US 3761957 A
A lens for a direct filament lamp assembly incorporates spaced rows of horizontal flutes having constantly varying prism angles which compensate for differing focal distances from the filament to the lens and varying angles between the emerging light rays and the lens normal thereby producing an illumination pattern of desired horizontal and vertical intensity.
Description (OCR text may contain errors)
United States Patent 1 Jarrett, deceased Sept. 25, 1973 DIRECT FILAMENT LAMP ASSEMBLY Robert O. Jarrett, deceased, late of Anderson, Ind. by Helen M. Jarrett, executrix Inventor:
Assignee: General Motors Corporation,
Filed: Sept. 13, 1971 Appl. No.: 180,024
US. Cl 313/110, 240/83, 240/413, 313/111, 313/116 Int. Cl. H0lk l/30, HOlj 5/16 Field of Search ..313/110,111, 116; 240/413, 8.3
References Cited UNITED STATES PATENTS 8/1933 Dickson 240/83 12/1936 Taylor 240/413 3/ 1969 l-lundley l/1967 Dahlke.....
6/1936 Peple 3/1958 Martin 240/83 Primary Examiner-Alfred L. Brody Attorney-J. L. Carpenter et al.
ABSTRACT A lens for a direct filament lamp assembly incorporates spaced rows of horizontal flutes having constantly varying prism angles which compensate for differing focal distances from the filament to the lens and varying angles between the emerging light rays and the lens normal thereby producing an illumination pattern of desired horizontal and vertical intensity.
3 Claims, 8 Drawing Figures Patented Sept. 25, 1973 2 Sheets-Sheet l 7F? INVENTOR.
Wm Qlzrretikceased BY Helen Mizrrezt eazirz'x Wmfl Patented Sept. 25, 1973 3,761,957
2 Sheets-Sheet 2 IN VENT OR.
foberkdvfirrettih'eami BY Helen M. Jarrett, xemir1k ATTOR EY DIRECT FILAMENT LAMP ASSEMBLY The present invention relates to lamp lenses and, in particular, to a lens for a direct filament lamp assembly having coorective optics for providing predictable illumination patterns.
Many of the smaller lamp assemblies currently used on motor vehicles are of the direct filament type wherein beam control is provided solely by an optical lens as contrasted to primary type lighting units such as headlamps wherein a reflector and lens cooperate to control and distribute the major portion of the illumination emitted by the filament. These direct filament lamps are generally used as indicator lights, such as marker lamps, or intermittent driving lights, such as backup lamps, all of which do not require strict beam control.
For such lamps, the illumination pattern can be conveniently obtained by using rows of horizontal flutes having a fixed or constant prism angle with respect to the lens surface. These constant angle flutes are generally effective in disciplining the central area of the beam and will establish a fairly predictable horizontal and vertical light distribution. However, light rays from a fixed light source, particularly at the outboard regions of the lens, strike a constant angle horizontal flute at varying angles and emerge through the outer face of the lens at varying angles from the lens normal thereby causing an upward or downward bending of the light pattern. The smile or frown as it is commonly called in the lighting art decreases the light intensity in these regions. For installations such as backup lamps which ideally have a light pattern with a wide horizontal spread and a narrow vertical spread, the smile" or frown reduces optical performance thereby requiring a larger unit to compensate for the resulting illumination loss.
The present invention provides a means of improving the lens optics for direct filament lamp assemblies such that the light beam is flattened out to eliminate the aforementioned smile" or frown." With this arrangement, the light intensity is increased at the wide angles of the desired pattern. More specifically, these features are achieved by providing each of the horizontal flutes with a constantly differing prism angle which compensates for changing focal distances and varying surface normals. Thus, for the center section of the lens, the flutes have a prism angle which places emerging light rays for optical compliance with the desired intensity in the central region of the illumination pattern. For each increment horizontally, the prism angle is appropriately revised to account for the longer focal distances and the incident angle of the impinging light rays with the resultant effect that the emerging light rays are directed for compliance with the desired illumination pattern.
This improved lens is made by forming horizontal grooves on the core of the lens mold which are mirror images of the desired flutes. The grooves are formed by a cutter which is selectively pivoted with respect to the core surface so as to provide a constantly varying depth of cut such that the prism lens in the molded lens will have properly fonned angles which will compensate for the changing optical relationships. The lens thus produced will eliminate the smile or frown, predictably vertically control the emerging light beam, and increase the overall light intensity in the horizontal plane.
The above and other features of the present invention will be apparent to one skilled in the art upon reading the following detailed description, reference being made to the accompanying drawings illustrating a preferred embodiment of the present invention in which:
FIG. 1 is a fragmentary front view ofa direct filament lamp assembly installed on a motorvehicle;
FIG. 2 is a view taken along line 22 of FIG. 1;
FIG. 3 is an enlarged view taken along line 33 of FIG. 1;
FIG. 4 is a view taken along line 44 of FIG. 1;
FIG. 5 is a schematic view illustrating the effect of filament and lens position on a light ray;
FIG. 6 is a top view of the cutter fixture for forming the lens core;
FIG. 7 is a front view of the cutter fixture of FIG. 6; and
FIG. 8 is a side view of the cutter fixture of FIG. 6.
Referring to FIGS. l and 2, there is shown a direct filament lamp assembly 10 of the type used as a backup lamp on a motor vehicle 12. However, it will become hereinafter apparent that the subject lamp will provide improved light in other automative and nonautomotive light applications. When functioning as a backup lamp, the lamp assembly 10 is mounted within a rectangular opening 14 of a rear vehicle panel 16 and is conventionally energized to provide illumination to the rear when the vehicle is conditioned for driving in reverse gear.
The lamp assembly 10 generally comprises a lens 18 and a housing 20 carrying a light source 22 having a horizontally disposed filament 24. The housing 20 is formed of a suitable material such as plastic and comprises a thin walled frontally opening base 26 terminating with a peripheral channel 28. The lens 18 includes a rearwardly extending peripheral lip 29 which registers with the channel 28. A thermoplastic cement 30 received in the channel 28 secures the lens 18 to the base 26.
A pair of laterally extending mounting ears 31 are formed at the sides of the channel. In assembly, the mounting ears 31 are secured by means of fasteners 34 to a bracket 32 fixed to the panel 16.
The light source 22 is mounted within an enclosed lamp envelope 38 defined by the interior surfaces of the housing 20 and the lens 18 by means of a conventional socket (not shown) carried on the base 26. Conventional means are provided for automatically connecting the light source 22 to a power source such as the vehicle battery to energize the filament 24 when the vehicle is in the reverse gear.
The lens 18 is formed of an optically transmissive material such as plastic and includes a generally rectangular and planar optical window 40 peripherally bounded by a rearwardly extending skirt 42 which is resiliently spaced from the panel 16 by an elastomeric gasket 44. The plane of the optical window 40 is generally parallel to the axis of the filament and downwardly inclined with respect to the panel 16.
Inasmuch as direct filament lamps typically do not use a reflector to confine the light beam, horizontal and vertical beam control is provided solely by optics in the lens. To this end, five linear warped light refracting prisms or flutes, representatively indicated by the numeral 50, are evenly vertically spaced and individually extend horizontally across the rear surface of the window 40.
Each warped flute 50 is rotated about its horizontal axis with respect to the filament 24 to present a constantly differing prism angle to the emitted light and the outer emerging surface 52 so as to incrementally compensate for the changing focal distance and angular relationship between the lens 18, the filament 24, and the emerging surface 52. As hereinafter described, the flutes 50 are individually prescribed in accordance with their position and the pattern sector they are designed to illuminate.
In lamp assemblies having a planar lens, the progressively increasing focal distance between the flutes 50 and the filament 24 toward the outer boundaries of the optical window 40 increases the incident angle of the emitted rays of the light source 22 on the prism while the angle between an emerging ray at the outer surface and a horizontal plane increases. Thus, with a constant prism angle lens, the emerging light rays are increasingly vertically refracted at the wider horizontal angles and the longer focal distances. This causes an upwardly curved image or smile" distribution for flutes whose surfaces are inclined upwardly toward the filament 24 and a downwardly curved image or frown" distribution for flutes whose surfaces are directed downwardly with respect to the filament 24.
The effect of prism angle, focal distance, and lens tilt is schematically illustrated in FIG. 5 wherein an emitted light ray 60 is projected from the filament 24 at an impingement Angle A (Alpha) with respect to a horizontal plane 62. The light ray 60 is refracted in the lens medium as internal ray 64 which exits and is refracted at the outer interface as emerging ray 66. For a flute 50 having a prism angle 9 (Theta), the impinging ray 60 will have a normal 72 to prism entrance angle B (Beta). The emerging ray 66 has an emergence angle F (Gamma) with respect to a line 74 perpendicular to the outer surface 52, the latter having a lens tilt angle A (Delta) with respect to a vertical plane 78. For further definition, the internal light ray 64 will then have a normal to prism refraction angle 5 (Epsilon) with respect to line 72, a normal to surface angle 4 (Zeta) with respect to line 74, and a refraction angle 1; (Eta) with respect to the horizontal plane 62.
In accordance with the above, a lens l8having flutes 50 formed in accordance with the present invention will predictably control the emergence angle I to achieve the desired intensity pattern as related to the prism angle 6 the impingement angle (1 (Alpha), and the lens tilt angle A More particularly, by conventional trigonometric relationships, the prism angle can then be expressed as follows:
9 tan (sin [a +A sin (sin F/N) ]lN cos [a+A-sin' (sin F/N sin [sin (F/N) Wherein N is the index of refraction for the lens medium.
Referring to FIGS. 2 and 3, it will be noted that inclination or prism angle 9 is greatest at the center section and progressively decreases outwardly thereof. This follows inasmuch as the increasing focal distance decreases the impingement angle a thereby requiring less optical correction to achieve the desired emergence angle I. Additionally, for a substantially constant horizontal intensity pattern, it will be noted the prism angle for each flute 50 will generally increase on either side of the horizontal centerline of the lens l8.
Using the above relationship, the design syntheses is initiated by establishing the illumination pattern for the middle flute at the center section of the lens. The above analysis is repeated for each horizontal flute at the center section and thereafter proceeds outwardly at given horizontal increments to obtain a continuous prescription for each flute. Of course, each horizontal shift changes the focal distance and results in a changing impingement angle a which will effect the horizontal and vertical emergence angle I. However, by prescribing the desired emergence angle F and conventionally geometrically determining the impingement angle a and the lens tilt angle A, the prism angle 6 can be calculated at each location. Further, by conventional trigonometric relationships, the prism angle 9 is rotated to a prescription normal to the outer surface 52. It will be appreciated the above design steps are well suited for solution by a digital computer to obtain a prescription for the prism angle 6 at each desired horizontal increment.
As conventional in the art, the flutes or optics for a lens are reversely formed on a die segment such that the flutes in the final molded article will then have the desired inclination for providing optical correction of the emitted light. In the present case, the cutter fixture 79 for forming a core segment 80 to produce a constantly varying prism angle 6 is shown in FIGS. 6 through 8 and generally comprises a cutter 82 including a conical cutting blade 84 and a cutter bed 86 pivotally connected to a stand 88 by pin 90. The stand 88 and the cutter bed 86 are provided with suitable drive means horizontally shifting the core segment 80 transversely with respect to the rotating cutter blade 84. An upwardly extending triangular bracket 91 is attached to the front of the cutter bed 86. The bracket 91 includes a projecting cam follower pin 92 at its apex which rides on the cam surface 96 of a flute template 98 fixedly mounted with respect to the cutter 82 and the stand 88.
The cam surface 96 is formed in accordance with the prescription outlined above for providing the desired prism angles by presenting to the cutter blade 84 an inclination of the core segment 80 which is inversely equal to the prism angle 8 at a particular horizontal lo cation. Therefore, as the cutter bed 86 translates with respect to the cutter 82 and the template 98, the core segment 80 will pivot with respect to the cutting blade 84 about the pins as controlled by the cam follower pin 92 riding over the cam surface 96. In this manner, a V-shaped groove corresponding to the prescribed constantly changing prism angle is reversely cut into the core surface such that the molded article will have projecting flutes which bear the desired inclination. For subsequent flutes, the cutter bed 86 or the cutter 82 is axially shifted with respect to the core segment 80 so as to be positioned complementary to the next adjacent flute. Thereafter, the machining of the grooves proceeds as outlined to form the remaining flutes of the core segment 80.
In the preferred form, the inner diametral edge of the cutter blade 84 is axially positioned at the axis of the pins 90 which additionally is coplanar with the surface of the core segment 80. Accordingly, the rotation of the cutter bed 86 about the pins 90 as prescribed by the cam surface 96 will cause cutter blade 84 to pivot about the diametral edge and cut the desired flute inclination in the core segment 80. Inasmuch as the prism angle for a backup lens is at a maximum at the center section and progressively decreases or flattens out toward the edges of the lens, the conical angle of the cutting blade 84 is equal to the prism angle of the center section of each flute. In this manner, the core segment 80 will be substantially horizontal at the center section and the template will have a generally sinusoidal shape to achieve the desired varying inclination.
Although only one form of this invention has been shown and described, other forms will be readily apparent to those skilled in the art. Therefore, it is not intended to limit the scope of this invention by the embodiment selected for the purpose of this disclosure but only by the claims which follow.
What is claimed is:
l. in a direct filament lamp assembly having a lens including optical means which increasingly vertically refract emitted light at the longer focal distances resulting in a deviation from a desired illumination pattern,
the improvement comprising:
optical means in the form of continuous warped light refracting flutes formed in parallel spaced relationship on the inner surface of the lens, each of said flutes having an incrementally differing angular position along the length thereof with respect to the filament and the outer surface of the lens so as to incrementally compensate for changing focal distances and angular positions of said outer surface thereby correcting said deviation and predictably distributing said emitted light from the lens in said desired illumination pattern.
2. In a direct filament lamp assembly having a housing and a light source including a filament mounted relative to the housing, a lens for uniformly distributing light rays from the filament in a predetermined intensity pattern, comprising: an optically transmissive window adapted to be positioned against the housing parallel to the filament; spaced rows of continuous refracting prisms formed on said window and extending individually across said window, each of said prisms having a linearly warped contour providing an incremental angular prism position with respect to the filament and the window, said incremental angular prism position compensating for the constantly changing focal distances therebetween and the constantly changing angles between emerging light rays and surface normals along the length of the prism such that light rays are refracted through the window for emergence in said predetermined intensity pattern substantially throughout the extent of said window.
3. A direct filament lamp assembly comprising: a housing; a light source having a filament with an axis mounted horizontally relative to the housing; and a lens for predictably uniformly horizontally distributing illumination from the filament in a predetermined intensity pattern, said lens having a planar optically transmissive body adapted to be positioned with respect to the housing with an outer surface generally parallel to the axis of the filament, and a plurality of equally spaced continuous horizontal prisms formed on the inner surface of the optically transmissive body and extending individually thereacross parallel to the axis of the filament, each of said prisms having a linearly warped contour which presents an incrementally differing inclination with respect to the filament and the body, said incrementally differing inclination compensating for changing focal distances and the angles be tween light rays entering the prism and normals to said outer surface such that light is refracted through the body for emergence in accordance with the position of the prisms in the window and the pattern sector the prisms are intended to illuminate.