Illumination – Plural light sources – With support
Reexamination Certificate
2000-10-02
2002-09-17
O'Shea, Sandra (Department: 2875)
Illumination
Plural light sources
With support
C362S244000, C362S246000, C362S294000, C362S373000, C362S218000, C362S225000, C362S800000, C362S084000
Reexamination Certificate
active
06450664
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to linear illumination. Such illumination is required, for example, for “web” applications in which items are conveyed on a web and are inspected by a camera. Typically, a linescan camera is used in this application. Since the linescan camera is inspecting a narrow strip of the web, transverse to its direction of motion, the requirement for the illumination source is that it illuminate the appropriate strip of material to the brightness level which is required. Exaiyiplics of web inspection are inspection of surfaces of ceramic tiles, print media, electronic components on printed circuit boards (PCBs), paper, and pharmaceutical packaging. It may also be used for sorting or grading of fruit and vegetables. Another application is scanners, in which the light source moves rather than the target.
PRIOR ART DISCUSSION
A typical machine vision system consists of an illuminator, a camera, and a control system for enabling the illuminator and camera in combination to capture an image of the object which is under inspection and for processing the image and initiaiting the desired action. The action may be marking the product as acceptable or rejected. The illuminator is an important part of the system, and properties of the illumination such as its brightness, its uniformity, its angle of incidence, its colour, and its degree of collimation can have a significant influence on the overall performance of the complete vision system. Additionally, the physical size of the illuminator is very important, as for some vision systems there is very little physical space available. For example, it may need to be located within a camera housing. Several types of illuminator have been used with machine vision systems, for example incandescent bulbs, fluorescent tubes, lasers, Xenon flash tubes, halogen bulbs combined with fibre light guides, and light emitting diodes (LEDs). Due to their relative cheapness, physically small size, long lifetime, fast switching speed and reasonable efficiency, LEDs have become increasingly popular.
In general linear illumination applications require that the camera have a fast shutter speed to stop the motion adversely affecting the image which is processed. Also, these applications require good intensity uniformity, ideally better than +/−5% along the line. A further requirement is a minimum intensity of illumination. This latter parameter depends upon factors such as the speed of the web, and typical mid-speed applications require at least 10 W/m
2
.
Heretofore, such illumination has been provided by light sources such as a halogen lamp or a gas discharge/fluorescent lamp (WO9801746 and U.S. Pat. No. 5,884,775). Such light sources are generally expensive to install and to maintain and a further problem is that they are quite bulky. Illumination has also been provided by lighting heads connected to halogen or other fibre-coupled sources. Such light sources are also quite bulky and expensive, and suffer from limited source lifetime and other problems.
Another type of linear illuminator consists of an arrangement of light emitting diodes arranged either within a housing, or on a printed circuit board within a housing. Each LED is in its own lensed package with a particular viewings angle. By viewing angle is meant the full width angle measurement in the far field at the half power points of the light which is being emitted from the package. In general, the light from these packages has a cross-sectional profile which is such that it can not be described by a smooth mathematical function such as a Gaussian function, but has undesirable intensity variations caused by the structure of the LIED chip and/or by the arrangement of the LED within the optical elements of the package. There is the potential for a lot of structure (non-uniformity) in the illumination pattern which is produced when these packaged LEDs are used to make an illuminator depending upon how well the beams from the individual LEDs mix. This can be improved by adding diffuser material either within the individual LED packages or on the exterior of the housing of the illuminator, but the effect in both circumstances is to reduce the efficiency of the system. The overall volume of the illuminator is very much determined by the size of the individual diode packages, which are typically several millimetres in diameter.
A problem with such illuminators is that the pointing accuracy of individual LEDs is poor, leading to lack of uniformity of the illuminated area. Also, the uniformity of the light produced by the individual LEDs is poor, and even after they mix on the target, the resulting uniformity is poor, perhaps +/−25%. Another disadvantage is that the overall brightness of the illumination is limited by the packing density of the individual packaged LEDs, which are typically housed in 5 mm or 3 mm diameter acrylic packages. For example, a 100 mm long line illuminator can include between 20 and 40 of these LEDs. Therefore, it is an object of the invention to provide a line illuminator which has some or all of the following advantages:
is compact, providing a high ratio of brightness to illuminator volume,
provides a uniform illumination intensity over the desired target area,
incorporates redundancy so that failure of some source!; does not result in darker areas within the illuminated area,
has a relatively high optical efficiency,
can be readily focused to adjust the size of the illuminated area,
allows configuration for a required application at the design and/or installation stages in a versatile manner.
SUMMARY OF THE INVENTION
According to the invention, there is provided a linear illumination Unit comprising a plurality of light sources mounted on a housing in a linear direction, and a lens mounted to focus light emitted by the light sources to provide a line of light, characterized in that,
the light sources are light emitting diodes having a viewing angle in excess of 60° whereby light is spread from the diodes to the lens and the lens allows spread in the linear direction and limits spread in a direction orthogonal to the linear direction.
In one embodiment, the light emitting diodes comprise an elongated mitting aperture extending in the linear direction.
In one embodiment, the light emitting diodes are integrated onto a monolithic integrated circuit.
In one embodiment, the light emitting diodes are covered by a continuous elongated body of epoxy.
In one embodiment, the epoxy contains a diffuser component.
In one embodiment, the epoxy contains a phosphor component.
In one embodiment, the illumination unit comprises a drive circuit for the light emitting diodes, the drive circuit comprising means for driving the light emitting diodes in groups each comprising distributed diodes whereby failure of a sub-circuit has a uniform impact on the illumination.
In one embodiment, the diodes adjacent to an end of the unit are more closely spaced than the other diodes.
In one embodiment, the lens is a Fresnel lens.
In one embodiment, the light emitting diodes are unlensed.
In one embodiment, the housing allows spread of light in the linear direction beyond the ends of the housing.
In one embodiment, the lens is mounted with respect to the light sources to allow light emitted by light sources of a neighboring illumination unit to enter the lens and be focused by it.
In one embodiment, the housing comprises means for mounting on a common support for a plurality of illumination units extending in the linear direction.
According to another aspect, there is provided a linear illumination system comprising a heat sink and a plurality of linear illumination units as described above mounted end-to-end in the linear direction on the heat sink.
According to another aspect, the invention provides a linear illumination unit comprising a plurality of light sources mounted on a housing in a linear direction, and a lens mounted to focus light emitted by the light sources to provide at line of light, characterized in that,
the light sources are light
DelGizzi Ronald E.
Jacobson & Holman PLLC
O'Shea Sandra
Stockeryale (IRL) Limited
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