negatively charged to approximately − 600 volts . thus , when the toner from cartridges 20 , 22 , 24 and 26 is brought into contact with a respective one of photoconductive drums 28 , 30 , 32 and 34 , the toner is attracted to and adheres to the portions of the peripheral surfaces of the drums that have been discharged to − 200 volts by the laser beams . as belt 36 rotates in the direction indicated by arrow 48 , the toner from each of drums 28 , 30 , 32 and 34 is transferred to the outside surface of belt 36 . as a print medium , such as paper , travels along path 50 , the toner is transferred to the surface of the print medium in nip 54 . the laser beam of each of print heads 12 , 14 , 16 and 18 impinges upon a respective one of sensor devices 56 , 58 , 60 and 62 , each of which is placed near the end of a scan line of the associated laser beam . one embodiment of a sensor device 56 is shown in fig2 as viewed in the direction of laser beam 38 . sensor device 56 includes a receiving surface 64 which , while being impinged upon by laser beam 38 moving in scanning direction 66 , transmits a voltage signal ( fig . 3 ) to a microcontroller 68 on a sign path 69 . a leading edge 70 of the voltage signal is caused by laser beam 38 intersecting a leading edge 72 of receiving surface 64 . similarly , trailing edge 74 of the voltage signal is caused by laser beam 38 intersecting a trailing edge 76 of surface 64 . sensor 56 is provided on a rigid frame 77 , illustrated schematically in fig1 . frame 77 supports at least one of photoconductive drums 28 , 30 , 32 , 43 , and at least one of printheads 12 , 14 , 16 , 18 . a time duration t d between the leading and trailing edges of the voltage signal which is sent from circuits 93 and 100 to microcontroller 68 varies with the position of laser beam 38 along process direction 46 . as is evident from fig2 the width of receiving surface 64 increases along process direction 46 . thus , given a constant speed of the impingement point of laser beam 38 across receiving surface 64 in scanning direction 66 , laser beam 38 will take a longer period of time to traverse the width of receiving surface 64 the further laser beam 38 is along process direction 46 . by measuring time duration td of the voltage signal between leading edge 70 and trailing edge 74 , microcontroller 68 can determine the position of laser beam 38 along process direction 46 . of course , the position of laser beam 38 along scanning direction 66 at the location of this sensor relative to the hsync sensor is also determined when laser beam 38 intersects leading edge 72 of surface 64 , the position of which is fixed . instead of calculating a position of laser beam 38 along process direction 46 for each time duration t d of the voltage signal , microcontroller 68 may use a look up table which describes the position of laser beam 38 along process direction 46 for selected values of time duration t d . microcontroller 68 can then interpolate between the values of the look up table in order to calculate the exact position of laser beam 38 along process direction 46 . of course , a separate look up table must be used for each possible scanning speed of laser beam 38 . due to saturation of sensor device 56 by amplification of the laser beam , the time duration t d of the voltage signal may also be a function of the optical power of the laser beam . thus , the optical power of the laser beam must be held constant during a scan along direction 66 and between scans . else , the optical power of laser beam 38 has to be factored into the calculation of the process location of laser beam 38 . another embodiment of a sensor device 78 ( fig4 ) includes two separate photosensitive sensor receiving surface portions 80 and 82 . sensor portion 80 has a standard hsync configuration , with both leading edge 84 and trailing edge 86 being perpendicular to scanning direction 66 . portion 82 , however , is oriented at an angle such that leading edge 88 and trailing edge 90 are parallel to each other , but are nonparallel to leading edge 84 and trailing edge 86 of portion 80 . with laser beam 38 scanning in direction 66 and intersecting leading edge 84 at time t 1 , a time at which laser beam 38 intersects leading edge 88 varies between t 2 and t 3 depending upon a process position of laser beam 38 along process direction 46 . receiving surface portions 80 and 82 produce respective photocurrent signals on signal paths 81 and 83 that are converted to respective voltage signals 85 and 87 by transimpedance amplifiers 93 . these voltage signals 85 and 87 are merged on common signal path 91 by connecting the open collector outputs of buffer comparators 100 . the common signal path 91 ( known in the art as a hard - wired logic gate ) is connected to microcontroller 68 . microcontroller 68 can then measure a time duration between laser beam 38 intersecting either of leading edge 84 and trailing edge 86 and laser beam 38 intersecting either of leading edge 88 or trailing edge 90 . that is , microcontroller 68 can measure a time duration between falling edge 92 and either of falling edge 94 or rising edge 96 . alternatively , microcontroller 68 can measure a time duration between rising edge 98 and either of falling edge 94 or rising edge 96 . this embodiment has the advantage that the more stable leading edge may be used for each sensor to determine the time interval related to beam process location . with each of sensor portions 80 and 82 having a fixed width in scanning direction 66 , microcontroller 68 can calculate a speed of laser beam 38 based upon the time duration of either of the two pulses in the voltage signal . thus , only one look up table , applicable for each possible scanning speed of laser beam 38 , is needed to calculate the process position of laser beam 38 in direction 46 . such a look up table could , for example , provide a list of values of the process distance as a function of the change in the time duration between falling edges 92 and 94 . alternatively , such a look up table could provide a list of values of the time duration between falling edges 92 and 94 as a percentage of the time duration of the first pulse between falling edge 92 and rising edge 98 . at initial factory set up , an initial time difference t d0 equal to the time duration between falling edges 92 and 94 is stored in the printer non - volatile random access memory . if , during operation , laser beam 38 drifts to a new process location , this change in location δy can be determined using the new time duration td , either from this look up table or from the following equation : wherein k is the gain describing the characteristics of the relationship between the process direction location and the time t d between the leading edges 92 and 94 . for example , if the angle 99 ( fig4 ) is a constant 45 °, then if this angle 99 were to vary over the length of edge 88 , then k would become a function of time duration t d . the above - described process can be repeated for each of the other laser beams 40 , 42 and 44 in order to determine drift in their respective positions in process direction 46 . the process positions of one or more of laser beams 38 , 40 , 42 and 44 can then be adjusted such that each of the colors of laser printer 10 can be printed in alignment along process direction 46 . the sensors are shown as producing signals with “ negative logic .” however , it is to be understood that the voltage signals can also be “ positive logic .” that is , the signals may be positive pulses with rising edges in place of falling edges 70 , 92 , 94 , and with falling edges in place of rising edges 74 , 96 , 98 . in the embodiments shown , each of the leading and trailing edges of the sensor device are linear . however , it is to be understood that it is also possible for the leading and trailing edges of the sensor device to be non - linear , such as curved . microcontroller 68 must factor in the particular geometry of the sensor device when calculating the process position of the laser beam . the present invention has been described herein as being used in conjunction with a laser printer . however , it is to be understood that it is possible for the present invention to be used in conjunction with any type of electrophotographic printer . while this invention has been described as having a preferred design , the present invention can be further modified within the spirit and scope of this disclosure . this application is therefore intended to cover any variations , uses , or adaptations of the invention using its general principles . further , this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which falls within the limits of the appended claims .