Chromatic distance sensor with highly sensitive measurement mode

Abstract

Method for operating a chromatic distance sensor (CRS) system to perform a workpiece height measurement of a workpiece surface with low reflectivity, the method comprising: providing a CRS that provides a valid wave peak value over an overall nominal measurement range uninterrupted between an upper and lower nominal measurement range limit when operated using an exposure time that is within a first nominal range of exposure times;Operating the CRS using a self-saturating exposure time that is longer than the exposure times in the first nominal range of exposure times, wherein: regardless of whether a workpiece surface is located within the total nominal measuring range or not, the self-saturating exposure time saturates at least one detector pixel of the CRS to at least one saturation limit level, preventing a valid wavelength peak value in an invalid peak value part of the total nominal measuring range, corresponding to the at least one detector pixel that is saturated by the self-saturating exposure time to at least the saturation limit level, thereby leaving valid wavelength peak value measurement positions only in a valid subset of the total nominal measuring range, which excludes the invalid peak value part;and provide, on a user interface portion of the CSR, a display of at least one of (a) a height measurement for a workpiece surface based on the use of the self-saturating exposure time, and (b) valid peak measurement positions contained in the valid subset of the total nominal measurement range corresponding to the use of the self-saturating exposure time.

Description

GENERAL STATE OF THE ARTTechnical FieldThis invention relates to precision measurement equipment and, more particularly, to chromatic range sensors and similar optical range finding devices and their use.Description of the Prior ArtIt is known to use chromatic confocal techniques in optical height or distance or range sensors. As described in U.S. Pat. No. 7,876,456 B2 (the '456 patent), which is hereby incorporated by reference in its entirety, an axial chromatic aberration optical element, also referred to as axial or longitudinal chromatic aberration, may be used to focus a broadband light source such that the axial distance to the focus changes with wavelength. Thus, only a single wavelength is accurately focused on a surface and the height or distance of the surface with respect to the focusing element determines which wavelength is best focused. Upon reflection from the surface, the light is re-focused onto a small detector aperture, such as a small hole or the end of an optical fiber. When reflected from the surface and returned through the optical system to the input / output optical fiber, only the wavelength that is well focused on the surface is well focused on the aperture. All other wavelengths are poorly focused on the aperture and therefore do not couple much power into the optical fiber. Therefore, in the case of the light returned through the optical waveguide, the signal level will be the greatest for the wavelength corresponding to the surface height or the distance from the surface. A spectrometer detector measures the signal level for each wavelength to determine the surface height.Certain manufacturers relate to practical and compact chromatic range sensing (CRS) systems operating as described above, suitable for use in an industrial environment as a chromatic point sensor (CPS) or chromatic line sensor or the like. A compact chromatically dispersive optical assembly used in such systems is referred to as an "optical pen" or "pen.". The optical pen is connected through an optical fiber to an electronic part of the chromatic point sensor, which sends light through the optical fiber to output it from the optical pen, and which provides a spectrometer that detects and analyzes the returned light. The returned light forms a wavelength-dispersed intensity profile received by the detector array of the spectrometer. Pixel data corresponding to the wavelength-scattered intensity profile is analyzed to determine the "dominant wavelength peak pixel coordinate" as indicated by a peak or centroid of the intensity profile, and the dominant wavelength peak pixel coordinate is used with a look-up table to determine the distance to the surface. The pixel coordinate of the dominant wavelength peak may be determined in subpixel resolution and may be referred to as the "distance-indicating pixel coordinate.".Also known in the art is a CRS, which is a "line sensor" that uses a slit aperture and focuses light along a line rather than on a point, thus providing the ability to measure a distance to a surface at a number of points along that line, as disclosed in U.S. Patent Publication US 2010 / 0 188 742 A1, which is hereby incorporated by reference in its entirety.SUMMARY OF THE INVENTIONThis summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.Some workpiece surfaces may have a particularly low reflectivity (e.g., matt, black or transparent workpieces) and provide a weak signal that is insufficient to generate a clear (valid) wavelength peak that can be well estimated in a resulting intensity profile received from the detector array of the spectrometer. One way to amplify the signal (e.g., the peak wavelength) is to increase the signal integration time, i.e., to increase an exposure time. Increasing the exposure time beyond a certain amount may cause considerable or complete saturation of some of the pixels in the detector array of the spectrometer, whether or not a workpiece surface is located to generate a corresponding wavelength. Saturation may be caused by internal (self) reflections within the CRS system, for example at various fiber coupling portions within the system, such as fiber couplers / connectors, fiber splitters, and fiber ends. Pixels saturated to at least a "saturation threshold" level cannot provide a valid wavelength peak signal due to insufficient peak height to support accurate peak position estimation. If a workpiece surface is at a distance at which a specific wavelength is focused and reflected back to a corresponding pixel that is substantially or fully saturated, then the system is unable to measure the distance. Thus, the operation of CRS systems is currently limited by a maximum exposure time chosen to avoid detector pixel saturation at or above a saturation threshold level (e.g., 80%-90% of full saturation at the upper signal limit, or more) that may correspond to a minimum sampling rate below which the sampling rate cannot be lowered. A typical minimum sampling rate is about 300 Hz. Thus, there is presently no solution or feature that can be used by relatively unskilled users of CRS systems to measure a workpiece surface that has insufficient reflectivity and produces a significant peak wavelength when using a maximum CRS exposure time selected to avoid significant detector pixel saturation.Various aspects of the present invention are directed to overcoming the technical problems or limitations described above.According to the invention, a method of operating a chromatic range sensor (CRS) system and a chromatic range sensor system according to the independent claims are proposed. The dependent claims relate to preferred embodiments.According to embodiments of the present invention, a method of operating a CRS system to perform workpiece height measurement of a workpiece surface with low reflectivity may be provided. In some embodiments, the CRS system may include a chromatic point sensor (CPS) system with an optical pen. In some example aspects, the CRS system preferably includes a high sensitivity measurement mode in which the CRS system may be operated at a lower sampling rate (e.g., lower than conventionally available sampling rates) to measure a low reflectivity workpiece surface. The CRS system operating at such a low sampling rate (i.e., using a longer self-saturating exposure time) may cause one or more of its detector pixels to be / become saturated at or above a saturation threshold level, but is preferably still capable of detecting a valid wavelength peak indicative of a height measurement for the workpiece surface when the workpiece surface is within a valid subset of a nominal total measurement range of the CRS system that excludes an invalid peak portion corresponding to the one or more detector pixels that is / are saturated at or above a saturation threshold level by the self-saturating exposure time.In particular, according to some example aspects, a method of operating a CRS system may be provided to perform workpiece height measurement of a workpiece surface with low reflectivity. The method may generally include three steps (but is not limited to the three steps). First, a CRS may be provided that preferably generates a valid wavelength peak over a nominal total measurement range that is uninterrupted between upper and lower nominal measurement range limits when operated using an exposure time that is within a first nominal range of exposure times. Second, the CRS may be operated using a self-saturating exposure time that is longer than the exposure times within the first nominal range of exposure times. Regardless of whether a workpiece surface is in the nominal total measurement range or not, the self-saturating exposure time preferably saturates at least one detector pixel of the CRS at or above the saturation threshold level that prevents a valid wavelength peak in an invalid peak portion of the nominal total measurement range corresponding to the at least one detector pixel saturated by the self-saturating exposure time at or above a saturation threshold level, such that valid wavelength peak measurement positions are left only in a valid subset of the nominal total measurement range excluding the invalid peak portion. Third, on a user interface portion (e.g., a screen) of the CRS, a display may be provided of at least one of (a) a height measurement for a workpiece surface based on the self-saturating exposure time, and / or (b) valid peak measurement positions included in the valid subset of the total nominal measurement range corresponding to use of the self-saturating exposure time.In some exemplary preferred aspects, the step of providing an indication of (a) a height measurement for a workpiece surface based on utilizing the self-saturating exposure time includes positioning the workpiece surface within the valid subset of the total nominal measurement range.In some exemplary preferred aspects, additionally or alternatively to the above, the step of providing an indication of (a) a height measurement for a workpiece surface based on the use of the self-saturating exposure time includes providing a plurality of respective height measurements for respective points on the workpiece surface.In some example preferred aspects, the display (b) may be provided on the user interface portion of the CRS, and / or the display (b) includes at least one of (b-i) a graphical representation of the valid peak measurement positions included in the valid subset of the total nominal measurement range and / or (b-ii) an indication that a workpiece surface is positioned in the invalid peak portion of the total nominal measurement range.In some example preferred aspects, the invalid peak portion includes two discrete segments of the total nominal measurement range.In some example preferred aspects, the valid peak portion comprises two discrete segments of the total nominal measurement range.In some example preferred aspects, the display (b) may be provided on the user interface portion of the CRS, and / or the display (b) includes at least one of (b-i) a display that displays the upper and lower limits of at least one uninterrupted portion of valid peak measurement positions included in the valid subset of the total nominal measurement range, and / or (b-ii) a first display state of a valid / invalid indicator that includes the first display state and a second display state that are displayed in response to measurements obtained in the valid subset of the total nominal measurement range and measurements obtained in the invalid peak portion, respectively.In some example preferred aspects, exposure times are preferably associated with respective sampling rates on the operator interface portion of the CRS, and / or the step of operating the CRS using a self-saturating exposure time comprises operating the CRS using a sampling rate corresponding to the self-saturating exposure time.In some example preferred aspects, respective exposure times in a first nominal range of exposure times are preferably associated with the corresponding sampling rates displayed on the user interface portion of the CRS in a first nominal set of sampling rates, and / or the step of operating the CRS using a self-saturating exposure time includes operating a sampling rate that is slower than the sampling rates included in the first nominal set of sampling rates.In some example preferred aspects, the first nominal sets of sampling rates are associated with a first normal mode of operation displayed on the user interface portion of the CRS, and / or the slower sampling rate is associated with a second high sensitivity mode of operation displayed on the user interface portion of the CRS.In some exemplary preferred aspects, the sampling rate corresponding to the self-saturating exposure time is at most 60 Hz. In particular, in some exemplary preferred aspects, the sampling rate corresponding to the self-saturating exposure time is at most 35 Hz.In some example preferred aspects, the step of operating the CSR using a self-saturating exposure time is preferably performed in a high sensitivity measurement mode selected by the user in the user interface portion of the CSR.In some example aspects, the method may further include: obtaining a self-saturating profile of the CRS that correlates the valid subset of the total nominal measurement range to the self-saturating exposure time in response to a user selection of an operating element on the user interface of the CRS, preferably prior to the step of operating the CRS using the self-saturating exposure time.In some example preferred aspects, the step(s) of providing an indication of at least one of (a) a height measurement for a workpiece surface based on the use of the self-saturating exposure time, and / or (b) valid peak measurement positions included in the valid subset of the total nominal measurement range corresponding to the use of the self-saturating exposure time may preferably use the obtained self-saturating profile that sets the valid subset of the total nominal measurement range for the corresponding self-saturating exposure time.In some example preferred aspects, the method may further comprise: obtaining a plurality of self-saturating profiles of the CRS, each preferably correlating a valid subset of the total nominal measurement range with a corresponding different self-saturating exposure time, wherein different self-saturating exposure times are preferably associated with different sampling rates, respectively; and / or displaying different sampling rates, preferably as selectable controls, on the user interface portion of the CRS.In some example preferred aspects, the method may further comprise: automatically positioning the workpiece surface in the valid subset of the total nominal measurement range with respect to the obtained self-saturation profile.According to other exemplary aspects, a CRS system for performing workpiece height measurement of a workpiece surface with low reflectivity may be provided. The CRS system may include an optics part, an electronics part, and / or a user interface part. The optical part can be configured to focus different wavelengths at different distances along a measurement axis close to a workpiece surface to be measured, and / or to direct reflected radiation from the workpiece surface onto a wavelength detector. The electronics portion may include the wavelength detector and may be configured to provide: (a) when the CRS system is operated using an exposure time that is within a first nominal range of exposure times, a valid wavelength peak over a nominal total measurement range that is uninterrupted between upper and lower nominal measurement range boundaries, and / or (b) when the CRS system is operated using an exposure time that is longer than exposure times in the first nominal range of exposure times, no valid wavelength peak in an invalid peak portion of the nominal total measurement range that corresponds to one or more detector pixels saturated by the self-saturating exposure time at or above a saturation threshold level, regardless of whether or not the workpiece surface is within the nominal total measurement range, such that valid wavelength peak measurement positions are left only in a valid subset of the total nominal measurement range excluding the invalid peak portion. The user interface portion may be configured to display at least one of (i) a height measurement for a workpiece surface that is within the valid subset of the total nominal measurement range based on the use of the self-saturating exposure time and / or (ii) valid peak measurement positions included in the valid subset of the total nominal measurement range corresponding to the use of the self-saturating exposure time.In some example preferred aspects, the CRS system may be included in a machine vision inspection system, preferably including a movable portion to which the optics portion is coupled to be movable along the X, Y, and Z directions relative to the workpiece surface, wherein the machine vision measurement system may preferably and optionally be configured to automatically position the workpiece surface relative to the optics portion in the valid subset of the nominal total measurement range.In some example preferred aspects, exposure times are preferably associated with respective sampling rates, and / or the user interface portion may display one or more sampling rates, each corresponding to one or more self-saturating exposure times.BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGSThe foregoing aspects and many of the attendant advantages of this invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: FIG. 1 is a block diagram of an embodiment of an example CRS system. FIG. 2 is a diagram of a system noise (bias) profile of a CRS illustrating the wavelength dependent voltage offset signal levels for the pixels in a detector array when no measurement surface is present. FIG. 3 is a diagram of a wavelength-dispersed intensity profile from a CRS representing a valid wavelength peak generated by a wavelength reflected from a measurement surface, where the pixel position of the peak corresponds to the measured distance of the measurement surface. FIG. 4A is a diagram of CRS distance calibration data correlating distance indicating (peak wavelength) pixel coordinates with known measurement distances to a measured workpiece surface. FIG. 4B is an example of a CSR distance calibration lookup table that relates wavelength peak pixel coordinates to corresponding measurement distances.FIGS. 5A-5E show a way of presenting respective self-saturation profiles that correspond to operating a CRS at five corresponding "high sensitivity" sampling rates (providing relatively long exposure times) when no measurement surface is present. Each self-saturation profile indicates a valid subset of the nominal total measurement range in which distance measurement is possible with a "high sensitivity" sampling rate. Some of the self-saturation profiles show a complementary invalid peak portion of inclusive detector pixels saturated at or above the saturation threshold level for those sampling rates corresponding to a self-saturating exposure time.FIGS. 6A and 6B show embodiments of user interface displays illustrating user selectable options (e.g., sampling rates) in a baseline setting embodiment (FIG. 6A ) and user selectable options (e.g., sampling rates and a display of corresponding self-saturation profiles) in a high sensitivity setting embodiment (FIG. 6B ) that provide enhanced options for low reflectivity workpieces. FIG. 7 shows an embodiment of a user interface display in the high sensitivity setting indicating a measurement distance and / or valid peak measurement positions in valid subsets of a total nominal measurement range of a CRS corresponding to different sampling rates. FIG. 8 is a diagram showing an embodiment of a machine image processing inspection system used for measuring a workpiece surface with low reflectivity. FIG. 9 is a flowchart illustrating a routine for operating a CRS system for performing height measurement of a workpiece surface with low reflectivity.DETAILED DESCRIPTIONFIG. 1 is a block diagram of an embodiment of a chromatic range sensor (CRS) system 100 that includes an optical element 120 (e.g., an optical pen), an electronics portion 160, and a user interface portion 171. The embodiment of the electronics portion 160 includes a signal processor 166, a memory portion 168, and a source and detector subsystem 161 having a wavelength detector 162, and a broadband light source 164. The CRS system 100 shown in FIG. 1 is a chromatic point sensor (CPS) system that always measures a single measurement point. In various embodiments, the wavelength detector 162 includes a detector array 163 of a spectrometer. The detector array 163 may include a plurality of pixels distributed along a measurement axis of the wavelength detector 162, the plurality of pixels receiving respective wavelengths and providing output spectral profile data. The electronics part 160 is coupled to the optical element 120 via a light path including an optical fiber cable 112. Optional or alternative aspects of the optical path are shown that include the fiber optic cable 112 having first and second segments 112A and 112B connected to a connector CONNECT-D in the fiber optic segment 112B and a coupler COUPLER-O connecting the segments 112B to the electronics portion 160. The light source 164, controlled by the signal processor 166, is connected to transmit a spectral profile of wavelengths into the optical element 120 through a path that transmits the optical signal to the optical element.To input the illumination fiber segment 165I, which includes 2x1 coupler COUPLER-E, CONNECT-E, and fiber optic cable 112. The optical element 120 includes an input / output optical fiber subassembly 105, a housing 130, and an optics portion 150. The input / output fiber optic subassembly 105 includes an input / output fiber optic 113 supported by and enclosed by the fiber optic cable 112 and a fiber optic connector 108. The input / output optical fiber 113 outputs an output beam through an aperture 195 and receives reflected measurement signal light through the aperture 195.In operation, light emitted from the fiber end through the aperture 195 is focused by the optics portion 150 which includes a lens that provides axial chromatic dispersion such that the focal point is at different distances along the optical axis OA depending on the wavelength of the light, as is known for CRS systems. During the measurement operations, the light is focused on a surface location 190 of a workpiece 170. Light reflected from surface location 190 is refocused by optics portion 150 onto aperture 195. Due to its axial chromatic dispersion, only a single wavelength will have the focal distance that coincides with the measurement distance "Z", which is the distance from a reference position RP fixed with respect to the optical element 120 to the surface location 190. The wavelength that is best focused at surface location 190 is the wavelength that is best focused at aperture 195. Thus, predominantly the best focused wavelengths pass through the aperture 195 and into the core of the optical fiber 113 of the optical fiber cable 112. The fiber optic cable 112 carries the signal light to a wavelength detector 162 which is used to determine the wavelength having the dominant intensity corresponding to the measurement distance Z to the surface location 190.In the illustrated embodiment, the reflected wavelength dependent light travels through the light guide path containing coupler COUPLER-E back to electronics portion 160 so that approximately 50% of the light is directed through fiber optic segment 165S to wavelength detector 162. The wavelength detector 162 receives the wavelength-dependent light intensity, converts it into an output spectral intensity profile (also referred to simply as an output spectral profile) distributed across an array of pixels along a measurement axis of the detector array 163, and operates to provide corresponding output spectral profile data based on pixel data output from the detector array 163.The subpixel resolution distance-indicating coordinate (DIC) of the profile data (see FIG. 3 ) is calculated by the signal processor 166 and the DIC (in subpixels) determines the measurement distance Z to the surface location 190 (in micrometers) via a distance calibration lookup table stored in the memory portion 168, as described below with reference to FIGS. 4A and 4B. The DIC may be determined by various methods (e.g., determining the center of gravity of the intensity profile data in a peak range). The profile data may be used to determine the subpixel resolution DIC, as described below.The user interface portion 171 is coupled to the electronics portion 160 and is configured to receive user input used for operation of the CRS system 100, such as a command to obtain a system noise (bias) profile of the CRS system 100, a user selection of sampling rates or other operating parameters, etc., via any suitable means such as a keyboard, touch sensor, mouse, etc. The user interface part 171 is also configured to display information on a screen, such as a distance successfully measured by the CRS system 100. The user interface portion 171 may also display valid peak measurement positions in a valid subset of a nominal total measurement range of the CRS system 100 (corresponding to operating at a lower or "high sensitivity" sampling rate) to direct the user to position a low reflectivity workpiece surface within the valid subset to perform a distance measurement as described below.FIG. 1 includes orthogonal XYZ coordinate axes as reference frames. The Z direction is defined to be parallel to the optical axis (OA) of the optical element 120, which is the distance measuring axis. In operation, as shown in FIG. 1, the workpiece 170 is deposited along the optical axis OA and may be mounted on a translation table 175, which may be advantageously oriented to translate along the Z-axis direction enforced by guide bearings 175A.The following description of FIG. 2 broadly describes certain known background signal processing and / or calibration operations that may be used in combination with this invention in various embodiments. The purpose is to emphasize that the inventive methods disclosed below are different from, but compatible with, these operations. FIG. 2 is a diagram 200 of a system noise (bias) profile of a CRS illustrating voltage offset signal levels Voffset(p) for the pixels in a detector array 163 when there is no measurement surface within the overall nominal measurement range of the CRS. In such a case, there is no intentionally reflected light and no clear or dominant peak wavelength in the resulting intensity profile. The voltage offset signal Voffset(p) is plotted in normalized volts for each of the 1024 pixels along the "wavelength" measurement axis. The "normalized volts" assign a value of 1.0 to the saturation voltage of detector array 163. Voltage offset signal Voffset(p) includes a bias signal level Vbias relatively invariable across the detector array and a background signal component Vback(p) shown variable across the detector array. The coordinate independent bias signal level Vbias may change due to voltage shifts associated with ambient temperature changes and heat generated by the electronics portion during operation. The variable background signal Vback(p) represents signals such as background light from wavelength dependent false (internal) reflections in the CRS as well as due to the dark current of the various pixels p. Weak but distinct internal reflections may occur, for example, at fiber optic coupling sections such as fiber optic couplers and connectors, fiber optic splitters, and fiber optic ends. Each CRS system typically also includes wavelength dependent variations in spectrometer responsiveness and / or broadband CRS light source that actually produces different intensities at different wavelengths. Thus, in various embodiments, to provide the best possible signal calibration or compensation, a system noise (bias) profile of the CRS may be obtained at different times in operation to track dynamic changes in the possible signal error components set forth above. In particular, the signal components Vback(p) may be stored as system noise (bias) profile data 169 for calibration or compensation of the pixel array of detector array 163. The system noise (bias) profile data 169 may be used to continuously compensate (e.g., by subtraction) subsequent profile data signals from each pixel p.According to various aspects of the invention, a system noise (bias) profile of the CRS operating at a relatively low sampling rate (or longer exposure time) may be used to build a self-saturation profile of the CRS at that sampling rate. A self-saturation profile indicates pixels saturated at or above a saturation threshold level and in turn indicates valid peak measurement positions included in a valid subset of a nominal total measurement range of the CRS along the optical axis OA of the CRS where distance measurement is still possible at this (relatively low) sampling rate. In other words, the valid subset of measurement positions (e.g., in microns) corresponds to the locations of the sub-pixels DIC where the remaining "unsaturated" detector array pixels (those that are not saturated above the saturation threshold level) are still capable of generating a valid wavelength peak even at this low sampling rate, as will be described in more detail below.The following description of FIGS. 3, 4A, and 4B broadly describes certain signal processing operations that determine distance-indicating coordinates (DIC) in subpixel resolution based on a valid wavelength peak generated in a wavelength-dispersed intensity profile by a CRS, and determine a measurement distance to a workpiece surface (e.g., in microns) based on the determined DIC. The operations roughly described herein are described in more detail in the '456 patent. The purpose of this description is to provide background information useful for a comprehensive understanding of the CRS distance measurements.FIG. 3 is a graph 300 of a wavelength-dispersed intensity profile of a CRS illustrating a valid wavelength peak 302 generated by a subset of measurement profile signals MS(p), indicating a wavelength focused on and reflected by a measurement surface. Each of the measurement profile signals MS(p) has the signal level (indicated in normalized volts) associated with each pixel p of the detector array (e.g., detector array 163). The valid wavelength peak 302 does not have pixels saturated at or above the saturation threshold level. It also has more than sufficient height (a good signal-to-noise ratio), is relatively symmetric, and allows a good estimate of the peak location or measurement distance indicating coordinate (DIC) 304 along the measurement axis of the detector array.FIG. 3 shows a bias signal level MVbias (in normalized volts), a peak pixel coordinate (ppc), and a data threshold MVthreshold defining the lower limit of a distance-indicating subset of measurement profile data MS(p) forming the wavelength peak 302.In short, in one embodiment, the measurement operations for determining a distance-indicating coordinate (DIC) (in pixels) and for determining a corresponding measurement distance (in micrometers) based on the determined DIC may include the following:• Positioning the target surface along the optical axis OA and detecting the resulting wavelength-dispersed intensity profile as in diagram 300.• Determine the peak pixel coordinate (ppc) which is the pixel having the highest signal.• Determining the measurement bias signal level MVbias at a certain sampling rate.• Determine the data threshold MVthreshold(e.g., as a percentage of peak height).• Determining the subpixel resolution distance-indicating coordinate (DIC) based on the distance-indicating subset of measurement profile data MS(p) forming the wavelength peak 302 having a value greater than MVthreshold.• Determine the measurement distance by correlating the DIC with a corresponding distance in the stored distance calibration data (e.g., a distance calibration curve as in FIG. 4A or a look-up table as in FIG. 4B).In the foregoing operations, a subpixel resolution DIC may be determined based on the distance-indicating subset of measurement profile signals MS(p) above MVthreshold. A DIC may be determined by various methods. In one embodiment, a DIC may be determined as the subpixel resolution coordinate of a centroid X C of the distance-determining subset of signals MS(p). For example, for a 1024 pixel detector, the centroid X C may be determined according to: whereinIn a specific example, in EQUATION 1 n=2. It is understood that EQUATION 2 limits the signals MS(p) used in the centroid calculation to a distance-indicating subset.FIG. 4A is a diagram 400A of CRS measurement distance calibration data 410A correlating subpixel resolution distance indicating coordinates (DIC) with known measurement distances (ZOUT) in micrometers along the optical axis (OA) of the CRS. The example shown in FIG. 4A applies to an optical element having a nominal total measurement range MR of 300 microns, which corresponds to distance-indicating coordinates (DIC) in the range of approximately 150 pixels - 490 pixels. However, if desired, the DICs may be calibrated over a larger pixel area of the detector array 163. An example laboratory calibration method for determining the CSR measurement distance calibration data 410A employs a mirror (e.g., representative of the surface at surface location 190 of FIG. 1 ) that is moved along the optical axis OA (e.g., in steps of 0.1 or 0.2 micrometers). For each current mirror position, the corresponding calibration DIC of the CRS system is determined based on the corresponding intensity profile data, as described above. The calibration DIC and corresponding current position (in micrometers along the optical axis) are then recorded to provide the calibration data 410A. During a workpiece measurement process, the measurement DIC obtained by a CRS system is related to the stored calibration data 410 to determine the measurement distance ZOUT corresponding to the measurement DIC.FIG. 4B is a diagram 400B of a look-up table shape of calibration data analogous to that described above. A schematic representation of a look-up table format is to be shown, and differences in the specific table values of FIG. 4B from those indicated in the specific calibration data 410A of FIG. 4A are insignificant or not relevant for this purpose. In FIG. 4B, the calibration DIC entries in the left column include the pixel coordinates from 1 to 1024 in increments of 0.1 pixels, and the corresponding measurement distances (in micrometers) are entered in the right column. In operation, the measurement DICs calculated by the CRS system are referenced to the stored calibration look-up table to determine the corresponding measurement distance (in microns). If the measurement DIC falls between adjacent calibration DIC values, then the measurement distance may be determined by interpolation.As described above, in a CSR system operating at a longer exposure time or integration time (i.e., at a lower sampling rate), some pixels in the detector array may be self-saturated or saturated above a saturation threshold level due to light occurring due to internal reflections (e.g., from fiber optic connectors, fiber optic splitters, fiber ends, and the like) even if a measurement surface is not present. According to various embodiments of the invention, a method of operating a CRS system includes determining a self-saturation profile of the CRS system operating at a particular (relatively low) sampling rate that indicates a valid subset of a nominal total measurement range of the CSR along the optical axis OA corresponding to DIC pixel positions at which the pixels are not saturated at or above the saturation threshold level (despite the low sampling rate) and thus may generate a valid wavelength peak. That is, a valid wavelength peak cannot be formed by a pixel saturated at or above a saturation threshold level, and thus the valid subset of a nominal total measurement range of the CRS corresponds to the DIC pixel positions excluding pixels saturated at or above the saturation threshold level (e.g., 80%-90% of full saturation at upper signal limit or more). A user can therefore perform a distance measurement of a workpiece surface at such a low sampling rate by placing the workpiece surface in the valid subset of the nominal total measurement range of the CRS system along the optical axis.FIGS. 5A-5E show a way of presenting respective self-saturation profiles according to operating a CRS at five corresponding "high sensitivity" sampling rates (providing relatively long exposure times) when no measurement surface is present. Each self-saturation profile may be determined based on system noise or bias profile (e.g., according to principles roughly described above) and indicates a valid subset of the nominal total measurement range in which distance measurement is possible at a corresponding "high sensitivity" sampling rate. Conversely, some of the self-saturation profiles may also be considered to show a complementary invalid peak portion of the total nominal measurement range corresponding to at least one pixel saturated at or above a saturation threshold level for those sampling rates corresponding to a self-saturating exposure time.FIGS. 5A-5E show respective diagrams 500A-500E, including: a horizontal axis representing a nominal total measurement range of the CRS along the optical axis of the CRS of approximately 0 μm-1200 μm; a vertical axis indicating the available measurement signal range at each position (or corresponding detector pixel), as described in detail below; a minimum measurement peak height threshold TH that allows sufficiently accurate estimation of a peak wavelength location or DIC; and an illustrative fictitious peak wavelength value 590 corresponding to the peak height threshold TH. In the illustrated example, TH is about 300 signal units.The nominal total measurement range (e.g., 0 μm-1200 μm) is the fixed uninterrupted measurement range of the CPS provided when using a conventional exposure time that is within a nominal range of exposure times (e.g., a nominal exposure time corresponding to a sampling rate of 300 Hz or higher). Under such conditions, it is relatively certain that no detector pixels at or above the saturation threshold level are saturated by internal reflections, and a valid wavelength peak (e.g., having a height exceeding the minimum measurement peak height threshold TH) and distance measurement may be provided over the specified measurement range. As described above with respect to calibration data, distance-indicating coordinates (DIC) in pixels along the measurement axis of the detector array correspond to the measurement positions in micrometers along the optical axis OA of the CRS system. Thus, in various descriptions herein, the measurement positions (along the optical axis) and pixel positions (along the detector measurement axis) may be used interchangeably or equivalently, unless any of these positions are specifically requested.The available measurement signal range shown along the vertical axis at each position may be considered, in various embodiments, as the available measurement signal at the detector pixel corresponding to the position or wavelength after all inner bias signal contributions (e.g., signal contributions from false inner reflections) have been subtracted from its maximum (or fully saturated) signal level. In various embodiments, a "self-saturation" signal threshold, or saturation threshold for short (not shown), is the level that, when subtracted from the maximum signal level, leaves an insufficient remaining available signal level to register a valid wavelength peak signal - e.g., a peak higher than the minimum measurement peak height threshold THthat can be used to estimate the location of the wavelength peak or DIC. In some embodiments, the sum of the self-saturation signal limit value and the minimum measurement peak height limit value THis approximately equal to the maximum signal level for a detector pixel. In other words, in some embodiments, subtracting a self-saturation signal that is at or exceeds the saturation threshold leaves behind a residual available signal that is at or below the minimum measurement peak height threshold TH. In some embodiments, the (self) bias limit may be set to 80%-90% or more of the full saturation at the upper signal limit. Conversely, in some embodiments, the minimum measurement peak height limit TH (for the remaining available signal at a pixel) may be set to 20% of the upper signal limit or 10% or less. However, these limits are merely exemplary and not limiting. In various embodiments, a saturation threshold and / or the minimum measurement peak height threshold TH may be selected at a desired level in view of various technical considerations, such as available signal levels, desired tolerances for read errors, noise considerations, expected workpiece reflection ranges, and the like.In each graph 500A- 500E, a "self-saturation" profile line 595 (e.g., 595A- 595E) shows the change in the available signal range for registering the signal contribution of light reflected from a workpiece surface without fully saturating the output of the corresponding detector pixel for different measurement positions across the total nominal measurement range. A major reason that the profile lines 595 differ between the diagrams is that their respective sampling rates and / or corresponding exposure times affect the accumulated amount of false internally reflected light that contributes to their respective bias signals. The units of the vertical axis may be considered any convenient label of photodetector signal units (e.g., a digital signal representing signal volts, or intensity units, or the like).The five "high sensitivity" sample rates shown in FIGS. 5A-5E are relatively lower than conventional sample rates used in a CRS to produce a longer exposure time to obtain a sufficient wavelength peak signal from a low reflectivity surface. It should be understood that the self-saturation profiles 595A- 595E are intended to be instructive and qualitatively illustrative, and the invention is not limited to the specific embodiments or the signal behavior illustrated in FIGS. 5A-5E.Referring to FIG. 5A, the diagram 500A shows a self-saturation profile 595A of a CRS operating at a sampling rate of 100 Hz, and that shows that for this particular sampling rate, there is a sufficient signal range (e.g., greater than TH) available for registering the signal contribution of light reflected from a workpiece surface for all measurement positions across the nominal total measurement range in this particular CRS system. For example, a signal or peak wavelength of the height such as 591 could be registered without fully saturating detector pixels at a wavelength corresponding to a position of about 25 μm. Similarly, a peak signal of height such as 592 could be registered without fully saturating detector pixels at a wavelength corresponding to a position of approximately 990 μm. The self-saturation profile 595A indicates that, for a 100 Hz sampling rate (or its corresponding exposure time) by this CRS system, actually no detector pixels are "self-saturated" (e.g., have a remaining available signal range at or below the minimum measurement peak height limit TH), and there is no invalid peak portion of the total nominal measurement range. Thus, for a sampling rate of 100 Hz, the valid subset 502A of the nominal total measurement range in which distance measurement is possible comprises the total nominal total measurement range.It should be appreciated that the hypothetically available measurement signal peak height at 100 Hz (e.g., as represented by the self-saturation profile 595A) may be lower than it would be at a conventional sampling rate (e.g., 300 Hz or more). However, the peak height actually obtained from a low reflectivity surface will be higher at the slower rate (e.g., exceeding TH) because the exposure time for accumulating the weak measurement signal will be longer. Although not actually "self-saturating" at or above a saturation threshold level, a 100 Hz sampling rate may not be included in the range of nominal sampling rates or conventional sampling rates for a CRS system because the signals from various normal or highly reflective surfaces may present a high risk of saturation of wavelength peak pixels at that sampling rate, which may prevent accurate estimation of the wavelength peak location DIC for normal or high reflectivity surfaces.Referring to FIG. 5B, the diagram 500B shows a self-saturation profile 595B of a CRS operating at a sampling rate of 60 Hz, and it shows that for this particular sampling rate in this particular CRS system, the signal range available for registering the signal contribution of the light reflected from a workpiece surface is lower for measurement positions than at 100 Hz, as expected. Specifically, the available signal range in the vicinity of the zero measurement position (i.e., in the invalid peak portion 504B) falls to approximately the minimum level TH. For this reason, for a 60 Hz sampling rate, it may be advisable to limit the valid subset 502B of the nominal total measurement range in which distance measurement is possible to include a single continuous subset over most of the nominal total measurement range, but to exclude positions near the zero measurement position.Referring to FIG. 5C, the diagram 500C shows a self-saturation profile 595 of a CRS operating at a sampling rate of 50 Hz. This closely approximates the 60 Hz sampling rate shown in Figure 5B, and the results are similar. For this particular sampling rate in this particular CRS system, the signal range available for registering the signal contribution of the light reflected from a workpiece surface for measurement positions is slightly less than 60 Hz, as expected. The available signal range falls below the minimum level TH in the vicinity of the 0 μm - 10 μm measurement positions (i.e., in the invalid peak part 504C). For this reason, for a 50 Hz sampling rate, it may be advisable to limit the valid subset 502C of the nominal total measurement range in which distance measurement is possible to include a single continuous subset over a large portion of the nominal total measurement range, but to exclude positions near the 10 μm measurement position and below.Referring to FIG. 5D, the diagram 500D shows a self-saturation profile 595D of a CRS operating at a sampling rate of 35 Hz. For this particular sampling rate in this particular CRS system, the signal range available for registering the signal contribution of the light reflected from a workpiece surface is significantly less than 50 Hz for measurement positions. The available signal range falls below the minimum level TH in the vicinity of the 0 μm - 75 μm measurement positions (invalid peak part 504D) and in the vicinity of the 640 μm - 920 μm measurement positions (invalid peak part 504D'). For this reason, for a sampling rate of 35 Hz, it may be advisable to restrict the valid subset of the total nominal measurement range in which distance measurement is possible to the two parts 502D and 502D', and exclude positions in the invalid peak parts 504D and 504D'.Referring to FIG. 5E, the diagram 500E shows a self-saturation profile 595E of a CRS operating at a sampling rate of 25 Hz. For this specific sampling rate in this specific CRS system, the signal range available for measuring positions for registering the signal contribution of the light reflected by a workpiece surface is then significantly less than 35 Hz. The available signal range falls below the minimum level TH over most of the total nominal measurement range (i.e., over the invalid peak portion 504E). Indeed, in this case, the pixels are fully self-saturated (e.g., from the 400 μm-960 μm measurement positions) throughout a large portion of the invalid peak portion 504E. For a 25 Hz sample rate, it may be advisable to limit the valid subset of the nominal total measurement range in which distance measurement is possible to section 502E and exclude positions in the invalid peak portion 504E.According to various embodiments of the invention, a CRS may be operated in two different settings - a default setting (first normal mode) and a high sensitive setting (second high sensitive mode). FIGS. 6A and 6B show example user interface display embodiments illustrating user selectable options (e.g., sampling rates) in a baseline setting embodiment (FIG. 6A ) and user selectable options (e.g., sampling rates and display of corresponding self-saturation profiles) in a high sensitivity setting embodiment (FIG. 6B ) that provides enhanced options for low reflectivity workpieces. The user interface display embodiments shown in FIGS. 6A and 6B may be shown on the user interface part 171 of the CRS system and may be used in the default setting and the high-sensitivity setting, respectively. Below the user interface tab 601 of the default setting, as shown in FIG. 6A, a user may be given a set of selectable sampling rates 602 that include commonly available sampling rates, such as 300 Hz, 500 Hz, 1000 Hz, 1500 Hz, and 2000 Hz, as shown. These sampling rates each correspond to exposure times within a nominal range of exposure times that can be used for "normal reflectivity" workpieces and do not cause self-saturation at any of the detector array pixels. Thus, at each of these user-selected sampling rates, the CRS system is capable of providing a valid peak wavelength over the entire nominal measurement range of the CRS. The user interface of the default may also include a system noise (bias) profile update key 604. The CRS system 100 obtains a system noise (bias) profile at the selected sampling rate (e.g., 1000 Hz in the example of FIG. 6A ) in response to the user selection of the update key 604, and stores the profile in the storage portion 168 for various calibration and compensation purposes as described above.When a user selects a high sensitivity setting tab 605, the user interface of the high sensitivity setting of FIG. 6B may be shown including a different set of selectable sampling rates 612 that include relatively lower sampling rates, such as 25 Hz, 35 Hz, 50 Hz, 60 Hz, 75 Hz, and 100 Hz. These sampling rates correspond in each case to the self-saturating exposure times which are longer than the exposure times in the nominal range of exposure times. Thus, when operated at any of these relatively lower sampling rates, the CRS system will have a valid subset of a nominal total measurement range in which elevation (distance) measurement with compatible low reflectivity surfaces is possible. The valid subset of the total nominal measurement range excludes an invalid peak portion in which height measurement is not possible, as described above with reference to FIGS. 5A-5E. The high sensitivity setting user interface may also include a system noise (bias) profile update button 614, similar to the baseline setting user interface. The CRS system obtains a system noise (bias) profile at the selected relatively low sampling rate (e.g., 50 Hz in the example of FIG. 6B ) in response to the user selection of the update button 614. Furthermore, in the high sensitivity setting 605, the CRS system may determine a self-saturation profile (see FIGS. 5A-4E ) based on or in conjunction with the system noise (bias) profile as described above to determine a valid subset within a total nominal measurement range of the CRS. The CRS system obtains a self-saturation profile at the selected sampling rate in response to the user selection of the update key 614 in the high-sensitivity setting 605, and may store the self-saturation profile or operation information derived therefrom in the storage part 168. In some embodiments, the update key 614 of the system noise (bias) profile appears in the high sensitivity setting 605, which obtains and stores a self-saturation profile based on or associated with a system noise (bias) profile, as does the update key 604 in the default setting 601, which only obtains and stores a system noise (bias) profile, such that any processing differences between the two settings are transparent to the user.It should be understood that the sample rates selectable in the default setting 601 and the high sensitivity setting 605 are intended to be illustrative only and not limiting. In various embodiments, each sampling rate at which a valid wavelength peak can be detected over a nominal total measurement range may be included in the baseline sampling rate set 602, while each sampling rate at which at least some of the detector array pixels are self-saturated to at least one (self) measurement threshold level (regardless of whether a workpiece surface is present in the nominal total measurement range), while other pixels are not self-saturated may be included in the high sensitivity sampling rate set.In some of the embodiments roughly described above, update key 604 or 614 initiates sampling of a system noise (bias) profile and / or a self-saturation profile at a current or selected sampling rate. In other embodiments, update key 604 or 614 may trigger sampling system noise (bias) profiles for a set of sampling rates that includes some or all of the possible sampling rates. Various such profiles (or operational information derived therefrom) may then be stored in the memory portion 168 such that when the user later selects one of the sampling rates whose corresponding profile is quickly usable by the CRS. The system noise (bias) profiles as well as the self-saturation profiles of a CRS are subject to environmental changes, device aging, and other time-dependent changes. Thus, in various implementations, the CRS may prompt the user to select the update button 614 at different times to obtain at least the self-saturation profile of a current sampling rate so that the profiles remain up-to-date.In the high sensitivity setting 605, a valid subset within a nominal total measurement range contains valid wavelength peak measurement positions along the optical axis OA of the CRS, as described above. Thus, if a workpiece to be measured is in the valid subset, the reflection from the workpiece surface will generate a valid wavelength peak to thereby indicate a measurement distance to the workpiece surface. FIG. 7 shows an example embodiment of a user interface display display in the high sensitivity setting that displays a measurement distance field 701 and / or possible valid peak measurement positions in a valid subset of a total nominal measurement range of a CRS corresponding to different sampling rates in a display portion 703 of the valid subset.A successfully determined measurement distance can be displayed in field 701 if the workpiece surface is in a valid subset range. If not, at least initially the workpiece surface must be moved manually, semi-automatically or automatically to be within a valid subset. To this end, a user may refer to the display portion 703 of the valid subset for guidance in positioning the surface in a valid subset for a particular sampling rate usable with the reflective features of the surface. In some embodiments, the valid subset display portion 703 may be automatically displayed in the user interface part 171 whenever a user makes a distance measurement in the high sensitivity setting, or displayed in response to user selection of a "display" button 616 for self-saturation profiles, as shown in FIG. 6B.The user interface, as shown in FIG. 7, may additionally or alternatively include a first indicator 705A, such as a green light, that indicates to the user that a surface is within a valid subset of the total measurement range of the CRS. Additionally or alternatively, a second indicator 705B, such as a red light, may be provided to indicate that a surface is in an invalid peak portion of the nominal total measurement range and prevents a valid measurement. More generally, any visual, audio, or other indication may be used that allows the user to distinguish between a first state in which the workpiece surface is within a valid subset and a second state in which the workpiece surface is within an invalid peak portion. It should be understood that the graphical representation of a valid subset of measurement positions shown in FIG. 7 is not limiting. Alternative representations are within the scope of the present disclosure. For example, a valid subset may be numerically represented for a particular sampling rate.In some embodiments, positioning a workpiece surface within a subset of a nominal total measurement range along the optical axis OA of the CRS may be automatically performed in a machine vision inspection system. FIG. 8 is a diagram of an embodiment of a machine vision inspection system 800 with a CRS system that includes an optical pen 100 used to measure a workpiece surface with a low reflectivity. When the machine vision inspection (CRS) system 800 is used to perform a height measurement for a low reflectivity workpiece surface, a user may select a lower sampling rate (longer exposure time) and the machine vision inspection system 800 may automatically position a workpiece surface to be measured within a valid subset of a nominal total measurement range along the optical axis OA of the CRS system.The machine vision inspection system 800 is a machine microscopic vision and inspection system having an optical imaging system 802, such as that described in commonly assigned U.S. Pat. Nos. 8,085,295 B2 and 7,454,053 B2, which are hereby incorporated by reference. Various aspects of image processing measurement machines and control systems are also described in more detail in commonly assigned U.S. Patent No. 7,324,682 B2 and published U.S. Patent Application No. 2005 / 0 031 191 A1, which are also hereby incorporated by reference.As described in more detail in the '682 patent and the '191 publication and illustrated in FIG. 8, the machine vision inspection system (MVIS) 800 may include an image processing controller 804 that may be used to retrieve acquired and stored workpiece inspection images, inspect, and analyze workpiece characteristics in such workpiece inspection images, and store and / or output the inspection results. A conventional camera portion of the MVIS may include an image acquisition optical system 802 that includes an objective lens having an optical axis OA" that enlarges and images the surface of a workpiece 806 to a camera (not shown) while the focal point is achieved by moving along a Z-axis guide bearing 802. The workpiece 806 on a measurement table 808 is movable on guide bearings 808A along the X and Y axes. The MVIS 800 can be roughly compared to those available from the QUICK VISION™ QV Apex Series of image processing systems from Mitutoyo America Corporation (MAC), located in Aurora, Ill. The MVIS 800 may be configured to support a CRS (e.g., in the form of a CPS optical pen 100) to move it along the Z-axis direction along with the image acquisition optical system 802 (in this particular embodiment) to be used in connection with various measurement functions. The optical fiber cable 112 connects the CPS optical pin 100 to the optical pin electronics portion 160. The machine image processing inspection system 800 includes a computer and a user interface 171 and the image processing system controller 804. The image processing system controller 804 may act as a host system for communicating with the electronic part 160 of the CPS optical pen (see also FIG. 1 ). The MVIS user interface 171 may also act as a CRS user interface and may provide some or all of the various CRS user interface features described above, particularly during learning mode operations of the MVIS 800. In various embodiments, the MVIS user interface 171 and the CRS user interface may be collapsed and / or indistinguishable.The (Z) measurement ranges of the image acquisition optical system 802 and the optical pen 100 may be calibrated or related to each other and to the MVIS-Z axis control coordinates. The optical pen electronics portion 160 and the image processing system controller 804 may be configured to exchange data and control signals according to known methods. Thus, the MVIS 800 may be used to automatically position a surface of the workpiece 806 to be measured within a valid subset of a nominal total measurement range of the CPS optical pen 100 for any current sampling rate.For example, the image processing system controller 804 may refer to a self-saturation profile obtained for a particular (lower) sampling rate and stored in the optical pen electronics portion 160 to determine the location of a valid subset within a nominal total measurement range of the optical CPS pen. The image processing system controller 804 may then guide the Z-axis position of the MVIS to automatically position the CPS optical pen 100 with respect to the workpiece 806, while the image processing system controller 804 may also move the workpiece 806 to various desired X-Y measurement positions (e.g., to profile a portion of the surface using the CRS).FIG. 9 is a flowchart illustrating a routine for operating a CRS system to perform height measurement of a workpiece surface with low reflectivity. At a block 900, a CRS system is provided that provides a valid wavelength peak over a nominal total measurement range along the optical axis OA of the CRS system when operated using an exposure time that is within a nominal range of exposure times. At a block 902, the CRS system is operated using a self-saturating exposure time that is longer than the exposure times in the first nominal range of exposure times, wherein regardless of whether or not a workpiece surface is positioned in the nominal total measurement range, the self-saturating exposure time saturates at least one detector pixel of the CRS to at least a saturation threshold level that inhibits a valid wavelength peak in an invalid peak portion of the nominal total measurement range that corresponds to the at least one detector pixel saturated to at least the saturation threshold level by the self-saturating exposure time, thereby leaving valid wavelength peak measurement positions only in a valid subset of the nominal total measurement range that excludes the invalid peak portion. At a block 904, a display is provided on a user surface of the CRS system, the display being at least one of (i) a height measurement for a workpiece surface based on usage of the self-saturating exposure time (e.g., as in the measurement distance field 701 in FIG. 7 ) and (ii) valid peak measurement positions included in the valid subset of the total nominal measurement range (e.g., the display portion 703 of the valid subset in FIG. 7 ).CRS methods and systems according to the present invention enable operation of a CRS system at longer exposure times (e.g., as provided by lower sampling rates, for example, in the range of about 25 Hz to 100 Hz) that have not been previously available for reliable operation (e.g., relatively unskilled users), thereby achieving distance measurement on workpiece surfaces having low reflectivity.Although preferred embodiments of the invention have been illustrated and described, numerous variations of the illustrated and described arrangements of features and operations will be apparent to those skilled in the art based on this disclosure. For example, a CPS with an optical pen has been described herein. However, a CRS system, such as a chromatic line sensor, may be configured to operate in accordance with the systems and methods disclosed herein. It is thus understood that various changes may be made to the embodiments disclosed herein without departing from the spirit and scope of the invention.The various embodiments described above may be combined to provide further embodiments. Aspects of the embodiments may be modified to provide still further embodiments. These and other changes may be made in light of the above detailed description. Features, components, and specific details of the structures of the above-described embodiments may be interchanged or combined to form further embodiments optimized for the respective application. Insofar as these modifications are apparent to one skilled in the art, they are implicitly disclosed by the above description without explicitly specifying any possible combination.

Claims

A method of operating a chromatic range sensor (CRS) system to perform a workpiece height measurement of a low reflectivity workpiece surface, the method comprising: providing a CRS that provides a valid wave peak over a total nominal measurement range uninterrupted between an upper and lower nominal measurement range limit when operated using an exposure time that is within a first nominal range of exposure times; operating the CRS using a self-saturating exposure time that is longer than the exposure times in the first nominal range of exposure times, wherein: regardless of whether a workpiece surface is in the nominal total measurement range or not, the self-saturating exposure time saturates at least one detector pixel of the CRS to at least a saturation threshold level that inhibits a valid wavelength peak in an invalid peak portion of the nominal total measurement range that corresponds to the at least one detector pixel saturated to at least the saturation threshold level by the self-saturating exposure time, thereby leaving valid wavelength peak measurement positions only in a valid subset of the nominal total measurement range that excludes the invalid peak portion; and providing, on a user interface portion of the CSR, an indication of at least one of (a) a height measurement for a workpiece surface based on the usage of the self-saturating exposure time, and (b) valid peak measurement positions included in the valid subset of the total nominal measurement range corresponding to the usage of the self-saturating exposure time.The method of claim 1, wherein the step of providing an indication of (a) a height measurement for a workpiece surface based on utilizing the self-saturating exposure time comprises positioning the workpiece surface in the valid subset of the total nominal measurement range.The method of claim 2, wherein the step of providing an indication of (a) a height measurement for a workpiece surface based on the self-saturating exposure time comprises providing a plurality of respective height measurements for respective points on the workpiece surface.The method of any one of claims 1 to 3, wherein the display (b) is provided on the user interface portion of the CRS and the display (b) comprises at least one of (b-i) a graphical representation of the valid peak measurement positions included in the valid subset of the total nominal measurement range and (b-ii) an indication that a workpiece surface is positioned in the invalid peak portion of the total nominal measurement range.The method of any one of claims 1 to 4, wherein at least one of the invalid peak portion or the valid subset of the total nominal measurement range comprises two discrete segments of the total nominal measurement range.The method of any one of claims 1 to 5, wherein the display (b) is provided on the user interface part of the CRS and the display (b) comprises at least one of (b-i) a display indicating the upper and lower limits of at least one uninterrupted sub-range of valid peak measurement positions included in the valid subset of the total nominal measurement range, and (b-ii) a first display state of a valid / invalid indicator comprising the first display state and a second display state displayed in response to measurements obtained in the valid subset of the total nominal measurement range and measurements obtained in the invalid peak part, respectively.The method of any of claims 1 to 6, wherein exposure times are associated with respective sampling rates on the user interface portion of the CRS, and the step of operating the CRS using a self-saturating exposure time comprises operating the CRS using a sampling rate corresponding to the self-saturating exposure time.The method of claim 7, wherein respective exposure times in the first nominal range of exposure times are associated with corresponding sampling rates in a first nominal set of sampling rates displayed on the user interface portion of the CRS, and the step of operating the CRS using a self-saturating exposure time comprises operating the CRS using a slower sampling rate than the sampling rates included in the first nominal set of sampling rates.Method according to claim 7, wherein the sampling rate corresponding to the self-saturating exposure time is at most 60 Hz, in particular at most 35 Hz.The method of any of claims 1 to 9, wherein the step of operating the CRS using a self-saturating exposure time is performed in a high sensitivity measurement mode selected by a user in the user interface portion.The method of any of claims 1 to 10, further comprising: obtaining a self-saturation profile of the CRS that correlates the valid subset of the total nominal measurement range to the self-saturating exposure time in response to a user selection of an operator on the user interface of the CRS, prior to the step of operating the CRS using the self-saturating exposure time.The method of claim 11, further comprising: obtaining a plurality of self-saturation profiles of the CRS that each correlate a valid subset of the nominal total measurement range with a corresponding different self-saturating exposure time, wherein different self-saturating exposure times are associated with different sampling rates, respectively; and displaying the different sampling rates as selectable controls on the user interface portion of the CRS.A chromatic range sensor (CRS) system for performing a workpiece height measurement of a workpiece surface having a low reflectivity, the CRS system comprising: (a) an optics portion configured to focus different wavelengths at different distances along a measurement axis proximate a workpiece surface to be measured and direct reflected radiation from the workpiece surface to a wavelength detector; (b) an electronics portion including the wavelength detector configured to, when the CRS system is operated using an exposure time that is within a first nominal range of exposure times, provide a valid wavelength peak over a total nominal measurement range that is uninterrupted between an upper and lower nominal measurement range limit, When the CRS system is operated using a self-saturating exposure time that is longer than the exposure times in the first nominal range of exposure times, not to provide a valid wavelength peak in an invalid peak portion of the nominal total measurement range that corresponds to one or more detector pixels that are saturated by the self-saturating exposure time to at least a saturation threshold level that inhibits the detector pixels from indicating a valid wavelength regardless of whether or not the workpiece surface is in the nominal total measurement range, such that valid wavelength peak measurement positions are left only in a valid subset of the nominal total measurement range that excludes the invalid peak portion; (c) a user interface portion configured to provide an indication of at least one of (i) a height measurement for a workpiece surface that is within the valid subset of the nominal total measurement range based on usage of the self-saturating exposure time, and (ii) valid peak measurement positions included in the valid subset of the nominal total measurement range corresponding to usage of the self-saturating exposure time.The CRS system of claim 13, wherein the CRS system is included in a machine vision inspection system having a movable portion to which the optics portion is coupled to be movable along X, Y, and Z directions relative to the workpiece surface, wherein the machine vision inspection system is configured to automatically position the workpiece surface relative to the optics portion in the valid subset of the nominal total measurement range.The CRS system of claim 13 or 14, wherein exposure times are associated with respective sampling rates, and the user interface portion displays one or more sampling rates each corresponding to one or more self-saturating exposure times.

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