Abstract:
A plurality of systems and methods for generating reproducible illumination by adjusting solid-state devices regulated by a control system that illuminate sample parts in a compensated, standardized manner. An illumination system includes an illumination source directed onto the optical axis of a light collection system. The light collection system includes a collection lens assembly and at least one CCD detector. The lens assembly and CCD detector perform the spatial imaging of the sample part. An optical element positioned between the illumination source and the sample part redirects a portion of the entire energy emitted from the illumination source to a monitoring detector. The monitoring detector measures the optical power illuminating the sample part and compares it to a previously measured reference illumination source level. Based on the results of the comparison and additional input from temperature, color and other sensor, the drive current to the illuminating source is adjusted to consistently illuminate the sample part within an instrument model line and over an extended time period.

Description:
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 μm. 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 optical output energy in a part of the spectral region where the CCD is photosensitive. An optical element is positioned between the illumination source and the sample part. It is within the scope of the systems and methods of this invention to use a dichroic or common beam splitter as the optical element. Any other known or later developed optical element capable of transmitting and reflecting the optical output energy from the illumination source simultaneously can also be used with the systems and methods of this invention. 
     The illumination source is also directed onto an optical axis of the light collection system. It is within the scope of the systems and methods according to this invention to illuminate the sample part in the transmission mode. Alternatively, the sample part can be illuminated in the reflective mode (specular or diffuse). In another aspect of the systems and methods according to this invention, the sample part can also be illuminated using a combination of the transmission and reflective modes. In yet another aspect of the systems and methods according to this invention, the sample part can also be illuminated obliquely, using the “dark field” illumination mode. 
     When the sample part is illuminated in the transmission mode, the optical element is placed between the illumination source and the sample part. As a result, a small portion of the entire optical output energy emitted from the illumination source is redirected to a monitoring detector. The remainder of the emitted optical output energy illuminates the sample part, from which the light collection system forms an image. 
     When the sample part is illuminated in the reflective mode, the optical element is also positioned between the illumination source and the sample part. As a result, a small portion of the entire emitted optical output energy from the illumination source passes through the optical element and is received by the monitoring detector. The remainder of the optical output energy is directed towards the sample part. The remainder of the emitted optical output energy is focused onto the sample part by a focusing or collecting lens. The focused optical output energy illuminates the sample part and is reflected or scattered back towards the focusing or collecting lens. The reflected or scattered optical output energy is imaged onto a CCD. 
     The systems and methods of this invention can be modified to include an additional collection device having multiple non-imaging detectors capable of measuring average spectral content. In particular, such non-imaging detectors do not need to measure spatial information. Such non-imaging detectors include, for example, color ratiometric detectors. Multi-element detectors with mosaic spectral filters could be used. Alternatively, a single element detector with a spectral filter wheel could be used. 
     The systems and methods of this invention are calibrated in situ to provide a consistent image of a sample part. In addition, the systems and methods of this invention require only a small drive current to drive the illumination source. The small drive current can be adjusted by the user to optimally illuminate colored sample parts. Further, the systems and methods of this invention provide a stable and standardized method for illuminating a sample part over a long period of time. 
     These and other objects of the invention will be described in or be apparent from the following description of the examplary embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in conjunction with the following drawings in which like reference numerals designate like elements and wherein: 
     FIG. 1 is a block diagram of one exemplary embodiment of an illumination system according to this invention that uses a transmission mode to illuminate a sample part; 
     FIG. 2 is a plan view of the illumination source illustrating an example of the solid-state devices; 
     FIG. 3 is a perspective view of the illumination source illustrating an example of the collection lens; 
     FIG. 4 is a block diagram of a second exemplary embodiment of an illumination system according to this invention that uses a reflective mode to illuminate a sample part; 
     FIG. 5 is a flowchart outlining one exemplary embodiment of a method for calibrating a reference illumination source according to this invention; 
     FIG. 6 is a flowchart outlining one exemplary embodiment of a method for generating a calibration table using a reference illumination source according to this invention; 
     FIG. 7 is a flowchart outlining one exemplary embodiment of a method for generating a lookup table using the illumination source according to this invention; and 
     FIG. 8 is a flowchart outlining one exemplary embodiment of a method for controlling a drive current according to this invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 is a block diagram of a system  100  using a transmission mode for illuminating a sample part  110 . The system  100  uses solid-state devices for illumination because of their stability and long-life. The solid-state devices may include light emitting diodes (LEDs). The solid state devices may operate in the visible and/or near-infrared regions of the spectrum. The solid-state devices operate in the spectral regions that charge coupled devices (CCDs) are known to be photosensitive in, such as, for example 360 nm-1100 nm. 
     LEDs are preferably used because LEDs are more amenable to precise optical power regulation than halogen lamps. This is due to the smaller drive currents needed to operate the LEDs. In addition, the discrete nature of LEDs allows the wavelength of the emitted light to be more flexibly selectable. Also, when driven electronically within the working parameters of the LEDs, the repeatability and reliability of the LEDs&#39; optical output energy are both very high. In addition, some LEDs are capable of emitting light in the ultra-violet A frequency range, which is used to improve the resolving power of imaging optics. 
     The solid-state devices used for the illumination source may be surface mounted or acrylic-encapsulated LED packages. For example, surface-mounted solid-state devices can be combined with collection and/or collimation lenses to form the illumination source. The illumination source can then multiplex among the individual solid-state devices to optimally illuminate the sample part. In addition, for multi-wavelength addressable devices, the illumination source can match or avoid the average spectral absorption properties of the sample part within the field of view to enhance the image contrast. The capability to standardize the light system with respect to color illumination selection within an instrument line is a valuable feature when inspecting colored parts. 
     As shown in FIG. 1, the system  100  illuminates a sample part  110  placed on a transparent plane  109 . The transparent plane  109  is movable in two orthogonal directions. The system  100  includes an illumination source  101  and data stored in a memory  112  representing a reference illumination source  108 . A light collection system  150  includes a collection lens assembly  104  and a CCD detector  111 . In particular, the CCD detector  11  has an optical axis  119  parallel to, and preferably coincident with, an axis of illumination  120  of the illumination source  101 . The illumination source  101  may be directed onto the optical axis  119  of the light collection system  150 . Illumination is made at or near a perpendicular direction to the plane  109  in the transmission mode. The light collection system  150  forms an image of the illuminated sample part  110 . 
     An optical element  102  is positioned between the illumination source  101  and the sample part  110 . The optical element  102  may be a dichroic plate or other beam splitter. Any other known or later developed optical element capable of simultaneously transmitting and reflecting optical output energy can also be used as the optical element  102 . The optical element  102  redirects a small percentage of the entire optical output energy emitted from the illumination source  101  to a monitoring detector  105 , as shown by the dotted lines in FIG.  1 . The remainder of the emitted optical output energy illuminates the sample part  110  from which the light collection system  150  forms an image. 
     The monitoring detector  105  measures at least one characteristic of the optical output energy emitted from the illumination source  101  before the CCD detector  111  captures an image of the sample part  110 . Furthermore, the monitoring detector  105  ensures that the sample part  110  does not influence the source monitor measurement and permits standardizing one illumination system with a plurality of similar illumination systems found in corresponding instruments within a product line. As such, the monitoring detector  105  is able to measure such characteristics as, for example, the intensity of light emitted by the illumination source  101 . The monitoring detector  105  is also able to measure the intensity of light emitted by a certain color from a color device within the illumination source  101  by electronically selecting the spectral bandwidth of the emitted light. The monitoring detector  105  is capable of measuring characteristics of the optical output energy other than those listed above. The system  100  stabilizes and standardizes the level of the optical output energy emitted from the illumination source  101  based, at a minimum, on one or more of the characteristics measured by the monitoring device  105 . 
     To standardize the output of the system  100 , a reference illumination source  108  is used. The reference illumination source  108  should be similar in nature to the illumination source  101 . Thus, if the illumination source  101  is an LED, the reference illumination source  108  should also be an LED. If the illumination source  101  is a combination of multi-color LEDs whose spectral bandwidths are electronically selectable, the reference illumination source  108  should also be a similar combination of LEDs, and so on. The reference illumination source  108  should be calibrated by a recognized or accredited organization, such as, for example, the National Institute of Standards and Technology (NIST). The characteristics of the optical output energy of the reference illumination source  108  are compiled into a calibration table stored in the memory  112 . If addressable narrowband illumination sources are used in the machine vision instrument, a calibration table is created for each source. Since the characteristics of the optical output energy of the reference illumination source  108  are used to calibrate the monitoring detector  105  when the machine vision instrument is manufactured, the optical output energy emitted from the illumination source  101  of the machine vision instrument can be compared with corresponding characteristics of the optical output energy from the reference illumination source  108  found in the calibration table. 
     In particular, the calibration table obtained for the monitoring detector  105  provides the output current of the monitoring detector  105  as a function of the optical output energy from the reference illumination source  108 . The calibration table resides in the memory  112  for each machine vision instrument. The calibration table correlates the optical output energy of the reference illumination source  108  input to the monitoring detector  105  and the current output by the monitoring detector  105 . 
     Once the calibration table has been generated, the illumination source  101  of the machine vision instrument permanently replaces the reference illumination source  108 . The optical output energy from the illumination source  101 , as measured by the monitoring detector  105 , can be compared to the optical output energy of the reference illumination source  108  in the calibration table stored in the memory  112 . A lookup table is generated by comparing the optical output energy from the illumination source  101  to corresponding values of the reference illumination source  108  to provide a starting drive current which is output on line  107  for feedback iterations. The initial datum of the starting drive current is used to set the illumination source  101  drive current when performing iterative adjustment. The adjustment is halted when the optical output energy level of the illumination source  101  is within a prescribed acceptable range. An initial drive current is desired because the optical output energy from the illumination source  101  changes due to temperature, age, device differences, etc. The lookup table resides in the memory  112  of the system  100  and can be updated as required, thereby reducing iteration convergence time. 
     A temperature probe  103  is positioned near the illumination source  101  and the monitoring detector  105 . Variations in the ambient temperature can affect the optical output energy levels of the illumination source  101  and the responsivity of the monitoring detector  105 . The temperature probe  103  provides a real-time monitor of the ambient temperature of the illumination source  101  and the monitoring detector  105 . The real-time ambient temperature can be used to compensate for variations in the optical output energy of the illumination source  101 . 
     Variations in ambient temperature can affect the optical output energy of the illumination source  101  by as much as 20% within a reasonable operating temperature range of 15° C.-35° C. Additionally, ambient temperature variations can affect the responsivity of the monitoring detector  105 . In one exemplary embodiment, photodiodes are used for the monitoring detector  105  because the photodiodes have a nominal variation within the above-described temperature range of about 10%. 
     The temperature probe  103  outputs the real-time ambient temperature of the illumination source  101  to the current controller  106  as a processed electrical signal. Accordingly, the temperature probe  103  helps the system  100  compensate for ambient operating conditions, such as, for example, temperature drifts in the optical output energy of the illumination source  101  that would otherwise negatively affect the stability and performance of the illumination source  101 . 
     The current controller  106  processes the electrical signal received from the temperature probe  103  and the output current received from the monitoring detector  105 . The current controller  106  outputs a compensated drive current on the line  107  based on the data from the temperature probe  103  and the monitoring detector  105 . The current controller  106  adjusts the optical output energy from the illumination source  101  using the compensated drive current output on the line  107 . This adjustment persists until the optical output energy achieves a desired target point in agreement with the corresponding optical output energy for the appropriate reference illumination source  108  stored in the calibration table. 
     The target point is the level of the optical output energy from the illumination source  101  that illuminates the sample part  110 , resulting in a consistent image of the sample part  110 . In essence, the target point defines an image quality based on an illumination level rather than a device drive current level. The target point can be subjectively chosen by the operator to correspond to an acceptable quality image with which to perform dimensional inspection. Alternatively, the target point can also be objectively chosen by a suitable standard of measurement to provide the acceptable quality image. Additionally, the target point can be provided using a graphical user interface, passed as a specified value contained within a “part program”, or determined from an appropriate algorithm. Thereafter, standardization and repeatability in establishing the same image brightness would follow a similar procedure. 
     To stabilize the system  100 , the current controller  106  can also compensate for optical power changes. Such changes may result from differences in optical coupling efficiency and/or component variance among systems. Additionally, changes may result from low-frequency temperature drifts in the ambient environment that affect the illumination source  101  and the monitoring detector  105  or from current source fluctuations in driving the illumination source  101 . 
     Further, a linearized scale of the illumination intensity level on the sample part  110  per selected wavelength may be provided to the user that is valid irrespective of the particular system  100 , the age of the illumination source  101 , the temperature of the illumination source  101  or the drive current supplied to the illumination source  101  on the line  107 . In practice, the optical output energy is a non-linear function of the drive current. As a result, the illumination source intensity levels may be user-adjustable within ranges that do not greatly alter the optical output energy. Since most illumination sources display non-linear behavior, the adjustment made by a user may be counter-intuitive, would not optimize adjustment resolution, and would not correspond in a linear fashion to the amount of optical output energy from the illumination source  101 . Further, a linearized, optical output energy, which is standardized via the reference illumination source  108 , provides to the user a new, intuitive setting of the illumination level whose adjustment resolution can be optimized to better match the performance of the illumination device  101 . 
     At least one characteristic of the optical output energy emitted by the illumination source  101  is measured by the monitoring detector  105 . The characteristic of the optical output energy measured by the monitoring detector  105  is compared to a corresponding characteristic of the reference illumination source  108  stored in the calibration table, as the responsivity of the monitoring detector  105  will not measurably change. Any discrepancy between the optical output energy of the illumination source  101  and that of the corresponding value for the reference illumination source  108  found in the calibration table is minimized by adjusting the current output on the line  107  from the current controller  106  to the illumination source  101 . The optical output energy emitted from the illumination source  101  is then remeasured by the monitoring detector  105 . Iterative adjustment to obtain agreement between that measured from the illumination source  101  and the desired reference illumination source  108  is made based on an appropriate standard of measurement such as difference, maximum, minimum, etc. 
     Based on this iterative scheme, the drive current output by the current controller  106  on the line  107  to the illumination source  101  is adjusted to yield an illumination level onto the sample part  110  in accord with the reference illumination source  108 . 
     The current controller  106  supplies a processed, compensated input drive current output on the line  107  to the illumination source  101  based on the real-time status of the optical output energy of the illumination source  101  via the monitoring detector  105  and the local environment temperature. Thus, the compensated input drive current signal output on line  107  is able to modify the optical output energy from the illumination source  101  so that it is in accordance with the reference illumination source  108 . Hence, the current controller  106  adjusts the optical output energy emitted from the illumination source  101  using the compensated drive current signal output on the line  107  until the optical output energy achieves the desired target point. 
     As shown in FIG. 2, one exemplary embodiment of the illumination source  101  uses solid-state devices  114 . The solid-state devices  114  can be, but are not limited to, surface-mounted LEDs or an acrylic-encapsulated LED package. FIG. 2 shows three LEDs  114   a ,  114   b , and  114   c  surface-mounted onto a substrate of the illumination source  101 . The LEDs  114   a ,  114   b , and  114   c  respectively operate, for example, in the red, green and blue spectral regions. Alternatively, some or all of the LEDs  114  could emit in the near infrared region of the spectrum, where better compatibility may be observed with some samples to be illuminated. This may be ideal for biological purposes, but is not limited to this use. 
     FIG. 3 is a perspective view of the illumination source  101  illustrating an example of the solid-state devices  114  being combined with a collection lens  113  to form the illumination source  101 . An advantage of combining the surface-mounted LEDs  114  and the collection lens  113  is the ability to multiplex the formed illumination source  101  among the individual LEDs to optimize the illumination of the sample part  110 . 
     FIG. 4 is a block diagram of a system  200  using a reflective mode for illuminating the sample part  110 . The system  200  uses solid-state devices for illumination, as previously described. The system  200  illuminates the sample part  110  placed on a transparent plane  209 . The transparent plane  209  is movable in two orthogonal directions. The system  200  includes an illumination source  201  and data stored in a memory  112  representing a reference illumination source  208 . A light collection system  250  includes a collection lens assembly  204  and a CCD detector  211 . The light collection system  250  forms an image of the illuminated sample part  110  onto the CCD detector  211 . 
     The illumination source  201  is directed onto an optical axis  219  of the light collection system  250  by an optical element  202 . Illumination is made at or near a perpendicular direction to the transparent plane  209  in the reflective type mode. The optical element  202  is positioned between the illumination source  201  and the sample part  110 . The optical element  202  directs a small percentage of the entire emitted optical output energy from one illumination source  201  onto a monitoring detector  205 , as shown by the dotted lines  230  in FIG.  4 . 
     As discussed previously, the monitoring detector  205  measures at least one characteristic of the optical output energy emitted from the illumination source  201  before the CCD detector  211  captures an image of the sample part  110 . In addition, the monitoring detector  205  is calibrated when the system  200  is manufactured using a corresponding reference illumination source  208 , as discussed above. The monitoring detector  205  is a stable, compensated photodetector and is used to standardize the system  200 . 
     To standardize the output of the system  200 , the reference illumination source  208  is used. As discussed above, the reference illumination source  208  should be similar in nature to the illumination source  201 . Also, the reference illumination source  208  should also be calibrated by a recognized or accredited organization, such as, for example, the National Institute of Standards and Technology (NIST). The characteristics of the optical output energy of the reference illumination source  208  are compiled into a calibration table residing in the memory  112 . If addressable narrowband illumination sources are used in the machine vision instrument, a calibration table is created for each source. Since the optical output energy of the reference illumination source  208  is used to calibrate the monitoring detector  205  when the machine vision instrument is manufactured, the optical output energy of the illumination source  201  can be compared with the corresponding characteristics of the optical output energy from the reference illumination source  208  found in the calibration table. 
     In particular, the calibration table obtained for the monitoring detector  205  provides the output current of the monitoring detector  205  as a function of the optical output energy from the reference illumination source  208 . The calibration table resides in the memory  112  for each machine vision instrument. The calibration table correlates the optical output energy of the reference illumination source  208  input to the monitoring detector  205  and the current output by the monitoring detector  205 . 
     Once the calibration table has been generated, the illumination source  201  of the machine vision instrument permanently replaces the reference illumination source  208 . The optical output energy from the illumination source  201 , as measured by the monitoring detector  205 , can be compared to the optical output energy of the reference illumination source  208  in the calibration table stored in the memory  112 . A lookup table is generated by comparing the optical output energy from the illumination source  201  to corresponding values of the reference illumination source  208  to provide a starting drive current which is output on line  207  for feedback iterations. The initial datum of the starting drive current is used to set the illumination source  201  drive current when performing iterative adjustment. The adjustment is halted when the optical output energy level of the illumination source  201  is within a prescribed acceptable range. An initial drive current is desired because the optical output energy from the illumination source  201  changes due to temperature, age, device differences, etc. The lookup table also resides in the memory  112  of the system  200  and can be updated as required, thereby reducing iteration convergence time. 
     A temperature probe  203  is positioned near the illumination source  201  and the monitoring detector  205 . Variations in the ambient temperature can affect the optical output energy levels of the illumination source  201  and the responsivity of the monitoring detector  205 . The temperature probe  203  provides a real-time monitor of the ambient temperature of the illumination source  201  and the monitoring detector  205 . The real-time ambient temperature can be used to compensate for variations in the optical output energy of the illumination source  201 . 
     Variations in ambient temperature can affect the optical output energy of the illumination source  201  by as much as 20% within a reasonable operating temperature range of 15° C.-35° C. Additionally, ambient temperature variations can affect the responsivity of the monitoring detector  205 . In one exemplary embodiment, photodiodes are used for the monitoring detector  205  because the photodiodes have a nominal variation within the above-described temperature range of about 10%. 
     The temperature probe  203  outputs the real-time ambient temperature of the illumination source  201  to the current controller  206  as a processed electrical signal. Accordingly, the temperature probe  203  helps the system  200  compensate for ambient operating conditions that would otherwise negatively affect the stability and performance of the illumination source  201 . 
     The current controller  206  processes the electrical signal received from the temperature probe  203  and the output current received from the monitoring detector  205 . The current controller  206  outputs a compensated drive current on the line  207  based on the data from the temperature probe  203  and the monitoring detector  205 . The current controller  206  adjusts the optical output energy from the illumination source  201  using the compensated drive current output on the line  207 . This adjustment persists until the optical output energy achieves a desired target point in agreement with the corresponding optical output energy for the appropriate reference illumination source  208  stored in the calibration table. 
     The target point is the level of the optical output energy from the illumination source  201  that illuminates the sample part  210 , resulting in a consistent image of the sample part  210 . In essence, the target point defines an image quality based on an illumination level rather than a device drive current level. The target point can be subjectively chosen by the operator to correspond to an acceptable quality image with which to perform dimensional inspection. Alternatively, the target point can also be objectively chosen by a suitable standard of measurement to provide the acceptable quality image. Additionally, the target point can be provided using a graphical user interface, passed as a specified value contained within a “part program”, or determined from an appropriate algorithm. Thereafter, standardization and repeatability in establishing the same image brightness would follow a similar procedure. 
     To stabilize the system  200 , the current controller  206  can also compensate for optical power changes. Such changes may result from differences in optical coupling efficiency and/or component variance among systems. Additionally, changes may result from low-frequency temperature drifts in the ambient environment that affect the illumination source  201  and the monitoring detector  205  or from current source fluctuations in driving the illumination source  201 . 
     The remainder of the optical output energy emitted from the illumination source  201  is redirected onto the optical axis  219  of the light collection system  250 . The redirected optical output energy is focused onto the sample part  110  using a focusing lens  204  to illuminate the sample part  110 . The redirected optical output energy focused onto the sample part  110  reflects and/or scatters from the sample part  110  onto the optical axis  219 . Some portion of the scattered energy from the sample part  110  is then gathered and recollected by the same focusing lens  204 . The recollected energy is then imaged onto the CCD detector  211 . 
     At least one characteristic of the optical output energy emitted by the illumination source  201  is measured by the monitoring detector  205 . The characteristic of the optical output energy measured by the monitoring detector  205  is compared to a corresponding characteristic of the reference illumination source  208  stored in the calibration table, as the responsivity of the monitoring detector  205  will not measurably change. Any discrepancy between the optical output energy of the illumination source  201  and that of the corresponding value for the reference illumination source  208  found in the calibration table is minimized by adjusting the current output on the line  207  from the current controller  206  to the illumination source  201 . The optical output energy emitted from the illumination source  201  is then remeasured by the monitoring detector  205 . Iterative adjustment to obtain agreement between that measured from the illumination source  201  and the desired reference illumination source  208  is made based on an appropriate standard of measurement such as difference, maximum, minimum, etc. 
     Based on this iterative scheme, the drive current output by the current controller  206  on the line  207  to the illumination source  201  is adjusted to yield an illumination level onto the sample part  110  in accord with the reference illumination source  208 . 
     The current controller  206  supplies a processed, compensated input drive current output on the line  207  to the illumination source  201  based on the real-time status of the optical output energy of the illumination source  201  via the monitoring detector  205  and the local environment temperature. Thus, the compensated input drive current signal output on line  207  is able to modify the optical output energy from the illumination source  201  so that it is in accordance with the reference illumination source  208 . Hence, the current controller  206  adjusts the optical output energy emitted from the illumination source  201  using the compensated drive current signal output on line  207  until the optical output energy achieves the desired target point. 
     The illumination source  201  is capable of illuminating a sample part  110  with optical energy emitted from a color-addressable, solid-state device which is switchable from, for example, 360 nm to 1100 nm. As such, the illumination source  201  can optimally match or avoid the absorption properties of surface pigments which coat the sample part  110 . The illumination source  201  can also provide radiation whose spectral content is sufficient to cover the visible region so that rudimentary color analysis within the field of view can be performed. Analysis of an independent measurement of the absorptive or reflective properties of the sample part  110  can establish the spectral region within which to optimally illuminate the sample part  110  to, for example, enhance the contrast In reflective or transmissive illumination, this measurement is accomplished on the collection device  150  or  250  side using color-ratiometric, opto-electronic detectors  214 . In the ultra-violet and near infrared regions, custom multi-element detectors with appropriate filters are required. 
     For white light produced by a filament source or by solid-state devices activated in parallel to produce the white light, some of the reflected or scattered optical output energy from the sample part may be further redirected by a second optical element  215  positioned within the optical path onto one or more ratiometric detectors  214 . The one or more ratiometric detectors  214  measure average scattered and reflected light from features in the field of view from the sample part  110  to estimate the red, green and blue components of color. Further, information indicating the color components allow the user to select the color of illumination used to optimize the image measurement. 
     FIG. 5 is a flowchart outlining one exemplary embodiment of a method for calibrating a reference illumination source according to this invention. Beginning in step S 100 , control continues to step S 200 , where a reference illumination source is provided. Next, in step S 300 , the reference illumination source is calibrated. 
     Then, in step S 400 , the characteristics of the optical output energy of the reference illumination source are compiled. Next, in step S 500 , each calibrated reference illumination source is stored to preserve integrity except when used to calibrate machine vision instruments. Then, in step S 600 , the reference illumination source calibration method ends. 
     FIG. 6 is a flowchart outlining in greater detail one exemplary embodiment of the method for generating a calibration table using a reference illumination source of FIG.  5 . Beginning in step S 1000 , control continues to step S 1100 , where the reference illumination source is temporarily installed into the machine vision instrument in the exact position the permanent illumination source is to be located. Next, in step S 1200 , a request drive current is chosen for the wavelength of the initial optical output energy to be emitted by the reference illumination source. Then, in step S 1300 , the ambient background light contribution and average temperature conditions are determined to obtain a temperature-compensated base line reading without sample illumination. Control then continues to step S 1400 . 
     In step S 1400 , the base line reading is stored. Then, in step S 1500 , the request drive current is output. Next, in step S 1600 , optical output energy is emitted by the reference illumination source. Then, in step S 1700 , a portion of the entire emitted optical output energy is measured to determine at least one characteristic of the emitted optical output energy. Control then continues to step S 1800 . 
     In step S 1800 , the measured optical output energy is converted to an output current. Next, in step S 1900 , the output current representing the emitted optical output energy is adjusted to compensate for the measured ambient conditions. Then, in step S 2000 , the compensated optical output energy, compensated reference illumination source optical input and the request drive current of the reference illumination source are stored in the calibration table. Control then continues to step S 2100 . 
     In step S 2100 , the request drive current is checked to determine if an incremental change in the request drive current can be made within the operating range of the reference illumination source. If so, control continues to step S 2200 . Otherwise, control jumps to step S 2300 . In step S 2200 , the request drive current is changed an incremental amount. Control then returns to step S 1500 . 
     If, in step S 2100  another incremental change in the request drive current cannot be made within the operating range of the reference illumination source, control jumps to step S 2300 . In step S 2300 , for a multi-color addressable reference illumination source, the next color is selected so that an identical calibration table can be generated for the new color. If another color can be selected, control continues to step S 2400 . Otherwise, control continues to step S 2500 . 
     In step S 2400 , the wavelength of the next color is selected, a request drive current is chosen and generation of a calibration table corresponding to the wavelength of the new color is started. Control then returns to step S 1300 . In step S 2500 , the method ends. 
     FIG. 7 is a flowchart outlining one exemplary embodiment of a method for generating a lookup table using the instrument illumination source. Beginning in step S 3000 , control continues to step S 3100 , where the reference illumination source is removed and stored. Then, in step S 3200 , the illumination source is permanently installed. Next, in step S 3300 , a request drive current is selected for the wavelength of the initial optical output energy to be emitted by the instrument illumination source. Control then continues to step S 3400 . 
     In step S 3400 , the ambient background light contribution and average temperature conditions are determined to obtain a temperature-compensated base line reading without sample illumination. Next, in step S 3500 , the request drive current is output to the illumination source. Then, in step S 3600 , optical output energy is emitted by the illumination source. Control then continues to step S 3700 . 
     In step S 3700 , a portion of the entire emitted optical output energy is measured to determine at least one characteristic of the emitted optical output energy. Next, in step S 3800 , the measured optical output energy is converted to an output current. Then, in step S 3900 , the output current representing the emitted optical output energy is adjusted to compensate for the measured ambient conditions of background light contribution and average temperature. Control then continues to step S 4000 . 
     In step S 4000 , the compensated optical output energy, compensated reference illumination source optical input and the request drive current of the reference illumination source are stored in the lookup table. Next, in step S 4100 , the request drive current is checked to determine if an incremental change in the request drive current can be made within the operating range of the instrument illumination source. If so, control continues to step S 4200 . Otherwise, control jumps to step S 4300 . 
     In step S 4200 , the request drive current is changed an incremental amount. Control then returns to step S 3500 . If, in step S 4100  another incremental change in the request drive current cannot be made within the operating range of the instrument illumination source, control jumps to step S 4300 . In step S 4300 , for a multi-color addressable instrument illumination source, the next color is selected so that an identical calibration table can be generated for the new color. If a remaining color can be selected, control continues to step S 4400 . Otherwise, control continues to step S 4500 . 
     In step S 4400 , the wavelength of the next color is selected, a request drive current is chosen, and generation of a lookup table corresponding to the wavelength of the new color is started. Control then returns to step S 3300 . In step S 4500 , the method ends. 
     It should be appreciated that steps S 3100  and S 3200  can be performed independently of steps S 3300 -S 4500 . Thus, steps S 3100 -S 3200 , if performed at some earlier time, and/or performed by another, can be omitted from the method of steps S 3300 - 4500  without changing the results of steps S 3300 -S 4500 . 
     FIG. 8 is a flowchart outlining one exemplary embodiment of a method for controlling a drive current of an instrument illumination source according to this invention. Beginning in step S 5000 , control continues to step S 5100 , where a target point is input Next, in step S 5200 , the target point is correlated to a measured characteristic of the optical output energy of a reference illumination source from the calibration table. Then, in step S 5300 , the previously measured reference characteristic from the calibration table is compared to a measured characteristic of the optical output energy of the instrument illumination source yielding an initial request drive current for the chosen color/wavelength. Control then continues to step S 5400 . 
     In step S 5400 , the ambient background light contribution and average temperature conditions are determined to obtain a temperature-compensated base line reading without sample illumination. Next, in step S 5500 , the initial request drive current is output to the illumination source. Then, in step S 5600 , optical output energy is emitted by the illumination source. Control then continues to step S 5700 . 
     In step S 5700 , at least one characteristic of the optical output energy emitted by the illumination source is measured. Next, in step S 5800 , a first compensation for the effect of the ambient temperature on the responsivity of the monitoring detector is performed. Then, in step S 5900 , a second compensation for the effect of the ambient temperature on the optical output energy of the illumination source is performed. Control then continues to step S 6000 . 
     In step S 6000 , the fully compensated measurement is compared to measurements from the reference illumination source stored in the calibration table. Then, in step S 6100 , the actual target point is compared to the desired target point. If the actual target point is not within a predetermined tolerance, control continues to step S 6200 . Otherwise, control continues to step S 6400 . 
     In step S 6200 , a differential drive current is determined. Next, in step S 6300 , a new drive current is determined from the differential drive current and the request drive current. Control then returns to step S 5500 . In contrast, step S 6400 , the method ends. 
     It should be appreciated that, if one or more of the temperature or other ambient condition sensors are not provided, then one or both of steps S 5800  and S 5900  can be omitted from the method outlined in FIG.  8 . 
     As shown in FIGS. 1 and 4, the current controller  106  and  206  supplies a processed, compensated drive current on the line  107  and  207  to the illumination source  101  and  201 . Thus, the current controller  106  or  206  can be implemented on a general purpose computer, a special purpose computer, a programmed microprocessor or microcontroller and peripheral integrated circuit elements, an ASIC or other integrated circuit, a digital signal processor, a hardwired electronic or logic circuit such as a discrete element circuit, a programmable logic device such as a PLD, PLA, FPGA or PAL, of the like. In general, any device, capable of implementing a finite state machine that is in turn capable of implementing the control routines shown in the flowcharts of FIGS. 5-8 and can be used to implement the current controller  106  or  206 . 
     While the invention has been described in conjunction with specific exemplary embodiments outlined above, it is evident that many alternatives, modifications and variations may be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention as set forth herein are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention.