U.S. patent number 3,869,205 [Application Number 05/392,581] was granted by the patent office on 1975-03-04 for illumination source for xerographic exposure.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to Charles F. Gallo.
United States Patent |
3,869,205 |
Gallo |
March 4, 1975 |
**Please see images for:
( Certificate of Correction ) ** |
ILLUMINATION SOURCE FOR XEROGRAPHIC EXPOSURE
Abstract
A zerographic system and process in which illumination is by
means of a light source emitting yellow light, preferably the
narrow emission bands of the low pressure sodium vapor lamp.
Inventors: |
Gallo; Charles F. (Penfield,
NY) |
Assignee: |
Xerox Corporation (Stamford,
CT)
|
Family
ID: |
23551181 |
Appl.
No.: |
05/392,581 |
Filed: |
August 29, 1973 |
Current U.S.
Class: |
355/67 |
Current CPC
Class: |
G03G
15/04036 (20130101) |
Current International
Class: |
G03G
15/04 (20060101); G03b 027/54 () |
Field of
Search: |
;355/3,4,17,67,71,83,88,37,70,113,114,115,116,117,118,119,120,121 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Horan; John M.
Attorney, Agent or Firm: Bird; Robert J.
Claims
What is claimed is:
1. A xerographic system including an object surface, projection
lens, an image surface in conjugate relationship to said object
surface, and illumination means to irradiate objects disposed on
said object surface for optimum object contrast,
said illumination means being a narrow emission band light source
emitting light in the yellow portion of the visible spectrum as the
result of a sodium vapor discharge, and
a yellow-sensitive photoreceptor at said image surface.
2. A xerographic system including an object surface, a projection
lens, a yellow-sensitive photoreceptor surface in conjugate
relationship to said object surface, and illumination means to
irradiate varicolored objects at said object surface to obtain
light-dark object contrast without color fidelity,
said illumination means being a narrow emission band light source
emitting light in the yellow portion of the visible spectrum as the
result of a sodium vapor discharge.
3. A xerographic system as defined in claim 2 in which said
photoreceptor surface includes a photoconductive material of
arsenic triselenide.
Description
BACKGROUND OF THE INVENTION
This invention relates to illumination in a xerographic system and
process, particularly illumination in a reflective imaging
process.
In the prior art, photocopy objects are typically illuminated by
light sources such as fluorescent lamps which emit either discrete
light of various wavelengths across the visible spectrum or white
light. Such illumination is, of course, perfectly satisfactory to
provide object contrast and the photoreceptors in general use are
sensitive to some or all of this radiation. Fluorescent sources,
however, have three inherent deficiencies. First, the efficiency of
the phosphor is usually on the order of 50 percent. Second, the
efficiency of the lamp degrades with increasing power, eventually
to the point of yielding less light at high powers than at lower
powers. Third, the efficiency of the phosphor degrades with time
and with heat, particularly at high powers. It should be noted here
that it is high powers that are required for most xerographic
applications.
The majority of documents are black-on-white, or, if in color, are
red-on-white, or blue-on-white. The reason is that these color
combinations have a high contrast in the spectral region where the
human eye is sensitive. That is, the spectral response of the human
eye to blue-on-white and red-on-white documents is high. The human
eye thus resembles a yellow sensitive photoconductor because it is
also most sensitive in the yellow. Conversely, yellow-on-white
documents are rare because they yield a low spectral response in
the human eye. Since most objects to be copied are purposely high
contrast objects to the human eye, the most useful photoconductor
is one whose spectral sensitivity resembles that of the human eye.
This is because most paper and ink combinations are designed for
maximum response in the human eye.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a light source for
xerography which provides suitable document contrast and emits
light in a spectral region corresponding to a desirable
"yellow"-sensitive photoreceptor, while being substantially more
efficient than fluorescent sources.
Briefly, this invention is practiced by the use of low pressure
sodium vapor light sources, directed onto a xerographic object.
For a better understanding of this invention, reference is made to
the following, more detailed description.
DESCRIPTION
A typical, xerographic photocopier includes an object surface or
platen, a projection lens, and an image plane or photoreceptor.
Light sources are provided to illuminate the object or document
which is imaged on the photoreceptor by the projection lens. This
is all well known and is given by way of background.
The spectral reflectivity of the object (i.e., paper and
characters) and the spectral sensitivity of the photoreceptor are
both important factors in creating and recording contrast.
Reflectivity of black ink is very low over a wide spectral range
and the reflectivity of white paper is high throughout the visible
range. Thus, there is high contrast in a black-white document
illuminated by any visible light. Colors other than black which are
commonly used in business documents are blue and red. The
reflectivities of these colors vary throughout the visible
spectrum, but both have relatively low reflectivity in the yellow
portion of the spectrum. Therefore, a document bearing blue or red
inks will be clearly visible (i.e., have good contrast) in yellow
light. This is a practical result because of the fact that blue and
red are the most common colors used in documentation, after black.
It should be pointed out here that there will not be color
fidelity; indeed, the black, blue, and red characters will all
appear black in yellow light. The purpose here, however, is to make
black-on-white copies of red-on-white, blue-on-white, and
black-on-white originals, and with yellow illumination there will
be contrast on all such originals to enable copying.
It has been found that the yellow emission of low pressure sodium
lamps produces good contrast of black, blue, and red characters on
white background. Conversely, a yellow ink on white background
provides little or no contrast and therefore is not copyable in
yellow light. Similarly, a green ink on white background provides
little contrast and will copy poorly when illuminated by sodium
emission. However, when limited to the black, blue, and red, the
most common document colors, the sodium lamp illumination provides
excellent contrast for black and white copying.
It has also been found that the more common photoconductive
materials such as, for example, arsenic triselenide and
modifications thereof, cadmium sulfoselenide, zinc oxide, polyvinyl
carbazole trinitro fluorenone, and tellurium, are yellow-sensitive
and therefore respond as well to the sodium vapor discharge as to
ordinary white light.
Finally, the low pressure sodium vapor lamp is advantageous over
prior art fluorescent sources, and is presently preferred as the
light source in the system described above, because being a vapor
discharge light source which directly emits the atomic spectrum, it
is inherently about twice as efficient as a fluorescent source
which must contend with the inefficiency of the phosphor (about 50
percent). The fluorescent lamps are also limited to undesirably low
powers.
The illumination by low pressure sodium lamps in a xerographic
photoreceptor as described provides high contrast for black and
white copying of the vast majority of common business documents, is
well matched to spectral sensitivity of many important
photoreceptors, and is much more efficient illumination than
fluorescent or incandescent sources can provide.
The foregoing description of this invention is given by way of
illustration and not of limitation. The concept and scope of the
invention are limited only by the following claims and equivalents
thereof which may occur to others skilled in the art.
* * * * *