Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
1998-12-24
2001-10-16
Lee, John R. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C315S158000
Reexamination Certificate
active
06303916
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to systems and methods that standardize reproducible illumination within an instrument line. In particular, the invention is directed to an illumination system that preferably uses solid-state devices regulated by a control system to illuminate sample parts. The illumination system is calibrated with a reference illumination source to provide stable, compensated standardized systems and methods that measure the intensity of light used to illuminate a sample part.
2. Description of Related Art
Current commercial metrology systems based on video inspection almost exclusively use tungsten filament lamps, e.g., halogen lamps, when performing measurements transmission, direct reflection or diffuse illumination modes. Halogen lamps are typically used because they have a reliable lifetime of approximately 2000 hours. These lamps also have sufficient energy in the visible portion of the spectrum and are relatively inexpensive. These lamps have characteristics similar to blackbodies near 2700 K-3200 K.
As a result of the broadband emission characteristics of halogen lamps, a majority of the commercial vision measurement system manufacturers spectrally limit the radiation emitted from the halogen lamps to exclude, for example, radiation in the wavelength range from 750 nm to 3.5 &mgr;m. This can raise the signal to noise ratio (SNR) for measurements performed in the visible region and permits a reduction in the sophistication required of the imaging optics. The spectral limitation also excludes a radiometrically sensitive region for silicon from being exploited for vision measurements.
However, incorporating hardware to operate halogen-lamp-based illumination systems is bulky, expensive and relatively unreliable compared to illumination systems using solid-state sources. Conventional vision systems have minimum requirements for spectrally unfiltered optical power on the order of 25 watts for illumination sources. Driving such illumination systems translates to source currents in the range of, for example, 0.6-2.0 amperes. Other vision systems can have even higher current drive requirements.
Typically, conventional illumination sources are remotely located because a significant amount of heat is generated by the illumination sources. If the heat generated by the illumination system is not accounted for, the accuracy of dimensional measurements in vision instruments could be compromised. The illumination sources are therefore spectrally low-pass filtered at, for example, a wavelength of about 750 nm.
Hence, an apparatus is needed to transport the light from the remote location of the illumination source to the point of use or measurement. Typically, this is accomplished using multimode glass fiber bundles. However, even low quality illumination bundles are known to be expensive, fragile and often not necessary for most users from a convenience standpoint.
SUMMARY OF THE INVENTION
The response time of conventional lamps to achieve steady state illumination after a step change in illumination level is on the order of seconds. This is due to the large thermal mass of the device, including primarily the filament and the glass envelope. Solid-state devices afford a tremendous advantage because they have high modulation capabilities and good frequency response characteristics. Thus, because of the advantages provided by solid-state devices, the solid-state devices can attain steady-state conditions faster than conventional lamps. This improves the value of the machine vision instrument by raising the instrument throughput.
The light output of any device is a function of many variables. Some of the variables include the instantaneous drive current, the age of the device, the ambient temperature, whether there is any dirt or residue on the light source, the performance history of the device, etc. Machine vision instrument systems typically locate objects within their field of view using methods which may determine, among other things, the contrast within the region of interest where the objects may be found. To some degree, this determination is significantly affected by the amount of incident light.
Automated video inspection metrology instruments generally have a programming capability that allows an event sequence to be defined by the user. This can be implemented either in a deliberate manner, such as programing, for example, or through a recording mode which progressively learns the instrument sequence. The sequence commands are stored as a program. The ability to create programs with instructions that perform a sequence of instrument events provides several benefits.
For example, more than one workpart or instrument sequence can be performed with an assumed level of instrument repeatability. In addition, a plurality of instruments can execute a single program, so that a plurality of inspection operations can be performed simultaneously or at a later time. Additionally, the programming capability provides the ability to archive the operation results. Thus, the testing process can be analyzed and potential trouble spots in the workpart or breakdowns in the controller can be identified. Without adequate standardization and repeatability, archived programs vary in performance over time and within different instruments of the same model and equipment. Illumination level variation can be effectively minimized and standardized by actively sampling a small percentage of the entire light output from each illumination source, comparing the light output to a target point level established through an instrument standardization process, and controlling the illumination sources based on the comparison.
This invention separately provides systems and methods that allow an illumination system using solid-state devices to be regulated using a control system to yield stable and standardized illumination of a sample part.
In one exemplary embodiment, the systems and methods according to this invention have the flexibility to measure light intensities in the visible and near infrared regions of the spectrum. In addition, the magnitude of a required drive current for the systems and methods according to this invention makes precise current adjustment easy, so that reproducible illumination within an instrument product line is achievable.
The systems and methods according to this invention use solid-state devices to illuminate the sample part. The solid-state devices require small drive currents to operate. It is thus easy to precisely adjust the drive currents of the solid-state devices. The precise nature of the solid-state devices allows for greater flexibility in selecting the output wavelength of the solid-state devices. Accordingly, the illumination source can be located near the illuminated sample part. As a result, the conventional glass fiber bundles are not necessary, making the systems and methods according to this invention compact, affordable and reliable. In addition, the solid-state devices provide very high optical repeatability and reliability when driven electronically within the working parameters of the solid-state devices.
The solid-state devices are a component of an illumination source that can illuminate a sample part along an axis of illumination that is perpendicular to a plane on which the sample part is placed. The solid-state devices usable in the systems and methods according to this invention may include, but are not limited to, light emitting diodes (LEDs). LEDs are selected because of their reliability and long-life. LEDs also have the ability to work in the ultra-violet, visible and near infrared regions of the spectrum.
A light collection system forms an image of the illuminated sample part In one mode, the light collection system has an optical axis that is coincident with the axis of illumination of the illumination source. In one exemplary embodiment, the light collection system preferably includes at least one charge coupled device (CCD) and at least one collection lens. The solid-state devices emit op
Lee John R.
Mitutoyo Corporation
Oliff & Berridg,e PLC
Pyo Kevin
LandOfFree
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