Improvement in Screens for Photo-mechanical Processes
Year: 1895

page_text: Date of Application, 11th Nov., 1895-Accepted, 21st Dec., 1895
Improvement in Screens for Photo-mechanical Processes
I, EDOUARD GASTON DANIEL DEVILLE, of the City of Ottawa, in the
Province of Ontario, in the Dominion of Canada, Surveyor General, do horeby
declare the nature of this invention and in what manner the same is to be
performed, to be particularly described and ascertained in and by the following
statement:
The object of my invention is to provide a new kind of screen for use in those¨
photo-mechanical processes where a screen is placed in front of a photographic plate
for the purpose of changing the continuous toues of an original into tones formed
of white and black dots.
My invention consists in a screen furnished with alternate opaque and transparent
squares, disposed like the squares of a chess board.
Reference is to be had to the accompanying drawings forming a part of this
Specification, in which similar characters of reference indicate corresponding parts
in all the figures.
Figure 1, is a face view of my improved screen, enlarged...
Figure 2, is a perspective view of a camera showing the manner of making the
Screen; and
Figure 3, is a face view of a modified form of my improved screen, enlarged.
The chess board screen produces twice as many dots on the print as the cross
lined screen from which it is made; a coarser screen must therefore be used' to
produce the same effect. For instance, a chess board screen made from plates"
ruled 90 lines to the inch gives the same fineness of grain as a cross lined screen
ruled 130 lines to the inch. The black and white squares forming the middle tone
of the print are parallel to the squares of the screen; they are not turned
around 45° as with the cross lined screen.
I make this screen from a cross lined screen ruled so that the thickness of the
dark and transparent lines is as nearly equal as possible, the transparent squares
covering one quarter of the surface of the screen. I take a long extension camera
and place the cross lined screen in A. B, Figure 2, in front of and close to a
photographic sensitive plate CD. Instead of a photographic lens, I use a metal
plate with two very minute holes E and F. On one of these holes, F for instance,
I form, by means of a condenser N, the image of a powerful source of light, like
the sun or an electric are light. The condenser must be perfectly clean and free
from defects and scratches. It may be a photographic lens of large aperture.
The screen projects a shadow over the plate, the only parts illuminated being
squares similar to those of the screen, shown in full lines on the figure.
If the plate were now developed, it would show opaque squares corresponding to
the transparent squares of the soreen, with transparent lines between them. But
before doing so, I bring the image of the source of light, by moving the condenser,
on the hole E, and give a second exposure equal to the first one.
The screen now casts another shadow on the plate which will cause another_set
of opaque squares after development. If a supposeil line joining E and F is
parallel to one of the diagonals of the squares, and if the distance of the two holes
is properly adjusted, the transparent square G H of the screen, which, with the
first exposure, produced the opaque square L K on the plate, now produces another
opaque square in L M. Each of the squares of the screen producing the same
effect, the plate shows, after developement, alternate opaque and transparent
squares like the squares of a chess board, as shown in Figure 1.

page_text: Designating by d the distance from the screen to the plate, by n the number of
lines of the screen to the inch, and by P, the distance from the photographic plate
P
to the diaphragm, the distance between the two holes E and F must be
If,
dn№ 2
for instance, the screen is ruled 90 lines to the inch and placed of an inch in
front of the plate and the diaphragm 100 inches from it, the distance of the holes
must be 3-13 inches.
Instead of a cross lined screen with transparent and dark lines of equal
width, I may employ oue in which the opaque lines are wider than the' transpäront
lines.
The opaque squares produced on the photographic plate do not. then quite join,
and the resulting screen consists of isolated opaque squares disposed in alternate
order on a transparent ground, as shown in Figure 3. I may also use a cross lined
screen in which the transparent lines are wider than the opaque lines. The opaque
squares, then formed on the photographic plate, overlap, and the screen produced
consists of alternate transparent squares on an opaque ground.
This process may be employed for simply copying screens by giving the exposure
through a single aperture, or it may be employed by combining the shadows cast
by exposure through various holes suitably disposed, to produce screens consisting
of minute geometrical patterns of various kinds.
Having now particularly described and ascertained the nature of my said
invention, and in what manner the same is to be performed, I declare that what
I claim is:
י.
"
T
1. A photo-mechanical screen for changing the continuous tones of an original
into tones formed of white and black dote, the said screen consisting of alternate
opaque and transparent squares disposed like the squares of a chess board,
substantially as specified.
2. A screen for photo-mechanical purposes, the same consisting of a transparent
ground having alternate isolated opaque squares disposed in chess board order
thereon, the said opaque squares having their corners adjacent, whereby the
transparent ground is practically divided into squares alternating with opaque
squares, substantially as specified."
Dated this 11th day of November 1895.
PHILIP M. JUSTICE,
Chartered Patent Agent, London,
For the Applicant.