Lamp with improved photometric distribution

A light bulb is provided has a lamp base electrically connecting a radiant light source and supporting the light source on a central axis. An envelope is supported from the base, comprising a generally tubular transmissive portion adjacent to the base and a generally reflective portion contiguous with the transmissive portion and completing enclosure of the light source. The reflective portion is in the form of a concave curve of revolution having a first focus at the light source and configured to direct the light from the source through the transmissive portion of the envelope at approximately a 45.degree. angle from the central axis, and wherein the 45.degree. angle is rotated to form a depending cone of light. A preferred embodiment has an envelope including a partially transmissive reflector and a tubular window as an optically transmissive light diffuser.

BACKGROUND OF THE INVENTION 
For many years the commonly used light bulb has consisted of little more 
than a clear or diffusing glass globe surrounding a compact light source. 
Reflector lamps have been provided with on-axis spherical, paraboloidal or 
elipsoidal reflectors to produce various beam widths. Control of the 
direct horizontal disability glare in lighting fixtures is commonly 
specified as a requirement in terms of VCP (Visual Comfort Probability). 
Similarly, reflected glare from near-vertical light striking a task 
surface has been specified in commercial lighting fixtures in terms of ESI 
(Equivalent Sphere Illumination). Both VCP and ESI characteristics have 
been relegated to light fixture designers and are therefore simply ignored 
by the designers and manufacturers of light bulbs. Since the ordinary 
incandescent light bulb by far the most common light source in residential 
lighting, there is little or no control over the photometric properties of 
the lighting in virtually every home. A typical example is the ordinary 
table lamp, in which a light bulb with spherical light destribution is 
placed within a shade that limits horizontal brightness and diffusely 
reflects some light downwards. If the shade has high transmissivity and 
efficiency the horizontal brightness is excessive, providing VCP so 
visually disabling that it would violate lighting standards for offices or 
schools. If the shade is has low transmissivity the lamp is a very 
inefficient room light, and the small circle of light near the base 
creates reflected glare. In this case the inefficiency and reflected glare 
would be unacceptable for business or schools, but is the state of the art 
in nearly every home. 
The purpose of the present invention is to provide a highly efficient light 
bulb that will profuce precisely controlled light distribution, with low 
horizontal brightness for high Visual Comfort Probability and low vertical 
reflected glare for improved Equivalent Sphere Illumination. A purpose of 
one preferred embodyment is to substantially remove the infra-red energy 
from the reflected beam by allowing it to pass through the reflector and 
thereby permit a cooler illuminated area. 
SUMMARY OF THE INVENTION 
A light bulb according to the invention comprises an enclosing transparent 
envelope disposed about a radiant light source, such as an incandescent 
filament or a gas discharge tube, and supported spaced from and on the 
centerline of a base, such as a screw base, bi-pin base or bayonet base, 
which includes a means for connection of the light source to external 
power. The envelope has a transmissive portion and generally reflective 
portion including the plane of the light source normal to the centerline 
of the base. The reflective portion of the envelope is a reflector in the 
form of a concave reflector of revolution im which a focus is at the light 
source and the light is reflected outside the envelope on an axis of 
approximately 45.degree. to the base centerline, wherein the 45.degree. 
axis is rotated to form a depending cone. 
One preferred embodiment of the invention provides a light bulb for such 
portable lamp applications as table lamps and floor lamps, in which a 
filament is axially spaced in an upward direction from a screw base and is 
at the first focus of a reflector comprising an elipse of revolution. The 
reflector is generally the enclosure of the distal (zenith) end of the 
bulb, has its major eliptical axis passing through the filament and 
inscribing approximately a 45.degree. half-angle cone in the downward 
proximal (nadir) direction towards the base, with a second focus 
inscribing a circle comprising the intersection of the depending cone with 
a horizontal plane outside the bulb. The reflector limits horizontal 
brightness by intercepting and substantially reflecting rays radiating 
from the source in the distal direction, and extends downward past the 
horizontal plane of the light source to approximately a 60.degree. 
half-angle from nadir. 
Another preferred embodiment of the invention provides a a light bulb for 
such portable lamp applications as table lamps and floor lamps, in which a 
filament is axially spaced in an upward direction from a screw base and is 
at the focus of a reflector comprising a parabola of revolution. The 
reflector is generally the enclosure of the distal end of the bulb, has 
its reflecion axis passing through the filament and inscribing 
approximately a 45.degree. half-angle cone in the downward proximal 
(nadir) direction towards the base. The reflector limits horizontal 
brightness by intercepting and substantially reflecting rays radiating 
from the source in the distal (zenith) direction and extending downward 
past the horizontal plane of the light source to approximately a 
60.degree. half-angle from nadir.

DETAILED DESCRIPTION OF THE DRAWINGS 
In FIG. 1 a light bulb 1 of a preferred embodiment is shown having a 
depending base 2 supporting and connecting a radiant light source 3, such 
as filament shown, to an external power source. A glass envelope 4 
encloses light source 3 and has a generally transmisive window 5, and a 
substantially reflective portion comprising a reflector 6 capable of 
completely or partially reflecting incident light from source 3. A 
preferred embodiment of reflector 6 is in the form of a mirror film or a 
commonly available dichroic coating on glass envelope 4, whereby reflector 
6 is substantially reflective at wavelengths shorter than 650 nanometers, 
and substantially transmissive at wavelengths longer than 650 nanometers. 
Reflector 6 is in the form of a portion of an elipsoid of revolution in 
which an elipse 7 has a major axis 8 passing through a first focal point 
at source 3 and a second focal poing 9 revolved at approximately a 
45.degree. half-angle cone about the nadir centerline 10 of base 2. 
In operation there are three sources of illumination from the bulb. First, 
direct rays 11 are radiated from light source 3 and transmitted directly 
through window 5 are distributed in a generally constant brightness cone 
of light distributed around nadir, following the inverse square law, and 
extending upwards to the optical cut-off angle of approximately 60.degree. 
from nadir by reflector 6. Second, direct rays 11a striking reflector 6 
become reflected rays 12 from reflector 6 also are transmitted through 
window 5 through focal point 9 to be destributed generally in the 
direction of major axis 8 of elipse 7 in a beam pattern having a peak 
intensity pattern in the form of a depending cone with a half angle of 
approximately 45.degree. from nadir. Third, some of the direct rays 11a 
from source 3 pass through reflector 6 and emerge from the bulb as 
filtered direct rays 14 of diminished intensity. 
In FIG. 2 light bulb 21 according to a second preferred embodiment of the 
present invention is shown having a depending base 22, illustrated as a 
bayonet base but equally functional as a screw base of a bi-pin base, 
supporting and connecting a radiant light source 23, such as filament 
shown, to an external power source. A glass envelope 24 encloses light 
source 23 and has a generally transmisive window 25 in the shape of a 
truncated cone, and a substantially reflective portion comprising a 
reflector 26 capable of completely or partially reflecting incident light 
from source 23. Reflector 26 is in the form of a portion of a parabola of 
revolution in which a parabola 27 has a major axis 28 passing through a 
focal point at source 23 and generally collimating rays 33 parallel to the 
parabolic axis 28, which is revolved at approximately a 45.degree. 
half-angle cone about nadir centerline 30. 
In operation there are three sources of illumination from the bulb. First, 
direct rays 31 are radiated from light source 23 and transmitted directly 
though window 25 are distributed in a generally constant brightness cone 
of light dirtributed around nadir, following the inverse square law, and 
extending upwards to the optical cut-off angle of approximately 60.degree. 
from nadir by reflector 26. Second, direct rays 31a striking reflector 26 
become reflected rays 33 from reflector 26 also are transmitted through 
window 25 to be distributed generally in the direction of major axis 28 of 
parabola 26 in a beam pattern having a peak intensity in the form of a 
depending cone with a half angle of approximately 45.degree. from nadir. 
Third, some of the direct rays 31a from source 23 pass through reflector 
26 and emerge from the bulb as filtered direct rays 34 of longer 
diminished intensity. 
The resulting light bulb exhibits excellent Visual Comfort Probability, 
having very low horizontal brightness in both bare bulb installations, 
such as chandeliers; and also in fixtures having relatively transparent or 
translucent diffusing shades. The bulb according to the invention also 
exhibits very high Equivalent Sphere Illumination, having low energy in 
the regions near nadir where reflected glare is not desireable. 
The present invention shows a preferred embodyment in the common screw base 
configuration for medium 110 v sockets and a bayonet base common for 12 v 
systems. Other standard bases, flanged, prefocussed, bi-pin and wire 
terminal bases are obvious applications for the bulb within the scope of 
the invention.