US2619581A - Street lighting reflector with parabolic upward curvature formation - Google Patents

Description

, 1952 P. H. MITCHELL 2,619,581

STREET LIGHTING REFLECTOR WITH PARABOLIC UPWARD CURVATURE FQRMATION Filed 001;. 22, 1948 2 SHEETS--SHEET l [rm/enter PelCiVa/ Elf/'12: 4e

NOV. 25, 1952 P MlTcHELL v 2,619,581

STREET LIGHTING REFLECTOR WITH PARABOLIC UPWARD CURVATURE FORMATION Filed Oct. 22, 1948 2 SHEETS-SHEET 2 [rm en for Perc/va/ H Ni/c/re/l Patented Nov. 25, 1952 STREET LIGHTING REFLECTOR WITH PARABOLIC UPWARD CURVATURE FORMATION Percival H. Mitchell, Toronto, Ontario, Canada Application October 22, 1948, Serial No. 56,010 13 Claims. (01. 24025) This invention relates to improvements in reflectors for use with incandescent lamps and particularly to annular reflectors with a vertical axis, for reflecting and concentrating light rays from flat ring filament lamps centrally located within the reflector.

The principal object of the invention is to providea novel reflector form which will, even when of very limited size, systematically obtain from a horizontal flat ring filament a maximum concentration of reflected light directed at relatively high angles in a vertical plane, whereby light with such concentrated intensity at high angles is directed downward to be incident adjacent the verges of the circular horizontal area illuminated by the filament to augment the light intensity at vertical axis whereby, from successive points upward on the reflector, the light emanating from substantial lengths on the circumference of a flat ring filament as light source, is reflected and directed in a beam centered on the vertical axis and within narrow angular boundaries in the vertical plane to obtain a maximum intensity of reflected light at the desired critical downward an.- gle of concentration.

This invention is applicable to inside mirrored lamps and to reflectors of metal or mirrored glass exterior to an incandescent lamp bulb. As in each application the refraction by the glass bulb of the incandescent lamp more or less affects the direction of the critical reflected rays another featureis' a simple means of compensation for such refraction adaptable to the system of determining the reflector curvatures.

Referring to the drawings,

Figure 1 shows an annular reflector with the scheme of development of the reflecting zones.

FigureZ is a diagram showing filament images as reflected by the reflectors in Figure 1.

Figure 3 shows an annular reflector of the type in Figure '1 external to a conventional incandescent lamp.

Figure 4 shows an incandescent lamp with inside mirrored surface as a reflector of the type in Figure 1. I I

Annular reflectors, in combination with incandescent lamps, are widely used for street lighting to give illumination on a circular area and in further combination with refractors or secondary reflectors the light rays may be concen trated within the circular area to effect asym-' metric distribution with relatively higher intensities on roadway areas. Incandescent lamp filaments have various forms and reflected light directed to the illuminated area is an overlapping of luminous images of the filament reflected frominnumerable reflecting points composing the reflector surface. When the filament is in the form of a flat ring lying in a plane normal to the lamp axis and centered on the axis the filament images, as reflected by reflecting points on the reflector surface, are either straight lines, when the reflecting point is in the plane of the filament, or ovals, from other reflecting points, above or below this plane, the ovals having relatively short minor axes. In conventional methods of describing the concentrating upward curvature of the reflectors for use with such lamps the centrev of the flat ring filament, at the intersection of the filament plane and the vertical optical axis, is taken as the focus for the curvature, which is generally parabolic. This has been acceptable when relatively large reflectors are used, such as with a diameter of ten times the filament circle diameter but for small reflectors, with diameters, say, of five times the filament circle diameter, it is notable that the light intensity obtainable by concentrating reflectors is relatively low. By the methods I employ the intensity obtainable by concentrating reflectors is substantially increased.

Flat ring filaments are standard construction for general service multiple lamps up to 200- watt size. The filaments are not complete circles but may be four-fifths of a circle, the open space being between the supporting conducting wires leading to the filament. The filament in plan may not be circular but may be four sides of a pentagon. For purposes of description'and design, in the following, the filament is considered as a complete circle and the effect of divergence from this shape is shown later. For any oval aspect of the filament as viewed from a point on the reflector the luminous intensity of any short length of filament is considered as directly proportionate to the whole circumferential length; this is acceptable as a general rule.

In street lighting luminaires for use with multiple incandescent lamps up to 200 watts size it is desirable to have small reflectors so that the whole device may be of relatively low cost. As suitable for such use I show reflectors nominally five inches in diameter. The same system of design may be applied to any size of reflector.

A conventional beam angle for concentration for street lighting purposes is at '75 degrees above the nadir or 15. degrees'below a horizontal plane and correspondingly the cut-off of the reflector bottom is at 80 degrees above the nadir which is measured at 10 degrees downward from the centre of the filament. The reflectors shown are based on these angles but other angles can be used as desired. While the best results are obtained at lamp settings for the desired angle of beam, by raising or lowering the lamp relatively to the reflector the beam will be lowered or raised and within a few degrees of shift the results are generally satisfactory.

In general, the light distribution of a street lighting luminaire, in a vertical plane, includes direct light from the lamp source from the nadir up to the cut-off and reflected light from the re flector, incident from the light source, from the cut-off up to the top of the reflector, the reflector top limit being controlled by the necessity of clearance of the neck of the lamp bulb. The lower part of the reflector contributes light to the beam and the upper part contributes light at angles from under the beam to part-way down to the nadir. The improvements shown are concerned with the lower zones of the reflector contributing light to the beam and at angles immediately above and below the beam angle.

Figure 1 shows the reflector curvature required for a specific condition of .a fiat ring filament in a horizontal focal plane and centered on a vertical optical axis with a filament ring diameter of one inch and the nominal diameter of the reflector bottom is five inches. The desired reflected ray concentration is maximum concentration over a one-degree zone between 74 /2 and 75 degrees above .the nadir; that is, the concentration desired is for a Iii-degree beam. The concentration over one degree is an arbitrary measure. In this figure refraction through a lamp bulb is not considered but as, in general, the rays will be diverted from their incident paths when passing through the glass bulb, means for compensating for this effect will be shown.

In Figure 1 the curved line extending from M up to S is composed of the joined parabolic arcs MN, NOP and PQR and a circular or elliptical arc RS. This curved line rotated about the vertical optical axis YY describes the annular reflector. ML is the reflector bottom and the diameter at ML is, in the example shown, 5 inches. XX is a focal plane normal to YY which intersects at C. A and B on the focal plane represent the two points on a flat ring incandescent filament as cut by the plane of the figure. The diameter of the flat ring filament in the example is 1 inch so that A is inch from C. The line MC is inclined degrees above ML intersecting YY at C, locating the focal plane and making a conventional SO-degree cut-off. Through A three lines ee', if and go are drawn inclined at 15 /2, 15 and Bi degrees respectively, to XX; these are parabolic axes for the parabolic arcs MN, NOP and PQR respectively, with the point A as focus common to all these arcs.

Commencing with the fixed point M, MN is a parabolic arc with as as axis and A as focus; this continues to N at which point lines N to A and N to B make the angle ANB equal to one degree. The parabolic arc NOP has ff as axis and A as focus, NA being a parabolic radius from A for both adjacent arcs MN and NOP with their parabolic axes at 15 /2 and 15 degrees to XX respectively. As viewed from N the whole ring filament appears as a much flattened oval with a minor axis equivalent to one degree and a major axis equivalent of about 24 degrees length, measured angularly. This parabolic arc NOP intersects the focal plane at O and as viewed from O the ring filament appears as a straight line. At

. P the angle between lines PA and PB is again,

one degree. The parabolic arc PQR has gg' as axis, inclined 14 degrees to XX and A as focus. The terminus of this are R; is determined by a line Ra drawn from R inclined at 14 degrees to XX so that-it passes under the reflector bottom at L on the opposite side with a clearance of the order of inch. The curved arc RS may be elliptical with one focus at A and a remote focus on Ra in the vicinity of and under L or may be an equivalent circular arc; it is only in the vicinity of R on this are that reflected rays contribute to the light concentration in the critical one de gree. The parabolic radius AN is common to the parabolic arcs MN and NOP but the parabolic axes and focal lengths differ; similarly the parabolic radius AP is also common to the adjacent parabolic arcs and the complete curvature of the joined arcs is continuously smooth. Q is a point on the reflector where lines from Q to A and C may be the angle AQC 1 /2 degrees; this is not constructional but is for reference in Figure2.

The lines Ma, Mb, Na, Nb etc. show paths of reflection of points A and B from reflecting points M, N etc. on the reflector, respectively, Ma and Ra being inclined at 15 and 14 degrees respeotively to XX.

Figure 2 is a diagrammatic view showing ring filament images (M), (N), (O), (P), (Q) and (R) as reflected from points M, N, O, P, Q and B respectively, on the reflector in 'Figure 1. These images are to scale angularly, that is, the

minor axes of the ovals represent or correspond to the angles between the lines from each respective reflecting point to A and B, and. the major axes represent or correspond to the diameter of the filament ring in the focal plane as viewed from the respective reflecting point. The points on the major axes corresponding to C in Figure 1 are indicated (C). The corresponding locations of A and B in each image, shown as (A) and (B), are at the termini of the minor axis and of C, shown as (C), at the intersection of the major and minor axes. The minor axes are coincident with the trace of the vertical optical axis (Y) (Y).

In the diagrams two datum lines TT and UU, located by reference to A, are at 7 1 and 75 /2 degrees respectively, and are shown to the same angular scale. It will be appreciated that since the light from A incident at R strikes a parabolic reflecting surface having A as focus and line gg' passing through A and inclined at 75 degrees to the nadir as axis, it will be reflected at 75 degrees. 1 Therefore datum line UU coincides with (A) in Figure 2. The portion of the filament between UU and TT contributes to a beam between 75 /2 and 74 degrees. The length of the filament image between these two lines relatively to the whole circumference of the oval thus is taken as the measure of the concentration effected. For comparison with the possible concentration using the point C in Figure 1 as the focusand a line through C inclined 15 degrees to XX as axis for describing the parabolic arc corresponding to ME in Figure l, the datum lines W and WW are drawn one-half degree above and below the location of (C) in each oval. Itwill be noted that datum lines are com.- mon to each system of reflector in (N), (O) and (P) and, in these, concentrations are equal. For oval images (M), (Q) and (R) the concentration when A is the focus is measured by the long arc of the side of the oval image and when C is the focus by the two short arcs at the ends of the oval image. For (M) the relative concentrations are of the order of 7 to 1; for (Q), 10 to 1; for (R), 8 to 1. As measured from A in Figure 1 the angular length of are from M to R is 32 degrees while the angular length of arc from N to P, in which the concentration is equal for both systems is approximately degrees. Outside this 5-degree anglethe concentration by using A as focus is greater than by using the. conventional focus at C.

It will be noted in these diagrams in Figure 2 that when the minor axis of image ovals is one degree or less in angular length the whole filament image is centered on the focus in line with the vertical optical axis YY. When the minor axis of image ovals is greater than one degree in angular length the critical portions of the filament length are similarly centered when my reflector is used, but if the focus C is used the twov critical portions of filament, at the ends of the ovals, are remote from the vertical optical system axis YY each being located up to half the major axis or some 12 degrees angularly away from this axis. In annular distribution this remoteness is of no consequence but when auxiliary devices such as secondary reflectors or refractors are used to obtain asymmetric distribution, centered on asymmetric axes, for example up and down a street, these widely spaced concentrations of filament light cannot satisfactorily contribute to the desired axial asymmetric concentration.

In Figure 2 it will be noted that while in (O) the straight line image and the ovals in (N) and (P) are well centered on a 75-degree beam axis the ovals below as represented in (M) with the lower terminus of the minor axis at 74 degrees have the major portion of the ovals above 75 degrees indicating that reflected light rays are directed upward to several degrees above 75 degrees. This is a desirable feature and is consistent with accepted practice. It is possible however to reverse these ovals from M to N so that the top of the ovals is at 75 degrees and.

all the reflected rays are directed at and below 75% degrees. In Figure '1 the point N on the reflector was located so that rays reflected from A and B would be directed at 74 /2 and 75 /2 degrees respectively. N is a point on a parabolic arc with axis inclined at 74 degrees to the nadir through A. Then N is also a point on a hypothetical parabolic arc with B as focus and axis through E inclined at 75 /2 degrees. The are MN can be made with B as focus with the parabolic axis hh through B inclined: at 75 /2 degrees so 6 that reflecting point between M and N on this arc reflect rays from B at 75 /2 degrees and from A at lower angles. The reflector arcs above N are made with A as focus and the arcs join to be continuously smooth. The effect of this can be shown in Figure 2 by locating the datum lines 8's and T'T at the top of the oval image (M) as from point ,M. Thus all reflected light from the reflector is at and below 75 /2 degrees with the same intensity in the critical one degree but with increased in-' tensity below this. There is then no reflected light above 75% degrees.

For any reflecting point between M and R, in Figure 1, with A as focus, the same point is on a hypothetical parabolic arc with B as focus and with a parabolic axis through B inclined to correspond with the angle of the reflected ray path of B as source. Then at any point between M and R the focus can be shifted from A to B, or vice versa, with a consequent variation in the scheme of concentration and vertical distribution. The actual amount of light incident on the zone from M toR, from the filament, is fixed in quantity. The rearrangement of this quantity of light between fixed angular boundaries and immediately.

outside these boundaries can effect a substantial range of distributions for practical results in street or area lighting.

Figures shows the reflector and filament as in Figure 1 but with a glass filament enclosing lamp bulb K, 3% inches in diameter, whichis standard for ZOO-wattgeneral service lamps; such bulbs have the centre for the herispherical bottom at F, of an inch below the focal plane XX'. The vertical axis is Y'Y' with intersection with XX at C. A and B show the filament as cut by the plane of the figure. Three reflecting points M, O and R are shown corresponding to M, O and R in Figure 1. The reflected ray paths of A, M'a', Oa' and Ra meet the glass bulb at m, o and 1. On passing through two curved glass Walls of the bulb to emerge from the bulb these rays will be refracted and the angular relation of the emerging paths to the incident paths may be calculated, or measured by simple laboratory means. The deviations may be of the order of 80 minutes and 40 minutes downward for rays from M and 0' respectively and 12 minutes upward for rays from R. A simple correction for these magnitudes of refraction is to locate a focus to be used instead of A. This focus may be located by drawing M'H inclined 80 minutes above MA and OI 40 minutes above O'A and the intersection of these two lines locates D; a line from R. to D is acceptably close to 12 minutes below RA. Using D as focus instead of A and with similar procedure in development of the series of parabolic arcs the paths of rays reflected from MO and with A as source, will be raised and the path from R lowered so that on emergence from the bulb these and intermediate paths will be substantially correct.

In Figure 3 the point Z locates the bottom edge of a 7-inch diameter reflector with SO-degree cut-off and rays from A reflected from a reflect ing point at Z will meet the glass bulb at z. This indicates a practical dimensional limitation for small reflectors with maximum directional control for this lamp bulb size as light rays from a corresponding point on a larger reflector meeting the glass bulb will be closer to the bottom of the bulb and cannot, satisfactorily, have compensation for refraction. It is well known that the bulb bottom, beyond this region, is in efiect M and pointsintermediate a "blind zone due to externalv and. internalv refiection and extremes of refraction.

Figure 4-shows a reflecting surface as in Figure 1 adapted to an inside mirrored incandescent lamp. The lamp L has a conventional external maximum diameter of 5 inches. The vertical axis is Y"Y" and the focal plane is X"X" with intersection at C". The mirrored interior surface of the glass enclosing bulb extends from M" located by a line drawn from C" inclined degrees below- 1:x". A and 3" represent thev one-inch diameter ring filament lying in the focal plane, as cut by the plane of the figure; this is centered on C". The lower portion of the glass bulb is shown, arbitrarily, as hemispherical with centre at E on Y"Y" where a line ZEZ drawn parallel with X"X" is normal to the curve above M" extended downward. When the curved surface from M" to R" is developed as in Figure 1, light rays reflected from a luminous point at A" will be directed from M" along Ma inclined at degrees to X"X"; from 0" along 0"a" inclined at 15 degrees to XX"; and from R" along Rfr" inclined at 14 degrees to X"X". These path lines meet the inner bulb surface at m, o and 1", respectively, and on passing through the curved glass are refracted, those through the glass at m and 0 being lowered and those at r being raised. The angle of change is slight, in this case, and may be of the order of minutes for those at m and r and only 3 minutes for rays at 0-. A substantial correction maybe made by drawing a line from M" inclined at an angle of 20 minutes above M"A intersecting X"X" at D then D is used as the common focus for the several parabolic arcs forming the curve from M" to R" with construction of the whole curvature from M to S" as described for Figure 1. Then from M" the ray M"A-" is reflected to be 20 minutes high, from O" the ray will be, as before, 3 minutes low and from R" will be 20 minutes low and on emission from the bulb will be substantially at the desired angle. Intermediate points on the curvature will reflect light rays from A" with intermediate angles of correction and there will be a systematic compensation for refraction through the bulb. For other practical forms of bulbs, including composite forms of the sealed beam type, corresponding correction means can be employed.

In this case an alternative can be used. If A is used as focus and the Whole reflector curvature from i to R. is one parabolic arc with the parabolic axis through A" and inclined 15 degrees to X"X" light rays from A" will be reflected at M" to be at 75 degrees but by refraction will be lowered approximately 20 minutes below l5 degrees; at O" the reflected rays will be lowered 3 minutes; and at R" the reflected rays will be raised, by refraction, 20 minutes. These rays then establish a zone of concentration with limits of 20 minutes above and below '75 degrees.

Figure 1 shows the system of reflector development using three parabolic arcs for the maximum concentration of light rays within one degree centered on a 75-degree axis. An alternative system is to use one parabolic are extending from M to R in Figure 1 with focus at A and with axis 1? inclined at 15 degrees through A. While for reflecting points in the region of the focal plane the concentration will be equal for both systems, at other reflecting points the reflected filament images contribute only to the one-half degreev above. or below. the .75edeg1tee. axis; and.

thus. the concentration: within one .degree will be less. concentration but concentration substantially greater than obtainable by using C .as the focus.;: In Figure 2 inv each oval it is obvious thattthe midway point on the minor axis between the .two

boundaries. of the critical one-degree, that is themidway point between UU. and TTisat or .corresponds to 75 degrees inclination, then for each oval a point on the focal plane, near A..but .to-..

ward C, in Figure 1 could have been. used. at.

the respective focus with a 75-.degree axis. In (R) in Figure 2 thispoint would .be approximately one-seventh of the distance from A .to C;

in v(Q), onethird; in (N) (O) and (P) it .could. be acceptably at A, C or any point between; in.

(M) it would be at one-quarter the distancesfrom A to C. A compromise common focus would. be located one-quarter the distance from. A to C, on the focal plane with M N O P Q R as one parabolic arc with axis at 75 degrees through.

the focus. Such a compromise would give sub stantial concentrations from a. reflecting zone extending from M to beyond Q and beyond this towards B there would be a greater lengthtof luminous filament contributing to the critical one degree than if the focus had been at C. As a further alternative compromise the one parabolic focus could be located midway between A. and C and over the whole parabolic arc there would be a greater concentrating effect than if C had been used. As a practical limitation in these alternatives the alternative focus would be closer to A than to C. 7

As stated, filaments of the incandescent lamps used in combination with these reflectors are not complete circles due to the gap between the conducting wires. When the open space, that is without a luminous filament, occurs in the position corresponding to the focus A in theFi'gures 1, 3 and 5, there is a deficiency in the intensity of reflected light in the critical one degree but generally with good intensity at other high angles. When the open space occurs in the position of B, in the same figures, there is good intensity in the critical one degree but a deficiency at other high angles. If A is the focus for arcs of the reflector surface above N,in Figure l, and B is the focus for the arc MN and theopen space in the filament occurs at A or B the deficiency in the critical one degree. is not so great. When in the horizontal distribution the deficiency is at right anglestov the street axis, in

street lighting applications, the effect is asym metric, which may be favourable. If the filament is oriented within the luminaire this effect can be used advantageously.

Most of the Well made commercial lamps have filaments reasonably closely conforming in plan to arcs of a circle so that the above analyses hold essentially true. When adherence to true circles is justified, one or more extra filament supports can be incorporated'into the lamp construction. When the filament conforms to four or five tangents to a circle there is, in effect, a systematic variation in beam intensity according to the different aspects of the filament from respective reflecting points on the reflector.

In the range of use of reflectors of this type, in certain applications the complete 360 degrees of annular reflector of the form described may not be required and only those sectors contributing to .asymmetric beams may be of these forms While adjacent or intervening sectors may There can be compromises .tov obtainless.

'be of other curvatures adapted to other distributions favourable to pavement illumination in areas outside the asymmetric beam paths. It is to be understood that while descriptions refer to annular reflectors of continuous form, reflectors incorporating sectors of limited annular extent having the described curvatures and as elements of reflectors of composite form are included within the scope of my invention.

What I claim as my invention is:

1. An annular reflector surface having a vertical axis and a focal plane in right angular relation thereto, a flat ring-type filament light source centered on the vertical axis and lying in said focal plane, said reflector surface extending below said filament and having a smooth upward curvature in any plane through said vertical axis, the lower portion of said upward curvature being formed of three joined lengths of parabolic arcs having foci lying adjacent the circumference of said ring filament, and the axis of each parabolic are being inclined angularly to the axis of the adjacent parabolic arc and'at an angle to the nadir greater than 65 degrees and extending in a direction of desired light concentration with the axes of the first and third parabolic arcs extended forming an acute angle therebetween determining the angular dispersion of reflected light concentrations to provide a light concentration whereby from successive points upward on the reflector the light emanating from substantial lengths on the circumference of said ring light source is reflected and directed in a beam centered on the vertical axis and within the narrow angular boundaries corresponding to the angle between the axes of the first and third parabolic arcs to obtain maximum intensity of reflected light at the desired downward angle of concentration. r I

2 A reflector as claimed in claim 1 in which the foci of said parabolic arcs are common and lie on the circumference of said ring filament.

3. A reflector as claimed in claim 1 in which the first of said parabolic arcs has its focus lying on the circumference of said ring'filament at a point on the remote side of said vertical axis and the third of said parabolic arcs has its focus on said ring filament at a point on the adjacent side of said vertical axis. v

4; The combination with a flat ring type incandescent filament, of an annular upwardly curved reflector adapted to concentrate light in a high angle beam having an angular dispersion of the order of one degree, said reflector having a vertical axis onwhich said filament is centered and a focal plane above the lower edge thereof -onwhich said filament lies, said reflector being generated by the revolution about the vertical axis of a smoothly curved line the lower part of which is defined by a plurality of parabolic contiguous arcs, said arcs having a common focus 'mediate arc symmetrical above and below: the

focal plane and of a length limited toits ter- -mini being'point areas reflecting light from points adjacentthenear and the remote side of -thefllament to be coincident with the boundary 10 angles whereby from successive points upward on the reflector the light emanating from substantial lengths on the circumference of said ring light source is reflected and directed in a beam centered on the vertical axis and" within narrow angular boundaries corresponding to the boundaries defined by the axes of said upper and lower parabolic arcs to obtain a maximum intensityof reflected light at the desired downward angle-of concentration.

5. A reflector as claimed in claim 4, in which said filament is enclosed in a glass bulb and the focus of said parabolic arcs is located intermediate the filament circumference and ..the vertical reflector axis to direct rays reflected from said reflector at an angle to emerge after refraction by .the curved glass of said bulb Within. the desired angular boundaries. '6. An annular reflector having a vertical'axis and a focal plane in right angular relation thereto, a flat incandescent ring filament centered on said axis and laying in said focal plane, said reflector having a smooth upwardcurvature in any plane through its vertical axis with the lower portion of said upward curvature being defined by a plurality of contiguous arcs, comprising a parabolic are extending either sideof said focal plane having a focus adjacent the circumference of said filament and a parabolic axis lying in said plane through the vertical axis and inclined at an angle below the focal plane substantially less thana quarter right angle equal to the desired angle of maximum light concentration, a parabolic are extending below said first-mentioned parabolic arc and having a focus adjacent the circumference of said filament and a parabolic axis lying in'said plane through the vertical axis and inclined at an angle of the order of a degree to the aforesaid parabolic axis to determine a lower angular boundary of maximum light concentration, and a parabolic are extending above said firstmentioned parabolic arc and having a focus adjacent the circumference of said filament and ,an axis lying in said plane through the vertical axis and inclined at a vertical angle of the order of a degree to said first-mentioned parabolic axis to determine an upper angular boundary of maximum. light concentration. a 7. A device as claimed in claim 6 in which the end points of said first-mentioned parabolic arc define with points on said filament on opposite sides of said vertical axis an angle corresponding to the angle between theboundaries of desire maximum light concentration. 4

8. An annular reflector surface for a ring type filament having a vertical axis and a focal plane in right angular relation thereto in which a ring filament is adapted to be located centered on said axis, said reflect-or surface extending below-said focal plane and having a smooth, upward curvature in any plane through said vertical axis, the lower portion of said upward curvature being constituted by three joined lengths of parabolic arcs each having its focus lying adjacent the circumference of said ring filament, the intermediate parabolic are having an axis inclined downwardly at an angle of the order of 15 to the focal plane and the parabolic arcs above and below said intermediate are having axes inclined downwardly the one at an angle of the order of 15 /2" and the other at an angle of the order of 14 to said focal'plane. 9. An annular reflector for a ring type filament having a reflect-or surface generated by the rotation of a composite curve about the axis of sym- 'metry of the filament, the generating curve of said surfaceincludingasection to formthe lower portion of the reflector surface made up of three joined parabolic arc lengths each having a focus approximately coincident with the circumference of said filament, the axis ofithe intermed'iate'of said parabolic arcs being inclined downwardly at an angle of the order of 15 toythepl-ane of said filament, the axes of the other parabolic arcs above and below said intermediate are ,being inclined downwardly to the plane of said. filament, theone atv an angle of the order of Li and the other at an angle .of the order of 15%; to said filament pl-ane.

10. An annular reflectorfor a ring type filament having azrefiector surface generated by the rotation of a composite curve about the axis of bolic .arc lengths each having a focus approximately coincident .with the circumferenc .of said fllamenttheaxis of the intermediate of said paraboliclarcs being inclined downwardly at an angle of the order of 15 to the plane of said filament, the axis .of the other parabolic .arcs above and below said intermediate are being inclined downwardlyto the plane .of said filament, the one atan angle of Li and the other at .an angle of theorder of 15%;" to said filament plane, and

said elliptical arc having a remote focus lying within the divergence of ,thenaxes of the first :and third parabolic arc lengths.

11. An annular reflector surface for a ring type filament having ;a vertical axisland a focal plane in right angular relation thereto in whichea ring filament is adapted .to be located centered on said axis, :saidreflector surface extending .below the focal plane and being described by the revolution of acurve about the vertical axis, said curve including a parabolic arc length centered on the focal plane and a parabolic arc length aboveland below said first arc length, said arclengths all having a focus approximately coincident with the circumference ofsaid filament and adjacent arc lengths having common parabolic radii to form=a continuous curvature but having different focal lengths, the first of said arcs having a parabolic axis intersecting said vertical axis and inclined below the focal plane at an angle substantially less than a quarter rightangle, the arc lengths either side of said first arclength having parabolic axes'inclined to'the aforesaid parabolic axis :at a small-acute angle and defining boundariesof light dispersion of a beam re- -fiectedfrom the reflector surface portion described by'said parabolic-arc lengths directed on the parabolic axis of said first parabolic arc length centered on-said focal'plane.

12.-Anannular street lightingbeam producing device, comprising an annular reflector arranged coaxially with a glass enclosed flat ring incandescent lamp filament and extending below and above the-focal plane in which the filament lies and having a reflecting surface described by the-revolution of a curve about the vertical axis, said curve including a parabolic arc length centered-on the focal'plane and a parabolicarc length above and below said first arc length, said are lengths-all having a focus approximately -coincident with the circumference of said filament, and adjacent arc lengths havingcommonparabolic radii to form a continuous curvature but having different focal lengths, the first of said arcs having a parabolic axis intersecting said vertical axis and inclined below the focal plane at an anglesubstantially less than a-quarter right angle, the arclengths either side of said first arc length having parabolic axes inclinedto the aforesaid parabolic axis at a small acute angle, said surface projecting a light beam with-a zone of'maximum intensity of light concentration along said parabolic axis of said first-mentioned parabolic arc at a-high angle above the nadir and belowa horizontal plane, saidbeam having the zone ofmaximum concentration of projected light contained between specific angularl-y; spaced zonal boundaries defined :by the parabolic axes of said parabolic arc lengths-above and below said first-mentioned arc length, the reflected images .of the luminous filament from zones of the reflector lying in the focal plane and-immediately. adjacent being projected -to be within the-said zonal boundaries, and reflected oval images of the luminous filament from reflecting points in said surfaces in zones below the immediate zone at the focal plane being projected-to have the lower edge of each image coincident with the lower zonal boundary andthe w-holeimage lying partly Within the zonal boundaries and partly above the zonal boundaries, and reflected oval images :of the luminous filament from reflecting points on said surface lying within a limited zone above th immediate zone at the focal plane being projected to have the upper edge of each image coincident with the upperzonal boundary and the whole image lying partly within the zonal boundaries and partly above.

13. An annular reflector having .a verticalaxis and ,a focal plane in right angular relation to the axis thereof, a flat ring-type filament light source centered ,on the vertical axis and, lying in said focal plane, said reflector having a smooth upward curvature in any plane through its vertical axis such that reflector points on the reflector lying in the focal plane reflecting images of the said filamentas straight-line imagesand reflecting points-on the reflector above and below the focal plane reflecting images of the said filamentas flattened ovals widening in their upward diameter measurable along their minor axes as the reflecting points recede from the focal plane,

saidsurface being described by the revolution about the vertical ,axis vof-a curve having at least the'lower portion COmDriSedby apluralityof contiguousparabolic arcs each are having its focus 10D the circumference'of; said flat ring type filament light sourceand ihavingits axis inclined angularly to the axis-of the adjacentparabolic arc,-said arcs comprising an intermediate arc with its parabolic axis inclinedqto the nadir at an angle greater than andzcorresponding to the desired angle of downward concentration'of light rays and substantially symmetrical with and extending above and below the focal plane to a respective zone in which a-reflectingpoint reflectsoval images of the said filamentlight source to have a diameter on its minor axis to provide an annular spread of substantially less than 10 ofthelzoneof effectedconcentration of light rays centered on the parabolic axis of said intermediate arc, and the axes of the adjacent parabolic arcs being inclined to the axis of the said intermediate arcto effect from successive points downward on the lower adj acentpa-rabolic. arc asuper- REFERENCES CITED The following references are of record in the file of this patent:

Number Number 14 UNITED STATES PATENTS Name Date Mygatt Nov. 5, 1912 Benford Feb. 21, 1928 Beck et a1 Oct. 23, 1934 Halvorson Mar. 1, 1938 FOREIGN PATENTS Country Date Great Britain May 14, 1931 Germany June 27, 1935

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