Abstract:
The invention relates to an electrographic printing or photocopying device and method. A toner mark ( 10 ) is provided on a photoconductive drum and printed with toner using a developing station. The toner mark ( 10 ) is scanned by a sensor at two points of measurement (a 1 ,a 2 ). The amount of toner in a developing mixture consisting of two components depends upon the difference (ΔU) or the quotient of the amount of the signals of the sensor at both points of measurement. A device which is also disclosed enables the exact length of the toner mark ( 10 ) to be determined.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a device for printing or copying in which least one toner mark on a toner carrier is inked with toner for monitoring and adjusting the toner surface coverage. The invention also relates to a method for printing or copying. 
     2. Description of the Related Art 
     A conventional printer or copier has a toner carrier, for instance a photoconductive drum, on which a latent image is generated, for instance by exposure of the drum with light. A developing station serves for inking the latent image with toner. This developer station contains a developer mixture of toner and a carrier, for instance magnetic iron particles, with an adjustable proportion of toner. In order to be able to check the toner surface coverage and adjust it, a toner mark is made on the toner carrier and is scanned, for instance using a reflex sensor. 
     The toner surface coverage in such a two-component developing system is essentially determined by the following factors: 
     a) the toner supply in the developer zone; i.e. at the contact surface of developer station and the surface of the toner carrier; 
     b) the toner concentration in the developer mixture; 
     c) the electrostatic charge behavior of the surface of the photoconductive drum; 
     d) the trieboelectric behavior of the developer mixture; that is, the force of the adhesion between carrier particles and toner; 
     e) the shape and size of the particles in the developer mixture; 
     f) the activity of the toner particles which are provided by the developer drum in the developer zone. 
     For a specific type of device, the factors c to f are relatively constant parameters. With respect to factors a and b, there is a mutual dependency; namely, the toner supply in the developer zone is dependent on the toner concentration, so that the contrast level of the latent image is determined by the toner concentration. This contrast level or the toner surface coverage area is proportional to the toner concentration. 
     In previous printers or copiers, the toner surface coverage has been adjusted by measuring the toner marks, for instance using a reflex sensor. The signals of the reflex sensor has then served as a measure of surface coverage; that is, the darker the inking of the toner marks with toner, the lower the signal level of the voltage of the receiver which picks up the reflected radiation. But this signal level is also dependent on the reflection behavior of the toner and of the character of the surface of the toner carrier, for instance of the photoconductor. Furthermore, the tolerances of the reflection sensor which scans the toner marks must be taken into account. It is therefore state of the art to perform an individual adjustment for each printer of the developer station, the toner carrier, and so on, to the toner material. When the toner type is changed and the toner carrier is replaced, this adjustment must always be performed again. In conventional printers, correction programs were installed to facilitate the adjustment work, which performed an automatic adjustment within particular bandwidths once the toner type and the type of photoconductive drum were inputted. But the result is frequently unsatisfactory despite these corrective measures, and a toner excess can set in, in which the toner layer on the toner carrier is thicker than necessary, which leads to excessive consumption of toner. 
     German Patent Document DE-A-39 38 354 teaches an image recording device in which a toner density sensor tests two toner marks on a photoconductive drum. The sensor contains two photoreceivers, which evaluate the degree of reflection of the two toner marks. The toner marks are arranged transverse to the direction of motion of the photoconductive drums, with one toner mark having a high density and the other toner mark, a low density. A bias voltage for the developer unit is set depending on the measurement result of the photoreceiver. 
     U.S. Pat. No. 5,410,388 teaches a device, or respectively, a method for electrographic printing or copying in which a densitometer determines the respective surface coverage at two consecutive measurement locations of an elongated toner mark using infrared radiation. When there is a difference between the surface coverage in the front region of the toner mark and the back region of the toner mark as expressed in a different degree of reflection, parameters of the development process are readjusted; for example, the toner concentration can be modified. If the difference in the reflection power or in the values of the surface coverage equals zero, then the developer process is left unchanged. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a device and a method which reduce the adjustment outlay and which achieve a relatively high-quality print result. 
     This object and others are achieved for devices by claims  1 ,  13 ,  22 ,  30  &amp; Sub. 
     The invention makes use of an effect that occurs in the inking of a solid surface with toner in the direction of motion of the toner carrier in two-component developer mixtures. As viewed in the direction of motion of the toner carrier, the toner supply within the developer zone changes. At first the toner supply is large, which leads to a high surface coverage. When this first supply of toner has been transferred to the toner carrier, then new toner must first be transported by the developer drums to replace it, which leads to a reduction of surface coverage given low the toner concentration. After the initial drop in toner supply, the supply of toner as seen across the length of the toner mark remains constant, and thus a constant surface coverage also sets in. In this regard, one refers to a depletion effect in the solid surface in the direction of motion of the toner carrier. This depletion effect manifests itself in a reduction of the surface coverage, until saturation is achieved; that is, within the depletion zone the surface coverage is 100% on the toner mark. Given this degree of surface coverage, the toner mark is covered with toner densely without gaps; given a black toner, increasing the layer thickness of the toner does not produce any additional blackening. The fluctuations in surface coverage along the toner mark are inversely proportional to the toner concentration. The higher the toner concentration, the smaller the differences in surface coverage, or respectively, contrast level on the toner marks. 
     The toner marks that have been inked with toner are now inventively scanned by at least one sensor at at least two consecutive measurement points as seen in the direction of the motion of the toner carrier, and the surface coverage at these measurement points is imaged as electrical signals. The proportion of toner in the developer mixture is adjusted dependent on the difference or the quotient of the absolute values of the signals at these two measurement points. Thus, it is not the absolute level of the signal of the sensor which is evaluated, but rather the difference in the signals, or respectively, the quotient of the signals, that is measured along the toner marks. This difference, or respectively, the quotient, is largely independent of the reflection power of the utilized toner, and it is therefore unnecessary to perform different adjustments for different types of toner. The reflection power of the surface of the toner carrier—for instance of the surface of a photoconductive drum, or that of a carrier material made of paper—on which the toner mark is printed and then scanned enters into the result only slightly, in particular when calibration is performed to the reflection behavior of the respective surface, as described below. 
     When the proportion of toner in the developer mixture is adjusted in such a way that the difference is close to zero or the quotient is close to one, then almost no fluctuation of the surface coverage occurs over the length of the toner mark. In this operative state, an optimal contrast level is guaranteed without a toner excess. 
     One exemplifying embodiment is characterized in that the difference or the quotient of the signals at the two measurement locations is compared to a desired value, and that, depending on the comparison, a controller actuates a conveyor mechanism that transports toner to the developer station. This creates a control system which ensures that the printer is always held in an optimal operating state with high-quality print results. A value close to zero is selected as desired value for a difference evaluation, and a value close to one is selected for a quotient evaluation. When the control process commences given a certain toner deficit—that is, the difference is greater than zero or the quotient is not equal to one—then the subsequent control process ensures that an operating state with a toner surplus does not set in, since a certain deviation to the desired value remains. A time-point controller is preferably used as the controller, which switches the conveyor mechanism back and forth between the ON and OFF states. 
     In the above described device and evaluation method, it is essential that the relative position of the measurement locations on the toner mark remain the same. Mechanical assembly tolerances of the toner carrier or the sensor can lead to changes in the distance between the toner carrier and sensor. Such position changes can also occur when the toner carrier or the sensor is replaced. When the toner mark is now scanned by the sensor following a predetermined delay period in which the toner mark that has been inked with toner is moved forward subsequent to the writing of the latent images until it reaches the sensor, not always the same locations of the toner mark are measured. As a result, the setting obtained from the electrical measurement signals of the sensors is no longer optimal. Accordingly, INSERT CLAIMS  22  &amp;  23  a device and a method are provide, which permit scanning of the toner mark at measurement locations whose position relative to the toner mark is constant. 
     In accordance with this aspect of the invention, the reference time at which a reference point on the toner mark passes the scanning sensor is fixed by a stationary scanning sensor, which is preferably the same sensor that scans the toner mark at the two measurement locations. The front edge or back edge of the toner mark is preferably used as a reference point. The additional times at which the two measurement locations are scanned can be specified dependent on this reference time given knowledge of the transport speed of the toner carrier. These measurement locations are then scanned in each toner mark with respect to the reference point at defined intervals from this reference point. 
     The described part of the invention is preferably used when the evaluation of the electrical signals occurs in accordance with the above described device and method. But it can also be used advantageously to fix the position of a toner mark that is scanned at two measurement locations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An exemplifying embodiment of the invention is detailed below with the aid of the drawing. 
     FIG. 1A is a plan view and FIG. 1B is a graph of the toner mark with two measurement locations and the characteristic curve of a sensor voltage over the length of the toner mark; 
     FIG. 2A is a graph of a characteristic field of the toner supply over the length of a solid surface in the principle of counter-directional development and FIG. 2B is side view of toner applying rollers 
     FIG. 3B shows a graph of a characteristic field as shown in FIG. 2, for the principle of synchronous development and FIG. 3B is a side view of the toner rollers; 
     FIG. 4A is a graph which shows characteristic field as shown in FIG. 2, for the principle of synchronous/counter-directional development and FIG. 4B is a side view of the rollers; 
     FIG. 5 is a graph of the relation between surface coverage and toner supply in the development zone; 
     FIG. 6 is a graph of the surface coverage over the length of a solid surface given different toner concentrations; 
     FIG. 7A is a graph of the relation of the surface coverage over the length of a toner mark for different toner concentrations and FIG. 7B is a plan view of two toner marks showing the scanning location shown in FIG. 7A; 
     FIG. 8 is a graph of the voltage delivered by a reflex sensor against the toner concentration for black and red toner; 
     FIG. 9A is a graph of the reflection power of different toner colors and FIG. 9B is a graph of the degree of surface coverage for these various toner colors given controlling of the toner concentration; 
     FIG. 10 is schematic side view of the structure of a printing device in which the invention is realized; 
     FIG. 11 is a graph of raster time at which the curve of the sensor voltage is scanned over the length of the toner mark and stored, and 
     FIG. 12 is a graph of the scanning process, when the back edge of the toner mark passes the reflex sensor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 a  shows a rectangular toner mark  10  whose longitudinal extent runs in the direction of motion of a photoconductive drum. The toner mark  10  which is provided with toner is scanned at two measurement locations a 1  and a 2 . By virtue of the longitudinal motion of the photoconductive drum, given a circular beam spot a planar extension of the measurement locations a 1  and a 2  arises in the manner of an elongated hole. The measurement location a 1  lies approximately in the middle of the first third of the toner mark  10 , and the measurement location a 2  lies approximately in the middle of the last third. 
     FIG. 1B shows a diagram of the curve of the voltage U of a radiation receiver over the length L of the toner mark  10 . The radiation receiver (not shown) captures the radiation that is reflected by the toner mark  10  and the surface of the photoconductive drum (likewise not shown) and converts it into a voltage U, possibly after amplification. In a first portion  12  of the curve, radiation of the reflex sensor is reflected by the blank surface of the photoconductor with a high reflection power, producing a maximum voltage level Um, which is used as a reference level. 
     When the toner mark  10  moves forward in the direction indicated by the arrow at velocity v, the radiation sensor captures the front edge  10   a  of the toner mark  10 , whereby the reflected radiation and thus the voltage U as well decrease. In section  14  a minimum of the voltage curve occurs when the beam spot lies completely within the toner mark  10  after the front edge  10   a  has passed. Section  14  is followed by section  16 , which is characterized by capture of the measurement location a 1 . The voltage curve rises slightly in this region. The reason for this is explained later. In section  18  the voltage U rises further. In section  20  the measurement location a 2  is scanned. In section  22  the measurement spot covers the back edge  10   b . By virtue of the high reflection of the surface of the photoconductive drum, the voltage U continues to rise until reaching the maximum value Um in section  24 . The difference value ΔU represented in the Figure is inventively evaluated. Preferably, the average voltage values U at the measurement locations a 1  and a 2  are advantageously taken into account. 
     FIG. 2A shows in a diagram that, in the principle of counter-directional development, the toner supply TA decreases over the length of a solid surface, for instance the toner mark  10 . In this principle of counter-directional development, photoconductive drum FLT and developer drum EW have opposite directions of rotation, as is illustrated schematically at FIG.  2 B. When the toner mark  10  reaches the developer drum EW with its front edge  10   a , then there are many toner particles immediately available for transfer onto the photoconductive drum FLT; that is, the toner supply TA is high. After the first toner particles are delivered, the toner supply TA grows depleted, and only the number of toner particles that are subsequently transported to the developer drum EW by the developer station are transferred. A drop occurs in the toner supply TA, as expressed by the three characteristic curves, which relate to high, average, and low concentrations of toner TK. A measurement location, for instance the measurement location a 1 , should be arranged in the region of this drop. After a particular length the amount of toner that is subsequently transported is constant; the characteristic curves run approximately parallel to a broken saturation curve  26 , at which 100% inking of the toner mark  10  with toner has occurred, which means that, given black toner, there is no additional blackening in the print even when the toner particles per unit area are increased. The measurement location a 2  is arranged in this region of the largely parallel characteristic curves. As can be seen in the characteristic field, the drop of the toner supply TA in the initial region is steeper, the lower the toner concentration TK is. Accordingly, the difference in the reflection behavior at the two measurement locations a 1  and a 2  is also greater, and as a result the difference voltage ΔU is also greater. 
     FIG. 3A shows a similar characteristic field as FIG. 2, though for synchronous development, in which the directions of rotation of the photoconductive drum FLT and the developer drum EW are the same. By virtue of the parallel rotation motion, an elevated toner supply occurs at the back edge  10   b  of the toner mark  10 , since the developer drum EW turns at a greater speed than the photoconductive drum FLT. Here, too, the measurement locations a 1  and a 2  are arranged one in the rectilinear part of the characteristic curve and one in the part of the curve that drops off relatively sharply. 
     FIG. 4A relates to characteristic curves of the toner supply TA over the length of the toner mark  10  in a principle of synchronous/counter-directional development, in which two developer drums EW are moved in opposite directions as shown in FIG. 4B. A falling curve of the toner supply TA occurs in the vicinity of the front edge  10   a  and the back edge  10   b . The measurement spot a 1 , or respectively, a 1 ′, is arranged in the decreasing region of the curves; the measurement location a 2  in the flat region. 
     FIG. 5 shows the relation between surface coverage FD at a solid surface, such as a toner mark  10 , and the toner supply TA in the developer zone. Given a low toner supply TA, the surface coverage FD is also low. This surface coverage increases up to 100% as the toner supply rises. A surface coverage of 100% means that the toner mark  10  is completely covered with toner, and there are no gaps enabling the surface of the photoconductive drum to show through. When additional toner layers are built up given a surface coverage of 100%, the blackening in the print is no longer elevated as a consequence. Interestingly, after a surface coverage FD of 100% is achieved and the toner supply TA has risen, in many printers a course corresponding to the curve  28  is detected, whereby the surface coverage FD decreases again. This may be attributable to a clumping and to irregularities in the toner layer structure, so that layers emerge which cancel out the complete surface coverage again. 
     FIG. 6 shows the arrangement of the measurement locations a 1  and a 2  for counter-directional development. As stated, one measurement location al must be arranged in the region of the falling curve, while the other measurement location a 2  must be arranged in the rectilinear region of the curve. The curve  30  shows points of intersection with curves of different toner concentrations TK, whose appertaining lengths L define the measurement locations for a 2 . For practical reasons, the measurement location a 2  is fixed to the right of the curve  30  at relatively large length L. 
     It can also be seen in FIG. 6 that, for very high toner concentrations TK, the curve for the surface coverage FD has a straight course; that is, the curve does not fall off but rather can even rise in the initial region, as indicated by section  32 . Thus, given very high toner concentrations, a uniformly dense surface coverage of approximately 100% over the length of a solid surface occurs. 
     FIGS. 7A and 7B show shows a practical example of the relation between toner supply TA and surface coverage FD over the length L given various toner concentrations TK, whereby the bottommost curve  34  has a low toner concentration. The curves  36 ,  38 ,  40  and  42  show increasing toner concentrations TK, with the curve  42  being related to a very high toner concentration TK, for instance 7 percent by weight or more. The toner mark  10  has a typical length 1 of 8 to 16 mm and a width b of 4 to 10 mm. Differences ΔTA in the toner supply occur at the measurement locations a 1  and a 2 , which differences increase with rising toner concentration TK. 
     The reflection behavior R T /R FLT  can be computed for every toner color and every photoconductive drum and then taken into account in an evaluation, for instance in the form of a correction table. The respective voltage difference ΔU can be corrected in order to account for different toner types. It has proven advantageous in practice that the quotient R T /R FLT  is very small, since the reflection power of the respective toner is negligible compared to the reflection power of the surface of the photoconductive drum. For instance, the value R T /R FLT  is approximately {fraction (1/300)} for black toner and {fraction (1/10)} for strongly reflected toner such as yellow or red toner. The error resulting from the different reflection powers of different toner colors is thus relatively small. 
     It is clear from FIG. 8 that for rising toner concentration TK, the voltage differenceU tends to zero. A practical control range RB emerges from approximately 2.3 to 6.6 percent by weight of toner. To the right of curve  44  there is a toner excess, whereby the voltage difference ΔU reverses itself. This region of toner excess is avoided when the control process is begun to the left of curve  44  and is continued up to a desired value slightly greater than zero. 
     FIG. 9A shows a comparison of the reflection power of different toner types as expressed by the curve  46 , where the surface coverage FDD is assumed to be nearly 100%. In FIG. 9B, the result of a control process accounting for the voltage difference ΔU is shown. A value at which the surface coverage FD is close to 100% is prescribed as a desired value. A relatively constant value of the surface coverage FD occurs for differently colored toners, regardless of the absolute reflection power and the absolute values of the voltage U that is generated by the radiation sensor. By contrast, the curve of the toner concentration TK fluctuates for different toner colors. 
     FIG. 10 shows the schematic structure of a printing device in which the invention is realized. A photoconductive drum FLT rotates in the printing process in the direction of the arrow P 1 , whereby a toner image is printed on individual pages  50 . A developer station  52  contains a receptacle  54  in which the developer mixture of toner and carrier is prepared. A developer drum  56  transfers the toner onto the surface of the photoconductive drum FLT. The photoconductive drum FLT and the developer station  52  function according to the principle of counter-directional development; that is, the directions of rotation of the developer drum  56  and the photoconductive drum FLT are opposite one another. The developer station  52  also includes a toner conveyor mechanism  58 , which feeds apportioned toner from a stock receptacle to a toner cross-conveyor  60 . This toner cross-conveyor  60  delivers the toner to the receptacle  54 . The toner conveyor mechanism  58  contains a drive motor, which is switched into the operative state ON or OFF by a two-position controller  62 . 
     A toner mark  10  is provided on the photoconductive drum FLT, which mark is scanned using a reflex sensor  64 . This reflex sensor  64  contains an LED  66 , which emits monochromatic infrared radiation. The advantage of using infrared radiation is that this radiation reacts less sensitively to the different toner colors, so that its reflection power enters less significantly into the result. Besides, it is possible to suppress unwanted white light more effectively using infrared light. The LED  66  is powered with the current I L  from a controllable current source  68 . Between the photoconductive drum FLT and the reflex sensor  64 , a glass covering  72  is arranged, which prevents contamination by toner particles. The emitted radiation bundle  74  is reflected various ways. The reflected radiation is composed of a portion  76 , which originates at the surface of the photoconductive drum FLT. Another radiation portion  78  is produced by reflection at the glass covering  72 . Finally, still another radiation portion  80  is produced, which originates from the reflection at the toner particles. The overall radiation that is reflected by the toner mark  10  is detected by a receiving device  82 , which contains a reception diode. The receiving device  82  forms the value ΔU=Ua 2 −Ua 1 . 
     The value ΔU is compared to a desired value Us at the controller  62 . When ΔU is greater than Us, the toner transport mechanism  58  is switched on, and toner is continuously conveyed until the deviation between ΔU and Us is controlled to approximately zero. 
     During an adjustment phase, the switch  84  is switched in the direction of the arrow  86 , whereby the controllable current source  68  is actuated via the controller  62 . In this adjustment phase, a scaling is performed to the reflection power of the black surface of the photoconductive drum FLT. Here, the black surface of the photoconductive drum FLT is irradiated by the reflex sensor  64 , and the appertaining voltage value U is measured in the receiving device  82 . The controllable current source  68  is now adjusted such that a constant maximum value Um sets in in the receiving device  82 . The toner mark  10  is then scanned later with this setting. What is achieved by this procedure is that the reflection power of the surface of the photoconductive drum is not so great a factor in the result, because the variable reflection behavior is scaled to the value Um. The values ΔU of different photoconductive drums are thus largely constant given otherwise identical toner scanning. When the photoconductive drum FLT is replaced by another, the control of the toner concentration does not change. The described adjustment phase can also be repeated at time intervals in order to correct a change in reflection power of the surface of the photoconductive drum FLT. 
     A character generator (which is not illustrated) that is arranged in front of the developer station  52  transverse to the direction of rotation P 1  of the photoconductive drum FLT writes the latent image or the latent images for one or more toner marks  10  on the surface of the photoconductive drum FLT. The line generator and the reflex sensor  64  are generally detachably installed, in which process tolerances arise. These can be summarized in that the distance along the perimeter of the photoconductive drum FLT between character generator and reflex sensor  64  typically fluctuates up to 2 mm. A time control is typically used for the scanning of the toner marks  10 . At the start of the writing of the latent image by the character generator, a starting time is specified. On the basis of the constant rate of rotation of the photoconductive drum FLT and of the known distance between line generator and reflex sensor  64 , a delay time tm is computed, from which the scanning instant for the toner mark  10  by the reflex sensor  64  derives. 
     FIG. 11 shows the voltage curve U when the toner mark  10  passes the reflex sensor  64 . The curve corresponds to that according to FIG.  1 . The toner mark  10  is scanned at times T 1  and T 16  within a time frame ZR. At each raster time T 1  to T 16  four scan values are obtained, which are fed to a computer control as digital values. The average value of the 16 scan values that are acquired at each time T 1  to T 16  is then used as average scan value. The average scan values are temporarily stored in a memory. 
     The time frame ZR begins at raster time T 1  when the delay time tm expires. In the method described above, averaged scan values for the times T 4  to T 7  and T 10  to T 13  are acquired as measurement values from which the difference, or respectively, quotient is calculated. The averaged scan values for the times T 4  to T 7  and T 10  to T 13  are in turn averaged in order to filter out the significant noise portion in the signals by average value formation. The values thus obtained for the measurement locations a 1  and a 2  are then processed. 
     As stated, the front edge  10   a  or the back edge  10   b  of the toner mark  10  is used as a reference point for detecting the position of the toner mark  10 . When the beam spot of the reflex sensor  64  impinges on this front edge  10   a  or back edge  10   b  in halves, then the voltage Uh is at least approximately 
     
       
           Uh= ( Uref−Utm )/2, 
       
     
     where Uref is the voltage given reflection of the radiation at the blank photoconductive drum FLT, and Utm is the voltage given reflection at the toner mark  10  at the times T 10  to T 13 . 
     In order to calculate the time Tref appertaining to the voltage Uh either at the front edge  10   a  or back edge  10   b  of the toner mark  10 , the time frame Zr is shifted with respect to the delay time tm, and the voltage U is respectively sampled at a time T 1 . This shift occurs iteratively per toner mark by a time interval between the times T 1  and T 2 . The number of shift steps required for determining the voltage Uh then indicates the amount by which the delay time tm must be corrected in order to scan the toner mark  10  at the measurement locations a 1 , a 2 , whose position has a definite distance from the front edge  10   a  or back edge  10   b  of the toner mark  10 . The scanning is selected at time T 1  because later times may vary chronologically owing to interrupt run times of the interrupt-controlled generation of the times T 1  to T 16 . It should also be noted that in FIG. 11 the voltage curve U is reproduced in a vertically compressed state; the voltage Uref is significantly higher relative to the voltage Utm than is represented in the curve. 
     FIG. 12 shows a graph in which the time frame ZR has been shifted up to time T 1  to calculate the voltage Uh. The number of shift cycles that are required for discovering the voltage Uh is a measure of the amount of delay time tm by which to correct in order to scan the toner mark  10  or marks at the predetermined measurement locations a 1  and a 2 . 
     The described method for fixing the exact position of the toner mark  10  is applied with every initial adjustment of the printer or copier. In this adjustment process, a number of toner marks are printer on the photoconductive drum FLT in order to achieve a high precision in the adjustment. The cited steps can also be applied at predetermined time intervals, for instance at intervals of one hour operating time, or after each new activation of the printer or copier ( after each new set up). 
     The described exemplifying embodiment can be modified in the scope of the invention. For instance, the sensor  64  can scan the toner mark  10  after the print transfer onto a carrier material, for instance paper. In this case, normalization can take place with reference to the reflex power of the carrier material. In another variant, a photoconductive strip can be used instead of a drum. Black toner material, colored toner material, a toner that is mixed together from toner materials with different primary colors, or a transparent toner material can be used here. This variant is described in published PCT international patent application WO98/39691 A1, for example. The contents of this WO publication are hereby incorporated into the present specification by reference. 
     Although other modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.