Illumination – Plural light sources – With modifier
Reexamination Certificate
2000-07-28
2002-09-24
O'Shea, Sandra (Department: 2875)
Illumination
Plural light sources
With modifier
C362S249070, C362S235000, C362S259000, C359S287000, C359S387000
Reexamination Certificate
active
06454437
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to illuminators used in industrial instrumentation in which it is desired to illuminate an object which is being inspected by a sensor or camera. An example is the illumination of objects on a production line which are being inspected by a machine vision system.
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 initiating 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.
FIG. A illustrates the main features of a typical prior art ring illuminator, such as for example displayed at the Volpi™ website, or in the Electronic Imaging Catalog of Edmunds Industrial Optics Inc™. The illuminator consists of an arrangement of light emitting diodes arranged either within a housing, or on a printed circuit board within a housing, with a hole in the centre of the housing and board through which a camera or sensor can view the objects which are being illuminated. Each LED is in its own package, usually lensed so as to provide a “viewing angle” of between 4 degrees and 120 degrees. 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 LED chip and/or by the arrangement of the LED within the optical elements of the package. It can be seen additionally from FIG. A that 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 a ring 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. It can also be seen from FIG. A that the overall volume of the illuminator is very much determined by the size of the individual diode packages, which are typically several millimeters 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 30 mm diameter ring of 5 mm LEDs can only hold about 16 of these lensed LEDs.
In order to produce a shorter working distance it has been proposed to arrange the LEDs at an angle to the optical axis. This means that the illuminator works well at a particular working distance, which is determined by the position at which the beams from the individual LEDs coincide. However, there is no facility for adjusting the working distance.
U.S. Pat. No. 5,822,053 describes a system for improving the uniformity and for matching the angle of illumination with the field of view. This consists in essence of bending the metal legs on the individual packaged LEDs to adjust the orientation of the beam from each LED. By observing the pattern on the target plane, it can then be ensured that the uniformity is improved to the extent that is possible, given the non-uniformity of the individual LEDs to start with. Following adjustment, the LED packages are fixed in place with epoxy. This technique can only improve uniformity up to a point, and does not address the other fundamental limitations of using a ring of lensed LEDs, such as limitations of brightness, compactness, and scaleability to small sizes.
U.S. Pat. No. 5,580,163, describes a mechanical adjusting system to enable the angle of incidence of the illumination from a ring illuminator consisting of the lensed LEDs to be varied. This is mechanically complex, and adds significantly to the manufacturing cost of the system and to its volume and mass. Also, it does not address uniformity and brightness problems.
Use of annular lenses has been described in the prior art. U.S. Pat. No. 4,567,551 describes a multidirectional surface illuminator which uses a Fresnel lens to redirect and focus the light from the light sources arrayed around the outside of an illuminator housing, so as to illuminate the object under inspection from a particular quadrant of the illuminator. It is very difficult to make a compact illuminator using this technique due to the arrangement of the light sources outside of the housing. Additionally, this arrangement does not achieve a desirable level of uniformity over a target area, and is not suitable for extension to very small sizes.
U.S. Pat. No. 5,690,417 describes a surface illuminator with a means for adjusting the orientation and inclination of the incident illumination, which includes light emitting diodes arranged in coaxial circles. Each coaxial circle of diodes may have their beam angles inclined at different angles to the angle of the axis of the housing. The diodes are energizable in pie-shaped sectors so that arcuate clusters of diodes may be illuminated to enable the orientation of the resultant illumination to be varied. The light emitting diodes are individually packaged so as to include a collimating lens and a Fresnel lens is used to focus and redirect the light towards the illuminated object. The illumination is similar to the prior art illuminator illustrated in FIG. A from the perspective of efficiency and uniformity. The only function of the Fresnel lens is to enable the variation of the angular inclination from the various segments, while using light emitting diodes mounted coaxially with the housing.
U.S. Pat. No. 5,897,195 describes an illuminator with a cylindrical or conical array of light emitting diodes producing collimated light beams that are redirected towards the illuminated object by a Fresnel-like diffuser. Again, this is a variation of the prior art illuminator shown in FIG. A which might have some advantages for producing particular angular inclinations. However, it is not very suitable for compactness, high uniformity or high efficiency.
These disadvantages with using discrete light sources such as LEDs have led to use of large illuminators for machine vision systems and for microscopy in order to achieve the required illumination intensity and distribution. These illuminators may, for example, comprise a ring-shaped fluorescent tube. These units are bulky and so impose design limitations on the instrument with which they are intended to be used, and they also are not efficient for producing monochromatic light. These illuminators may also comprise “cold light” sour
Choi Jacob Y.
Jacobson & Holman PLLC
LandOfFree
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