Patent Publication Number: US-7904008-B2

Title: Method of using multiple developing members in a single-component developing system

Description:
FIELD OF THE INVENTION 
     The present invention is directed generally to electrophotography, more particularly, to a non-contact, single-component developing system employing a plurality of ancillary developing members that enables high speed development of electrostatic images and facilitates the consistent reproduction of high quality output images. 
     BACKGROUND OF THE INVENTION 
     Electrophotographic imaging process employs a charge-retentive, photosensitive member to form an electrostatic latent image. The latent image is rendered visible by depositing toner particles thereon. The developed particles are then transferred from the latent image to a transfer material such as paper. The resultant powder image deposited on the transfer material is permanently affixed thereto by applying heat and/or pressure, or with solvent vapor. 
     Color electrophotographic development is achieved by sequentially repeating the development process described above for each color and superimposing the developed images onto one another. 
     Various developing methods for visualizing electrostatic images are known in the art. One method is known as “non-contact” or “gap-jumping” development, wherein a thin layer of toner particles adhering to a toner-carrying member separated by a “gap” from a latent image-bearing member is brought into the developing region between the toner-carrying member and the latent image-bearing member. A high voltage associated with the latent image on the latent image-bearing member exerts electrostatic forces that direct the toner particles towards the latent image. The electrostatic forces are often of insufficient magnitude to overcome the adhesion forces holding the toner particles in the thin layer on the toner-carrying member. It has been proposed to apply a high AC bias voltage to the developing region in order to overcome the adhesion forces. The AC voltage is of sufficient magnitude to peel the toner particles from the toner-carrying member and allow the toner particles to “jump” the gap between the toner-carrying member and the latent image-bearing member. The toner particles land on the latent image-bearing member to form a developed image. The reciprocal nature of an AC voltage in turn frees the toner particles adhering to the latent image-bearing member from the latent image-bearing member and exerts electrostatic forces that direct the toner particles back to the toner-carrying member. This process is repeated until the latent image area moves far away from the developing region and the toner particles settle on the latent image area. The use of an AC bias has adversary effects on print speed because the rate at which the toner particles oscillate in the developing region (i.e., AC frequency) must be significantly greater than the rate at which the latent image on the latent image-bearing member moves (i.e., the surface moving speed of the latent image-bearing member) and an increase in AC frequency often results in undesirable artifacts such as poor reproducibility of thin character and line images. Furthermore, color developing in the presence of an AC bias requires one latent image-bearing member for each color and an accumulator or some other intermediate transfer member that stores each developed “tone” image, increasing the complexity and cost of the electrophotographic imaging system. 
     It has been proposed to eliminate the need for AC bias and/or an accumulator using various methods (e.g., by effectively reducing the impact of toner adhesion forces on the development process and thereby facilitating toner jump, or by increasing the thickness of a latent image-bearing member and thereby increasing the potential difference between latent image areas and non-image areas). 
     It has also been proposed to employ multiple developing rollers to enable high speed, high quality development. However, developing rollers were either sequentially activated, or disposed to repetitively perform substantially the same functions of a single developing roller. 
     In order to enable high speed development and eliminate the need for an accumulator while fully taking advantage of toner adhesion forces without employing rollers that perform substantially identical functions, a novel developing method capable of further improving the performance and functionality of a single developing roller by employing ancillary developing members is expected. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to a non-contact, single-component developing system for electrophotographic devices that enables high speed, high quality development and color development without an accumulator, or some other intermediate transfer member by employing a plurality of ancillary developing members such as rotating drums or belts operating in tandem with a toner-carrying member to facilitate toner detachment from the toner-carrying member. The term “non-contact” used in the present invention means that the toner-carrying member is separated by a prescribed gap from the toner-receiving surface of a latent image-bearing member and does not necessarily mean that none of the ancillary developing members are disposed in contact with the latent image-bearing member. 
     The gap between the toner-carrying member and the latent image-bearing member is larger than a typical gap in a non-contact, single-component developing system and consequently the toner-carrying member is biased (DC only) to a potential that is larger in magnitude (but not large enough to cause arcing between the toner-carrying member and the latent image-bearing member) than a typical DC bias used in such a developing system. The DC bias applied between the toner-carrying member and the latent image-bearing member produces an electrostatic force of insufficient magnitude to overcome toner adhesion forces that adhere toner particles to the toner-carrying member. 
     The ancillary developing members are strategically disposed and biased to exert supplemental electrostatic forces on toner particles adhering to the toner-carrying member that tend to free the toner particles from the toner-carrying member and direct the toner particles towards the ancillary developing members. Toner particles that are freed from the toner-carrying member due to the combined electrostatic forces fly towards either the latent image-bearing member, or the ancillary developing members, depending on the strengths of the electrostatic forces. The toner particles travel towards the latent image-bearing member if the electrostatic forces exerted in the gap between the toner-carrying member and the latent image-bearing member are of greater magnitude than those exerted in the gap between the toner-carrying member and the ancillary developing members. Conversely, the toner particles travel towards the ancillary developing members if the electrostatic forces exerted in the gap between the toner-carrying member and the latent image-bearing member are of smaller magnitude than those exerted in the gap between the toner-carrying member and the ancillary developing members. Thus, toner particles develop on latent image areas only. When non-image areas on the latent image-bearing member are brought into the region between the toner-carrying member and the ancillary developing members, toner particles either remain on the toner-carrying member, or fly towards and arrive at the ancillary developing members. Therefore, the developing system of the present invention provides improved image quality and eliminates background development. Furthermore, the combined electrostatic forces are sufficient to overcome toner adhesion forces and cause toner jump without the use of AC bias. This enables color development without an accumulator or some other intermediate transfer member. 
     Once toner particles arrive at the surface of the latent image-bearing member, or the ancillary developing members, the toner particles remain on the destination surface and do not return to the toner-carrying member. Electrostatic forces exerted on the toner particles do not direct the toner particles back to the toner-carrying member. Furthermore, adhesion forces adhering the toner particles to the destination surface cause the toner particles to remain on the destination surface. This advantageous exploitation of adhesion forces and the use of high electrostatic forces to facilitate toner jump enable high speed development that is less sensitive to the surface moving speed of the latent image-bearing member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of a non-contact, single-component developing system of the present invention that employs a single ancillary developing member. 
         FIG. 2  is a schematic illustrating the forces acting upon a toner particle during the development process. 
         FIG. 3  is a graph showing electric field lines between two cylindrical surfaces. 
         FIG. 4  is a graph showing the combined electrostatic force as a function of toner-carrying member rotation angle. 
         FIG. 5  is a schematic of a non-contact, single-component developing system of the present invention that employs two ancillary developing members. 
         FIG. 6  is a schematic illustrating the forces acting upon a toner particle during the first stage of the development process. 
         FIG. 7  is a schematic of a non-contact, single-component color developing system in accordance with the present invention that employs two ancillary developing members for each color. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The non-contact, single-component developing system of the present invention enables high-speed development of an electrostatic image and facilitates consistent high quality image reproduction. More particularly, the developing system of the present invention utilizes high electrostatic forces without AC voltages and exploits adhesion forces that adhere toner particles to a toner-carrying member by employing a plurality of ancillary developing members such as rotating drums or belts, whose electrostatic force contributions enable toner particles of various sizes and charges to jump from the toner-carrying member to a latent image-bearing member. Detached toner particles are directed and move in outward directions (i.e., from the toner-carrying member to the latent image-bearing member or the ancillary developing members), causing toner particles to jump the gap only once. Once toner particles are freed from the toner-carrying member, they move rapidly due to the electrostatic forces acting on the toner particles that are of high enough magnitude to overcome toner adhesion forces. This rapid one-way trip to the destination surface allows the developing system to be less sensitive to print speed (i.e., surface moving speed). 
     Referring in detail to the figures,  FIG. 1  shows a non-contact, single-component developing system in accordance with the present invention. The developing system includes a latent image-bearing member  10  such as a photosensitive drum or belt. An electrostatic latent image is formed on the surface of the latent image-bearing member  10  by a latent image forming mechanism or latent image forming means (not shown) and the surface of the latent image-bearing member  10  moves in the direction of an indicated arrow. 
     The developing system also includes a toner container  40 , a toner-carrying member  20 , and an ancillary developing member  22 . Hereinafter, the ancillary developing member  22  will be referred to as the “post-developer.” A metering blade  42  as a toner layer regulating means operates to create a thin layer of toner particles  41  and to charge the toner particles  41  on the toner-carrying member  20 . As described in detail below, toner particles that are freed from the toner-carrying member  20  arrive at the latent image-bearing member  10  or the post-developer  22 . A cleaning blade  32  removes toner particles transferred to the post-developer  22  and the removed toner particles are collected into the toner container  40 . 
     The surface moving speeds (i.e., circumferential speeds) of the toner-carrying member  20  and the post-developer  22  are substantially identical to the surface moving speed of the latent image-bearing member  10 . 
     The post-developer  22  and the toner-carrying member  20  are disposed to form a gap between the latent image-bearing member  10  and the toner-carrying member  20 , a gap between the latent image-bearing member  10  and the post-developer,  22  and a gap between the toner-carrying member  20  and the post-developer  22 . Furthermore, the post-developer  22  and the toner-carrying member  20  are disposed in such a way that an electrostatic latent image recorded on the latent image-bearing member  10  passes the toner-carrying member  20  first and then the post-developer  22 . The gap between the latent image-bearing member  10  and the toner-carrying member  20  is in the range from about 800 to 2,500 microns, preferably about 1600 microns. The gap between the latent image-bearing member  10  and the post-developer  22  is in the range from 20 to 300 microns, preferably about 100 microns. The gap between the toner-carrying member  20  and the post-developer  22  is in the range from about 500 to 2,000 microns, preferably about 800 microns. 
     The surface of the latent image-bearing member  10  is initially uniformly charged by a charging mechanism or charging means (not shown) to preferably about −700 V (DC). After exposure of the latent image-bearing member  10  to light to form an electrostatic latent image, the potential of the latent image-bearing member  10  is reduced to approximately −50 V (DC). 
     A DC bias voltage is applied between the toner-carrying member  20  and the latent image-bearing member  10 . The toner-carrying member  20  is biased to a potential in the range of approximately −2,000 to −8,000 V (DC), preferably about −4,500 to −6,000 V (DC), desirably about −5,500 V (DC). The post-developer  22  is biased to a potential between the image area potential and the non-image area potential (i.e., between −700 V and −50 V (DC)), preferably about −400 V (DC). Alternatively, an AC bias voltage having a peak-to-peak value of preferably about 100 V and a frequency of preferably about 5,000 Hz may be applied between the post-developer  22  and the latent image-bearing member  10  in superposition with the DC bias applied therebetween in order to facilitate toner detachment from the toner-carrying member  20 . 
     Although negative DC voltages are used above, it will be understood that positive voltages can also be used in accordance with the present invention by recording positive charge images on the latent image-bearing member  10  and reversing the polarities of DC bias voltages. 
     As shown in  FIG. 2 , as the toner-carrying member  20  carries the toner near the latent image-bearing member  10  and subsequently towards the post-developer  22 , a toner particle  44  experiences electrostatic forces E 1  and E 2  that tend to free the toner particle  44  from the toner-carrying member  20  and an adhesion force A that adheres the toner particle  44  to the toner-carrying member  20 . The electrostatic force E 1  directed towards the latent image-bearing member  10  results from the potential difference between the toner-carrying member  20  and the latent image-bearing member  10 , and is of insufficient magnitude to overcome the adhesion force A even in the region where the distance between the latent image-bearing member  10  and the toner-carrying member  20  is the shortest (i.e., when E 1  is the strongest). Thus, no toner develops until the toner reaches a region that is substantially close to the post-developer  22 . The electrostatic force E 2  results from the potential difference between the toner-carrying member  20  and the post-developer  22  and directs the toner particle  44  towards the post-developer  22 . For the toner particle  44  to be caused to leave the surface of the toner-carrying member  20 , the combined electrostatic force (i.e., E 1 +E 2 ) must be sufficient to overcome the adhesion force A. 
     In general, force is directly proportional to electric field and charge. The electrostatic forces E 1  and E 2 , and the adhesion force A, therefore, may be expressed in terms of electric field. The electrostatic forces E 1  and E 2  are then approximately 
                 E   ⁢           ⁢   1     =             V   dev     -     V   PC         L   ⁢           ⁢   1       ⁢           ⁢   and   ⁢           ⁢   E   ⁢           ⁢   2     =         V   dev     -     V   post         L   ⁢           ⁢   2           ,         
where V dev  is the DC bias applied to the toner-carrying member  20 , V PC  is the surface potential of the latent image-bearing member  10 , V post  is the DC bias applied to the post-developer  22 , L 1  is the field line length between the toner-carrying member  20  and the latent image-bearing member  10 , and L 2  is the field line length between the toner-carrying member  20  and the post-developer  22 . A field line between two surfaces represents a passage along which a charged particle will travel, as illustrated in  FIG. 3 .
 
       FIG. 4  shows an example plot of the combined electrostatic force E 1 +E 2  as a function of rotation angle θ of the toner-carrying member  20  (see  FIG. 2 ) where V PC =−50 V (DC). The combined electrostatic force E 1 +E 2  is of sufficient magnitude to overcome the adhesion force A when the rotation angle θ is approximately 27° in  FIG. 4 . Incidentally, the field line lengths L 1  and L 2  are approximately equal when θ=27°. But the line representing L 1 =L 2  may shift left and right and/or the curve E 1 +E 2  may shift up and down depending on the values of the parameters discussed above such as the DC bias voltages. 
     According to the example illustrated in  FIG. 4 , the toner particle  44  is freed from the toner-carrying member  20  when the rotation angle θ is 27° due to the combined electrostatic force E 1 +E 2  that exceeds the adhesion force A and flies towards either the latent image-bearing member  10 , or the post-developer  22 , depending on the strengths of the electrostatic forces E 1  and E 2 . The toner particle  44  travels towards the latent image-bearing member  10  if the surface potential of the latent image-bearing member  10  is approximately −50 V (DC) (i.e., image area) because the potential difference between the latent image-bearing member  10  and the toner-carrying member  20  is greater than that between the toner-carrying member  20  and the post-developer  22  (i.e., E 1 &gt;E 2  because L 1 =L 2 , see the equations above). Conversely, the toner particle  44  travels towards the post-developer  22  if the electrostatic force E 2  exerted in the gap area between the toner-carrying member  20  and the post-developer  22  is of greater magnitude than the electrostatic force E 1  exerted in the gap area between the latent image-bearing member  10  and the toner-carrying member  20 . Since E 2 &gt;E 1  when the surface potential of the latent image-bearing member  10  is approximately −700 V (DC), a non-image area does not attract and receive the toner particle  44 . 
     In addition to the electrostatic forces E 1  and E 2 , the potential difference between the post-developer  22  and the latent image-bearing member  10  creates an additional electrostatic force that directs the toner particle  44  towards the latent image-bearing member  10  if the surface potential of the latent image-bearing member  10  is approximately −50 V (DC), or towards the post-developer  22  if the surface potential of the latent image-bearing member  10  is approximately −700 V (DC). Therefore, the DC bias applied to the post-developer  22  further ensures that toner particles develop on latent image areas only. When non-image areas are brought between the toner-carrying member  20  and the post-developer  22 , toner particles simply fly towards and arrive at the post-developer  22 . 
     From  FIG. 4 , a toner particle that exhibits a stronger adhesion force is detached from the toner-carrying member  20  when the rotation angle θ is greater than 27° and the particle is subject to the identical developing mechanism as described above. That is, the particle will develop on an image area recorded on the latent image-bearing member  10 , or land on the post-developer  22 , but will not be attracted to a non-image area on the latent image-bearing member  10 . Thus, the developing system of the present invention is capable of consistently producing high quality output images. 
     Once toner particles leave the toner-carrying member  20 , due to the high electrostatic forces exerted on those particles, they tend to travel rapidly and quickly arrive at the surface of the latent image-bearing member  10 , or the post-developer  22 . Adhesion forces that adhere the transferred toner particles to the destination surface and electrostatic forces exerted on the transferred toner particles tend to hold the toner particles to the destination surface. Therefore, the developing system of the present invention does not allow toner particles that leave the toner-carrying member  20  and arrive at a destination surface to return to the toner-carrying member  20 . High electrostatic and adhesion forces cause the transferred toner particles to remain on the destination surface. This advantageous exploitation of adhesion forces and rapid toner travel to the destination surface enable high speed development of electrostatic images that is less sensitive to an increase in the surface moving speed of the latent image-bearing member  10 . 
     The developing system of the present invention also eliminates wrong-signed toner development. Wrong-signed toner development causes various undesirable artifacts such as foggy background (i.e., non-image area development). Because of the high negative DC bias applied to the toner-carrying member  20 , wrong-signed toner particles (i.e., positively charged particles) remain on the surface of the toner-carrying member  20 , completely eliminating the possibility of developing those positively charged particles. 
       FIG. 5  shows another preferred embodiment of the present invention. The non-contact, single-component developing system shown in  FIG. 5  includes a latent image-bearing member  110  such as a photosensitive drum or belt, a toner-carrying member  120 , and two ancillary developing members  121  and  122 . Hereinafter, the first ancillary developing member  121  will be referred to as the “pre-developer,” and the second ancillary developing member  122  as the “post-developer.” 
     The developing system also includes a toner container  140 , a metering blade  142 , and two cleaning blades  131  and  132 . The metering blade  142  operates to create a thin layer of toner particles  141  and to charge the toner particles  141  on the toner-carrying member  120  from the toner container  140 . As described in detail below, toner particles that are freed from the toner-carrying member  120  develop on the latent image-bearing member  110 , or land on the pre-developer  121  or the post-developer  122 . The cleaning blades  131  and  132  remove toner particles transferred to the pre-developer  121  and post-developer  122 , respectively, and the removed toner particles are collected into the toner container  140 . 
     The surface moving speeds (i.e., circumferential speeds) of the toner-carrying member  120 , and the post-developer  122  are substantially identical to the surface moving speed of the latent image-bearing member  110 . The surface moving speed of the pre-developer  121 , however, is significantly greater than that of the latent image-bearing member  110 , and is preferably about twice the surface moving speed of the latent image-bearing member  110 . 
     The latent image-bearing member  110 , the pre-developer  121 , the post-developer  122 , and the toner-carrying member  120  are disposed to form a gap between the latent image-bearing member  110  and the toner-carrying member  120 , a gap between the latent image-bearing member  110  and the pre-developer  121 , a gap between the toner-carrying member  120  and the pre-developer  121 , a gap between the latent image-bearing member  110  and the post-developer  122 , and a gap between the toner-carrying member  120  and the post-developer  122 . Furthermore, the pre-developer  121 , the post-developer  122 , and the toner-carrying member  120  are disposed in such a way that an electrostatic latent image recorded on the latent image-bearing member  110  passes the pre-developer  121  first, then passes the toner-carrying member  120 , and lastly passes the post-developer  122 . Preferably, the gap between the latent image-bearing member  110  and the toner-carrying member  120  is approximately 1,600 microns; the gap between the latent image-bearing member  110  and the pre-developer  121  is approximately 100 microns; the gap between the toner-carrying member  120  and the pre-developer  121  is approximately 1,500 microns; the gap between the latent image-bearing member  110  and the post-developer  122  is approximately 100 microns; and the gap between the toner-carrying member  120  and the post-developer  122  is approximately 800 microns. 
     The surface of the latent image-bearing member  110  is initially uniformly charged by a charging mechanism or charging means (not shown) to preferably about −700 V (DC). Subsequently, an electrostatic latent image is formed on the surface of the latent image-bearing member  110  by a latent image forming mechanism or latent image forming means (not shown). After exposure of the latent image-bearing member  110  to light, the potential of the latent image-bearing member  110  is reduced to approximately −50 V (DC). 
     A DC bias voltage is applied between the toner-carrying member  120  and the latent image-bearing member  110 . The toner-carrying member  120  is preferably biased to a potential in the range of approximately −4,500 to −6,000 V (DC). The pre-developer  121  and the post-developer  122  are biased to potentials between the image area potential and the non-image area potential (i.e., between −700 V and −50 V (DC). Preferably, the potential of the pre-developer  121  and the post-developer  122  is approximately −400 V (DC). 
     As the toner-carrying member  120  carries the toner  141  towards the pre-developer  121 , toner particles that are adhered to the toner-carrying member  120  by relatively weak adhesion forces are freed from the toner-carrying member  120  and transferred to the pre-developer  121 . The pre-developer  121  carries the transferred toner particles into the gap between the latent image-bearing member  110  and the pre-developer  121 . The difference between the bias voltage on the pre-developer  121  (−400 V) and the image area potential (−50 V) on the latent image-bearing member  110  may exert a force of sufficient magnitude on the transferred toner particles to overcome the weak adhesion forces and cause the toner particles to jump the gap between the pre-developer  121  and the latent image-bearing member  110 . The difference between the bias voltage on the pre-developer  121  (−400 V) and the non-image area potential (−700 V) exerts a force on the transferred toner particles that directs the transferred toner particles towards the pre-developer  121  and hence causes the transferred toner particles to remain on the pre-developer  121 . Thus, the transferred toner particles develop on image areas on the latent image-bearing member  110 , but not on non-image areas. Alternatively, provided that only a negligible amount of toner development occurs in the gap between the latent image-bearing member  110  and the pre-developer  121 , the pre-developer  121  may rotate in the opposite direction of an indicated arrow in  FIG. 5 , in which case toner particles that are transferred to the pre-developer  121  will not be carried into the gap between the latent image-bearing member  110  and the pre-developer  121  and will be brought back towards to toner container  140  and scraped off into the toner container  140 . 
     Particles that are not transferred to the pre-developer  121  are brought towards the latent image-bearing member  110 . As shown in  FIG. 6 , a toner particle  144  that is exposed to both the pre-developer  121  and the latent image-bearing member  110  experiences electrostatic forces E 11  and E 12  that tend to free the toner particle  144  from the toner-carrying member  120  and an adhesion force A 1  that adheres the toner particle  144  to the toner-carrying member  120 . The electrostatic force E 11  directed towards the latent image-bearing member results from the potential difference between the toner-carrying member  120  and the latent image-bearing member  110 . The electrostatic force E 12  results from the potential difference between the toner-carrying member  120  and the pre-developer  121  and directs the toner particle  144  towards the pre-developer  121 . If the combined electrostatic force (i.e., E 11 +E 12 ) is not sufficient to overcome the adhesion force A 1 , the toner particle  144  remains on the toner-carrying member  120  and subsequently the toner-carrying member  120  carries the toner particle  144  towards the post-developer  122 . On the other hand, If the combined electrostatic force (i.e., E 11 +E 12 ) is sufficient to overcome the adhesion force A 1 , the toner particle  144  is caused to leave the surface of the toner-carrying member  120 , and the strengths of the electrostatic forces E 11  and E 12  determine its destination surface. That is, if the electrostatic force E 11  is greater than E 12 , the particle  144  flies towards the latent image-bearing member  110 , else it flies towards the pre-developer  121 . 
     In addition to the electrostatic forces described above, the high surface moving speed of the pre-developer  121  creates a windage that directs the toner particle  144  towards the latent image-bearing member  110 , facilitating the development of the particle  144  when the electrostatic force E 12  is slightly greater than the electrostatic force E 11  and the surface potential of the latent image-bearing member  110  is approximately −50 V. 
     In essence, the pre-developer  121  develops toner particles that are held by relatively weak adhesion forces (e.g., improperly charged particles and particles with small contact areas), or transfers those particles to the pre-developer  121 . This eliminates the possibility of undesirable development when toner particles are carried into the region between the toner-carrying member  120  and the latent image-bearing member  110 , and hence removes undesirable artifacts such as foggy background (i.e., non-image area development). 
     Particles that are brought near the post-developer  122  are subject to the same developing mechanism described in the previous embodiment shown in  FIG. 1 . When the sum of the electrostatic forces resulting from the potential difference between the toner-carrying member  120  and the latent image-bearing member  110  and from the potential difference between the toner-carrying member  120  and the post-developer  122  is sufficient to overcome the adhesion forces that adhere the toner particles to the toner-carrying member  120 , the toner particles are caused to leave the surface of the toner-carrying member  120 . The toner particles travel towards the latent image-bearing member  110  if the surface potential of the latent image-bearing member  110  is approximately −50 V (i.e., image area). Conversely, the toner particles travel towards the post-developer  122  if the surface potential of the latent image-bearing member  110  is approximately −700 V (i.e., non-image area). Also, the potential difference between the post-developer  122  and the latent image-bearing member  110  creates an additional electrostatic force that directs the toner particles towards the latent image-bearing member  110  if the surface potential of the latent image-bearing member  110  is approximately −50 V, or towards the post-developer  122  if the surface potential of the latent image-bearing member  110  is approximately −700 V. Therefore, the DC bias applied to the post-developer  122  further ensures that toner particles develop on latent image areas only. When non-image areas are brought between the toner-carrying member  120  and the post-developer  122 , the toner particles simply fly towards and arrive at the post-developer  122 . 
     The two ancillary developing members  121  and  122  operate in tandem with the toner-carrying member  120  to develop toner particles on image areas only. The developing system improves print quality and reproducibility by employing a double-pass development scheme. During each pass, the combined electrostatic forces are sufficient to cause toner particles to jump from the toner-carrying member  120  without the use of an AC bias. This enables color development without an accumulator or some other intermediate transfer member. 
       FIG. 7  shows a non-contact, single-component color developing system in accordance with the present invention. The color developing system includes a latent image-bearing member  210  such as a photosensitive drum or belt, four toner-carrying members  220   y ,  220   m ,  220   c , and  220   k , four pre-developers  221   y ,  221   m ,  221   c , and  221   k , and four post-developers  222   y ,  222   m ,  222   c , and  222   k.    
     The first toner-carrying member  220   y  carries yellow toner (not shown) into the first developing region defined by the latent image-bearing member  210 , the first pre-developer  221   y , the first toner-carrying member  220   y , and the first post-developer  222   y ; the second toner-carrying member  220   m  carries magenta toner (not shown) into the second developing region defined by the latent image-bearing member  210 , the second pre-developer  221   m , the second toner-carrying member  220   m , and the second post-developer  222   m ; the third toner-carrying member  220   c  carries cyan toner (not shown) into the third developing region defined by the latent image-bearing member  210 , the third pre-developer  221   c , the third toner-carrying member  220   c , and the third post-developer  222   c ; and the fourth toner-carrying member  220   k  carries black toner (not shown) into the fourth developing region defined by the latent image-bearing member  210 , the fourth pre-developer  221   k , the fourth toner-carrying member  220   k , and the fourth post-developer  222   k.    
     The pre-developer  221   y  and the post-developer  222   y  operate in tandem with the toner-carrying member  220   y  to develop the yellow toner on the latent image-bearing member  210 ; the pre-developer  221   m  and the post-developer  222   m  operate in tandem with the toner-carrying member  220   m  to develop the magenta toner on the latent image-bearing member  210 ; the pre-developer  221   c  and the post-developer  222   c  operate in tandem with the toner-carrying member  220   c  to develop the cyan toner on the latent image-bearing member  210 ; and the pre-developer  221   k  and the post-developer  222   k  operate in tandem with the toner-carrying member  220   k  to develop the black toner on the latent image-bearing member  210 . 
     The four toner-carrying members  220   y ,  220   m ,  220   c , and  220   k , the four pre-developers  221   y ,  221   m ,  221   c , and  221   k , and the four post-developers  222   y ,  222   m ,  222   c , and  222   k  are substantially similarly DC biased as in the previous embodiment shown in  FIG. 5 . 
     The color developing system also includes four charging means  201   y ,  201   m ,  201   c , and  201   k , and four latent image forming mechanisms or latent image forming means  202   y ,  202   m ,  202   c , and  202   k  such as LED arrays. 
     The color developing system operates as follows. The first charging means  201   y  uniformly charges the latent image-bearing member  210  to approximately −700 V (DC). Subsequently, an electrostatic latent image is formed on the surface of the latent image-bearing member  210  by the first latent image forming mechanism  202   y . The potential of the exposed portions of the latent image-bearing member  210  is reduced to approximately −50 V (DC). As the latent image on the latent image-bearing member  210  is brought into the first developing region, the yellow toner particles develop on the image areas as described in the previous embodiment shown in  FIG. 5 . The second charging means  201   m  then uniformly charges the latent image-bearing member  210  again to approximately −700 V (DC) and the second latent image forming mechanism  202   m  shines light onto specific portions of the latent image-bearing member  210  that require the inclusion of the color magenta. The potential of those exposed portions of the latent image-bearing member  210  is reduced to approximately −50 V (DC) if no toner is deposited thereon, and to approximately −200 V (DC) if toner particles are already deposited thereon. Thus, the potential difference between light exposed portions of the latent image-bearing member  210  and the second toner-carrying member  220   m  is reduced by approximately 150 V (DC) when magenta toner particles are to be deposited on top of yellow toner particles. However, the reduction in potential difference is of insignificant magnitude when compared against the high DC bias applied between the second toner-carrying member  220   m  and the latent image-bearing member  210 . In fact, the reduction in potential difference (150 V) corresponds to less than 4% of the DC bias applied between the toner-carrying member  220   m  and the latent image-bearing member  210  and hence the corresponding reduction in electrostatic forces from the reduction in potential difference has little impact on the development of magenta toner particles. After magenta toner particles are developed, the process is repeated for the two remaining colors. 
     Once toner particles land on the latent image-bearing member  210 , or on toner particles already deposited on the latent image-bearing member  210 , electrostatic forces exerted on the toner particles and adhesion forces adhering the toner particles to the latent image-bearing member  210  and to each other cause the toner particles to remain on the latent image-bearing member  210 . This advantageous exploitation of electrostatic forces and adhesion forces enables efficient, reliable accumulation of different color toner particles on the latent image-bearing member  210 , resulting in consistent production of high quality output images. 
     The foregoing description of the various embodiments and principles of the inventions have been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many alternatives, modifications, and variations will be apparent to those skilled in the art. Moreover, although various inventive concepts have been presented, such aspects need not to be utilized in combination, and various combinations of inventive aspects are possible in light of the various embodiments provided above. Accordingly, the above description is intended to embrace all possible alternatives, modifications, combinations and variations that have been discussed or suggest herein, as well as all others that fall within the principles, spirit and broad scope of the invention as defined by the claims.