Improvements in Photometers for Measuring Intensity of Light in the Photographic Camera.
Year: 1888

page_text: Date of Application, 10th Aug., 1888
Complete Specification Left, 7th May, 1889-
Complete Specification Accepted, 8th June, 1889.
Improvements in Photometers for Measuring Intensity of Light in
the Photographic Camera.
1, 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 to improvements in the photometer for photographic purposes
for which I obtained former Letters Patent dated October 1st 1887, No. 13,332,
by which the intensity of the light focussed on the screen of the camera is estimated
(in order to deduce therefrom the proper time of exposure) by means of translucent
materials of varying thickness.
In the specification of my previous patent above referred to, it was stated that in
order to estimate the intensity of light it was necessary to view the luminous points
which successively come opposite the small aperture, from a distance of about ten inches
which is inconvenient under some circumstances and especially for outdoor work.
To avoid this, I provide a small lens A (see figure 1 of the accompanying drawing)
of suitable focal length through which the luminous points can be accurately observed
by applying the eye close to the instrument. The instrument is otherwise the same
as described in the specification before referred to and is held by one hand against the
focussing screen while with the other the central knob C is rotated to bring the
gradually darkening luminous points in succession opposite the eye-piece.
Figure 2 represents a modification of the instrument in which, instead of a series of
translucent sectors of a rotary disc requiring the use of both hands to operate it, a
stationary disc divided into squares or spaces B of different degrees of opacity, each
bearing a letter or other sign for comparison with the table giving the proper times of
exposure. Each letter or sign may consist of a through aperture or be transparent on
a black ground and upon them is applied the translucent material. The disc B thus
formed and mounted in a tube D provided with an eye piece and lens is applied like
the instrument before described against the focussing screen. When viewed through
the lens some of the letters will be visible and others invisible, the last letter visible
being that which indicates the time of exposure.

page_text: In this case, the transparent letters form a substitute for the small perforations in
the first arrangement, and they may also be formed of a series of small holes. Or,
instead of reference to a table, I may use figures denoting directly the proper time of
exposure.
The lens tube D is made telescopic so as to enable the focal distance to be varied for
different views and also allow of its being readily removed and used as an ordinary
focussing eye-piece.
J. DECOUDUN.
By A. M. & Wm. Clark,
53, Chancery Lane, London,
Patent Agents.

page_text: Improvements in Photometers 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 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 improvements in the photometer for photographic purposes
for which I obtained former Letters Patent dated October 1st 1887 No. 13,332,
by which the intensity of the light focussed on the screen of the camera is estimated
(in order to deduce therefrom the proper time of exposure) by means of translucent
materials of varying thickness.
In the specification of my previous patent, above referred to, it was stated that in
order to estimate the intensity of light, it was necessary to view the luminous points
which successively come opposite the small aperture, from a distance of about ten inches
which is inconvenient under some circumstances and especially for outdoor work.
To avoid this, I provide a small lens A (see figure 1 of the drawing accompanying
my Provisional Specification) of suitable focal length, through which the luminous
points can be accurately observed, by applying the eye close to the instrument.
The instrument is otherwise the same as described in the specification before
referred to, and that illustrated in face view and in diametrical cross-section in figure 1
is held by one hand against the focussing screen while with the other the central
knob C is rotated to bring the gradually darkening luminous points in succession
opposite the eye-piece.
Figure 2 represents diametrical and cross-sections of a modification of the
instrument, in which, instead of a series of translucent sectors of a rotary disc
requiring the use of both hands to operate it, a stationary disc B is employed,
divided into squares or spaces of different degrees of translucency, each bearing a
letter or other sign for comparison with the table giving the corresponding periods
of exposure. Each letter or sign may be formed by transparent material surrounded
by, or of a through aperture in, an opaque ground, the medium of variable degrees
of translucency being applied over the whole series. The disc B thus formed and
mounted in a tube D provided with an eye-piece and lens, is applied like the
instrument before described, against the focussing screen. When viewed through
the lens, some of the letters will be visible and others invisible, the letter which is
visible to the least extent being that which indicates the time of exposure.
In this case, the transparent letters form a substitute for the small perforations in
the first arrangement, and they may also be formed of a series of small holes. Instead
of employing letters or signs for comparison with a table, I may use figures denoting
directly the proper time of exposure.

page_text: The lens tube D is made telescopic so as to enable the focal distance to be varied,
to suit different sights and also allow of its being readily removed and used as an
ordinary focussing eye-piece. The form and dimensions of the apparatus may be
varied and the eye-piece be provided with one or more lenses.
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 :-
1st. The combination with an instrument for determining the time of exposure in
photographic cameras such as forms the subject of former Letters Patent No. 13,332
of 1887, of an eye-piece and lens A as hereinbefore described and represented in
figure 1 of the drawing, for the purpose specified.
2nd. In an instrument for determining the time of exposure in photographic cameras
by means of translucent materials, the combination with a stationary disc B divided
into spaces possessing different degrees of translucency, of an eye-piece and lens
including the whole disc in its field of view, as herein before described and illustrated
in figure 2.