Chromatic Confocal Probe Research Articles

Intensity response model and measurement error compensation method for chromatic confocal probe considering the incident angle

Chromatic confocal measurement technology is extensively applied in precision surface profiling within the industrial domain. However, due to limited measurement space and the complex shape of surfaces, it becomes challenging to ensure that the optical axis is aligned with the normal direction of the surface, particularly in regions with steep slopes. Although the reflected light can be captured when the probe is tilted within the numerical aperture angle, it introduces measurement uncertainty. In this paper, we investigate the reflective characteristics of incident light and develop a chromatic confocal light intensity response model that accounts for the incident angle. This model is based on the Rayleigh-Sommerfeld diffraction and dispersion theory. Simulation results demonstrate that the tilt angle results in a shift in the peak wavelength of the reflected spectrum. To compensate for the measurement error caused by this wavelength shift, we establish a calibration curve representing the angle and wavelength variation for the chromatic confocal measurement system. Experimental findings reveal that after compensation, the measurement error attributable to the incident angle is reduced by approximately 72 %.

A double-sided surface scanning platform for sapphire substrate quality assessment

With excellent mechanical and optical properties, sapphire substrate becomes a very important industrial material especially for LED manufacturing. Its quality control requires two tasks, double-sided surface profiling and defect detection. In this paper, a 3D surface scanning system is proposed as a platform for sapphire substrate inspection. During the scanning process, the height information of both upper and lower surfaces is measured by a chromatic confocal probe. For lateral positioning feedback, an in-house developed sensor, namely, image grating, is introduced in this paper. For these inspection processes, there are three challenges: 1) the motion error in the vertical direction, 2) simultaneous measurement of both upper and lower surfaces, and 3) lateral positioning error in a large scanning area. To address these challenges, three key technologies were developed: 1) real-time motion error compensation based on a fringe interferometer, 2) a double-sided measurement for transparent specimen without the pre-knowledge of its refractive index, and 3) planar image grating pair for accurate 2D displacement feedback. The experimental results showed that the proposed measurement system was able to achieve 50 nm accuracy of the standard deviation when measuring the step height of gauge blocks. When the system measured 4-inch double-sided polished sapphire substrate, the measurement repeatability of TTV (Total Thickness Variation) was within 60 nm; and that of warpage was within 10 nm. As a side product, refractive index can also be measured using the proposed system, and experimental results showed respectable consistency.

Development of an on-machine measurement system with chromatic confocal probe for measuring the profile error of off-axis biconical free-form optics in ultra-precision grinding

Ultra-precision grinding of large-scale free-form optics made of hard brittle material is faced with several thorny problems, such as the grinding process is complex, hard to estimate the wear of the grinding wheel and the on-machine measurement technique of profile error is immature, which seriously affects the rapid convergence of profile accuracy. The on-machine measurement of profile error can effectively avoid the re-installation error introduced into the compensation period, and then the effectiveness and machining efficiency of compensation grinding are enhanced. Therefore, we developed an on-machine measurement system with a chromatic confocal probe for measuring the profile error of off-axis biconical free-form optics in ultra-precision grinding. First, the on-machine measurement system was developed on a 4-axis ultra-precision machine tool, and the scanning principles and position calibration were studied. Then, the scanning strategy was systematically researched, which included the centering approach, scanning path planning, scanning parameter calculation, and the evaluation principle of profile error. Finally, the on-machine measurement experiments were carried out on the off-axis biconical free-form optics with a width of 220 mm and height of 100 mm. The sub-micron variation range of measurement results is achieved by measuring the full-aperture profile error under different sampling parameters. The developed on-machine measurement system and scanning strategy can significantly improve the profile accuracy and machining efficiency of the large-scale free-form optics made of hard brittle material, and reduce the production cost.

Influence of Surface Tilt Angle on a Chromatic Confocal Probe with a Femtosecond Laser

This paper presents an intentional investigation of the effect of the object tilt angle on the tracking local minimum method (TL method), which is the one for detecting the measurement target position of the object optical axis, in a chromatic confocal probe employing a differential dual-fiber-detector optical system with a mode-locked femtosecond laser as the light source. The effect of the object tilt angle on dual-detector confocal probes, and even chromatic confocal probes, has not been investigated in detail so far, although the effect of object tilt angle on scanning confocal probes has been studied. At first, to examine the influence of the object tilt angle on the TL method, a theoretical model is established, and numerical simulations are performed based on the established theoretical equation. Then, the effect of aberrations in confocal optics on the confocal response curve is investigated in experiments. Finally, investigations on the effect of the object tilt angle on the TL method are demonstrated in experiments.

Calibration of beam vector deviation for four-axis precision on-machine measurement using chromatic confocal probe

On-machine measurement based on chromatic confocal principle is an effective method for quality control in precision manufacturing. However, angular misalignment between the measurement axis and the optical axis of probe will introduce cosine error, and the probe offset will cause rotational deviation. These alignment errors, which negatively affect measurement accuracy, are represented by beam vector deviations. In this paper, the influence of the beam vector deviation is first quantitatively analyzed. Then, an effective calibration method using standrad parts is proposed. The accuracy of the calibration method is verified by measuring standard sphere profile, and the uncertainty considering indication error, random error, sampling strategy and fitting optimization algorithm is evaluated. The feasibility of the proposed method is demonstrated by practical on-machine measurement experiments of a typical high-gradient freeform surface. Trajectory deviation is corrected based on virtual rotation center planning. The results agree well with offline measurements.

On-machine chromatic confocal measurement for micro-EDM drilling and milling

On-machine surface measurement of micro-scale components after micromachining becomes crucial to satisfy the increasing quality requirements. It implements a seamless measurement activity without interrupting manufacturing actions and hence facilitates the closed-loop quality control inline. Chromatic confocal probe sensing is a promising technique for on-machine integration thanks to its capability of assessing the surface with high accuracy and high speed. This paper presents the development of an on-machine measuring system using a chromatic confocal sensor, integrated in a micro-electrical discharge machining (micro-EDM) machine. The effects of scanning speed are qualitatively investigated in relation to outliers and quantitatively reflected through surface height parameters. In addition, the effects of surface gradient and surface contamination upon the measurement quality are studied. Two study cases, i.e. an array of micro-dimples and a micro-gear which are drilled and milled by micro-EDM, respectively, have been implemented to evaluate the developed system and to compare with off-line commercial confocal microscope. This comparative study demonstrates the system capability for on-machine areal measurement with sub-μm accuracy and reduced inspection time of 64%.

Measurement Range Expansion of Chromatic Confocal Probe with Supercontinuum Light Source

Confocal probes have been widely adopted in various industries owing to their depth-sectioning effects. A dual-detector differential chromatic confocal probe using a mode-locked femtosecond laser source is proposed herein, and the measurement range expansion of the probe using a supercontinuum light source is discussed. Supercontinuum light has an extremely wide spectrum. A simulation based on wave optics is performed to evaluate the detection sensitivity and measurable range by considering the chromatic aberration of the lens materials. Additionally, an experimental setup is constructed using a supercontinuum light source, and its feasibility is validated. A measurable range of 200 μm is adopted in the experiment, and three-dimensional surface profile measurements are performed. However, the developed confocal probe has not been used for surface topography measurements. Experiments are conducted to verify the performance of the developed probe.

Enabling precision coordinate metrology for universal optical testing and alignment applications

Optical designs for the next generation space science instruments call for unconventional, aspheric, and freeform (FF), prescriptions with tight tolerances. These advanced surfaces enable superior-performance, compact, and lower cost systems but are more challenging to characterize and, hence, to fabricate and integrate. A method was developed to characterize a wide range of optical surfaces, without requiring custom-made correctors, and to align them to each other for a high-performance optical system. A precision coordinate measuring machine, equipped with a non-contact, chromatic confocal probe, was used to measure numerous optics including large convex conics, high-sloped aspherics, several FF surfaces, and grazing-incidence x-ray optics. The resulting data were successfully reduced using custom-developed, advanced surface fitting analysis tool, to determine the optic’s alignment relative to the global and local coordinate systems, surface departure from design, and the as-built optical prescription. This information guided the modeling and the alignment of the corresponding as-built optical systems, including a flight system composed of a three-mirror anastigmat.

A trial toward sub-100 nm motion error compensation of ultra-precision machine tool using on-machine metrology system with chromatic confocal probe

A trial toward sub-100 nm motion error compensation of ultra-precision machine tool using on-machine metrology system with chromatic confocal probe

An Off-Axis Differential Method for Improvement of a Femtosecond Laser Differential Chromatic Confocal Probe

This paper presents an off-axis differential method for the improvement of a femtosecond laser differential chromatic confocal probe having a dual-detector configuration. In the proposed off-axis differential method employing a pair of single-mode fiber detectors, a major modification is made to the conventional differential setup in such a way that the fiber detector in the reference detector is located at the focal plane of a collecting lens but with a certain amount of off-axis detector shift, while the fiber detector in the measurement detector is located on the rear focal plane without the off-axis detector shift; this setup is different from the conventional one where the difference between the two confocal detectors is provided by giving a defocus to one of the fiber detectors. The newly proposed off-axis differential method enables the differential chromatic confocal setup to obtain the normalized chromatic confocal output with a better signal-to-noise ratio and approaches a Z-directional measurement range of approximately 46 μm, as well as a measurement resolution of 20 nm, while simplifying the optical alignments in the differential chromatic confocal setup, as well as the signal processing through eliminating the complicated arithmetic operations in the determination of the peak wavelength. Numerical calculations based on a theoretical equation and experiments are carried out to verify the feasibility of the proposed off-axis differential method for the differential chromatic confocal probe with a mode-locked femtosecond laser source.

On-machine noncontact scanning of high-gradient freeform surface using chromatic confocal probe on diamond turning machine

Inspecting a high-gradient freeform surface is a challenging task in manufacturing automation due to its complexity of the surface and high-aspect-ratio. On-machine measurement has shown remarkable achievements in ultra-precision manufacturing. In this paper, an on-machine noncontact scanning method using chromatic confocal probe for high-gradient freeform surface is proposed. First, a coordinate sampling model of scanning based on an ultra-precision diamond turning machine is established. Then, the steepness angle is defined to guide the scanning curves distribution, and global mesh scanning lines are generated by controlling planning deviations. Segmented motion trajectory of the probe is designed considering the vector direction of optical axis and sensor offset error. Finally, the scanning parameters are determined and the NC inspection program is generated. The proposed method can make a balance between accuracy and efficiency. Its effectiveness is verified by the on-machine inspection experiments.

A new signal processing method for a differential chromatic confocal probe with a mode-locked femtosecond laser

This paper presents an improved data processing method, which is referred to as the tracking intersection method, for a fibre-based dual-detector differential chromatic confocal probe with a mode-locked femtosecond laser source. In the dual-detector differential confocal system, two fibre detectors of an optical spectrum analyzer are employed to detect the spectra of the confocal signals. One of the fibre detectors, referred to as the measurement detector, is located at the confocal position of the confocal system, and the other fibre detector, referred to as the reference detector, is located at a position with a certain offset from the confocal position. In the tracking intersection method, the Z-directional displacement of a measurement object is derived from tracking the wavelength at the intersection point of the two fibre detector outputs. A new algorithm is proposed to generate a normalized output with a sharp peak at the intersection wavelength, from which the intersection wavelength can be detected with a high-sensitivity while the influence of the non-smooth spectral distribution of the femtosecond laser source can be removed. Compared with the previous tracking local minimum method, the new tracking intersection method is more robust to noises in the fibre detector outputs and consequently can realize better performance such as higher resolution and larger range in measurement of the displacement of the object. The advantages of the new method are demonstrated by simulation and experiment.

Theoretical investigation on the effect of misalignment error in a chromatic differential confocal probe

Theoretical investigation on the effect of misalignment error in a chromatic differential confocal probe

Investigation and Improvement of Thermal Stability of a Chromatic Confocal Probe with a Mode-Locked Femtosecond Laser Source

An intentional investigation on the thermal stability of a mode-locked femtosecond laser chromatic confocal probe, which is a critical issue for the probe to be applied for long-term displacement measurement or surface profile measurement requiring long-time scanning, is carried out. At first, the thermal instability of the first prototype measurement setup is evaluated in experiments where the existence of a considerably large thermal instability is confirmed. Then the possible reasons for the thermal instability of the measurement setup are analyzed quantitatively, such as the thermal instability of the refractive index of the confocal lens and the thermal expansion of mechanical jigs employed in the probe. It is verified that most of the thermal instability of the measurement setup is caused by the thermal expansion of mechanical jigs in the probe. For the improvement of the thermal stability of the probe, it is necessary to employ a low thermal expansion material for the mechanical jigs in the measurement setup and to shorten the optical path length of the laser beam. Based on the analysis result, a second prototype probe is newly designed and constructed. The improved thermal stability of the second prototype probe is verified through theoretical calculations and experiments.

A Method for Expansion of Z-Directional Measurement Range in a Mode-Locked Femtosecond Laser Chromatic Confocal Probe

A method is proposed to expand the Z-directional measurement range of a fiber-based dual-detector chromatic confocal probe with a mode-locked femtosecond laser source. In the dual-detector chromatic confocal probe, the Z-directional displacement of a measurement target is derived from the peak wavelength in the normalized intensity ratio from the two light intensities obtained by the two identical fiber detectors. In this paper, a new method utilizing the main-lobe and side-lobes of axial responses acquired from both the normalized intensity ratio Ia and the invert normalized intensity ratio In, which is the inverse of Ia, is proposed to obtain the seamless relationship between the peak wavelength and the Z-directional displacement of a measurement target. Theoretical calculations and experimental investigation are carried out to demonstrate the feasibility of the proposed measurement range expansion method.

Theoretical investigation on measurement range expansion of a femtosecond laser chromatic confocal probe

Theoretical investigation on measurement range expansion of a femtosecond laser chromatic confocal probe

Application of a chromatic confocal measurement system as new approach for in-line wet film thickness determination in continuous oral film manufacturing processes

The key parameter of the oral film production process is the wet film thickness since it regulates the active pharmaceutical ingredient (API) content of the finished product. There is no general recommendation on how to adjust the gap height of the coating knife during the film manufacturing process to obtain the target content. Therefore, trial and error approaches are common to determine the surplus of drug for every newly developed formulation. This wastes resources, money and time and calls for an adequate in-line tool for wet film thickness measurement during the film manufacturing process to ensure consistent quality. In this work, a chromatic confocal optical probe was implemented into a continuous oral film manufacturing process on a pilot-scale coating bench. The optical probe allows a non-destructive and contactless wet film thickness measurement. The validation of the method showed good results. Linearity was demonstrated over a wide range of film thicknesses (R2 = 0.999). A good precision between different films was revealed by a coefficient of variation smaller than 2%. The robustness investigations showed that the method is applicable for transparent and non-transparent film forming masses. Furthermore, coloring agents, particles in the polymer mass and different viscosities do not influence the thickness measurement.

A chromatic confocal probe with a mode-locked femtosecond laser source

The high spatial coherence and high stability of a mode-locked femtosecond laser source make it ideal for chromatic confocal imaging. Nevertheless, the high non-smoothness of spectrum of the mode-locked laser source often restricts its application in chromatic confocal probes for a wide range of depth measurement. To eliminate the spectral non-smoothness of the mode-locked laser source, we propose a new chromatic confocal setup in such a way that the reflected laser beam is divided into two sub-beams which are then made to pass through two optical paths with different confocal settings where two identical fiber detectors are placed at the focal position and a defocus position, respectively, for acquiring two confocal signals. An axial response is then obtained from the intensity ratio of the two confocal signals for depth measurement. With the proposed confocal setup, we can expand the working spectral range of a mode-locked laser source to its whole spectrum. Theoretical and experimental investigation have revealed that the developed chromatic confocal probe with the specific mode-locked laser source employed in experiments has a measurement sensitivity of about −4 nm/μm, a depth measurement range of about 40 μm, and a depth resolution of better than 30 nm, respectively.

An on-machine error compensation method for an ultra-precision turning machine

On-machine error compensation (OMEC) is efficient at improving machining accuracy without increasing extra manufacturing cost, and involves the on-machine measurement (OMM) of machining accuracy and modification of program code based on the measurement results. As an excellent OMM technique, chromatic confocal sensing allows for the rapid development of accurate and reliable error compensation technique. The present study integrated a non-contact chromatic confocal probe into an ultra-precision machine for OMM and OMEC of machined components. First, the configuration and effectiveness of the OMM system were briefly described, and the relevant OMEC method was presented. With the OMM result, error compensation software was then developed to automatically generate a modified program code for error compensation. Finally, a series of cutting experiments were performed to verify the validity of the proposed OMEC method. The experimental results demonstrate that the proposed error compensation method is reliable and considerably improves the form error of machined components.

Non-contact on-machine measurement using a chromatic confocal probe for an ultra-precision turning machine

On-machine measurement (OMM) involves the interruption of the machining process and the subsequent measurement of the workpiece without its removal from the machining tool. Chromatic confocal sensing is a well-known measurement technique that is able to evaluate the position of a point on an object surface along the optical axis of the system with high accuracy. The present study integrates a chromatic confocal measurement probe with an ultra-precision diamond turning machine to achieve the non-contact OMM of machined components. The procedure for establishing the position of the rotary axis of the spindle based on dual standard spheres is first described in detail, and the relevant OMM procedure for machined components of different surface topographies is explained. Then, a 50-μm quartz step height standard is employed to investigate the linear measurement accuracy of the chromatic confocal probe. Finally, the measurement accuracy of the proposed OMM method is compared experimentally with that of the stylus method. The results show that the estimated form error value of the OMM method agrees well with the value obtained by the stylus method. The proposed OMM method feasibly achieves non-contact OMM with a nanometer-level accuracy for an ultra-precision turning machine and is capable of reconstructing the 3D surface topography of flat, spherical, and aspheric surfaces. After integrating the OMM method, the ultra-precision turning machine can realize the function of processing-measurement integration.