IMAGE PROCESSING OF FIM AND IAP MICROGRAPHS

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IMAGE PROCESSING OF FIM AND IAP MICROGRAPHS

M. Miller

To cite this version:

M. Miller. IMAGE PROCESSING OF FIM AND IAP MICROGRAPHS. Journal de Physique Col-

loques, 1986, 47 (C7), pp.C7-477-C7-482. �10.1051/jphyscol:1986780�. �jpa-00225975�

JOURNAL D E PHYSIQUE

Colloque C7, suppl6ment au n o 11, Tome 47, Novembre 1986

IMAGE PROCESSING OF FIM AND IAP MICROGRAPHS

M . K . MILLER

Metal and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A.

Abstract

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A system has been developed to process sequences of field-ion micrographs, and imaging atom probe micrographs and elemental maps. The system consists of a commercial video digitizer capable of digitizing standard RS-170, or RS-330 video input and a frame memory that can store the digitized image in memory.

The video digitizer is an additional single-board subsystem incorporated in the O W L atom probe IBM-PC AT microcomputer. The resolution of the system is 512 horizontal by 480 vertical with 12 bits of memory associated with each pixel. The system can digitize data either from real time images of from recorded images on video tape or from prints or negatives.

INTRODUCTION

The use of image processing is not new to field-ion microscopy as illustrated by the optical color superposition of micrographs to reveal atoms that were removed or displaced from a platinum - 10% iridium alloy after bombardment with slow helium ions [I]. A computerized imaging system has recently been developed by Schiller et al. [2] that allows superposition of field-ion images to be performed in real time.

In this paper a real time image processing system is described and some preliminary results on the metallurgical applications of the system are presented.

HARDWARE AND SOFTWARE

The hardware consists of a single-slot real-time image processing system (Series loo), manufactured by Imaging Technologies Inc., that is incorporated into the ORNL on-line atom probe control IBM PC AT microcomputer [3], as shown in figure 1. The system has a resolution of 512 horizontal by 480 vertical with 12 bits assigned to each pixel. A special video lock capability allows images to be acquired from a Video Cassette Recorder in addition to standard RS-170, or RS-330 compatible video cameras. The analog input video signal is digitized to 256 gray levels. The output signal is sent to a standard monitor and to a dual purpose flat screen monitor for obtaining hard copies.

A key feature of this board is the feedback/input Look Up Table, LUT, that allows images in the frame memory to be processed in real time, figure 2. Depending on the operation desired, this LUT can be used either as a single 4096 by 12 bit LUT or as 16 separate 256 by 8 bit LUTs. The inputs can come from the video source, the frame memory, or the LUT select register. This allows several modes of operation including adding, subtracting or averaging two images to 6 bits of accuracy; adding or subtracting a constant from a full screen image; transforming the image prior to storage; transforming previously stored images; arithmetic operations; and implementing graphic overlays, display windows, and area-of-interest processing.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986780

JOURNAL DE PHYSIQUE

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Figure 2. Schematic diagram of the video digitizer single board subsystem. Note the feedbacklinput Look Up Table.

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MICROCOMPUTER ^{*- RECORDER} Figure 1. Schematic diagram of the video system.

The hardware can zoom up to 8x and pan the image non-destructively so that the image can be examined in more detail.

A comparison of a conventionally recorded field-ion micrograph and a digitized micrograph of the same specimen revealed only minor degradation in the quality of the image from the digitizing process at this 512 by 480 resolution. The conventional micrograph was recorded on Ilford FP4 film by a Nikon FE 35mm camera directly from the FIM screen. The digitized image was recorded with a Sony CCD video camera, type AVC-Dl, on a Sony VO-5800H video cassette recorder using 3 / 4 inch video tape, then digitized by the Series 100 system, and finally output to a flat screen monitor.

The system has an interactive program (Image-pro 100) to analyze either real-time or previously stored images. This program can incorporate user modules, as illustrated by the mesh and contour plots below, to enhance its performance for a particular application.

IMAGE ANALYSIS

Several examples of applying convolution and non-convolution filters to a micrograph of a modulated two phase microstructure are shown in figure 3 . The low-pass filter blurs the image. The high-pass filter sharpens the image with high frequency components and brings out the edges. The Laplacian filter removes low contrast components and leaves only the edges of the image. Horizontal edge enhancement brings out the edges and removes all other parts of the image. The median filter smooths the image and lowers the contrast. The dilation filter thickens the lighter elements and thereby brightens the image. The erosion filter thins the lighter elements and dulls the image. The Sobel filter provides extremely high contrast edges.

The standard techniques, such as the horizontal edge, high-pass, Laplacian, and Sobel filters, for detecting edges (i.e. interfaces between phases) in images were not very effective for field-ion micrographs. A more effective technique was found to be the use of contour and threshold functions as shown in figure 4. However, the non-uniformity of intensity across the micrograph degraded these functions. By combining these contour maps from a series of successive micrographs that were taken a known distance apart a true 3 dimensional reconstruction of the microstructure can be made.

Unfortunately several factors influence the uniformity of the field-ion image thus complicating image analysis. These factors include the ring structure of the poles in the micrograph, different intensities at the various zones, the presence of

"bright spots", and the gain and intensity variations in the microchannel plate and phosphor screen assembly. The influence of some of these factors can be minimized by careful selection of imaging conditions and using techniques such as averaging a series of micrographs while the specimen is slowly evaporating.

IMAGE MEASUREMENT

Images can be processed to extract characteristic distances present in the microstructure. Lines and areas can be measured to provide distance between points and areas within traced curves. Interactive calibration provides measurements in real units. An example of an intensity line scan across a spinodally decomposed two-phase Fe - 28.6 wt.% Cr

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10.6 wt.% Co alloy aged for 8 h at 600°c is shown in figure 5. The two phases are indicated by the regions of low and high intensity.

The intensity profile can also be examined in the frequency domain to detect periodic components such as the wavelength of a modulated microstructure. It should be noted that the true measurement of precipitate size or thickness is limited because of local magnification differences between different phases [ 4 ] .

The intensity line scan can be extended to provide a complete 3 dimensional representation of a section of the surface as shown in figure 6 for the same two

C7-480 JOURNAL

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PHYSIQUE

Original Xicrograph Lo-pass filter (5x5) High-pass filter (5x5) Fe-28.6 wt.%Cr-10.6 wt.% Co Convolution Convolution

Aged 8h at 600'~.

Laplacian filter (5x5) Horizontal edge filter Median filter (5x5)

Convolution (5x5) Convolution Non-convolution

Dilation filter (5x5) Erosion filter (5x5) Sobel filter (3x3) Non-convolution Non-convolution Non-convolution Figure 3. Examples of applying convolution and non-convolution filters to a

field-ion micrograph.

Figure 4. Contour and Threshold functions applied to the field-ion micrograph shown in figure 3. The contour function indicates the interface between the two phases and the threshold function indicates the extent of the two phases.

INTENSIN SCAN ACROSS FIM MICROGRAPH

DISTANCE

Figure 5. Intensity line scan across a field-ion micrograph of a spinodally decomposed Fe

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28.6 wt.% Cr - 10.6 wt.% Co alloy aged 8 h at 6 0 0 ~ ~ . The regions with the higher intensity are the iron-rich phase and those regions with the lower intensity are the chromium-enriched phase.

The insert indicates the position of the line scan on the field-ion micrograph.

JOURNAL DE PHYSIQUE

Figure 6. Three dimensional intensity line scan of the surface of a field-ion micrograph of a spinodally decomposed Fe

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28.6 wt.% Cr

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10.6 wt.% Co alloy aged 8 h at 600'~.

phase modulated material described above. In addition the coordinates of a traced line can be measured from a micrograph. By combining this type of datr, from a series of micrographs the reconstruction of a boundary plane or interface is possible.

Acknowledgment

This research was sponsored by the Division of Materials Sciences, U.S. Department of Energy, under contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc .

REFERENCES

[I] E.W. Mueller and T.T. Tsong, "Field Ion Microscopy: Principles and Applications", 1969, Elsevier, New York, p146.

[2] T. Schiller, U. Weigmann, S. Jaenicke and J.H. Block, J. de Physiclue,

a

479, (1986).

[3] M.K. Miller, J. de Physique,

a,

493 (1986); C 2 , 499 (1986).

[4] M.K. Miller, C..J. Miller and S.S. Brenner, Proc. 28th International Field Emission Symposium, Portland, 1981, p195.