Image analysis – Applications – Surface texture or roughness measuring
Reexamination Certificate
2000-04-07
2003-06-24
Johns, Andrew W. (Department: 2621)
Image analysis
Applications
Surface texture or roughness measuring
C382S152000
Reexamination Certificate
active
06584215
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an apparatus and a method capable of ascertaining and displaying deformations or shifts in test objects in real time.
BACKGROUND OF THE INVENTION
In some measurement tasks, the deformation or shifting, particularly of diffusely scattering object surfaces, must be detected precisely, for instance to examine the influence of an exertion of force on a test object. There is often a need to be able to follow the deformation of the test object or its surface directly, while the surface structure or shape itself occasionally does not matter. This is especially pertinent to practical conditions, that is, close to production and with the deformation of the surface being displayed in the clearest, most readily apparent and readily evaluatable possible form.
For detecting surface displacements or expansions of a test object whose surface scatters diffusely, strike projection methods, such as the moiré method, and interferometric methods are known, such as electronic speckle pattern interferometry (ESPI method) or the shearing method. While the moiré method is more useful for larger deformations or shifts, the interferometric methods are used particularly for measuring lesser travel distances but with greater resolution. There are no restrictions to object size in the speckle method.
From U.S. Pat. No. 4,660,978, a shearing interferometer with a fixedly adjusted mirror tilt is known for determining distortions of a wave front of an optical beam. The shearing interferometer has a beam splitter that splits the arising beam, which possibly has a curved wave front, into two split beams and reunites them. The two split beams are each reflected by mirrors. One of the mirrors is connected to an inclination device, while the other mirror is connected to a displacement device. The split beams reunited by the beam splitter are carried to a camera. For different mirror inclinations of the inclinable mirror, the displaceable mirror is then adjusted incrementally. From the brightness or intensity values that result at the camera, the phase relationship of the beam can be determined, and from this data pertaining to the curvature of the wave front can be calculated.
Similar devices can be used to examine deformations of object surfaces. For instance, two states of a test object are compared for the purpose, in which the object is picked up in two different load states and the interferograms of the two states are subtracted. The result is a differential interferogram, which, depending on the measurement principle employed, shows either the displacement or the expansion of the object between the two states in the form of lines of interference. The amount of displacement or expansion at a pixel of the differential interferogram can then be determined, for instance by counting the interference lines beginning at a pixel of known displacement or expansion and taking the wavelength of the light employed into account.
If the measuring head, in a similar way to the shearing method described above, is equipped with a phase shift unit, then an expanded evaluation can be performed by the principle of the phase shift method (W. Osten, “Digitale Verarbeitung und Auswertung von Interferenzbildern” [Digital Processing and Evaluation of Interference Patterns], Chapter 6, Akademieverlag, ISBN 3-05-501294-1). In it, phase images are generated that assign a certain phase angle to each pixel. If the phase images of two states of the object are subtracted, the result is a phase differential image. In contrast to the aforementioned differential interferogram, the phase differential image does not have sinusoidally modulated interference lines but instead directly indicates the phase difference angle between the two states of the object.
In the phase shift method, the object to be examined, if successive images of the same object's state are to be picked up in succession with a different phase relationship, must remain absolutely still.
To aid in this, German Patent DE 38 43 396 C1 discloses a method known as “direct phase measurement” or as the “spatial phase shift method”. All that this method needs for calculating 2&pgr;-modulated phase images is a grating projection and a camera image.
It is an assumption here that the period length in the striped pattern corresponds to a constant number of pixels, and that the background intensity of adjacent pixels is identical. This represents a rough approximation of actual conditions and leads to phase errors.
In testing technical objects, it is important to facilitate the evaluation of differential interferograms, so that defects in an object or other special features of the test object that can be detected by the deformation can be made clearly apparent. U.S. Pat. No. 5,091,776, teaches reconfiguring an obtained differential image before displaying it, or in other words, finding the amount of the difference pixel by pixel. The absolute value of the difference generated can furthermore be multiplied by a constant factor via an amplifier, and thus the overall image contrast can be varied.
Locally fluctuating light conditions can lead here to restricted evaluatability of the images obtained.
SUMMARY OF THE INVENTION
With this as the point of departure, it is the object of the invention to create an apparatus and a method capable of real-time ascertainment and display of deformations of test objects that furnish an improved image quality. This object is attained by the method and measuring system of the present invention.
In the method of the invention, in a first method step, the initial step, a normalization data set is generated from a set of phase-shifted images of the surface region of interest on the object; this data set contains normalization information specific for each pixel. This normalization data set is specific for the region of a object surface that is located in the viewing field of a picture taking device. The normalization data set is stored in memory and held in readiness for using the further post-treatment of the images taken of the same surface region. For each pixel, the normalization data set contains information about the amplitude with which the image brightness or intensity varies if a phase displacement is performed. Thus the normalization data set represents an amplification or normalization factor that is specific for each pixel. By applying this normalization data set to the differential images generated, an image suitable for display on a monitor is obtained that has equally good contrast at every detail of the image.
The method is suitable for measuring object deformations both by the stripe method and by speckle modulation, for instance. In both cases, a marked improvement in image quality compared to an uncorrected method is obtained.
In order to display a deformation of a test object compared with an initial state, it is expedient to store a reference image in memory that is utilized together with an image picked up currently for generating a differential image. It is possible to pick up moving object surfaces. If the measurement object moves during the period required by a camera to read out its image sensor, this does not fundamentally impede the picture taking. It is therefore possible to observe the test object continuously with the camera; the difference between the currently arriving image and the reference image stored in memory is formed pixel by pixel. These differential values calculated pixel by pixel are normalized using the applicable pixel-specific amplitude value of the stripe or speckle modulation and are displayed as a normalized differential value image.
In many cases, it is expedient to generate a quantitative value image pixel by pixel from the differential image. This improves the display. The zero crossover of the sinusoidal course of intensity of a speckle becomes black, and both positive and negative values are represented by gray values.
To determine an image to be displayed of the current state of the test object, only a current camera image and the reference
Johns Andrew W.
Kunitz Norman N.
Nakhjavan Shervin
Venable LLP.
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