Developing process for two-colored electrophotography

A developing process for two-colored electrophotography comprising: (1) charging the surface of a photoreceptor having a conductive substrate and two photosensitive layers successively formed on the conductive substrate, said photosensitive layers having different spectral sensitivities, (2) charging the surface of said photoreceptor with a different polarity from the charging polarity in step (1), (3) exposing a two-colored original to form electrostatic latent images, which have different polarities corresponding to the two-colored original, on the surface of said photoreceptor, (4) transferring a first color toner charged with a different polarity from the charging polarity of one of the electrostatic latent images on said electrostatic latent image to develop said electrostatic latent image iwth the first color toner, (5) exposing the surface of said photoreceptor to eliminate electric charges with the same polarity as the first color toner which are induced on the surface of said photoreceptor in the vicinity of said electrostatic latent image developed by the first color toner, and (6) transferring a second color toner charged with a different polarity from the charging polarity of the other electrostatic latent image on said other electrostatic latent image to develop said other electrostatic latent image with the second color toner.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an electrophotographic method for the formation 
of two-colored images. More particularly, it relates to a developing 
process for the formation of distinct images having two colors by 
minimizing the halo-effect. 
2. Description of the Prior Art 
In order to emphasize particular portions in documents, conference 
materials or the like printed by a color ink such as black, corrections, 
comments and underlines are made on the original document material or the 
like by the use of colored pencils (e.g., red pencils) which are 
distinguishable from the basic color (i.e., black) of the original. 
However, once the original documents or materials are copied by a copying 
machine to distribute to subscribers, conference members, etc., such 
corrections, comments and underlines are reproduced in black only, so that 
the corrections, comments and underlines no longer appear to be 
emphasized. The colors to be used for such corrections, comments or 
underlines do not usually have to correspond to the colors in the 
original, but they are required to be distinguishable from the basic color 
of the original. 
For this purpose, various methods for the formation of two-colored images 
have been proposed in, for example, U.S. Pat. No. 4,189,224 and U.S. Pat. 
No. 4,413,899 both of which are patented to Ricoh Co. Ltd., Japan. 
According to these proposed methods, on a photoreceptor comprising a 
conductive substrate and a photoconductive layer formed on the conductive 
substrate, two kinds of electrostatic latent image having different 
polarities from each other which correspond to the two colors in the 
original are simultaneously formed, first. To the electrostatic latent 
images, two kinds of color developer charged with different polarities are 
adhered resulting in two-colored images, which are then subjected to a 
charging treatment to have the same polarity and transferred to a transfer 
paper followed by a fixing treatment. As the photoreceptor mentioned 
above, there have been two kinds of photoreceptor, one of which has a 
single photosensitive layer and an insulating layer on the photosensitive 
layer and the other of which has a photosensitive composite composed of 
two photosensitive layers of different spectral sensitivities. In the case 
where the photoreceptor is composed of a photosensitive composite, the 
electrostatic latent images having different polarities are formed by two 
charging treatments with different polarities and an exposing treatment, 
followed by a developing using a positively charged developer from a first 
developing means and a negatively charged developer from a second 
developing means, resulting in toner images having two colors. 
In the developing step, as shown in FIGS. 5 and 6, the toner 200 from the 
second developing means is attached to the vicinity of the toner image 100 
developed by the toner from the first developing means, resulting in a 
halo-image, namely a halo-effect. This phenomenon is assumed to result 
from electric charges located in the end portion of the electrostatic 
latent image on the photoreceptor. These electric charges which are 
charged with a different polarity from the charging polarity of the said 
electrostatic latent image, are induced from the ground to make toner to 
adhere to the surface of the photoreceptor with the electrostatic force of 
attraction thereof. The haloeffect is unavoidable in the formation of 
two-colored electrophotography as shown in FIG. 5(B), when the toner image 
100 is a red color and the toner 200 located in the vicinity of the toner 
image 100 is a black color the red-image tends to be visually emphasized 
so that the halo-image with the black-colored toner is deemphasized. 
However, as shown in FIG. 5(A), when the toner image 200 is black and the 
toner 100 around the toner image 200 is red, the edge portion of the 
black-toner image 200 appears to be indistinguishable from the other. 
SUMMARY OF THE INVENTION 
The developing process for two-colored electrophotography of this invention 
which overcomes the above-discussed disadvantages and other numerous 
drawbacks and deficiencies of the prior art, comprises: (1) charging the 
surface of a photoreceptor having a conductive substrate and two 
photosensitive layers successively formed on the conductive substrate, 
said photosensitive layers having different spectral sensitivities, (2) 
charging the surface of said photoreceptor with a different polarity from 
the charging polarity in step (1), (3) exposing a two-colored original to 
form electrostatic latent images, which have different polarities 
corresponding to the two-colored original, on the surface of said 
photoreceptor, (4) transferring a first color toner charged with a 
different polarity from the charging polarity of one of the electrostatic 
latent images on said electrostatic latent image to develop said 
electrostatic latent image with the first color toner, (5) exposing the 
surface of said photoreceptor to eliminate electric charges with the same 
polarity as the first color toner which are induced on the surface of said 
photoreceptor in the vicinity of said electrostatic latent image developed 
by the first color toner, and, (6) transferring a second color toner 
charged with a different polarity from the charging polarity of the other 
electrostatic latent image on said other electrostatic latent image to 
develop said other electrostatic latent image with the second color toner. 
When the first color toner is non-photoconductive, the exposure in step (5) 
is carried out using a lamp. 
When the first color toner is photoconductive, the exposure in step (5) is 
carried out using a filter transmitting a light therethrough having a 
wavelength which makes photoconductive both the surface of said 
photoreceptor and the first color toner thereon. 
Thus, the invention described herein makes possible the objects of (1) 
providing a developing process for two-colored electrophotography in which 
the attachment of the colored toner to the vicinity of the color toner 
image is minimized to thereby form distinct images having two colors with 
a suppressed halo-effect; and (2) providing a developing process for 
two-colored electrophotography which consists of simple steps.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As a photoreceptor, any of photoreceptors on which electrostatic latent 
images with positive and negative polarities are formed and maintained by 
a known exposure treatment can be used with this invention, an example of 
which is a photosensitive composite. The photosensitive composite 
comprises photosensitive layers having different spectral sensitivities 
and is made of, for example, inorganic photosensitive substances such as 
amorphous selenium, zinc oxide, etc., or organic photosensitive substances 
such as polyvinylcarbazole, etc., but is not limited thereto. When the 
photosensitive layer is made of amorphous selenium, it is positively 
charged due to the p-electroconductivity of the amorphous selenium. When 
it is made of zinc oxide, it is negatively charged due to the 
n-electroconductivity of zinc oxide. 
As a conductive substrate which supports the photosensitive layers thereon, 
any conductive substrate known to be useful for photoreceptors can be 
used, examples of which are an electroconductive metal such as aluminium, 
a substrate prepared by disposing an electroconductive substance on a 
plastic film base by vacuum evaporation deposition, or the like. 
A dielectric can be used, instead of the photoreceptor, on which 
electrostatic latent images are formed with positive and negative 
polarities by means of pin electrodes or the like. 
Exposure is carried out between the developing treatment of one of the 
electrostatic latent images with the first color toner and the developing 
treatment of the other with the second color toner, thereby minimizing the 
influence of electric charges induced on the surface of the photoreceptor 
in the vicinity of the first color toner image upon the development of the 
said other latent image with the second color toner. This exposure is 
carried out using a lamp when the first color toner is 
non-photoconductive. As the non-photoconductive color toner, any color 
toner known to be useful for two-colored electrophotography can be used, 
an example of which is composed of color toner particles having a diameter 
in the range of from 1 to 50 .mu.m, which are prepared by dispersing 
pigments into a resin binder together with, as desired, a charge-control 
agent and/or a toner-blocking agent. The color toner is used, as a toner 
of a dual-component developer, with a magnetic carrier, but it can be used 
as a mono-component developer when it contains a magnetic powder therein 
such as magnetite, ferrite, etc. The pigments are made of, for example, 
carbon black when they are a black color; and red iron oxide, cadminum 
red, fast red, etc., when they are a red-color. Examples of the resin 
binder are styrene resins, acrylic resins, polyester resins, etc. Examples 
of the charge-regulating agent are an oil-soluble dye such as nigrosine 
base (C.I.5045), oil black (C.I.26150), spirane blue; metal naphthenates; 
metallic fattiate soaps; resinate soaps; and azo dyes containing metals. 
Examples of the toner-blocking agent are silica, alumina, talc, etc. 
Alternatively, when the first color toner is photoconductive, the 
above-mentioned exposure is carried out using a filter transmitting light 
therethrough having a wavelength which makes photoconductive both the 
surface of the photoreceptor and the first color toner thereon. A 
photoconductive pigment used for the photoconductive color toner, is at 
least one selected from an inorganic pigment such as zinc oxide, titanium 
oxide, etc., and an organic pigment such as a phthalocyanine pigment 
(e.g., non-metallic .beta.-phthalocyanine (C.I.74100)), a 
bisbenzoimidazole pigment (e.g., perylene (C.I.71105)); an indigo pigment 
(e.g., thioindigo (C.I.73360); an azo pigment (e.g., diamine blue 
(C.I.21180)); a perylene pigment; a quinacridone pigment, etc. The 
photoconductive pigment in the color toner is in the range of from 10 to 
35% by weight, preferably 12 to 30% by weight. If it is over 35% by 
weight, the mechanical strength of the color toner decreases resulting in 
deterioration of the color toner so that the durability thereof will be 
shortened. If it is less than 10% by weight, the decay rate of electric 
charges on the toner in the exposure step decreases. As the resin binder, 
any resin binder known to be useful for a photoconductive toner can be 
used, examples of which are natural resins such as balsam resins, rosins 
or the like; synthesized resins such as vinyl resins, acrylic resins, 
styrene resins, polyamide resins or the like; or mixture thereof, which 
exhibit an adhesiveness by a heat or pressing treatment. The resins used 
herein are either thermoplastic resins or thermosetting resins. Additives 
are used as desired. The pigment are, for example, photoconductive zinc 
oxide or titanium oxide particles, on the surface of each of which a 
sensitizing coloring matter such as acridine orange, Rhodamine, 
erythrosine, etc., is bound in a single layered or laminating form. Dyes 
are used as desired. 
The second color toner to be used together with the photoconductive color 
toner as the first color toner is not required to be photoconductive, but 
it shoulder be a toner of a mono- or dual-component system. 
When the first color toner is photoconductive, the exposure is carried out 
using a filter transmitting a light therethrough having a wavelength which 
makes photoconductive both the surface of the photoreceptor and the first 
color toner thereon, thereby minimizing the influence of electric charges 
induced on the surface of the photoreceptor in the vicinity of the first 
photoconductive color toner image upon the development of the 
electrostatic latent image with the second color toner. 
EXAMPLE 1 
FIG. 1 shows a developing apparatus for this invention which comprises a 
photoreceptor drum 1, a charging means 2 disposed near the drum 1, a first 
exposing means 3 near the drum 1 down stream of the charging means 2, a 
first developing means 4 near the drum 1 down stream of the first exposing 
means 3, a second developing means 5 near the drum 1 down stream of the 
first developing means 4, and a second exposing means 6 such as a 
discharging lamp near the drum 1 between the first and the second 
developing means 4 and 5. The photoreceptor drum 1 is composed of the 
first and the second photosensitive layer 12 and 13, having different 
spectral sensitivities, which are laminated on a conductive substrate 14. 
The developing apparatus operates as follows: 
As shown in FIG. 2, the photoreceptor 11, which is constituted by the first 
and the second photosensitive layers 12 and 13 and the conductive 
substrate 14, is subjected to a first charging treatment with, for 
example, a positive polarity by the charging means 2 (FIG. 2(a)), followed 
by a second charging treatment with a different polarity (i.e., a negative 
polarity) from the charging polarity in the first charging treatment FIG. 
2(b)) to obtain the first photosensitive layer 12 charged with a positive 
polarity and the second photosensitive layer 13 charged with a negative 
polarity, resulting in a double-electric layer consisting of different 
polarities on the photoreceptor 11. The charging process is, of course, 
carried out depending upon the charging characteristic of each of the 
photosensitive layers 12 and 13. The charging means 2 comprises, for 
example, an AC corona charger generating either positive electric charges 
or negative charges, or a pair of corona chargers generating positive and 
negative electric charges, respectively. 
Then, on the photoreceptor 11 having a double-electric layer, an exposure 
corresponding to two colors of the original 33 is carried out by means of 
a first exposing means 3 (FIG. 2(c)), causing photoconduction in the 
corresponding region of each of the photosensitive layers 12 and 13 
depending upon each of the two color wavelengths. For example, the region 
on the photosensitive layers 12 and 13 corresponding to a white color 
region W (which corresponds to a white background) becomes photoconductive 
so that potential thereon becomes zero, while the region R (which 
corresponds to a red color in the original) on the second photosensitive 
layer 13 which has a sensitivity to the red color wavelength alone becomes 
photoconductive decreasing in the negative electric charges on the second 
photosensitive layer 13. The positive electric charges still remain on the 
first photosensitive layer 12. Since the light does not reach the region B 
in the photoreceptor 11 corresponding to the black region in the original, 
the electric resistance thereof is kept at the same level as that prior to 
the exposure, resulting in electrostatic latent images having different 
polarities on the photoreceptor 11 which correspond to the two colors in 
the original 33. The above-mentioned first exposing means 3 comprises a 
light source 31 and an optical system 32 which makes an optical image 
corresponding to the original 33 on the photosensitive layer 12. 
The electrostatic latent images having the different polarities are then 
subjected to a developing treatment with a color toner by means of a first 
developing means 4 which comprises a developer container 42, the first 
color toner 41 and a carrier contained in the container 42, and a sleeve 
43. The container 42 is made of, for example, a non-magnetic resin such as 
pholyethylene/terephthalate or the like. The sleeve 43 is made of a 
non-magnetic and conductive material with respect to at least the surface 
thereof, at the back of which a magnet or the like is disposed. The first 
color toner 41 and the carrier are mixed by an agitation means within the 
container 42 such that friction therebetween induces an electrostatic 
charge having e.g., a negative polarity on the first toner 41. On the 
sleeve 43, the charged first color toner 41 forms a magnetic brush 44, 
which is in contact with the photoreceptor 11 so that one of the 
electrostatic latent images thereon is developed by the first color toner. 
The development with the first color toner is not limited to a known 
contact development, but is carried out by a known non-contact development 
such as a "jumping" development, a touch-down development, etc. In the 
surrounding area of the electrostatic latent image developed by the first 
color toner, electric charges having a different polarity from the 
charging polarity of the said latent image (i.e., the same polarity as the 
first color toner) are induced. 
In order to minimize the induced charges, then, the surface of the 
photoreceptor 11 is subjected to an exposing treatment by means of the 
second exposing means 6, in such a manner that the induced charges in the 
surrounding area of the above-mentioned latent image disappear or are 
minimized without influence on the charges of the latent image. 
Then, the other latent image remaining on the photoreceptor 11 is developed 
with the second color toner by means of the second developing means 5 
(FIG. 2(d)) which has the same structure and developer as the first 
developing means 4 except the color of the toner. Since the 
above-mentioned induced charges do not exist in the surrounding area of 
the first-color toner image, the second color toner 51 is not bound to the 
surroundings of the first-color toner image so that the halo-effect can be 
significantly suppressed. 
In order that the resulting two-colored images are realized to be clear and 
distinct, it is preferable that the first color toner 41 is black and the 
second color toner 51 is red. The colors of the color toners 41 and 51 are 
not, of course, limited to black and red respectively. But these colors 
show the greatest reduction in the halo-effect. 
The two-colored toner images obtained are then charged with the same 
polarity (e.g., a positive polarity) by a charging means (not shown) 
positioned down stream of the second developing means 5, followed by an 
electrostatical transferring treatment to transfer the toner images onto 
the back of a transfer paper, resulting in a two-colored (e.g., red and 
black) image which corresponds to the colors in the original and in which 
the two colors are easily distinguishable. 
EXAMPLE 2 
FIG. 3 shows another developing apparatus for this invention which has the 
same structure as that in shown in FIG. 1, except that a discharging means 
60 is used instead of the second exposing means 6. The discharging means 
60 comprises a discharge lamp 61 and a filter 62 transmitting therethrough 
a light from the lamp 61 having a wavelength which makes photoconductive 
both the surface of the photoreceptor 11 and the photoconductive color 
toner 41. 
As shown in FIG. 4, the photoreceptor 11 is subjected to the first charging 
treatment with, for example, a positive polarity (FIG. 2(a)), followed by 
the second charging treatment with a different polarity (i.e., a negative 
polarity) from the charging polarity in the first charging treatment (FIG. 
2(b)) in the same manner as in Example 1, resulting in a doubleelectric 
layer consisting of different polarities on the photoreceptor. 
Then, on the photoreceptor 11 having a double-electric layer, an exposure 
corresponding to two colors of the original 33 is carried out by means of 
the exposing means 3 (Example 2(c)) in the same manner as in Example 1, 
causing photoconductive in the corresponding region of each of the 
photosensitive layers 12 and 13 depending upon each of the two color 
wavelengths, and resulting in electrostatic latent images having different 
polarities on the photoreceptor which correspond to the two colors in the 
original. 
The latent images are then subjected to a developing treatment with a color 
toner by the first developing means 4 in the same manner as in Example 1. 
The first photoconductive color toner 41, which is electrostatically 
charged, in advance, within the developer container 42 forms a magnetic 
brush 44, which is in contact with the photoreceptor 11 so that one of the 
latent images thereon is developed by the first photoconductive color 
toner 41. The development is not limited to a known contact development, 
but is carried out by a known non-contact development such as a "jumping" 
development, a touch-down development, etc. 
In order to minimize the charges which are induced in the surrounding area 
of the first color toner image, the surface of the photoreceptor 11 is 
exposed through the discharging means 60, so that the first 
photoconductive toner 41 on the latent image is rendered photoconductive, 
resulting in the migration of the negative electric charges of the 
photoconductive color toner 41 to the positive electric charges of the 
latent image, and the disappearance of both electric charges thereof. As 
the charges in the skirt area of the latent image disappear, the induced 
charges having a different polarity from that of the latent image (i.e., 
the same polarity as the first photoconductive color toner 41) are 
attracted below the first photoconductive color toner 41, and no longer 
exist in the surrounding area thereof. 
Then, the other latent image remaining on the photoreceptor 11 is developed 
with the second color toner 51 by the second developing means 5 (FIG. 
2(d)) in the same manner as in Example 1. Since the above-mentioned 
induced charges no longer exist in the surrounding area of the first 
photoconductive color toner 41, the second color toner 51 does not adhere 
to the skirt area of the toner image so that the halo-effect can be 
suppresed. 
The second color toner 51 is not required to be photoconductive. In order 
that the resulting two-colored toner images are realized to be clear and 
distinct, the colors of the first and the second color toners 41 and 51 
are preferably black and red, respectively, but are not limited thereto. 
The two-colored toner images are then charged with the same polarity (e.g., 
a positive polarity) by a charging means (not shown) positioned downstream 
of the second developing means 5, followed by an electrostatical 
transferring treatment to transfer the toner images on to a transfer 
paper, resulting in a two-colored (e.g., red and black) image which 
corresponds to the two colors in the original and in which the two colors 
are easily distinguishable. 
It is understood that various other modifications will be apparent to and 
can be readily made by those skilled in the art without departing from the 
scope and spirit of this invention. Accordingly, it is not intended that 
the scope of the claims appended hereto be limited to the description as 
set forth herein, but rather that the claims be construed as encompassing 
all the features of patentable novelty which reside in the present 
invention, including all features which would be treated as equivalents 
thereof by those skilled in the art to which this invention pertains.