New or Improved Method of and Apparatus for Ascertaining the Requisite Time of Exposure in Photography.
Year: 1890

page_text: Date of Application, 14th Jan., 1890
Complete Specification Left, 4th Oct., 1890-Accepted, 27th Dec., 1890
PROVISIONAL SPECIFICATION.
New or Improved Method of and Apparatus for Ascertaining the
Requisite Time of Exposure in Photography.
I, EDWARD GEORGE BALLARD of 1, Eversley Park, Chester in the County of
Chester, H.M. Inspector of Alkali Works, do hereby declare the nature of this
invention to be as follows:-
A ready method of measuring the amount of actinic rays proceeding from any
landscape or object has for a long time been a thing much desired by photographers,
both amateur and professional. Many instruments with this view have been from
time to time brought out, but none of them have proved very satisfactory. The
same may be said of tables, which, although of great assistance in open landscape,
have this drawback, that, wherever the quality of the light is interfered with, that
is, in forest scenery, deep glens with overhanging trees where the very air appears to
be green, interiors of churches, etcetera, where the light which enters perhaps passes
through coloured glass, and so forth, you are compelled to guess at the exposure
after all.
The apparatus about to be described purposes not only to measure the intensity
of the rays of light proceeding from any object, but also the quality of the same.
The principle upon which the instrument is constructed is based upon (1) that
luminous calcium sulphide or Balmain's luminous paint is only excited by those
rays of light which usually act upon the photographic plate: (2) that when
luminous paint is excited by exposure to light, either daylight or other white light,
such as a burning piece of magnesium wire, and the source of light is suddenly
removed, it will be observed that the bluish light emitted from the luminous paint
gradually fades. The rate and manner in which this fading occurs are employed
by me in this invention as a means for ascertaining the intensity of the light
proceeding from any object, landscape or sea view.
This is arrived at by noting the time required for the luminous paint to acquire
the same degree of intensity as a portion of the light proceeding from the object,
landscape or sea view in question, and referring to a table specially constructed
for that purpose.
In carrying out my invention into practice, one form of apparatus which I have
found very successful is as follows, and is given by way of illustration only
-:
A wooden or other opaque tube, coloured dull black inside and open at both
ends and of convenient size and shape at one end to adjust to the eye, is provided
at the other end with a second and similar but shorter tube hinged to it so as to
form a lid. This lid is glazed next the tube with blue glass capable of cutting off
all rays except those corresponding with those which proceed from the luminous
paint. A second glass is slipped into this, coated as regards about half its area
with an opaque layer of calcium sulphide paint (Balmain's luminous paint) above
this a number of layers of tissue paper or other uniform translucent material is
placed, and a covering is added of ordinary or cobalt blue glass. The number of
sheets of tissue paper to be inserted is found by experiment for each variety of dry
plates. Their use is to lessen the intensity of light passing through to the degree
requisite to give correct results with the instrument, this once found is constant for
that variety of plate.
To use the instrument, allow the hinged portion to fall and so get the side with
the luminous paint fully exposed to daylight for 30 seconds.
Place the eye at the end. Then quickly close the hinged portion. A. blue
uminous ground and a dark ground will be seen side by side.
Count the number of seconds it takes for the blue luminous side to acquire the

page_text: same tint as the dark side, that is note when the difference in blue disappears.
Refer to the table mentioned, and, opposite the number of seconds required for the
two halves to appear alike, will be found the correct exposure to be given to the
photographic plate to which the instrument is adjusted.
The instrument I have constructed is adjusted to a plate of about 18 Warneker
sensitometer value, working with a stop or diaphragm in the lens of one-sixteenth
of the focus of the same.
The actinometer can easily be adjusted to a plate of any degree of speed by
removing or adding to the layers of tissue paper behind the glass carrying the
luminous paint.
In adjusting the actinometer, any of the published tables of exposure may be
employed, or better still that excellent little calculating machine for photographic
exposures called the "actinograph" invented after much labour and study by
Dr. Ferdinand Hurter and Mr. Driffield of Widnes. Or in very perfect instruments a
clockwork, or its equivalent, can be set in action by closing the hinged portion, and
arrested by opening it, this can be arranged with a scale to show the requisite
exposure in seconds.
Dated this 13th day of January 1890
WM. P. THOMPSON & Co.,
6, Lord Street, Liverpool, Patent Agents for the Applicant.
COMPLETE SPECIFICATION.
New or Improved Method of and Apparatus for Ascertaining the
Requisite Time of Exposure in Photography.
I, EDWARD GEORGE BALLARD, of 1 Eversley Park, Chester, in the County of
Chester, H.M. Inspector of Alkali Works, 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 :-
This invention relates to improvements in the method of and instruments for
measuring the relative intensity of the actinic rays present, or reflected from
any object or landscape, at different times or under different atmospheric or other
conditions.
A ready method of and simple instrument for measuring the relative intensity
of such actinic rays has long been a great desideratum, especially by photographers.
Various instruments have, from time to time, been brought before the public with
the object of supplying this want, but none of them have, as far as I am aware,
proved very satisfactory. The same may be said of the various tables which are
mainly based upon data obtained by experimenters with their respective instruments.
Some of such tables in the form of instruments have been complicated in con-
struction and tedious to manipulate, and most of them, although of great assistance
in open landscape, have this drawback, that in situations where the quality of the
light is interfered with by the various coloured mediums through which it passes-
as for example, in forest scenery, deep glens with overhanging trees, interiors of
churches (especially those having stained glass windows) etcetera―the duration of
the requisite exposure becomes, after all, very much a matter of guess-work.
The object of my present improvement, as applied to photography, is to furnish
a simple and reliable instrument adapted to measure rapidly the relative intensity
of the actinic rays which are reflected from the object or landscape at any given
time and under any conditions, atmospheric or otherwise, existent thereat.
The principles upon which the instrument is constructed are mainly based upon
the following data:-
1. Certain luminous or phosphorescent substances, such as luminous calcium
sulphide (commercially known as Balmain's luminous paint) are excited by and
emit only the actinic rays of light which act upon photographic plates, that is, the
blue or violet rays.

page_text: 2. When such luminous substances are exposed to the light for a comparatively
short time, a point of maximum intensity of luminosity appears to be attained, which
cannot be exceeded either by extending the period of exposure or by increasing the
intensity of the light.
3. When such luminous substances are excited as above described and the
light suddenly cut off from them, the bluish rays emitted therefrom gradually
fade.
The rate at which this fading takes place is employed by me in this invention
(when applied as an aid to photography) as a means for ascertaining, at any given
time and under any conditions thereat existing, the relative intensity of the actinic
rays reflected from any given object or landscape, with a view to determine exactly
what length of exposure it is necessary to give to a photographic plate of a specified
type directed towards such object or landscape at approximately the same time and
practically under the same conditions.
The length of exposure necessary for the plate is determined by, first, noting
(either by clockwork or otherwise) the time required for the luminosity of the
excited material in the instrument to fade from its maximum intensity to that of a
portion of the actinic rays which are reflected from the object or landscape, and pass,
by preference, through an obscuring medium so as to enter the interior of the
instrument at a reduced intensity; and, secondly, by referring the time so noted to
a specially constructed table (such as that hereafter referred to) for the purpose
of ascertaining the exact amount of exposure required for the type of plate to
be employed.
The accompanying drawing shows one form of instrument, constructed in accord-
ance with my invention and giving very good results.
Figure 1 being a longitudinal section, and Figure 2 a similar section of a portion
of the instrument showing a slight modification.
A, is an opaque tube or chamber of any convenient size and shape, having its
interior of a dull black colour. One end of the tube is provided with a sighting
orifice A¹ and, by preference, with an eye-piece A². On the opposite end of the
tube A, a similar but shorter tube or frame B is mounted in such a movable manner
that one end of it may alternately be exposed to the light (either artificial or
natural) and presented to the darkened interior of the tube A. To this end, the
frame B may be conveniently hinged to the tube A, as at b, so that it forms a kind
of lid thereto.
The frame B is glazed at its rear end (that is, the end next the tube A) with a
piece of ground glass C, or it may be with a piece of blue glass as hereinafter
described. In front of this is a piece of transparent glass D having a portion of its
rear surface (say half its area) covered with an opaque layer E of luminous calcium
sulphide or its luminous equivalent. If the luminous material is not sufficiently
opaque in itself, a corresponding opaque backing may be applied on either side of the
lass D. The remainder of the glass, as, for instance, a circular space d in the
entre thereof, is left transparent. Or, in lieu of the glass D, the luminous material
may be supported upon a backing of opaque material, such as cardboard, having its
centre or other suitable portion cut away for the admission of the external
actinic rays into the tube A. Or the luminous material with an opaque backing D¹
may even be applied directly to the forward face of the adjacent ground glass C, as seen
Figure 2, the glass D being then superfluous. Moreover I find it preferable to
place the ground surface of the glass C next the interior of the tube A, to prevent
the image of the eye-piece or the observer's eye being reflected therefrom and thus
nterfering with the observation.
Behind the glass backing D is a piece of blue glass F, which is capable of cutting
if all the rays entering the tube A from the exterior, except the actinic rays
corresponding in colour to those emitted by the luminous material E. The kind
of glass which I have found to give very good results has a composition very similar
o ordinary cobalt glass, with the addition, however, of a little oxide of copper.
On one or both sides of the blue glass F is placed a series of layers G of a

page_text: uniformly translucent material, such, for example, as tissue paper. The paper may
be conveniently wrapped round the glass, as shown. This paper serves to lessen the
intensity of the rays passing through the unobstructed portion of the glass backing D
to such a degree as is necessary to give correct results with the instrument. The
required degree of obscurity is constant for each type or variety of photographie
plate. It is found in each case by actual experiment with the instrument and is
regulated by the number of layers or folds of the paper G.
H is a plate of plain or ground glass placed in front of the tissue paper and blue
glass, as a protection. The parts CE DG FH may be arranged close one against
the other within the frame B and may be held therein by the shoulder and
removable lugs or other suitable fastening devices ². In cases where it is difficult.
to obtain blue glass of exactly the right colour, a second piece of blue glass may
be placed in the rear of the luminous material E (either in addition to or instead
of the ground glass C) in order that the rays passing through the first blue glass F
and those emitted from the material É may appear to the user to be of
approximately the same colour.
The mode of operation is as follows
The hinged frame B is opened as shown in dotted lines, so that the luminous
material is fully exposed to light (whether natural or artificial) for a period
sufficient to excite it to the maximum degree, say for 30 seconds in ordinary cases.
The eye is then placed against the eye-piece A2, the instrument is directed towards
the object to be photographed, and the hinged part B quickly closed. There will
then be visible to the user a blue luminous ground surrounding a darker spot.
Immediately on closing the frame B, the user begins to count, or notes, on a suit-
able chronometer, the number of seconds which it takes for the blue luminous
portion to acquire the same tone as the darker central portion, that is, he notes
how long it takes for the difference in the tones of the two portions to disappear
and the whole surface to become of a uniform tone. The user now refers to a table
(such, for instance, as the one given by way of illustration near the end of this
Specification) and, in line with the number of seconds so noted on the instrument,
he finds the correct exposure to be given to a photographic plate of the type to
which the instrument is adjusted.
The actinometer can easily be adjusted so as to give correct readings for plates
of any degree of sensitiveness by removing one or more folds or layers of tissue
paper G, or by adding thereto. Or the translucency of the paper may be kept
constant so as to give correct results for one variety of plate, and the table may
set forth the necessary allowance to be made to obtain correct results with other
varieties of plates and with stops or diaphragms of different sizes. Or separate
tables may be constructed for each variety of plate and size of stop. The particular
instrument employed by me is adjusted to plates of about 18 Warnerke's sensito-
meter value, working with a stop or diaphragm in the lens of one-sixteenth of the
focus of the same, commonly known as
f
16
In adjusting the actinometer, any of the published tables of exposure may be
employed, or, better still, that excellent little calculating machine for photographic
exposures, called the "actinograph" and invented, after much labour and study, by
Dr. Ferdinand Hurter and Mr. Driffield of Widnes. Or, in very perfect instru-
ments, a clockwork, or its equivalent, can be set in action by closing the hinged
portion, and arrested by opening it. The clockwork may also be arranged with a
scale to show the requisite exposure in seconds.
It will be obvious that the instrument is not limited, in its application, to
photography, as it may be employed in many other cases where the relative
intensity of actinic rays is to be ascertained. Also, in some cases, the instrument
may be successfully used without the employment of an obstructive medium, such
as G. Also, in cases where the actinic rays alone are present, the blue medium F

page_text: may be dispensed with. Moreover, a simple medium, having the proper colour and
degree of translucency could be employed, instead of the compound medium F G,
as examples, I may mention bluish tissue paper and blue ground glass.
The following is a copy of a book of instructions, which I issue to customers, and
which is embodied in this Specification merely by way of example. It contains
the table herein referred to.
INSTRUCTIONS FOR USING BALLARD'S ACTINOMETER.
Place the instrument to the eye, and direct it towards the subject you are about
to photograph.
Allow the hinged portion to fall so that it shall be well exposed to full daylight
or to the light emitted from burning one inch of magnesium wire for 30 seconds;
then close it rapidly. A dark spot on a blue ground will be seen.
From the moment of closing the hinged portion, begin to count the number of
seconds it takes for the dark spot to vanish. Refer to column marked Actinometer
Seconds in the table suited to the plate in use, and, opposite, will be found, stated
in seconds of time, the exposure necessary.
N.B. A focussing cloth thrown over the head will greatly assist in making the
observation.
Under ordinary conditions, it only takes a few seconds to determine an exposure.
THE TABLES.
These are constructed on the system of comparing the speed of the various
makes of plate with that of the wet plate, the rapidity being expressed in terms of
so many times (5, 10, 20, 60, times, etcetera); this being, at present, the recognised
mode of stating the sensitiveness of a plate, though perhaps not a very satisfactory
one. However, one of the tables given will doubtless be found to suit the plate
in use, and they are adapted to any latitude, time of day, or month of year.
Judgment as to the condition of the light (which is always so deceptive to the
eye) is entirely dispensed with. The instrument indicates with rapidity the actual,
not the probable, exposure required for any subject, and is so constructed that it
measures only those rays which usually act on the photographic plates.

page_text: When other sized stops are used it is necessary to multiply or divide the exposure
indicated in the tables by the following numbers:-
F
For
4
stop divide by
F
8
"
"
22
F
"
113
"
3
"
F
"
22-6
"
F
22
32
F
دو
multiply by
16
4
2
2
4
"
"
"
45.2
"
"
39
F
"
64
"
"
99
16
For plates of greater sensitiveness than 125 times, the tables for 15, 20, 25, etc.,
may be employed by merely shifting the decimal point to the left one place. Thus,
with a 250-times plate-Suppose the spot vanishes in 14 seconds, the exposure
would be 168 second, instead of 168, which the table for 25-times plates
indicates.
F
The actinometer is sent out set to a 30-times plate, working with 6
stop, so that

page_text: the number of seconds required for the spot to vanish, divided by 10, will be the
correct exposure for such plates, as will be seen from the table.
Should the user of the instrument desire it, he can similarly adjust the actino-
meter to any other speed of plate by simply removing the brass front of the hinged
portion, and cutting off or adding to the layers of tissue paper which will be found
wrapped round a piece of blue glass inside. For plates of greater sensitiveness
than 30 times, remove one or more layers of the tissue paper; and for plates of less
sensitiveness than 30 times, add one or more layers as the case may require. But
this is not necessary when the tables are used. The actinometer must then be left.
as sent out by the manufacturer.
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 1
claim is:
-
1. The method of measuring the relative intensity of the actinic rays of light
present at any given time or place or under any conditions, which consists in causing
a portion of such rays, at a suitable intensity, to be viewed in juxtaposition to rays
of approximately the same colour emitted from a layer of fully excited luminous
material, and in noting the time taken by the said emitted rays to fade to the
intensity of the said first named rays, substantially as and for the purpose
described.
2. The method of ascertaining the proper amount of exposure to be given to a
photographic plate at any given time and under any conditions existent thereat, the
said method consisting in (1) exposing a layer of luminous material of the kind
described to the light till fully excited, (2) cutting off the light therefrom, (3) noting
the time taken for the actinic rays emitted from such excited material to fade from
their maximum intensity to that of a portion of the actinic rays reflected from the
object to be photographed and viewed, at a suitable intensity, in juxtaposition to
the rays emitted from the luminous material, and (4) referring the time so noted
to a specially constructed table for the purpose of ascertaining the requisite amount
of exposure to be given to the plate, substantially as described.
3. In an actinometer, the combination of a darkened chamber having a sighting
orifice, an opaque or opaque-backed layer of luminous material of the kind described,
arranged opposite said orifice in juxtaposition to a space adapted to permit the
entrance of external rays into the chamber, a suitably coloured transparent medium
located beyond said space and exposed to the external light, means for reducing the
intensity of the rays passing through said space, and a device whereby the luminous
material may be alternately exposed to and cut off from the light, substantially as
described.
4. In an actinometer, the combination, with a darkened chamber A having a
sighting orifice A¹, of an opaque or opaque-backed layer of luminous material of
the kind described located opposite said orifice in juxtaposition to a space adapted
to permit the passage of actinic rays into said chamber, whereby said rays may be
viewed in juxtaposition to the rays emitted from the luminous material, substantially
as and for the purpose described.
5. The chamber A having sighting orifice A', opaque or opaque-backed layer E
of luminous gradually fading material having a central unobstructed space, blue
transparent medium F, translucent medium G, and means for alternately exposing
said luminous material to the light and presenting it to the sighting orifice of the
darkened chamber as described, in combination with a specially constructed table
setting forth the various amounts of photographic exposure corresponding respectively
with the various readings of the instrument, substantially as described.
D: 91 1.
1000
WM. P. THOMPSON & Co.,
Of Liverpool, Manchester, Nottingham, and London, Patent Agents.