Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~24~76
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LOW FIELD ELECTROPHOTOGRAPHIC PROCESS
FIELD OF THE INVENTION
This invention relates in general to
electrophotography and in particular to a novel low
field electrophotographic process. More
specifically, this invention relates to a low field
electrophotographic process employing a
photoconductive insulating element which exhibits
high quantum efficiency at low voltage.
BACKGROUND OF THE INVENTION
Photoconductive elements comprise a
conducting support bearing a layer of a photo-
conductivP material which is insulating in the dark
but which becomes conductive upon exposure to
radiation. A common technique for forming images
with such elements is to uniformly electrostatically
charge the surface of the element and ~hen imagewise
expose it to radiation. In areas where the photo-
conductive layer is irradiated, mobile charge
carriers are generated which migrate to the surface
of the element and there dissipate the surface
charge. This leaves behind a charge pattern in
nonirradiated areas, referred to as a latent electro-
static image. This latent electrostatic image can
then be developed, either on the surface on which it
is formed, or on another surface to which it has
been transferred, by application of a liquid or dry
developer composition which contains electroscopic
marking particles. These particles are selectively
attracted to and deposit in the charged areas or are
repelled by the charged areas and selectiveiy
~9$
.
~L249~7~
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deposited in the uncharged areas. The pattern of
marking particles can be fixed to the surface on
which they are deposited or they can be transferred
to another surface and fixed there.
Photoconductive elements can comprise a
single active layer, containing the photoconductive
material, or they can comprise multiple active
layers~ Elements with multiple active layers
(sometimes referred to as multi-active elements)
have at leas't one charge-generating layer and ~t
least one charge-transport layer. The charge-
generating layer responds to radiation by generating
mobile charge carriers and the charge-transport
layer facilitates migration of the charge carriers
to the surface of the element, where they dissipate
the uniform electrostatic charge in light-struck
areas and form ~he latent electrostatic image.
l'he photoreceptor properties that determine
the radiation necessary to form the latent image are
the quantum efficiency, the thickness, the
dielectric constant, and the existence of trapping~
In the simplest case, where trapping can be
neglected, the exposure can be expressed as:
k ~ ~V
Le~ J
where E is the exposure in ergs/cm2, ~ the
relative dielectric constant, L the thickness in cm,
e the electronic charge in esu~ A the wavelength in
nm, ~ the quantum efficiency, k a constant equal to
5.2 X 10 133 and ~V the voltage difference between
the image and background area, Vi - Vb. The
quan~um efficiency, which'cannot exceed unity,
represents the fraction of incident photons that are
absorbed and result in free electron-hole pairs.
, ~.
.
a99~76
-3-
For electrophotographic processes known
heretofore, ~V is typi-cally ~00-500 V. ~ssuming
typical values oE ~ = 3.0, ~ = 500 nm, and ~ =
10- cm, the above equation predicts an exposure
energy of 11.8 to 14.7 ergs/cm . This assumes
that there is no trapping and is based on the
absorbed radiation. In practice, the radiation is
not completely absorbed, and the exposure is
correspondingly larger. Thus, most photoreceptors
require exposures in the range of 20-100 ergs/cm2
to form an electrostatic image. These are
equivalent to ASA ratings between 0.1 and 0.02. In
contrast, the exposure required to form a latent
image in conventional silver halide photography is
15 in the range of 10 2 to 10 1 ergs/cm2, or
less, and, accordingly, the radiation sensitivity of
electrophotography is less than that of conventional
silver halide photography by a factor of at least
While increases in electrophotographic
sensitivity can be realized by increases in
thickness or quantum efficiency, these effects are
limited. Increases in photoreceptor thickness tend
to result in trapping, which gives rise to a sharp
decrease in sensitivity. Since the quantum
efficiency cannot exceed unity, increases in
efficiency are limited. For the example discussed
in the preceeding paragraph, the maximum increase in
sensitivity would be a Eactor of about 5. In
practice, absorption and reflection losses,
photogeneration efficiencies of less than unity,
etc., would limit the increase to probably no more
than a factor of about 3. Consequently, if the
sensitivity is to be significantly increased, the
magnitude of the voltage difference between the
image and background areas must be reduced.
, :.
.
~2~47i~;
--4--
.
Moreover, if the sensitivity is to be increased
without a concurrent increase in electrostatic
noise, the magnitude of Vb must also be reduced,
since a reduction in ~ without a corresponding
reduction in Vb results in a very low signal to
- noise (S/N) ratio.
A reduction in both ~V and Vb requires
that the photoreceptor be initially charged to very
low voltages, e.g., VO = 10 volts. However, with
photoconductive elements of both the
single-active-layer and mult;ple-active layer types,
the quantum efficiency typically decreases sharply
with decreasing voltage. [See D. M. Pai and R. C.
Enc~, Phys. Rev. 11, 5163, (1975); P. J. Melz,
J. Chem. Phys. 57, 1694, (1972); and P. M.
Borsenberger and D. C. Hoesterey, J. Appl. Phys. 51,
4248 (1980)]. As a result, electrophotographic
processes typically employ a high initial voltage,
such as 500 volts, and electrostatic latent image
formation typically requires exposures oE the order
of 20 to 100 ergs/cm2.
It is toward the objective of providing a
high speed electrophotographic process which
exhibits minimal electrical noise, and, in
particular, a low field process employing a very low
initial voltage, such as a voltage of 10 volts, that
the present invention is directed.
SU~IARY OF THE INVENTION
. . . _
The electrophotographic process of this
invention comprises the steps of:
(1) providing a photoconductive insulating
element comprising:
(a) an electrically-conductive
support,
476
--5--
(b) a barrier layer `overlying the
support,
and (c) a photoconductive stratum
overlying the barrier layer which comprises a layer
of intrinsic hydrogenated amorphous silicon in
- electrical contact with a layer of doped
hydrogenated amorphous silicon and in which the
doped layer is very thin in relation to the
thickness of the intrinsic layer;
(2) uniformly electrostatically charging
the element to a surface voltage in the range of
from S to S0 volts,
and (3) lmage-wise exposing the element to
activating radiation to thereby form a latent
electrostatic image on the sur~ace thereof.
The term "activating radiation" as used
herein is defined as electromagnetic radiation which
is capable of generating electron-hole pairs in the
photoconductive insulating element upon exposure
thereof.
Use of a very low initial voltage in the
process of this invention, that is a voltage in the
range of 5 to 50 volts, in combination with use of
an amorphous silicon element of the particular
25 structure described herein has been unexpectedly
found to provide the desired characteristics of very
high electrophotographic sensitivity without
excessive electrical noise. The low Vb and low ~V
which characterize the process are rendered feasible
30 by the unique electrophotographic properties of the
aforesaid element, which provides high quantum
efficiency at low voltage.
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BRIEF l)ESCRIPTIO~ OF-THE D~AWINGS
,
FIGURE 1 is a logarithmic plot of quantum
efficiency versus electric field for a
photoconductive insulating element that is useful in
- 5 the process of this invention and for a control
element.
FIGU~E 2 is a ~-logE plot for the test
element and control element of FIGU~F~ 1.
I)Esc~Ipl~IoN OF THE PREFERRED ~ sODll~ENTS
The preparation of thin films of amorphous
silicon, hereinafter referred to as ~ -Si, by the
glow discharge decomposition of silane gas, SiH4,
has been known for a number of years. (See, for
example, R. C. Chittick, J. H. Alexander and ~. F.
Sterling, J. Electrochem. Soc., 116, 77, lg69 and
R. C. Chittick, J. N-Cryst. Solids, 3, 255, 1970).
It is also known that the degree of conductivity and
conductivity type of these thin films can be varied
by doping with suitable elements in a manner
20 analogous to that observed in crystalline
semiconductors. (See, for example, W. E. Spear and
P. G. LeComber, Solid.State Commun., 17, 1193,
1975). Furthermore, it is widely recognized that
the presence of atomic hydrogen plays a major role
25 in the electrical and optical properties of these
materials (see, for example, M. H. Brodsky, Thin
Solid Films, 50, 57~ 1978) and thus there is
widespread current interest in the properties and
uses of thin films of so-called "hydrogenated
30 amorphous silicon," hereinafter referred to as
O~-Si(H~.
The field oE electrophotography is one in
which there is extensive current interest in the
utilization of thin films of ~-Si(H). To date, the
~;~4~3~7~ii
-7-
art has disclosed a wide variety of photoconductive
insulating elements, comprising thin films of
intrinsic and/or doped oC-Si(H), which are adapted
for use in electrophotographic processes. (As used
5 hereing the term "a doped oc-Si(H) layer" refers to
-- . a layer of hydrogenated amorphous silicon that has
been doped with one or more elements to a degree
sufficient to render it either n-type or p-type-).
Included among the many patents and published patent
applications describing photoconductive insulating
elements containing layers of intrinsic and/or
doped oc-Si(H) are the following:
Misumi et al, U. K. Patent Application No.
2 018 446 A, published October 17, 1979.
15Kempter, U. S. patent 4,225,222, issued
September 30, 1980.
Hirai et al, U. S. patent 4,265,991, issued
May 5, 1981.
Fukuda et al, U.S. patent 4,359,512, issued
20 November 16, 1982.
Shimizu et al, U. S. patent 4,359,514,
issued November 16, 1982.
Ishioka et al, U. S. patent 4,377,628,
issued March 22, 1983.
25Shimizu et al, U. S. patent 4,403,026,
issued September 6, 1983.
Shimizu et al, U. S. patent 4,409,308,
issued October 11, 1983.
Kanbe et al, U. S. patent 4,443,529, issued
30 April 17, 1984.
As hereinabove described, the present
invention makes use of a particular type of
photoconductive insulating element, characterized by
the presence of both doped and intrinsic layers of
o~~Si~H)~ in an electrophotographic process in
which the element is electrostatically charged to a
~Z~76
--8--
low surface voltage, that is a voltage in the range
of from 5 to 50 volts. More specifically, the
photoconductive insulating element utilized in the
electrophotographic process of this invention
5 comprises:
- (a) an electrically-conductive
support, by which is meant a support material which
is itself electrically conductive or which is
comprised of an electrically-insulating material
10 coated with an electrically-conductive layer,
(b) a barrier layer overlying the
support, by which is meant a layer which serves to
prevent the migration of charge-carriers from the
support into the photoconductive layers of the
15 element,
and (c) a photoconductive stratum
overlying the ba~rier layer which comprises a layer
of intrinsic o~-Si(H) in electrical contact ~ith a
layer of doped oC-Si(H) and in which the doped layer
20 is very thin in relation to the thickness of the
intrinsic layer.
It is critical to the invention that the
photoconductive stratum comprise both an intrinsic
oG-Si(H) layer and a doped ~-Si(H) layer, since
25 use of an intrinsic ~-Si(H) layer alone would not
be an effective means of generating the necessary
charge carriers when employing a low surface
voltage; while use of a doped v~-Si(H) layer alone
would result in too high a dark conductivity for the
30 element to be useful in the low field process of
this invention. It is also very important that the
doped layer be very much thinner than the intrinsic
layer9 since, if this were not the case, the dark
conductivity would be excessively high for use in
35 the low field process of this invention.
9 ~Z4~6
It is also critical to the invention that
the element be electrostatically charged to a very
low surface voltage, that is a voltage in the range
of ~rom 5 to 50 volts. Only by the use of such a
low voltage is it possible to achieve very high
electropho~ographic sensitivity -- a sensitivity
which is so high that the element can be reason~bly
characterized as a camera-speed material -- without
the generation of e~cessive electrical noise. It is
this use of very low voltage which specifically
distinguishes the process of this invention from
conventional electrophotographic processes ~hich
utilize much higher voltages.
Photoconductive insulating elements,-
whether of the single-active-layer or
multiple-active-layer types, typically exhibit a
quantum efficiency at low voltage which is much less
than they exhibit at high voltage. However, the
photoconductive insulating elements described herein
exhibit a quantum efficiency at low voltage which is
substantially the same a~ that at high voltage. It
is this characteristic which renders them especially
suitable for use in the novel low field
electrophotographic process of this invention.
The elements employed in the process of this
invention utilize an electrically-conductive support,
and such support can be either an electrically-
conductive material or a composite material
comprised of an electrically-insulating substrate
coated with one or more conductive layers. The
electrically-conductive support should be a
relatîvely rigid material and preferably one that
has a thermal expansion coefficient that is fairly
close to that of a layer of ~c-Si(H). Particularly
35 useful materials include aluminum, steel, and glass
that has been coated with a suitable conductive
7~
- 10 -
coating. Preferably, the support is fabricated in a drum or tube
configuration, since such configurations are most aperopriate for
use with a relatively brittle and fragile material such as
-Si(H).
A particularly important ~eature of the photoconductive
insulating element employed in the process of this invention is
the barrier layer. It serves to prevent the injection of charge
carriers from the substrate into the photoconductive stratum.
Specifically, it prevents the injection of holes from the
substrate when the photoreceptor is charged to a negative
potential, and it prevents the injection of electrons from the
substrate when the photoreceptor is charged to a positive
potential. Either positive or negative charging can, of course,
be used in the process of this invention, as desired. Inclusion
of a barrier layer in the element is necessary in ordee for the
element to provide adequate charge acceptance.
A number o~ materials are known to be useful to form a
barrier layer in an amorphous silicon photo~onductive insulating
alement. For example, useful materials include oxides such as
silicon oxide (SiO) or aluminum oxide (A1203). Preferably,
the barrier layer is a layer of -d -si ~H) which has been heavily
doped with a suitable doping agent. The term "heavily doped", as
used herein, is intended to mean a concentration of doping agent
of at least 100 ppm.
The term "a photoconductive stratum" is used herein to
refer to the combination of an intrinsic ~ -Si(~) layer and a doped
~ -Si(H) layer in electrical contact therewith. Since the
essential requirement is merely that the activating radiation be
incident upon the doped layer, the
`:
Z~4~6
particular order of these layers in the
photoconductive stratum-is not ordinarily critical.
For example, the doped layer can be the outermost
layer and the exposure can be from the front side of
the element, or the order of the doped and intrinsic
- layers can be reversed and the exposure can be from
the rear side.
The layer of intrinsic o~-Si(H) can be
formed by processes which are well known in the
art. Most commonly, the process employed is a gas
phase reaction, known as plasma-induced
dissociation, using a silane (for example SiH4) as
the starting material. The hydrogen content of the
intrinsic G~-Si(H) layer can be varied over a broad
15 range to provide particular characteristics as
desired. Generally, the hydrogen content is in the
range of 1 to 50 percent and preferably in ~he range
of 5 to 25 percent (the content of hydrogen being
defined in atomic percentage~.
The layer of doped ~-Si(H) can be formed
in the same manner as the layer of intrinsic
~G Si(H), except that one or more doping elements
are utilized in the layer forming process in an
amount sufficient to render the layer n-type or
p-type. (Doping elements can also be used in the
formation of the intrinsic layer since a layer of
hydrogenated amorphous silicon, as typically
prepared by the plasma-induced dissociation of
SiH4, is slightly n-type and a slight degree of
p-doping is typically employed to render it
intrinsic.) The hydrogen concentration in the doped
layer can be in the same general range as in the
intrinsic layer.
Many different doping agents are known in
the art to be of utility in advantageously modifying
the chaFacteristics of a layer of o~~Si(H).
~ . .
~ .. . . . . .
~24~76
-12
Included among such doping agents Hre the elements of
Group VA of the Periodic Table, namely N, P, As, Sb
and Bi, which provide sn n-type layer - that is,
one which exhiblts ~ preference for conduction of
5 negative charge carriers (electrons) - Rnd the
elements of Group IIIA of the Periodic T~ble, namely
B, Al, Ga, In and Tl, which provide a p-type
layer - that is one which exhibits a preference for
conduction of positive charge carriers (holes). The
10 preferred doping agent for forming an n-type layer is
phosphorus, and it is conveniently utilized ~n the
plasma-induced dissociation in the form of phosphine
~as (PH3). The preferred doping a~ent for forming a
p-type layer is boron, and it is conveniently utili~ed
15 in the plasma-induced dissociatlon in the form of
diborane gas
(B2H6 ) .
The concentration of doping ~gent employed in
forming the doped d~-Si(H) layer can be varied over a
20 very broad r~nge. Typically, the doping agent is
employed in sn amount of up to about 1,000 ppm in the
gaseous composition used to form the dopPd layer, and
preferably in an amount of about 15 to about 150 ppm.
When a doped o~-Si(H) layer is util~zPd as the
25 barrier layer ~n the element, it is typically a
heavily doped layer, for example, A layer formed from
a composition containing 500 to 5,000 ppm of the
doping agent.
A particularly advantageous process, for use
30 in formins the doped ~ -Sl~H) l~yer that is an
essential component of the photoconductive insulating
element employed in the method of this invention, is
the process described in United States Patent
4,540,647, entitled "Method For The Manufacture Of
35 Photoconductive Insulating Elements With A Broad
-13- ~24~76
Dynamic Exposure Range," by P. M. Borsenberger. As
described in this patent, a ma~or improvement in the
process of forming ~ doped ~-Si(H) lsyer by
plssma-induced dissociatlon of a gaseous mixtu~e of a
5 silane and a doping agent is achieved by controlling
the temperature of the dissociation process ~o ~hat an
initial ms~or portion of the layer of doped
d~-Si(H) is formed at a temperature in the range of
from 200 C to 400 C and a final minor portion of
10 the layer of doped ~-Si(H) is formed at a
temperature in the range of from 125C to 175C.
This improvement in the manufacturing process leads to
the important benefit of a greatly extended dyn~mic
exposure r~nge.
The dynamic exposure range is a very
important factor in electrophotographic processes.
The usual method for evaluating this range is b~sed on
a technique employed in conventionfll photography.
This technique involves the following steps:
(l) The surface potential in volts is
plotted versus the logarithm of the exposing radiation
for a given initial potential, VO~ to thereby
provide a V-logE curve.
(2) The derivative of the curve is then
25 determined ~nd plotted on the same exposure axis. The
derivatiYe is expressed in units of volts/logE and
defined as the contrast,2r.
(3) The dynamic exposure range, in units of
logE, is then defined as the ratio of the initial
30 potential, VO~ to the maximum contrast, armax
Defined in this manner~ the experimental values of the
dynamic exposure rsnge very closely approximAte
476
the range of optical densities that can be
accurately reproduced by the photoreceptor surface
potential.
Photoconductive ins~lating element~ -
5 compris1ng a layer of doped d~-Si(H) exhibit a
r~ther high contrsst ~nd thus a rather nsrrow
dynamic exposure range, typically a range of ~bout
0.7 to about 0.8 logE. While values of this
m~gnitude are usually sufflcient for the
10 reproduction of digital information (line copy, for
example), they are not sufficient for continuous
tone reproduction (pictorial informa~ion, for
exflmple). The invention disclosed in the aforesAid
copending patent application is capable of extendin~
15 the dynamic exposure range to a value of flS high as
1.4 logE, or higher, and thus greatly enhances the
utility of the resulting element.
In a preferred example of the process of
the aforesaid United States Patent 4,540,647, an
-Si(H) layer that is doped with boron is prepared
by incorporating 15 ppm of diborane g~s in the
s~lane gas, and the temper~ture of the deposition
process is controlled so that ~bout eighty percent
of the thickness of the doped d~-Si(H) layer is
25 formed st a temperature of about 250 C and the
remaining twenty percent is formed at a temperature
of about 150C. It ls not known with certainty
why ~uch process provides the benefit of extended
dynamic exposure range. The initial 3tep in
30 pla~ma-induced dissociation reactions is the
transfer of the plasma energy to the gas phase.
Provided the plasma energy is sufficiently hlgh, new
chemical species are formed that are the
intermediate pecies in ~he formation of more ~able
35 compounds. In the dissociation of SlH4, the
intermediate spec~es are believed to be the positive
~Z~76
ion fragments SiH, SiH2 and SiH3. Control of
the temperature in the aforesaid manner may result
in the formation of a "hydrogen profile," that is a
variation in hydrogen concentration across the
thickness of the layer, or it may alter the relative
proportions of intermediate spécies that are formed
and thereby alter the character of the layer that is
deposited.
The thickness of the various layers making
10 up the photoconductive insulating elements employed
in the process of this invention can be varied
widely. The barrier layer will typically have a
thickness in the range of from about 0.01 to about
5 microns, and preferably in the range o~ from about
15 0.05 to about 1 microns. The intrinsic ~--Si(H)
layer will typically have a thickness in the range
of from about 1 to about 50 microns, and pre~erably
in the range of from about 3 to about 30 microns.
The doped ~-Si(H) layer will typically have a
20 thickness in the range of from about 0.01 to about
0.2 microns, and preferably in the range of from
about 0.02 to about 0.1 microns.
The doped o~-Si(H) layer must be
. sufficiently thin to provide the element with a high
25 degree of dark resistivity, generally a dark
resistivity of at least 1011 ohm-cm, and most
typically in the range of 1011 to 1014 ohm-cm.
While the exact ratio of the thickness of the doped
layer to the thickness of the intrinsic layer is not
30 critical, the doped layer is typically very thin in
relation to the thickness of the intrinsic layer.
It is preferred that the ratio of the thickness o~
the doped v~-Si(H) layer to the thickness of the
intrinsic dG-Si(H) layer be less than 0.01 and
35 particularly preferred that it be in the range of
from 0.001 to 0.005.
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-16-
As previously indicated, the preferred
doping agent for forming an n-type layer is
phosphorus, and the preferred doping agent for
forming a p-type layer is boron. These agents are
preferably utilized in the doped layer at a
concentration of about 15 to about 150 ppm.
The amount of doping agent utilized needs
to be carefully controlled to achieve optimum
results. For example, an amount of doping agent
lO which is too low will result in an undesirably low
quantum efficiency, while an amount of doping agent
that is too great will result in an excessively high
dark conductivity.
In addition to the essential layers
15 described hereinabove, the photoconductive
insulating elements employed in the process of this
invention can contain certain optional layers. For
example, they can contain anti-reflection layers to
reduce reflection and thereby increase efficiency.
20 Silicon nitride is a particularly useful material
for forming an anti-reflection layer, and-is
advantageously employed at a thickness of about 0.1
to about 0.5 microns.
In the process of this invention, the
25 photoconductive insulating element is
electrostatically charged to a surface voltage of 5
to 50 volts, and most preferably of 10 to 20 volts.
Charging to this low voltage provides the basis for
a very high speed electrophotographic process. The
30 process is also advantageous in that the element has
an extremely fast response time, exhibits
sensitometry which is essentially temperature
independent, and can be readily adapted to provide
panchromatic sensitivity through appropriate control
35 of the hydrogen content.
~Z~L9~76
- 17 -
The invention is further illustrated by the following
e~ample of its practice.
A photoconductive insulating element was prepared with
the following layers arranged in the indicated order:
(1) a glass substrate,
(Z) a vacuum-deposited layer of aluminum,
(3) a barrier layer consisting of a 0.15 micron thick
D layer of SiO,
~) a 10 micron thick layer of intrinsic ~ -Si(H),
and (5) a 0.03 miceon thick layer of ~-Si(H) which had been
doped with phosphoeous by incorporating phosphine gas at a
concentration of 100 ppm in the silane composition used to form
the layer.
Using a positive surface potential and exposure to
activating radiation at a wavelength of 400 nm, the quantum
efficiency was determined in relation to the magnitude of the
surface potential. (The quantum efficiency is defined as the
O ratio of the decrease in the surface charge density to ~he
absorbed photon flux, assuming the charge density is related to
the surface voltage by the geometrical capacitance.) The results
are shown in Figure 1, which also provides the results for an
other~ise identical control element which did not have the dnped
-Si(~) layer. In the figure, which is a loga~ithmic plot of
quantum efficiency (~) versus electric field, the results for the
test element of the invention are shown by open circles, while
those for the control element are shown by solid circles. As
shown in Figure 1, the quantum efficiency of the control element
O decreased substantially with decreasing surface voltage, while the
quantum efficiency of the test element was substantially
.~,.
~L2~9~7~
.
-18-
- independent of surface voltage over a wide range of
voltages. With both the control and test elements,
the quantum efficiency at high voltage was unity.
As demonstrated by Figure 1, the thin layer of
doped ~C-Si(H) is a critical component of the
photoconductive insulating elements which are useful
-~ in the method of this invention, as this layer
strongly reduces the field dependence of the
photogeneration efficiency and thereby gives rise to
the high sensitivity that is observed at low fields.
The exposure dependence of the surface
voltage for the control and test elements described
abov~, with an initial potential of 10 volts, is
shown in Figure 2. In obtaining these data, the
exposure wavelength was 400 nm, the exposure
duration was 160 microseconds, and the voltage was
sampled 0.5 seconds after the cessation of
exposure. As shown by Figure 2, the control element
exhibited discharge from VO to Vo/2 with an
exposure of 0.29 ergs/cm , corresponding to an ASA
rating of about 12, while the test element required
only 0.11 ergs/cm2, corresponding to an ASA rating
of about 30.
The invention has been described in detail
with particular reference to preferred embodiments
thereof, but it will be understood that variations
and modifications can be effected within the spirit
and scope of the invention.
....
~ . ~' . ~ r ~ _ = _ ~ ~, , _ , _ ~ _ _ _ , , __ ,
