An Improvement in the Manufacture of Sensitive Plates for Photographic Purposes.
Year: 1891

page_text: Date of Application, 7th Dec., 1891
Complete Specification Left, 3rd Oct., 1892-Accepted, 19th Nov., 1892
PROVISIONAL SPECIFICATION.
An Improvement in the Manufacture of Sensitive Plates for
Photographic Purposes.
I, JOHN TYACK SANDELL of 10 Pall Mall, London, S.W., General Manager of
R. W. Thomas & Co. Limited, Photographic Manufacturers, do hereby declare the
nature of this invention to be as follows:-
My invention relates to an improvement in the manufacture of photographic
plates and it has for its object to avoid the defects of halation and solarization or
Liability to reversal of the image under excessive exposure which are incidental to
such photographic plates as now made.
My invention consists in coating the plate by the superposition of two or more
sensitive emulsions of different degrees of sensibility to actinic light, the underneath
being of the least and the uppermost of the greatest degree of sensibility so that
portions of the same subject which are in strong contrast as regards strength of
illumination will virtually be photographed upon strata of different degrees of
sensitiveness of the compound film or medium, the effect produced upon the haloid
salt of the lower strata, thus compensating for the deficiency due to over-exposure
of the corresponding portion of the upper stratum. The prevention of halation is
due to the greater opacity and lesser sensibility of the lowermost stratum. The
emulsions composing the different strata would contain any of the usual haloid salts
of silver commonly employed. For instance the first or underneath coating or
stratum may be composed of an ordinary bromide of silver emulsion, the next may
be of bromide or bromo-iodide or chloro-bromo-iodide of silver emulsion but of
greater sensitiveness obtained by subjecting the emulsion to heat or treating it with
alkali in the ordinary way of treating emulsions. Similarly as regards the third and
subsequent coatings, that is to say if more than two are used, they would be of
successively greater degrees of sensitiveness. Generally two or three coatings.
would afford strata of sufficiently different degrees of sensitiveness to meet the
ordinary requirements of any kind of landscape work or of interiors. It is of course
to be understood that each coating is to be allowed to thoroughly set and dry before
another is applied.
Dated this 7th dovr of December 1901
J. T. SANDELL,
By A. M. & Wm. Clark,
53, Chancery Lane, London, Chartered Patent Agents.
COMPLETE SPECIFICATION.
An Improvement in the Manufacture of Sensitive Plates for
Photographic Purposes.
I, JOHN TYACK SANDELL of 10 Pall Mall, London, S.W., General Manager of
R. W. Thomas and Company, Limited, Photographic Manufacturers, do hereby
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:-
My invention relates to an improvement in the manufacture of sensitive gelatine
plates for photographic negatives, and it has for its object to diminish the liability
to halatiou and solarization (or reversal of the image) which are incidental to
photographic plates as now made when subjected to a comparatively slight excess
of exposure.
By employing plates made according to my invention, it is possible to produce
J
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page_text: photographs in which all the details of subjects presenting strong contrasts of light
and shade are perfectly brought out. For example, the interiors of churches and
the like where the windows and other apertures are illuminated by a strong external
light are exceedingly prone to halation and solarization of those brilliantly
illuminated portions coupled with insufficient exposure of the less strongly
illuminated parts of the subject, but by my invention these defects are avoided as a
much greater latitude in the duration of exposure is admissible without injury to the
resulting negative by reason of the fact that, owing to the compound structure and
differential sensibility of the improved plate, what would be over-exposure in the case
of an ordinary plate of similar rapidity is compensated as hereafter described..
My invention consists in coating the plate substantially as hereinafter set forth
with two or more superposed coatings or layers of gelatine emulsion having different
degrees of sensibility to actinic light, so that different portions of the picture which
are in strong contrast as regards strength of illumination will be photographed
virtually upon layers or strata of different degrees of sensitiveness, the effect pro-
duced upon the haloid silver salt or salts of the less sensitive stratum thus
compensating for the deficiency of intensity due to over-exposure of the corre-
sponding portion of the more sensitive stratum. The coating which is first applied
to the plate and which constitutes the underneath or rearmost stratum, should
possess the least degree of sensitiveness and the front or uppermost one should
possess this quality in the highest degree, the intermediate layer or layers (in
the case of more than two) progressing in regular gradation between the two
extreme degrees of sensitiveness. The prevention of halation is due to the greater
opacity and lesser sensibility of the lowermost stratum which has the effect of
absorbing and so obstructing the transmission of the most powerful rays of light to,
and their reflection from, the surface of the support upon which the layer is
applied.
The gelatine emulsions composing the different layers or strata would contain any
of the usual haloid salts of silver commonly employed. For instance, the first or
underneath coating or stratum may be composed of an ordinary bromide of silver
emulsion, the next may also be of bromide or it may be of bromo-iodide or chloro-
bromo-iodide of silver emulsion, or all the coatings may be composed of an emulsion of
the same haloid salt, but in any case the second and subsequent coatings will be of
progressively increasing degrees of sensitiveness, such difference of sensitiveness
being obtained in the preliminary preparation of the emulsion by subjecting it to
heat or treating it with alkali in the ordinary way of treating emulsions for the
purpose of increasing their sensitiveness to light. Similarly as regards the third
and subsequent coatings (that is to say; if more than two are used) they would be
of successively greater degrees of sensitiveness produced in the same manner by
heating or treating with alkali. Generally, two coatings would afford strata of
sufficiently different degrees of sensitiveness to meet the ordinary requirements of
any kind of landscape work whilst three coatings are preferable for the majority
of interiors.
It is of course to be understood that each coating is to be allowed to thoroughly
set and dry before another is applied.
In order to ensure the most satisfactory results in working this process it is
essential that the different strata although varying in sensitiveness to light should
be perfectly uniform in respect to the quality of hardness and the consequent
liability to shrinkage in drying. To attain this essential uniformity is is important
to observe that the gelatine from which the different emulsions are prepared should
be of the same make and quality; the different emulsions should be prepared as
nearly as possible at the same time, should be subjected to similar and contem-
poraneous treatment during "ripening," and should be hardened to equal extents by
the addition of alum or chrome alum in the usual way so that (save in respect of the
differences in preliminary treatment by cooking or with alkali upon which their
respective sensitiveness to light depends) the different emulsions to be superposed
will be as nearly as possible alike in their purely physical qualities.

page_text: In the manufacture of a doubly coated plate for general purposes I prefer for the
undermost layer, or that next the glass, an emulsion prepared by the boiling method
of great fineness of grain which is more amenable to development than the more
sensitive upper coating. The sensitiveness of this undermost filin should be about
from 15 to 18 on what is known as Warnerke's sensitometer and the percentage
composition of the dried film as calculated from the formula adopted is-
and these quantities in ounces would represent the solid matter contained in
770 ounces of fluid emulsion made up to that quantity with alcohol 30 fluid ounces
and water.
The emulsion for the upper layer consists preferably of a mixture of one part of
a boiled emulsion made by means of the centrifugal separator which is found to
have greater opacity and is consequently less prone to halation, and two parts of
emulsion made by what is known as the ammonia method. These emulsions have
the same composition as that of the undermost layer except that they contain
only 33 part iodide of silver and have the greatest degree of sensitiveness which it
is possible to obtain at the present time and by the present methods.
The undermost film or layer of a trebly coated plate is composed preferably of
an emulsion prepared by heat alone and brought to a sensitiveness represented by 5
on the Warnerke sensitometer. In composition it resembles the previously
described emulsions, except that iodide of silver is omitted from, and citric acid
1 part added to, the formula already given. The upper layer of a trebly coated
plate would be the same as described for the upper layer of the doubly coated plate,
and the intermediate layer of the trebly coated plate would be the same as the
undermost layer of the doubly coated plate.
Having now particularly described and ascertained the nature of the said inven-
tion and in what manner the same is to be performed, I declare that what I
claim is-
1. The preparation of sensitive photographic plates having two or more super-
posed coatings or layers of gelatine emulsion which possess in regular gradation
from the undermost to the uppermost coating or layer, progressively increased
degrees of sensitiveness to light, substantially as and for the purposes described.
2. In the manufacture of sensitive photographic plates, the combination of two
or more superposed coatings or layers of gelatine emulsion possessing in regular
gradation from the undermost to the uppermost coating or layer progressively
increased degrees of sensitiveness to light so as thereby to lessen the liability to
solarization and halation, as specified.
Dated this 3rd dor of October 1802
J. T. SANDELL,
By A. M. & Wm. Clark,
53, Chancery Lane, London, Chartered Patent Agents.