Exposure control in electrostatic photocopying processes

Claim 2

In an electrostatic photocopying process, controlling the exposure of a charged film of photoconductive material by turning on a light source to initiate exposure of said film with a light image, to thereby produce a latent electrostatic charge image on said film, detecting the average charge of said film from at least the time exposure is initiated; detecting the rate of change of said average charge and terminating exposure of said film to said light image when a rate of change of said average charge is detected from one rate to a lower rate.

Claim 3

In an electrostatic photocopying process of the type using a photoconductive film charged uniformly to a high potential to produce a latent electrostatic charge image by exposing said uniformly charged film to a light image, the improvement of controlling the time of exposure of said film to said light image for optimum contrast exposure by turning on a light source to initiate exposure of said film with said light image; detecting the average charge of said film and producing a first electrical signal proportional to the average charge detected, passing said electrical signal through two differentiating circuits in cascade to produce a signal that is the second derivative of said first electrical signal, thereby producing a pulse at the time the change of said average charge changes from a first rate characteristic of an under-exposed condition to a second rate characteristic of an over-exposed condition, and turning off said light source in response to said pulse, thereby terminating exposure of said film to said light image when said film has been exposed for optimum contrast.

Claim 4

In an electrostatic photocopying process of the type using a photoconductive film charged uniformly to a high potential to produce a latent electrostatic charge image by exposing said uniformly charged film to a light image, the improvement of controlling the time of exposure of said film to said light image for optimum contrast exposure by turning on a light source to initiate exposure of said film with said light image; detecting the average charge of said film and producing a first electrical signal proportional to the average charge detected, passing said electrical signal through two differentiating circuits in cascade to produce a signal that is the second derivative of said first electrical signal, thereby producing a pulse at the time the change of said average charge changes from a first rate characteristic of an under-exposed condition to a second rate characteristic of an over-exposed condition, and turning off said light source in response to said pulse, thereby terminating exposure of said film to said light image when said film has been exposed for optimum contrast.

Claim 5

In electrostatic photocopying apparatus having means for exposing a uniformly charged film of photoconductive material to a light image, thereby to discharge illuminated areas on the surface of said film and leave a latent electrostatic charge image for development, the improvement comprising means for continually sensing the average surface charge of said film from at least the time said exposing means is turned on, means for producing a signal proportional to the average surface charge sensed as exposure continues, means for differentiating said average surface charge signal to obtain a first derivative signal proportional to the rate of change of said surface charge signal, and therefore proportiOnal to the rate of change of said surface charge, means for differentiating said first derivative signal to obtain a second derivative signal proportional to the rate of change of said first derivative signal, thereby producing a pulse in said second derivative signal when the rate of change of said average surface charge signal changes from one rate during initial exposure to a second rate characteristic of an over-exposure condition of said film surface, and means responsive to said pulse for turning off said exposing means.

Claim 6

In electrostatic photocopying apparatus having means for exposing a uniformly charged film of photoconductive material to a light image, thereby to discharge illuminated areas on the surface of said film and leave a latent electrostatic charge image for development, the improvement comprising means for continually sensing the average surface charge of said film from at least the time said exposing means is turned on, means for producing a signal proportional to the average surface charge sensed as exposure continues, means for differentiating said average surface charge signal to obtain a first derivative signal proportional to the rate of change of said surface charge signal, and therefore proportiOnal to the rate of change of said surface charge, means for differentiating said first derivative signal to obtain a second derivative signal proportional to the rate of change of said first derivative signal, thereby producing a pulse in said second derivative signal when the rate of change of said average surface charge signal changes from one rate during initial exposure to a second rate characteristic of an over-exposure condition of said film surface, and means responsive to said pulse for turning off said exposing means.

Claim 7

In electrostatic photocopying apparatus of the type in which a photoconductive film is first uniformly charged to a high potential and then exposed to a light image to produce on said film a latent image that may be developed by comminuted, triboelectric material, the combination comprising: a controlled light source, means for turning on said light source in response to an electrical control signal of predetermined value, means for initiating said electrical control signal, means responsive to light from said source for producing said light image, means for directing said light image onto said photoconductive means, means for producing a first electrical signal proportional to the average charge of said photoconductive film during exposure to said light image, means responsive to said first electrical signal proportional to the average charge of said photoconductive film for producing a second electrical signal proportional to the rate of change of said first signal, means responsive to said second electrical signal proportional to the rate of change of said first signal for producing a third electrical signal proportional to the rate of change of said second signal, thereby producing a pulse when said first signal changes from a first rate of change during initial exposure to a second rate of change characteristic of an over-exposed condition of said film, and means for terminating said electrical control signal in response to said pulse, thereby turning off said light source when said charged conductive film has been optimumly exposed to said light image.

Claim 8

In electrostatic photocopying apparatus of the type in which a photoconductive film is first uniformly charged to a high potential and then exposed to a light image to produce on said film a latent image that may be developed by comminuted, triboelectric material, the combination comprising: a controlled light source, means for turning on said light source in response to an electrical control signal of predetermined value, means for initiating said electrical control signal, means responsive to light from said source for producing said light image, means for directing said light image onto said photoconductive means, means for producing a first electrical signal proportional to the average charge of said photoconductive film during exposure to said light image, means responsive to said first electrical signal proportional to the average charge of said photoconductive film for producing a second electrical signal proportional to the rate of change of said first signal, means responsive to said second electrical signal proportional to the rate of change of said first signal for producing a third electrical signal proportional to the rate of change of said second signal, thereby producing a pulse when said first signal changes from a first rate of change during initial exposure to a second rate of change characteristic of an over-exposed condition of said film, and means for terminating said electrical control signal in response to said pulse, thereby turning off said light source when said charged conductive film has been optimumly exposed to said light image.

Claim 9

Apparatus as defined in claim 4 wherein said means for producing said first electrical signal proportional to the average charge of said photoconductive film comprises a transparent conductive sheet over said photoconductive film, a conductive plate under said photoconductive film and a high input impedance connected to said transparent sheet and said conductive plate to amplify the potential between said sheet and said plate.