Illumination Photometer.
Year: 1898

page_text: PROVISIONAL SPECIFICATION.
Illumination Photometer.
CHARLES WILLIAM SCOTT CRAWLEY, 16, Red Lion Street, Clerkenwell, E.C.,
Electrical Engineer, do hereby declare the nature of this invention to be as
follows:-
My invention is for the purpose of measuring the illumination of a street,
room, or other place. It consists of a box or tube containing a lamp, candle, or
other similar standard, which illuminates a small white or coloured surface at the
other end of the box. This surface is capable of rotation by means of an outside
handle. The surface is rotated in such a way as to vary the angle at which the
Above this
light from the standard falls upon it, and thus vary the illumination.
surface is another white or coloured surface, and the instrument is placed so that
this second surface is at the spot whose illumination is to be measured. In this
upper screen is a small hole, or other suitable orifice, through which the surface
of the lower screen can be seen. If the illumination of the upper screen is greater
than that of the lower this spot appears dark.
If the illumination of the upper
screen is less than that of the lower the spot appears bright. The illumination of
the lower screen is then adjusted until the spot appears neither bright or dark, but
of the same illumination as the rest of the screen; then the illumination of the
upper screen is equal to that of the lower. Attached to the lower screen is a handle
moving over a scale, calibrated in terms of any convenient unit. Then when
a balance is obtained the illumination of the surface to be measured can be read
off at once in terms of this unit. As standard lamp I prefer to use an amyl-
acetate one of any
convenient form, and to regulate it by bringing the point of the
flame up to a given mark inside, which can be read from above by means of a
mirror in the side or end. The light from the standard lamp is usually more
yellow than the lights illuminating the surface to be measured, making it some-
what difficult to get a balance. In order to obviate this difficulty I generally
tint the lower screen slightly with blue, or which produces the same effect, I
tint the upper screen with yellow.
In order to obtain a more even scale for the calibration I generally curve the
lower screen.
1000
I, CHARLES WILLIAM SCOTT CRAWLEY, of 16, Red Lion Street, Clerkenwell, E.C.,
Electrical 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 is for the purpose of measuring the illumination of a street,
room, or other place and it is illustrated in the annexed drawings wherein,

page_text: Figure 1 is a sectional elevation.
Figure 2 is an end view with the end plate a¹ removed.
Figure 3 is a side elevation, and
Figure 4 is a plan view.
;
My apparatus consists of a box or tube a having end plates a¹, a² and supported
through a suitable pillar or the like a and base at. The said box or tube a contains
al lamp, candle, or other similar standard b which illuminates a small white or
coloured surface c at the other end of the box. This surface is capable of rotation
by means of an outside handle d. The surface c is rotated in such a way as to
vary the angle at which the light from the standard 6 falls upon it, and thus vary
the illumination. Above this surface is another white or coloured surface e,
and the instrument is placed so that this second surface is at the spot whose
illumination is to be measured. In this upper surface or screen
e is a small
hole or slit f, or other suitable orifice, through which the surface of the lower
screen c can be seen.
If the illumination of the upper screen e is greater than that of the lower one c
this spot appears dark and if the illumination of the upper screen e is less than
that of the lower one the spot appears bright. The illumination of the lower
screen is then adjusted by operating the handle d until the spot appears neither,
bright or dark, but of the same illumination as the rest of the screen, and the
illumination of the upper screen is equal to that of the lower one.
Attached to the lower screen is a pointer moving over a scale h calibrated in
terms of any convenient unit. Then when a balance is obtained the illumination
of the surface to be measured can be read off at once in terms of this unit.
As standard lamp I prefer to use an amyl-acetate one of any convenient form,
and to regulate it by bringing the point of the flame up to a given fixed point i
inside which can be read from above by means of a mirror j located in the side
or end a¹ of the box.
The light from the standard lamp is usually more yellow than the lights
illuminating the surface to be measured, making it somewhat difficult to get a
balance. In order to obviate this difficulty I generally tint the lower screen
slightly with blue, or (which produces the same effect) I tint the upper screen e
with yellow.
In order to obtain a more even scale for the calibration I generally curve the
lower screen as shown.
The lamp can be raised or lowered by being turned in the screen k, the tube
at the top of the lamp and which contains the wick sliding through the tube m
which is fixed.
f
n, n are two diaphragms between the lamp and the screen to prevent screen
illumination by side rays.
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:
1. In an illumination photometer a curved screen for the purpose of obtaining
a more even scale substantially as set forth.
2. The illumination photometer constructed substantially as described with
reference to the accompanying drawings.
Dated this 7th day of June 1800
A. F. SPOONER,
323, High Holborn, London, Agent for the Applicant.