Linear lighting system

Details Linear lighting system Linear lighting system

2001-07-02

2003-08-05

O'Shea, Sandra (Department: 2875)

Illumination

Elongated source light unit or support

With light modifier

C362S216000, C362S227000, C362S249070, C362S241000, C362S247000, C362S296040, C362S016000, C362S011000, C362S260000

Reexamination Certificate

active

06601970

ABSTRACT:

The present invention relates to a lighting system used in such equipment as imaging devices for recognizing and/or examining an object using a camera, and particularly to a lighting system having a linear light source and a reflector suitable for the light source.
BACKGROUND OF THE INVENTION
In order to obtain an intended image using a camera, it is necessary to select an optimal lighting system suitable for that purpose. One practical example of such lighting systems is an LED lighting system including a linear light source having plural LEDs (light-emitting diodes) arranged in a row and a reflector (or reflectors) having a cylindrical reflective surface whose section is concave. The LED lighting system illuminates the longitudinal area of an object.
FIG. 7
is a section of a practically used LED lighting system, taken vertical to the longitudinal direction. In
FIG. 7
, numeral
1
a
denotes an LED packaged with resin or glass, and numeral
1
b
denotes a longitudinal board equipped with plural LEDs
1
a
. The LEDs
1
a
are arranged in a row on the board
1
b
at proper intervals. Thus, the LEDs
1
a
and the board
1
b
construct a linear light source
1
. Numeral
2
denotes a reflector for reflecting a part of the light emitted from the linear light source
1
. The optical axis of the linear light source
1
coincides with that of the reflector
2
. Numeral
2
a
denotes a reflective surface of the reflector
2
, whose section is concave. The reflective surface
2
a
is mirror-finished by a vapor deposition or plating of metal such as aluminum, or by plastering a tape or the like. The form of the concave section is generally an aspherical quadratic curve such as an ellipse or parabola. In the example of
FIG. 7
, the section is elliptical, and the linear light source
1
is disposed at or proximate to one (F
1
) of the two focuses of the ellipse. Numeral
3
denotes an irradiation plane disposed proximate to another focus F
2
of the ellipse. Such an optical construction is based on the optical characteristic of an ellipse that all the light emitted from one focus is reflected by the elliptical surface and converges to another focus. The irradiation plane
3
is set to face the linear light source
1
and the reflector
2
. When the concave section is designed parabolic and the light source is located at the focus of the parabola, the parabolic surface reflects the light and yields a parallel beam of light.
In the above LED lighting system, the light emitted from the linear light source
1
radially spreads like a solid angle around the optical axis. As the light spreads broader, the optical aberration of the LED
1
a
increases. Therefore, in general, the light within a preset solid angle around the optical axis is used as an effective light. In
FIG. 7
, the range of the effective light (effective emission angle) is shown as 2&thgr;. Also, when an LED is designed for illuminating not a large area but a limited area of an object, the intensity of light emitted from the LED decreases as the spreading angle of light around the optical axis increases. Therefore, practically, the light utilized for illumination is mostly composed of a high intensity light within a narrow angle range around the optical axis, and a low intensity light surrounding the high intensity light is utilized merely supplementarily. As a result, the high intensity light directly illuminates the irradiation plane
3
, and the surrounding low intensity light is first reflected by the reflector
2
and then illuminates the irradiation plane
3
. In
FIG. 7
, the former is shown as a direct light
4
a
and the latter as a reflected light
4
b.
Generally, the direct light
4
a
is a diverging light, so that the direct light
4
a
illuminates a broadened area on the irradiation plane
3
. Thus, it is only the light within a limited angle around the optical axis that effectively illuminates a desired area, while most of the direct light
4
a
illuminates outside of the desired area, thus being wasted. Further, the light illuminating the outside area is reflected by walls around and turns into a scattered light (which is called “stray light”). The scattered light often badly influences the examination or the like, so that it must be eliminated by some means. Therefore, for example, a shielding plate having a narrow aperture is disposed close to the irradiation plane
3
. In another example, the lens of the LED package is designed so that the emitted light converges only onto a desired area. The LED, however, lacks universal availability because it is designed for a particular distance between the linear light source
1
and the irradiation plane
3
and for a particular illumination area. When it is desired to locate the irradiation plane
3
as far from the linear light source
1
as possible, or when it is desired to reduce the illuminated area on the irradiation plane
3
as small as possible, the amount of wasted part of the direct light
4
a
increases. In this case, the amount of part of the direct light
4
a
reaching the irradiation plane
3
decreases, so that the luminance on the irradiation plane
3
decreases. Thus, in the illumination by the direct light
4
a
, some light wastage is inevitable.
The reflected light
4
b
, on the other hand, is a converging light, and all the light reflected by the reflective surface
2
a
converges to the irradiation plane
3
. Therefore, in the illumination by the reflected light
4
b
, no light is wasted.
Thus, the conventional LED lighting system utilizes the direct light
4
a
that has a high intensity but is scattered and the reflected light
4
b
that is converged but has a low intensity, so that the efficiency is low.
The above problem might be solved by increasing the amount of the reflected light
4
b
while minimizing the amount of the wasted part of the direct light
4
a
. In order to attain that objective, however, it is necessary to greatly increase an effective diameter of the reflective surface
2
a
. Such a design is impractical because the reflector
2
would be so large that it would extend toward the irradiation plane.
One possible improvement to the prior art is to locate the linear light source
1
to face the reflector
2
so that the light emitted from the linear light source
1
around the optical axis is introduced to the reflector
2
.
FIG. 8
shows a section of an LED lighting system, taken vertical to the longitudinal direction, where the optical axis of the linear light source
1
coincides with that of the reflector
2
and the linear light source
1
is set to face the reflector
2
. In this system, all the light spreading within a narrow angle around the optical axis and having a high intensity (i.e. the light propagating within the effective emission angle) is introduced to the reflector
2
. However, part of the light within the effective emission angle, particularly the central part of the light including the optical axis and having a very high intensity, is obstructed by the linear light source
1
and/or the board
1
b
after being reflected by the reflector
2
. As a result, that part of the light cannot reach the irradiation plane
3
and is wasted. In
FIG. 8
, the part around the optical axis where no hatching is done corresponds to the wasted part of the light. Thus, contrary to the expectation, the luminance on the irradiation plane
3
decreases, which prevents a practical use of the system.
A method of efficiently using the direct light
4
a
is known where a cylindrical lens is employed instead of the reflector.
FIG. 9
shows a section of a lighting system using a cylindrical lens, taken vertical to the longitudinal direction. Numeral
5
denotes the cylindrical lens, which is formed to have an aspherical section so that it receives an effective light from the linear light source
1
within the angle 2&thgr; around the central axis and effectively converges the light to the irradiation plane
3
. The production cost of the lens
5
, however, is very high whether it is manufactured by a grinding of glass materials o

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