1. FIELD OF THE INVENTION
This invention relates to electrophotography and more particularly to a method for controlling the intensity of exposure of an electrophotographic continuous tone film to accurately reproduce a desired optical density range.
2. DESCRIPTION OF THE PRIOR ART
Electrophotographic image reproduction systems have been in existence for a number of years. In general, such systems operate as follows. An imaging element conprising a photoconductive layer that upon exposure to actinic radiation becomes conductive allowing an accumulated charge on the element surface to selectively bleed through a conductive path is first charged with a uniform charge layer by passing such element under a source of ionizing radiation, e.g., a scorotron or other such corona charging device. The charged surface is then exposed to imagewise modulated actinic radiation, rendering the photoconductor layer conductive and discharging the accumulated charge. The term "actinic radiation" is construed to encompass not only photochemical activity but also the photoelectric effects described herein and the like.
In a continuous tone system, as contemplated herein, the amount of charge left on the imaging element surface is inversely proportional to the amount of actinic radiation received by the element. In this manner a pattern of electrostatic charges is produced on the imaging element forming a latent image corresponding to the imagewise modulated actinic radiation incident on the element. The magnitude of the electrostatic charge at any one point on the imaging element is inversely proportional to the intensity of the exposing actinic radiation.
The latent image may now be rendered visible by development using colored particles which preferably bear a static charge and which are attracted to the charge pattern on the imaging element. Depending on the desired result, the colored particles may bear a charge of the same polarity as the charge originally placed on the imaging element or an opposite polarity. If the charge polarities are the same and an appropriate bias electrode used the colored particles are preferentially attracted to the areas from which the original charge has been bled away, producing a "dark" or "colored" area of intensity proportional to the original exposure. If the charge polarities are opposite, then the areas that received the least exposure to actinic radiation will attract the most particles. In the first instance there is an image reversal; the light tones appear dark and the dark tones appear light. In the second instance the image tones are reproduced the same as the original.
The colored particles may be in dry form or may be supplied in a dispersion in a carrier liquid. Generally referred to as toners, the colored particles or dispersions are well known in the art. Liquid toners tend to produce higher image resolution and are sometimes preferred for that advantage.
Following toning, the image may be viewed as such, dried, fused or transferred onto a receiving element or any combination of the above, as is well known in the art.
In recent years the widespread use of computers and their ability to store and manipulate large amounts of data has resulted in image handling systems that employ image enhancement in applications such as radiography, printing, etc. In radiography, for instance, a radiogram may be split into a number of digitally encoded picture elements, or "pixels", transmitted through telephone lines, stored on a disk, retrieved at will, contrast enhanced, and displayed for diagnostic purposes. Typical display media are cathode ray tubes, silver halide film, electrostatic display, etc.
At present the display of high resolution diagnostic quality images is inadequate. Cathode ray tube displays have limited resolution and dynamic range. Reproduction on a silver halide film, while offering excellent resolution and dynamic range, is expensive, usually time consuming and requires darkroom facilities. Electrophotography is very promising since it reproduces high resolution images of sufficient dynamic range rapidly without the need for dark room development and complicated chemical processes. However, in order to obtain the required diagnostic quality in the finished product the exposure intensity level must be controlled to compensate for the electrostatic charge-retaining characteristic response of an electrophotographic film and for the toner electrostatic response. To complicate matters neither the response of the film nor of the toner is linear, and both tend to vary with time, usage and/or environmental conditions.
Accordingly, in view of the foregoing, it is believed advantageous to provide a system for the accurate reproduction of the tonal range in a continuous tone image.