Improvements in Reflectors for Lamps and other purposes.
Year: 1891

page_text: Date of Application, 19th Aug., 1891-Accepted, 21st Nov., 1891
COMPLETE SPECIFICATION.
Improvements in Reflectors for Lamps and other purposes.
I, ALFRED JOHN LOFTUS, (Phrá Nidesa Jalahdi) K.C.C.S. of Wat Chang,
Bangkok, Siam, Captain and Hydrographer in His Siamese Majesty's Navy, do
hereby declare the nature of my said invention and in what manner the same
is to be performed to be particularly described and ascertained in and by the
following statement :-
Reflectors for lamps are generally made without much attention being given to
the exact form of their curve and consequently we generally find that only a small
fraction of the light that might be reflected is so reflected as to be rendered
serviceable.
The curve produced has hitherto been either circular, that is to say part of a
sphere, or parabolic.
Now my invention consists in a combination in one reflector of two or more curves
so arranged as to produce when a luminous flame is placed in or about its focus a
fairly divergent beam of light, and so that when the lamp is looked at from a
distance in front every part of the reflector appears luminous, and the flame entirely
fills the whole reflector.
The reflectors hitherto in use do not "fill" with light in so perfect a manner as
do those made according to my invention, and although the difference in the
sectional curve is slight the gain in illuminating power is very great, a very small
difference in the angle of reflection making as is well known a great difference to
the course of the reflected rays.
In carrying my invention into practice 1 draw the curve, for any given set of
conditions, as hereinafter indicated, and then cut a templet to correspond accurately
with the curve described, and this is used by the workmen who fashion or form the
reflectors of metal or glass in the usual manner, working accurately to the curve
supplied them. In this way I have succeeded in obtaining excellent copper
reflectors spun in a lathe and weighing very much less and consequently at very
much less cost both in respect of labour and metal, and yet much more effectual
than the ordinary copper and other reflectors now generally in use for signal lamps
and the like.
I class my reflectors into three divisions namely 1st those in which the focal
point falls within the cavity of the reflector, that is to say, nearer to the surface of
the reflector than a line passing diametrically from one edge of the reflector to the
opposite edge. 2nd, those in which the focal point falls on or near such line, and
further from the
3rd, those in which the focal point falls without, that is to say,
surface of the centre of the reflector than such line. These I term respectively
1st, 2nd, and 3rd grade reflectors, and shall so refer to them throughout this
Specification.
In order to set out the curve or series of curves for a reflector of given dimensions
I proceed as follows, as illustrated in the accompanying sheets of drawings and
indicated by tie figures and letters of reference.
Fig. 1, represents the method of setting out the curve for a 1st grade reflector.
Fig. 2, the like for a 2nd grade reflector; and
Fig. 3, the same for a 3rd grade reflector.
Having decided on the diameter of the reflector I am about to design I draw two
parallel lines A and B at a distance apart equal to the said diameter and midway
between them Araw another line Fc parallel thereto to represent the optical axis
of the reflector.
If now I in. nd to draw a 1st grade reflector I take as radius the distance
between the 1' 3 Fc and A that is to say I take half the diameter of the reflector.

page_text: If it is a second grade reflector I am about to design I take a radius and have found
that the following proportion gives a good result namely about th greater, and for
a third grade reflector in like manner a good result is obtained by taking a radius
about rd greater than for a first grade.
With such radius, from any convenient point C on the line F c, I describe an arc 5
touching or cutting, as the case may be, the lines A and B.
From the centre C I draw CE and CG at from 20° to 30° on each side of the
line C F, I prefer an angle of 27° and I produce these lines beyond C. The
points E and G are the limits of the curve struck from the centre C the curve
beyond being struck from other centres.
I next draw tangents to the arc touching the arc at E and G respectively, that
from E cutting the line A. at H.
I next seek a point on the line A between the arc and the tangent to indicate the
edge of the reflector and I select for trial any point, say J. I then draw a straight
line J E to represent a chord of the intended arc, I bisect such line in K and
from K draw a line perpendicular to J E cutting EC in "L," then with L E as
radius from the centre L I describe the arc E J. I produce L K to cut the arc
in N. I then test the curve in the following manner. I draw O. E. parallel to A
falling on the curve "EG" at "E." From "E" I draw ER" making the
angle "REC" equal to OE C. Then R is the focus of that part of the curve,
and approximately for all parts between E and G. From R as a centre I draw a
circle the diameter of which is less than, say about one half of the diameter of the
flame of the lamp to be used.
66
I draw other lines M N etc. parallel to A and draw lines from the points of their
intersection with the arc, for instance the point N making the angle L N P equal
to the angle M. N L. and I produce N. P. to cut the line F. C. If now the
line N. P. falls within the circle drawn round the point R. the curve is satisfactory.
If however the lines drawn as above described from points in the curve E J do not
fall within the circle round R a fresh point J must be taken and the process
repeated until a curve is found which satisfies the condition of focussing all the
rays within a small circle about the point R or round some point near R on the
line F C.
A direct method by which the point J.may be approximately determined is as
follows:-
Produce C F to S making F S equal to F R. From S draw S T perpendicular
to RS and cutting line A in T. Then on the line A T make T D equal to D R.
Then J, the point sought, will be between D and H, distant from D about one third
of the distance from D to H.
An approximately similar but elliptic curve may be described from two foci on
each of the lines E C, G C.
On the produced line E C I mark a point U making C U equal to half E C, and
on the same line another point V making CV equal to E C.
L
With the points U and V for foci at the distance E I describe the elliptic
curve E J continuous with the arc G F E.
Similarly I produce the radius G. C, beyond C, and from similarly determined
foci describe an eliptic curve from G, to J₁ on the line B, corresponding
with E J.
Then J. E. F is one half of the curve to which the reflector is to be made.
The reflectors which I have described may be used with advantage by dentists,
photographers, and generally where a strong light is required without any lantern
or case.
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
claim is :-
1. Constructing reflectors for signal and other lamps and for other purposes
substantially as described and set forth.
1

page_text: 2. Constructing reflectors for signal and other lamps and other purposes in such
manner that in transverse section the central portion has a circular curve and the
portion beyond has an elliptical curve, the two curves being continuous and having
a common tangent at their junction, the elliptical portion focussing parallél rays of
light on or about the same spot as does the circular portion, or in place of the
elliptical curve an approximation thereto formed of a circular arc and such curve
or curves being arrived at and arranged substantially as hereinbefore described and
set forth.
Dated the 10th love of A vonat 1901
ALFRED JOHN LOFTUS.