An Improved Photometer for Measuring Intensity of Light in the Photographic Camera.
Year: 1887

page_text: Date of Application, 1st Oct., 1887
Complete Left, 2nd Mar., 1888
Complete Accepted, 27th Apr., 1888
PROVISIONAL SPECIFICATION.
An Improved Photometer for Measuring Intensity of Light in the
Photographic Camera.
I, JULES DECOUDUN of 8 Rue Saint Quentin, Paris, France, Civil Engineer do
hereby declare the nature of this invention to be as follows:-
My Invention relates an improved photometer for measuring the intensity of the
light in a photographic camera in order to ascertain the proper time of exposure.
Attempts have been made to estimate the intensity of the light which is focussed
on to the ground glass, by means of three or four plates of transparent and more or
less tinted glass, but this means of estimation is difficult and gives only vague
indications.
I have found, by experiment, that the quantity of light cannot be accurately
measured by the use of transparent media but only by using translucent materials of
varying thicknesses. If a translucent material through which the light can barely
pass, such as a piece of tracing paper for example, be applied either upon or at a short
distance from the ground glass upon which the photographic image is focussed, a
blurred image is perceived and by superposing successive additional thicknesses of
the paper, the blurred image becomes a mere confused mass of dark and light spots
which finally becomes more and more obscured until it is reduced to a nearly uniform
dark tint, a greater or less number of superposed thicknesses being needed to produce
this result according as the light is more or less strong. The number of superposed
sheets or the thickness of the translucent medium is thus directly proportional to the
quantity of light falling upon the ground glass, so that by counting them or measuring
the thickness, an indication of the intensity of the light is obtained.
My Invention consists in a mode of measuring the intensity of the light focussed
upon the ground glass screen of a photographic camera by the employment of a
translucent medium of, a progressively varying degree of thickness and therefore of
translucidity, such as sheets of paper of different thicknesses or superposed in varying
number, plates of porcelain, celluloid, tinted gelatine, ground glass, plastic compo-
sitions, and tissues, and generally, any translucent material which is variably translucid
at different thicknesses.
The apparatus which might be devised for carrying out the principle of this

page_text: (
invention may be greatly varied in form and arrangement, and I will here describe
an example only. This apparatus consists of a disc divided in sectors of progressively
decreasing degrees of translucidity through which, when applied against the focussing
screen, the image is examined in succession until one is found through which the
image, which appears blurred through the thinnest, becomes reduced to a uniform
dark tint whence the proper time of exposure is estimated. In using this photometer,
the operation therefore, consists in finding through which sector of the disc the light
begins to be extinguished, but in consequence of the various colors of the image a
certain amount of judgment is necessary.
Having remarked that if between the focussing screen and the photometric disc, a
piece of wire gauze or perforated metal, or a sheet of paper divided by lines into
squares, be placed through which the image was visible in small luminous points, it
was easier to judge when the light became confused through the thinner and when it
became extinguished through the thicker sectors; and having observed that when
using for this purpose a perforated -plate having holes of different sizes the light
through the smaller apertures seemed to disappear earlier than that through the larger
ones; I am enabled to rapidly estimate the intensity of the light by selecting a sector
through which the luminous points have nearly disappeared at the small apertures
whilst still visible at the larger ones.
The accompanying drawing represents an example of a photometer based on this
experience, but I would have it understood that this is given merely as illustrative of
the principle, and that I do not limit myself to the particular construction or to the
details of form and dimension.
Figure 1 is a face view of the rotary disc of the photometer. Figure 2 is a rear
view and Figure 3 a cross-section, of the instrument.
The instrument comprises a translucid disc B of variable and progressively
increasing opacity, formed by glueing on a perforated disc A sectors of translucid
paper or other medium in progressively increasing numbers of layers. The perfora-
tions of the disc A are in radial lines through the middle of each sector and comprise
a number of small holes a and a large one b so that the lines of apertures are covered
by a series of media of progressively decreasing translucency around the circle.
This disc is mounted on the spindle of a knob C and enclosed in a box D having
corresponding apertures E, G, in its front and rear covers through which the different
sectors may be viewed in succession by light transmitted through the perforations,
the apertures being of a size to permit only one line of perforations to be seen at a
time, so as to prevent the others confusing the eye. In the front cover there is also
a second aperture F bearing a reference number or letter corresponding to the sector
for the time being visible through the apertures E, G.
The instrument is used by applying it against the focussing screen of the camera
and looking through the aperture G, whilst by turning knob C the different sectors
are brought in succession opposite said aperture, commencing at the thinnest or most
transparent, the thickness or opacity of the medium being gradually increased until
the small apertures nearly disappear whilst the larger one remains visible.
reference number or letter then shewn at the aperture F, denotes by reference to a
table, the proper amount of exposure to be given, the table being calculated according
to the different degrees of rapidity of the plates in use.
-Dated this 1st day of October 1887.
The

page_text: Photographic Camera.
I, JULES DECOUDUN, of 8, Rue Saint-Quentin, Paris, France, Civil Engineer, 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 improved photometer for measuring the intensity of
the light in a photographic camera in order to ascertain the proper time of exposure
to be given to the plate or film. As the light which acts on the sensitized surface is
identical with that focussed on to the ground glass, any means whereby the intensity
of the light received upon the focussing screen may be accurately ascertained, will.
enable the operator to determine the proper time of exposure. Attempts have
accordingly been made, to estimate the intensity of the light, by means of three or
four plates of transparent and more or less tinted glass, applied on the focussing
screen, but this means of estimation is difficult and gives only vague indications.
I have found, by experiment, that the quantity of light cannot be accurately
measured by the use of transparent media, but only by using translucent materials of
varying thicknesses. If a translucent material through which the light can barely
pass, such as a piece of tracing paper for example, be applied either upon, or at a
short distance from, the ground glass upon which the photographic image is focussed,
a blurred image is perceived and by superposing successive additional thicknesses of
the paper, the blurred image becomes a mere confused mass of dark and light spots.
which finally become more and more obscured until they are reduced to a nearly
uniform dark tint, a greater or less number of superposed thicknesses being needed
to produce this result according as the light is more or less strong. The number of
superposed sheets, or the thickness of the translucent medium, is thus directly propor-
tional to the quantity of light falling upon the ground glass, so that by counting the
sheets, or measuring the thickness of the translucent material, an indication of the
intensity of the light is obtained.
My Invention consists in a mode of measuring the intensity of the light focussed upon
the ground glass screen of a photographic camera, by the employment of a trans-
lucent medium of a progressively varying degree of translucidity, such as sheets of
paper of different thicknesses or superposed in varying number, plates of porcelain,
celluloid, tinted gelatine, ground glass, plastic compositions and tissues, and generally
any material which is variably translucid at different thicknesses.
The apparatus which might be devised for carrying out the principle of this

page_text: invention, may be greatly varied in form and arrangement, and I will here describe
an example only, which consists of a disc divided in sectors of progressively
decreasing degrees of translucidity, through which when applied against the focussing
screen, the image is examined in succession until one is found through which the
image (which appears blurred through the thinnest sector) becomes reduced to a
uniform dark tint, whence the proper time of exposure is. estimated. In using this
photo-meter therefore, it is simply necessary to ascertain through which sector of the
dise the light begins to be extinguished, but in consequence of the various colors of
the natural image, a certain amount of judgment is necessary.
Having remarked that if between the focussing screen and the photometric disc, a
piece of wire gauze or perforated metal, or a sheet of paper divided by lines into
squares, be placed, through which the image was visible in small luminous points, it
was easier to judge when the light became confused through the thinner, and when it
became extinguished through the thicker, sectors; and having observed that when
using for this purpose, a perforated plate having holes of different sizes, the light
through the smaller apertures seemed to disappear earlier than that through the
larger ones; I am enabled to rapidly estimate the intensity of the light, by selecting
a sector through which the luminous points have nearly disappeared at the small
apertures whilst still visible at the larger ones.
The drawing accompanying my provisional specification, represents an example of
a photometer based on this experience; but I would have it understood that this is
given merely as illustrative of the principle, and that I do not limit myself to the
particular construction shewn, or to the details of form and dimension.
Figure 1 is a face view of the rotary disc of the photometer.
view, and figure 3 a cross-section, of the instrument.
Figure 2 is a rear
The instrument comprises a translucid disc B of variable and progressively
increasing opacity, formed by glueing on a perforated opaque disc A, sectors of trans-
lucid paper or other medium, in progressively increasing numbers of layers. The
perforations of the disc A are in radial lines through the middle of each sector, and
comprise a number of small holes a and a large one b, so that the lines of apertures
are covered by a series of media of progressively decreasing translucency around the
circle. This disc is mounted on the spindle of a knob C and enclosed in a box D
having corresponding apertures E, G, in its front and rear sides, through which the
different sectors may be viewed in succession, by light transmitted through the per-
forations, the apertures being of a size to permit only one line of perforations to be
seen at a time so as to prevent the others confusing the eye. In the front cover there
is also a second aperture F at which is exhibited a reference number or letter
corresponding to the sector for the time being visible through the apertures E, G.
The instrument is used by applying it against the focussing screen of the camera
and looking through the aperture G, whilst by turning knob C, the different sectors
are brought in succession opposite said aperture, commencing at the thinnest or most
transparent, the thickness or opacity of the medium being gradually increased until
the small apertures nearly disappear whilst the larger one remains visible. The
reference number, or letter, then shewn at the aperture F in the front of the box,
denotes by reference to a table, the proper amount of exposure to be given, the table
being calculated according to the different degrees of rapidity of the plates in use.
The aperture G-may be provided with a small tubular eye piece for the purpose of
excluding the external light, furnished if desired_with_a_ lens, to facilitate
examination.
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. A photometer for measuring the intensity of the light received on the focussing
screen of a camera in order to deduce the proper time of exposure; consisting of a
series of media of progressively graduated translucency applied over series of

page_text: perforations in an opaque support and movable together therewith past a sight
aperture in an enclosing casing, in the manner specified.
2. The herein described construction of photometer for the purpose specified, con-
sisting of an opaque disc with groups of perforations, covered with a series of inedia
of progressively graduated translucency pivoted within a case provided with a sight
aperture opposite which the groups of variably translucent perforations may be
brought in succession as shown and described.