In general, two types of electrophotographic toning systems exist, which may be described and termed, in an electrical sense, as "floating" and "hard-wired" or "biased" toning systems, each having its particular advantages, and each having been used in known commercial systems and machines.
Technically, a hard-wired system offers the most control over the photographic process since it satisfies the requirements of electron movement within the image copying system. In practice, however, commercial systems are often based upon some form of a "floating" system since floating systems tend to produce quality images from several different types of image mediums, each of which have distinct critical parameters as to the electrophotographic processing or copying of the same.
Floating systems may be classified into three main categories, which are as follows:
(1) No electrode-edge effect; PA0 (2) Floating conducting layers; and PA0 (3) Floating electrode.
To the knowledge of the present inventor, the floating electrode type system was first used within a specific electrophotographic copying machine in order to reduce the "dark-banding" effect when a charging and toning operation was accomplished simultaneously, and when paper exhibiting poor surface conductivity was being employed, such system later being refined in its use in other electrophotographic machines.
In most systems known to the inventor, "floating" systems were used to correct a variety of problems of image quality copying upon various image media while still providing the users of such systems the additional benefits of good "fill-in" and automatic exposure control capability. One problem with a "floating" system used within a known system is its inability to correctly render images from an image medium having a high percentage of black areas, such as, for example, a photograph, which appear as being "washed out".
Such "floating" systems are usually referred to in the field of the invention as systems having floating sensitometry, which systems have difficulty in reproducing images having characteristically large dense areas, such as, for example, the above-mentioned photographs, and some types of forms, or the like. This difficulty arises from the fact that "floating" sensitometry systems always tend to deposit the same amount of toner upon the copy medium for the entire copy, and since such large dense areas require more toner than the remaining areas of the copy, this characteristic of the "floating" systems tends to reproduce such dense areas in a weak or "washed-out" manner since the toner is averaged or equally distributed over a large area. Such systems have also been described as having a self-adjusting sensitometry, and are exemplified by the inventor's own U.S. Pat. No. 3,964,436.
As stated above, the present invention provides a significant improvement over the above-disclosed types of systems, and provides the user with the benefit of self-compensating or self-adjusting sensitometry. The present invention also provides better control of large density percentage images, and a simple and practical method and means for achieving the satisfactory inherent results of a "hard-wired" system.
In order to fully understand the above-described significant improvement of the present invention, it is necessary to understand how toning takes place in the above-identified types of electrophotographic toning systems.
With respect to "floating" systems, when toner is deposited upon an electrophotographically charged pattern image, an equal and opposite polarity charge is deposited upon, or supplied to, the development electrode. This opposite charge causes "floating" systems to have self-control or be self-adjusting. For example, within a known unit having an external capacitance which is very small, the capacitance dictates a 10% criteria for the size of the electrode, and as toning occurs, the deposition of the aforenoted opposite polarity charge causes a charging of this small capacitance so that after a small toner deposition, and thus a small charge deposition, the voltage upon such electrode reaches that of the maximum charged area upon the image, whereupon toning ceases since there is no longer any field to cause particle migration.
Accordingly, if large areas are being toned, then this occurs before a high-density is reached, and thus, a light image results therefrom. This is further complicated by the fact that the low-charge areas are also part of such external capacitance and further tend to modify the amount of toning which occurs.
On the other hand, within "hard-wired" systems, the voltage level is maintained, and toning thus continues, so as to produce high densities regardless of the size of the dense area being toned. However, in systems and applications where for best results, a variable voltage level is desired, the "hard-wired" system produces variable and undesirable results. Thus, the present inventor has found and duly noted that the field of electrophotographic copying machines and systems has a need for a means and a method for an electrophotographic toning system which yields the benefits of both "floating" and "hard-wired" toning systems. The presently disclosed invention satisfies this and other needs within the field of electrophotographic copying machines and systems thereof.
The closest prior art known to the inventor is that disclosed within U.S. Pat. No. 4,045,217, assigned to RICOH COMPANY, LTD., wherein it is noted that it has previously been proposed to use a constant voltage diode connected between a developing electrode and the ground, and which has a characteristic breakdown voltage slightly exceeding the maximum value of the residual potential within the background portion of the image. In distinct contrast thereto, the present invention, as disclosed hereinbelow, uses a back or reverse-biased diode means which is connected between a developing electrode means and a high voltage D.C. power supply means which is employed as the bias voltage supply source for such reverse-biased diode means. Furthermore, although the present invention does use such reverse-biased diode means as a constant voltage diode, the bias voltage level being easily variable by varying the voltage level of the bias voltage supply source, contrary to the abovementioned disclosure of U.S. Pat. No. 4,045,217, the diode means of the present invention is a non-zener diode which is not directly connected between the developing electrode and ground, and which allows the usage of voltages beyond those levels normally capable of being accommodated by commercially available zener diodes. In this regard, the types of films useable with the presently disclosed invention require voltages on the approximate order of 800 volts. Even further, the diode means of the instant invention does not have a characteristic breakdown voltage slightly exceeding the maximum value of the residual potential within the background portion of the image.