Electrophotographic apparatus

An electrophotographic apparatus switchable between a mode for forming a positive image from a negative original and a mode for forming a positive image from a positive original, comprising:

• means or forming an electrostatic latent image on an image bearing member;
• first development means for developing said latent image with toners charged in the same polarity as said image bearing member is charged;
• second development means for developing said latent image with toners charged in the opposite polarity to said charged polarity of said image bearing member;
• developing bias voltage generating means for applying a bias voltage of a determined polarity to said first and second development means;
• operating means for setting a density of the image on said image bearing member;
• first control means for controlling said developing bias voltage generating means such that said bias voltage decreases when the image density is set to become lower by said operating means;
• second control means for controlling said developing bias voltage generating means such that said bias voltage increases when the image density is set to become lower by said operating means;
• and
• selection means for selecting one of said first and second control means in response to said switching of mode to change said bias voltage applied to said development means by the selected control means.

An image forming apparatus in which an original image is exposed onto a photosensitive member to form an electrostatic latent image thereon, and said lateent image is developed to be transferred onto a recording medium, comprising:

• development means for developing said latent image with toners selected from those charged in the same polarity as said photosensitive member is charged and those charged in the opposite polarity to the charged polarity of the photosensitive member;
• developing bias voltage source means for applying a bias voltage of a determined polarity to said development means;
• indication means for indicating a density of image to be formed on the recording medium;
• first control means for controlling said developing bias voltage source such that said bias voltage decreases when the image density is indicated to become lower by said indication means;
• second control means for controlling said developing vias voltage source such that said bias voltage increases when the image density is indicated to become lower by said indication means;
• and
• selection means for selecting one of said first and second control means in accordance with the polarity of toners for developing the latent image on the photosensitive member to charge the bias voltage applied to said development means by the selected control means.

An image forming apparatus comprising:

• means for forming an electrostatic latent image with a first polarity on an image carrier;
• first developing means for depositing a first developer charged with a second polarity, different from the first polarity, onto the electrostatic latent image for performing direct developing;
• second developing means for depositing a second developer charged with the first polarity onto the electrostatic latent image for performing reverse developing;
• mode signal generating means for selectively generating either of a first mode signal for forming a positive image from a positive original and a second mode signal for forming a positive image from a negative original;
• first bias means for applying a first bias voltage to said first developing means for performing direct developing by said first developing means;
• second bias means for applying a second bias voltage of the same polarity as that of the first bias voltage to said second developing means for performing reverse developing by said second developing means;
• means for selectively actuating either of said first and second bias means in accordance with the mode signals generated by said mode signal generating means;
• setting means for setting a density of the image;
• bias control means for controlling bias voltages of said first and second bias means in accordance with the density set by said setting means;
• and
• transfer means for transferring onto a sheet the developed image which is formed on said image carrier by one of said first and second developing means, said transfer means providing a charge in the first polarity or in the second polarity in accordance with the mode signals.

An apparatus for reading and printing microfilm images comprising:

• means for exposing an original image of the microfilm onto a photoreceptor having a charged surface with a first polarity to form an electrostatic image on the charged surface corresponding to the original image;
• first developing means for depositing a first developer charged with a second polarity, different from the first polarity, onto the electrostatic latent image for performing direct developing;
• second developing means for depositing a second developer charged with the first polarity onto the electrostatic latent image for performing reverse developing;
• mode signal generating means for selectively generating either of a first mode signal for forming a positive image from a positive microfilm and a second mode signal for forming a positive image from a negative microfilm;
• first bias means for applying a first bias voltage to said first developing means for performing direct developing by said first developing means;
• second bias means for applying a second bias voltage of the same polarity as that of said first bias voltage to said second developing means for performing reverse developing by said second developing means;
• means for selectively actuating either of said first and second bias means in accordance with the mode signals generated by said mode signal generating means;
• setting means for setting a density of the image;
• bias control means for controlling bias voltages of said first and second bias means in accordance with the density set by said setting means;
• and
• transfer means for transferring onto a sheet the developed image which is formed on said photoreceptor by one of said first and second developing means, said transfer means providing a charge in the first polarity or in the second polarity in accordance with the mode signals.