IMPROVEMENTS IN THE METHOD AND APPARATUS FOR DISTRIBUTING AND DIFFUSING POWERFUL LIGHTS
Year: 1883

page_text: LETTERS PATENT to Alexander Pelham Trotter of Furnival's Inn in the
County of Middlesex for an Invention of "IMPROVEMENTS IN THE METHOD
AND APPARATUS FOR DISTRIBUTING AND DIFFUSING POWERFUL LIGHTS
PROVISIONAL SPECIFICATION left by the said Alexander Pelham Trotter
at the Office of the Commissioners of Patents on the 15th January 1883.
ALEXANDER PELHAM TROTTER of Furnival's Inn in the County of Middlesex
"IMPROVEMENTS IN THE METHOD AND APPARATUS FOR DISTRIBUTING AND
DIFFUSING POWERFUL LIGHTS 33
When a light such as that of an electric lamp or of a brilliant burner is so
powerful that it cannot comfortably be looked at, it is usual to soften it by the use
of opal or other partly opaque glasses, by which a large portion of the light is
wasted. Moreover when such powerful lights are employed for illuminating large
areas, the space in the immediate neighbourhood of the light may be brilliantly
lighted while the moro distant portions of the space are comparatively dark. My
invention relates to improved means of softening powerful lights without great loss
of light and of distributing the illumination with comparative uniformity over
considerable areas, and I effect both these objects by the use of transparent glass
so shaped or moulded on its surface as to diffuse or to distribute or both by
refraction and reflection the rays proceeding from the source of light. I will
describe, as an example, a form of glass lantern which I employ for distribution of
such a light as that from an electric lamp. The general shape of the lantern is
that of an inverted cone, the light being situated at or near the centre of its base.
The one surface of the cone, which may be the exterior, is circularly grooved so as
to present a series of steps each of which has its surface so curved that the rays of
light passing through it are distributed by refraction uniformly over the desired
extent of area extending around the lamp. The stepping may be formed on the
interior surface of the glass or partly on the interior and partly on the exterior.
Above the cone I provide a reflector of such curvature that the rays are uniformly
distributed by reflection from it. Between the base of the cone and the reflector a
glass cylinder may be placed having its surface circularly grooved liko that of the
[Price 6.1

page_text: cone to give uniform distribution. Instead of employing a cone, a glass of
approximately spherical or hemi-spherical form, or generally shaped as a solid of
revolution, may be employed, the curvature of its steps being varied to suit the
particular angles of incidence of the rays. Instead of employing a conical or
spherical form having a circle for its base, the form may be modified so as to be a
pyramid by generating it on a polygonal base. When it is desired to distribute
principally in two opposite directions as for example for illuminating a foot pave-
ment or railway platform, I make the lantern of an elliptical or approximately
elliptical form in plan; and generally the lantern may be shaped in plan as required
for distributing the light over areas of various forms.
When it is desired to diffuse or soften the light as well as to distribute it
uniformly, I break up the steps into a number of segments which project from the
general surface with curvatures arranged in plan as well as in section to effect
distribution. When diffusion only without regard to distribution of the light is
desired, projections from the surface or depressions in the surface are formed as
solids of revolution or conoids having curvature such as to refract the rays or to
reflect them uniformly through a considerable angle in all directions. I prefer to
combine the arrangements for distributing and diffusing by forming the distributing
steps on the exterior and forming diffusing flutings on the interior of the glass
or conversely, and these flutings may be formed so as to operate by reflection as well
as by refraction.

page_text: DESCRIPTION OF THE DRAWINGS
Fig. 8 is an external elevation of a lantern of the form of an inverted cono or
pyramid having the light L situated in the centre of its base. The oblique surface
of the cone or pyramid is made with a number of steps or zones so shaped that the
light passing through them is uniformly distributed on the plane below. The forms
of those steps.or zones are determined in the following manner. Referring to Fig. 1
I assume that a number of uniformly distributed rays proceeding from the light L
meet the interior surface of the glass, then I should form the exterior surface E of
such a curvature that these rays would be refracted in the directions required for
uniform distribution on a horizontal plane below. But as this would involve the
use of glass of great thickness, I divide the exterior surface into a number of steps
as indicated by the dotted lines e, each step having the same curvature as the part
of the curve E to which it corresponde. In this manner I determine the forms of
the successive steps or zones as shewn in Fig 2, each step being adapted to refract
one set of rays and direct them on a portion of the area to be illuminated in such
manner that the illumination of the whole is approximately uniform. Fig. 3 shows
a construction of the curve for one step, and Fig. 4 shews the whole section with
its successive steps. In order farther to give distribution horizontally to several
sets of rays I determine a curve such as is shewn in Fig. 5, in which I assume the
rays from the source of light to be parallel, and I employ this curve in several ways,
that is to say, first I cut the steps or zones determined as previously described into
a number of parts each having the form of Fig. 5 in horizontal section; or, secondly,
I keep the steps or zones entire on the exterior of the glass, and I form the interior
of the glass with vertical flutings such as that shewn in Fig. 5; or, thirdly, I place
together two glasses the one with the horizontal zones and the other with the
vertical flutings. It will be understood that on looking at the light when the
horizontal zones alone are used, it will be seen as a diffused band of light elongated
vertically, when the vertical flutings alone are used it will be seen diffused
horizontally, and when both are used together it will appear as a sheet of uniform
brightness. Instead of employing a curve such as Fig. 5, which operates only by
refraction, I may employ a curve such as is shewn by Fig. 6 which operates partly
by refraction and partly by reflection. As in most cases the rays of light which
are directed upwards are useless for illumination, I generally place above the
lantern a reflector such as is shewn in Fig 7, of a form determined so as to give
uniform distribution by reflection downwards. When a portion of the light
directed upwards is required ns for example to light a ceiling, the reflector instead
of being opaque may be of opaline glass so as to reflect part of the light and to
allow part of it to pass through. Instead of a reflector such as is shewn in
Fig 7, a hemispherical reflector may be used, each ray being reflected back to the
source of light and proceeding thence along with the direct rays through the
glass of the lantern. When the space to be illuminated is not of regular circular
shape but of other forms, the shape of the lantern and the curves of its flutings are
varied to suit the form. For example, in the case of a lantern to light a foot
pavement or a railway platform, I make the curvature of the flutings such as to
direct the light mostly along the line of the pavement or platform. In many cases
the lights are arranged as shewn in Fig. 9, alternately spaced on opposite sides of a
street, so that each has to illuminate a triangular area In such a case a lantern of
triangular form would be suitable. For small lights, such as those of incandescent
electric lamps, the glass lantern may be made in one piece. For larger lights, it is
preferable to make it as a polygonal framing with separate glass panes.
Having thus described the nature of my invention and in what manner the same
is to be performed, I claim :-
First. Constructing a lantern glass with a number of horizontal steps or zones sc
formed that the light rays passing through them are refracted in directions such

page_text: that the light is distributed uniformly over a given area, substantially as herein
described.
Second. Constructing a lantern glass with a number of vertical flutings so formed
that the light rays are refracted or partly reflected and partly refracted in such
directions that the light is uniformly diffused horizontally, substantially as herein
described.
Third. In a lantern glass the combination of horizontal steps or zones for
uniform distribution with vertical flutings for uniform horizontal diffusion, this
combination being effected by forming the zones and flutings on one surface or on
both the surfaces of the glass, or by forming the zones on one of two glasses and
the flutings on the other, substantially as herein described.
In Witness whereof I the said Alexander Pelham Trotter have hereunto sot
my hand and seal this second day of July in the year of our Lord One thousand
eight hundred and eighty-three.
ALEXANDER PELHAM TROTTER (L.S.)