WO2017103927A1

WO2017103927A1 - Method and apparatus for inspection of substrates

Info

Publication number: WO2017103927A1
Application number: PCT/IL2016/051336
Authority: WO
Inventors: Alan Michael THEN; Steven Craig HEADLEY; James Richard HOGLUND
Original assignee: Advanced Vision Technologies (A.V.T.) Ltd.
Priority date: 2015-12-16
Filing date: 2016-12-15
Publication date: 2017-06-22

Abstract

Color measurement device includes an optical-mixing-chamber, at least one light-source, a measurement-sensor and a reference-sensor. The optical-mixing-chamber includes a semi-toroidal section one side of which is open toward a dome-section extending therefrom. The optical-mixing-chamber further includes at least one diagonal-cylindrical-cavity diagonally penetrating the semi-toroidal section outer wall and extending through the semi-toroidal section inner wall towards a target location, and a central-cylindrical-cavity along the axis of symmetry thereof, optically isolated from the diagonal-cylindrical-cavity. Each light-source is located in a respective one diagonal-cylindrical-cavity and directs a diverging-light-beam through the semi-toroidal section toward the target-location. The projection of the diverging-light-beam on the inner wall extends beyond the opening of the cavity at the inner wall. Thus a portion of the light is reflected within the optical-mixing-chamber and the remaining portion being directed toward the target-location. The measurement-sensor receives light from the target-location. The reference-sensor receives light from within the optical-mixing-chamber.

Description

METHOD AND APPARATUS FOR INSPECTION OF SUBSTRATES

FIELD OF THE DISCLOSED TECHNIQUE

The disclosed technique relates to color inspection of substrates in general, and to methods and devices for color inspections with light sources exhibiting variations in the characteristics thereof, in particular.

BACKGROUND OF THE DISCLOSED TECHNIQUE

In the printing industry, a need exists to determine the quality of printing on a substrate. Some of the quality issues may include whether the correct colors have been printed and whether the plurality of image and/or text blocks have been properly registered on the substrate. While the present invention may be described in terms of the inspection of printed materials, it is to be understood that the inspection methods and systems may have applications in other areas that need inspection, such as fabrics, Printed Circuit Boards (PCBs), semiconductor wafers, dyed biological materials, and any area where color is used as a means to convey or evaluate meaningful information. In the past, inspection was performed manually but had obvious limitations. Subsequently, automated inspection devices were introduced.

One such device is a product called Spectralab and is manufactured and sold by Advanced Vision Technology (A.V.T Ltd), the assignee of the present invention. The SpectraLab device, shown in part in Figure 1 herein, employs a unit marked in Figure 1 as the spectrophotometer unit, also known as a Spectral Measurement Unit (SMU), which rides on a rail back and forth across a web of printed material moving in a transverse direction to the rail. The SMU includes a flash unit which may include a Xenon bulb, which is timed to flash at specific locations determined by the system that controls the Spectralab. The system controlling the location and timing of the "flash measurement" is generally a camera (or other supplemental sensor). The purpose of the flash unit is to capture a specific portion of the moving printed web in order to check, for example, the color thereof against a standard programmed into the system. The target area to be illuminated and measurement captured is typically less than 5mm in width, while the measurement aperture of the device exhibits an area smaller than the target and typically about 2mm. The printed web can typical reach speeds as high as 1000m/minute. The SpectraLab device utilizes a high quality 31 channel spectrophotometer which provides accurate spectral measurements that can be transformed into meaningful measurement of ink density, L^*a^*b^* color measurements, and the like. Due to the nature of the device and measurements needed to produce accurate spectral measurement values, this current system operates with a measurement frequency of 30 Hz. While accurate and reliable, there is a significant benefit for a system which may operate and measure at much higher measurement frequencies while maintaining acceptable colorimetric accuracy.

SUMMARY OF THE PRESENT DISCLOSED TECHNIQUE

It is an object of the disclosed technique to provide a novel color measurement device. In accordance with the disclosed technique, there is thus provided a color measuring device. The color measurement device includes an optical mixing chamber, at least one light source, a measurement sensor and a reference sensor. The optical mixing chamber includes a semi-toroidal section and a dome section extending therefrom. One side of the semi-toroidal section is open toward the dome section. The dome section is attached to the semi-toroidal section. The optical mixing chamber further includes at least one diagonal cylindrical cavity diagonally penetrating the semi-toroidal section outer wall and further extending through the semi-toroidal section inner wall towards a target location. The semi-toroidal section further includes a central cylindrical cavity along the axis of symmetry thereof. The central cylindrical cavity is optically isolated from the diagonal cylindrical cavity. The light source is located at the outer wall side of a respective one of the at least one diagonal cylindrical cavity and directs a diverging light beam through the semi-toroidal section toward the target location. The projection of the diverging light beam on the inner wall extends beyond the opening of the cavity at the inner wall. Thus a portion of the light is reflected within the optical mixing chamber and the remaining portion being directed toward the target location. The measurement sensor, is located within the central cylindrical cavity, directed toward reflection received from the target location and the other sensor. The reference sensor is located within the central cylindrical cavity, directed toward reflections received from within the optical mixing chamber. BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed technique will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which:

Figure 1 is a schematic illustration of an automatic inspection unit which is known in the art;

Figure 2, is a schematic illustration of a color measuring device in accordance with an embodiment of the present invention is illustrated;

Figure 3A is a schematic illustration of the dome section of the color measuring device of the disclosed technique;

Figure 3B is a schematic illustration of the semi-toroidal section of the color measuring device of the disclosed technique;

Figure 4 is a schematic illustration of the mounting of the color measuring device into a PCB and integration into a housing;

Figure 5 is a schematic illustration of an exemplary graphs depicting the spectral channels and responses; and

Figure 6 is a schematic illustration of a graph depicting the change in LED illumination intensities over a period of time.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The disclosed technique overcomes the disadvantages of the prior art by providing a color measuring device that employs a "dual beam" configuration, which "internally" splits the light emitted from the illumination source into two beams, a measurement beam and a reference beam, prior to the illumination of the target area to be measured. The color measuring device according to the disclosed technique includes a sensor for measuring the measurement beam (the measurement sensor) and another separate sensor for measuring the reference beam (the reference sensor). The measurement beam is projected on to the target area to be measured, and is referenced to the reference beam. The optical path of the reference beam to the reference sensor is optically isolated from the optical path of the measurement beam to the measurement sensor. In such a case, the dual beam optical configuration can compensate continuously for variations in the characteristics of the light source or sources, specifically when the measurement sensor and the reference sensor are closely matched to each other.

Reference is now made to Figure to Figure 2, which is a schematic illustration of a color measuring device, generally referenced 10, in accordance with an embodiment of the present invention is illustrated. Figure 2 illustrates color measuring device 10 (also referred to herein as unit 10), which includes two half units, a dome section 12 and a semi-toroidal section 14 disposed on either side of a PCB board 16. In other words, one side of said semi-toroidal section 14 is open toward said dome section 12, while dome section 12 is attached to semi-toroidal section 14 with a PCB between the sections. Dome section 12 and semi-toroidal section 14 together form an optical mixing chamber. It is to be understood that the embodiment shown illustrates portions of the unit 10 above and below a PCB board but the unit may be arranged in any other manner suitable for mounting purposes. Unit 10 also includes one or more LEDs 17 which are mounted at an angle and directed to a focal point or color target area illustrated as 18. This target area may be, by way of example, a color portion on a printed moving substrate/web 19. In Figure 2, the LEDs are mounted at an approximately 45 degree angle but may be oriented at other angles depending on the space available and the desired results. This arrangement is known in the industry as a 45/0 arrangement as sensor 34 is mounted above the target area at 0 degrees. In the unit shown in Figure 2, there are four LEDs illustrated, but less or more than four LEDs may be implemented. The reason for the provision of four LEDs is to make the unit 10 compliant in accordance with ISO standard 13655. Nonetheless, it should be obvious that both a lesser or greater number of LEDs and various spatial arrangement are possible within the context of this design concept. The LEDs 17 are mounted in light tubes 20 which direct the light to color target 18 through one of more focusing lenses 22. It is noted that the light tubes 20 are not continuous in structure but rather are interrupted by a toroidal cutout volume 41 in the semi-toridial section 14, seen in Figure 2 as a cutaway, side view. In other words, the optical mixing chamber further includes at least one diagonal cylindrical cavity (i.e., light tubes 20), which diagonally penetrates the other wall of semi- toroidal section 14 and further extending through the inner wall of semi- toroidal section 14 towards the location of target 18. At least one light source, such as LEDs 17 is located at the outer wall side of a diagonal cylindrical cavity.

In operation, the one or more LEDs are excited and emit light from and through the light tubes 20 and towards the color target 18 through tube ends 24. However, not all the light from the LEDs reaches the color target 18 due to the discontinuity of the light tubes 20. The light which reaches the color target 18 is illustrated as light path 50, shown in the figure as yellow. The light which reaches color target 18 is reflected off the color target 18 (which in our discussion may be a color portion on a printed moving web 19) and directed through lens 26, light tube 28 and then lens 30. A sensor 34, referred to as the measurement sensor, is mounted to the underside of the PCB board 16, to be discussed in further detail below. Measurement sensor 34 is oriented to receive light indirectly from the LEDs 17 as transmitted through the light tubes 20 and reflected from the color target 18, as focused through the lenses 26 and 30. The light path just discussed is illustrated as light path 54 shown in purple in the figure. In addition, a second sensor 36, referred to as the reference senor, is mounted on the alternate side of the PCB 16 to that of measurement sensor 34. Reference sensor 36 is optically isolated from the light path 54, and will not receive any of the light having interacted with the color target 18. Instead the reference sensor 36 receives a portion of the light directly emitted from the LEDs 17, as re-directed and mixed within the toroidal chamber 41 and half-integrating dome 44 (i.e., within the optical mixing chamber). In other words, semi-toroidal section 14 further including a central cylindrical cavity along the axis of symmetry thereof, where the central cylindrical cavity is optically isolated from the diagonal cylindrical cavity or cavities of the light sources.

Furthermore, the angle of incident of light striking the reference sensor 36 can be restricted by the blocking aperture 38, through a defined opening 40 so as to optimize the sensor response. Thus, light emitted from the LEDs 17 at any given time is split by a cutout in light tube 28 into two independent optical paths. The shape and size of the cutout, and any optics in the light path, determines the amount of light which goes to each path. Along the first optical path, light is guided to target 18 and reflected light from target 18 is reflected to the measurement sensor 34. Along the second optical path, some of the light which is emitted from the LEDs 17 is trapped into the toroidal chamber 41 which reflects some of the trapped light into the dome 44 and to the reference sensor 36. The first and second optical paths are optically isolated in such a way that although fed by the same light source simultaneously, measurement sensor 34 measures only light reflected from target 18 while reference sensor 36 measures only light which has not been reached or reflected from target 18.

In order to keep the LEDs 17 relatively cool they may be mounted on a finned aluminum mounting unit 42 and coupled with a thermal transfer agent. The dome section 12 of color measurement device 10 may include an integrating domed area 44 illustrated as well in Figure 3A which is in contact with the toroidal area 41 . Figure 3B illustrates the shape and structure of the toroidal area 41 which, as with dome 44, may be coated with white reflectance material. As seen in Figure 2, some of the light output from the LEDs, due to the discontinuity of tubes 20 will be directed to the interior of the toroidal structure 41 , as can be seen as light path 52, shown in light blue. This light will be reflected within the toroid as illustrated and directed up to the dome 44 where it will "bounce around" (i.e., reflect internally) and be "mixed". Some of this light will ultimately be directed into narrow aperture opening 40. The toroidal chamber 41 and half-integrating dome 44 are ideally coated with a highly diffusive, color neutral, reflective coating known to those skilled in the art of optics. The coating is typically a barium sulfate base and is exemplified by a product such as Spectraflect® from LabSphere. The effect of the coating is to uniformly mix the various light sources so that the reference sensor 36 samples an accurate representation of the incident light that will strike the color target 18. Alternatively, light can be delivered to the toroidal chamber 41 from a central located light source via a fiber optic cable. Finally, some combination of these two light input schema could be employed.

An additional benefit of this "toroidal mixing chamber" is the ability to employ differing LEDs or light inputs, having for example different spectral characteristics. The desirability of this basic arrangement lies in the ability to employ perhaps a red, green, and blue LEDs which could be triggered simultaneously to produce a white light, or alternatively to be triggered independently to isolate the response of the sample to various lighting conditions. With appropriate electronics control, the nature of the incident light can have its spectral power distribution altered both in wavelength and intensity.

To review, when the LEDs 17 are illuminated, some of the light will travel through the tubes to the target 18, be reflected up through lenses 26 and 30 and light tube 28 and then to sensor 34. However, not all the light which emanates from the LEDs is directed to sensor 34. Some of the light, representing the input light values, from the LEDs will be reflected within the toroidal structure 41 , in the illustration of Figure 2, "upwardly" and into the dome 44. Some of the light which reaches dome 44 will, after being reflected within the dome 44, be directed through narrow opening 40 and then be sensed by sensor 36.

Reference is now made to Figure 4, which is a schematic illustration of the mounting of the device 10 into PCB 16 and integration into housing 22. This housing also may include the electronics pertinent to operation and control of the operation of the device 10. The housing 22 may be mounted on a rail that is oriented and spaced from a moving printed web. PCB 16 includes openings in the section located within the optical mixing chamber which allow light to pass from semi-toroidal section 14 to dome section 12. The sensors 34 and 36 may be sourced from a number of companies. One company in particular which provides a suitable sensor is named MAZet and the sensor may be model MMCS6CS. The purpose of using two sensors will now be explained.

A prior art optical system that employs a "single beam" illumination source essentially illuminates the target area to be measured, and measures the reflected light from the target, without compensating for the effect of changes in the optics, the illumination source, or the detector characteristics. This is the simplest and least expensive measurement geometry to implement, but is also the least accurate technique.

However, an optical system according to the disclosed technique, which employs a "dual beam" configuration as outlined in the present invention, "internally" splits the light emitted from the illumination source into two beams, prior to the illumination of the target area to be measured. The measurement beam is projected on to the target area to be measured, and is referenced to the second (reference) beam that is optically isolated within the instrument. In the instrument described herein, there exists a sensor for the measurement beam and a separate sensor for the reference beam. In this case, the dual beam optical configuration can compensate continuously for variations in the illumination source. In general, if the sensors used can be closely matched to each other, as is the case in this implementation, the dual beam optical configuration can also compensate continuously for variations in the sensors.

A simple example can illustrate the concept. Were sensor 34 to measure the light from output measurement light 54 to decrease in intensity, one has limited certainty as to the source of that decreased intensity. It may result from changes in the color target 18 which reflects less light, or changes in the LEDs 17 which illuminated the target. The dual beam arrangement of the present invention allows such potential erroneous readings to be eliminated with a high degree of certainty, and indeed to be correct for on any given measurement. Without such an arrangement as provided by the present invention, additional system controls are required, for example, to periodically add additional calibration measurement against a known stable reference standard.

To illustrate the operation of the integrating and corrections functions made with the present invention, reference is now made to Figure 6, which is a schematic illustration of a graph depicting the change in LED illumination intensities over a period of time. As can be seen and as mentioned above, the LED light source(s) may not be stable over a period of time. The blue lines show the change in illumination levels from the LEDs which, looking from left to right, can be seen to vary in intensity of level. The pinkish colored line shows the signal as acquired by the measurement sensor 34. As can be seen, fluctuations in light source intensity immediately appear in the measured signals. These fluctuations are noise originated from the light source and do not reflect any change in the target. Unless removed, this noise may cause false readings of the target and may lead to incorrect conclusions regarding printing quality or color or may result in incorrect feedback to the printer. The yellow line shows a corrected measured signal as was acquired by measuring sensor 34 but also after the elimination of the signal variation (e.g., noise) recorded by reference sensor 36. Without the second reference sensor 36 and the integration of light from light source (i.e., which may exhibit different spectral characteristics) of the present invention, the light illumination measured based on the blue line would be as shown in the pink line wherein the signal varies in accordance with the change in the LED illumination. As mentioned, this may be due to changes in the LED output and the temperature of the sensor and the LEDs themselves. Without compensation, the pink line shows variations corresponding to the LED illumination levels in the blue line that might indicate a variation in color of the target 18 where there is no such variation at all. The present invention includes, as discussed, a second sensor 36 which receives light reflected or "bounced around" in the dome. This light is not light reflected from target 18. With corrections made for variations in sensor temperature and LED output, the signal measured, shown in the yellow line on Figure 6, shows a compensated, steady measured signal. The following describes how the above described dual beam is employed for compensation. One implementation is to capture a measurement, or a series of measurements, with the optical system described, on a standard white reflectance target with a known reflectance, preferably with a high reflectance values across the visual spectrum. If several measurements are captured, the measurement signals from both the measurement sensor and the reference sensor are averaged, or combined utilizing another relevant statistical method, for each measurement wavelength across the visual spectrum. Ratios are then created, on a wavelength by wavelength basis, between the measurement and reference sensor values for each wavelength.

DualBeamCorrectionRatios(A) = Ref_std(A) / Meas_std(A)

These ratios are the correction factors used to correct further measurements. Variations in the illumination spectral power distribution are apparent in the reference beam, and in the incident beam illuminating the target to be measured. These variations cannot be discerned in the reflected measurement from the target as the target modulates the incident illumination beam. However, the variations are discernable in the reference beam that is not modified by the influence of the measured target. Using the Standard ratios (per wavelength) calculated above, subsequent measurements can be corrected for variations in the illumination by multiplying the inverse ratio of the measurement sensor and the reference sensor values on a wavelength by wavelength basis to get a corrected set of measurement values that are corrected for measurement illumination variations.

MeasuredReflectance(A) = [Ref_std(A) / Meas_std(A)] ^* [Meas_current(A) / Ref_current(A)] Thus, by essentially eliminating the variations caused by differences in LED output as well as differences in the temperature of the sensor, a "true" reading is obtained such that the compensated measured signal accurately reflects the color of the substrate sampled. The system illustrated in Figure 2 may include one or more temperature sensors mounted in the dome or mounted in the vicinity of the sensor to measure the temperature of the sensor and to feed that information into the electronics for integration of signals from both sensors.

Unlike the existing SMU containing a xenon lamp which flashes, as discussed above, in the present invention the LEDs are illuminated either continuously or semi-continuously (i.e., a series of alternating "light on" and "light off" periods, where a plura of measurements are acquired during each "light on" period). As also discussed above, one of the goals of inspection of printed substrates is to determine whether the color(s) printed are true to some standard. The variations in light output from the LEDs, the heating of the LEDs and the sensor may ultimately affect the colors being read by device 10 and sensor 34. The desire, of course, is that the color of the light delivered to sensor 34 be interpreted as the "true" color reflected from the printed substrate and to reduce level of noise due to artifacts originated in the light source. The light which is reflected within the toroidal dome 44 and which is read by the sensor 36 may provide a different reading of the color than that which is read by sensor 34 and by integrating the readings of sensors 34 and 36, a value that reflects the true color reflected may be obtained. The benefit of the toroidal dome 44 is that there is very little loss of light and the light read by sensor 36 may be used to implement a process to correct the readings of sensor 34.

At least one advantage of utilizing LEDs instead of a flashing xenon lamp is that semi-continuous readings can be made at very high speeds. There is limited need to synchronize the light source to the sensor and the pulse rate of the lamp is no longer a limiting factor. Whereas in the prior system the practical read speed was limited to about -30 Hz, with the present invention speed readings as high as 30 kHz or higher may be made, based on overall system signal to noise ratio. Because the system of the present invention utilizes a 6 color channel solid state sensor versus a 31 channel grating spectrophotometer in the previous system, higher capture rates and analysis speeds are achieved (see illustrations in Figure 5). It is also possible with the addition of additional optical elements, such as a prism, to implement an embodiment containing two such solid state sensors with differing filter, effectively double the available channels. Thus, it should be understood that solid state sensors with less than 6 color channels or with more than 6 color channels may also be utilized for the realization of the present invention. A color channel in a solid state sensor may have different light frequency acceptance widths. According to one example of the present invention there is provided a sensor having a 50nm FWHM width channels. However, other widths and variable widths may also be utilized in other embodiments of the invention. Such devices having differing number and differing channel widths are currently and readily available with from such companies as Pixelteq, and are generally referred to as multispectral photodiode arrays. As can be seen in Figure 5, a 6 channel reading or any given of readings, may then be converted using an algorithm known to those skilled in the art into a spectral reflectance measurement. The higher the number of color channels in the solid state sensor, the more accurate the spectral reflectance measurement becomes. In addition, the prior system needed to "target" a specific area to flash xenon lamp requiring complex and costly timing synchronization of the flash to measure the intended moving color target. This need is greatly relieved with the present invention because of its ability to achieve very high measurement frequencies, thus eliminating the need to specifically target an area along the run direction of a moving web.

It will be appreciated by persons skilled in the art that the disclosed technique is not limited to what has been particularly shown and described hereinabove. Rather the scope of the disclosed technique is defined only by the claims, which follow.

Claims

1 . A color measuring device including:

a optical mixing chamber, said optical mixing chamber including a semi-toroidal section and a dome section extending therefrom, one side of said semi-toroidal section being open toward said dome section, said dome section being attached to said semi-toroidal section, said optical mixing chamber further including at least one diagonal cylindrical cavity diagonally penetrating said semi-toroidal section outer wall and further extending through said semi-toroidal section inner wall towards a target location, said semi-toroidal section further including a central cylindrical cavity along the axis of symmetry thereof, said central cylindrical cavity being optically isolated from said diagonal cylindrical cavity;

at least one light source located at the outer wall side of a respective one of said at least one diagonal cylindrical cavity, directing a diverging light beam through said semi-toroidal section toward said target location, the projection of said diverging light beam on said inner wall extending beyond the opening of said cavity at said inner wall, thereby a portion of said light being reflected within said optical mixing chamber and the remaining portion being directed toward said target location;

a measurement sensor, located within said central cylindrical cavity, directed toward reflection received from said target location; and

a reference sensor, located within said central cylindrical cavity, directed toward reflections received from within said optical mixing chamber.

2. The color measuring device according to claim 1 , wherein said measurement sensor and said reference sensor are mounted on opposite sides of a Printed Circuit Board, said printed circuit board includes openings in the section located within the optical mixing chamber thereby allowing light to pass from said semi-toridial section to said dome section.

3. The color measuring device according to claim 1 , wherein said at least one light source is a Light Emitting Diode.

4. The color measuring device according to claim 1 , wherein said central cylindrical cavity includes an aperture opening toward said dome section.

5. The color measuring device according to claim 1 , wherein the inner walls of said optical mixing chamber are coated with a diffusive, color neutral, reflective coating.

6. The color measuring device according to claim 5, wherein said coating is barium sulfate.

7. The color measuring device according to claim 1 including at least two light sources.

8. The color measuring device according to claim 7, wherein the spectral characteristics of one of said at least two light sources is different from the spectral characteristics of at least one other light source.

9. The color measuring device according to claim 1 , wherein a color measurement is compensated according to the following: MeasuredReflectance(A) = [Ref_std(A) / Meas_std(A)] ^* [Meas_current(A) /

Ref_current(A)]

wherein, Ref_std(A) relates to a standard measurement form said reference sensor, Meas_std(A) relates to a standard measurement form said measurement sensor of a standard white reflectance target, Meas_current(A) relates to the measurement form said measurement sensor of the target and Ref_current(A) relates to a measurement form said reference sensor of the light employed during the acquisition of Meas_current(A).