Image Acquisitionand Proces

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Image Analysis 191 The Resting Preview VI provides image and transducer verification, and allows focus and finger placement adjustments to be made while familiarizing the subject with the procedure. The Resting Data VI collects 30 seconds of analog input data, providing baseline measurements of the pulse volume and resting finger force. From these measurements, the Resting Mean global variable is populated, and target force values for subsequent use during the nail press are computed from this value. As images are not acquired here, data is gathered at a high collection rate (1024 Hz) and provides an additional resource of data for any subsequently desired frequency analysis measurements requiring higher resolution. The Nail Press VI utilizes its front panel interface to instruct and prompt the subject accordingly. The test consists of two sequences but starts only after the occurrence of a hardware trigger. This trigger starts a counter on the DAQ card, which generates a square pulse wave that serves to initiate each buffer of the image acquisition. During the initial sequence (the first 10 seconds), the interface provides an audible prompt and visual instructions to “Rest and Relax” while resting images and analog input data are gathered at 128 Hz. The subject is then instructed to “press and hold” in order to reach a desired target force, calculated by adding a load cell calibrated force to the RESTING MEAN global value. Once the target is exceeded, a countdown of 10 seconds begins, during which the subject maintains the target force level. The subject is then prompted to “rest and relax” for 20 additional seconds of data collection. A browser (Figure 7.37) is created which displays the images acquired during the press for verification of analysis results, and all images and raw data are written to file for postprocessing and analysis. The generation of time stamped data for timing verification purposes was essential, as this VI was expanded and elaborated upon for functionality. The use of software synchronization was not problematic at the low image acquisition rate (1 Hz); however, the use of hardware synchronization, which provides resolution in the order of nanoseconds, will be used when higher collection rates are desired. 7.5.2.2 Postprocessing and Analysis Characteristic force and blood volume responses during the nail press are shown in Figure 7.38. The nail press sequence may be reviewed within the Image Analysis VI, which cycles through the acquired images at a user-defined speed (Figure 7.39). As each image is displayed, a histogram is generated and data from this report is unbundled and saved for further analysis. Although acquired as RGB images, the histograms are generated from their 8-bit representations. From the histogram data, selections are made for those classes demonstrating the most significant change during the press, and an evaluation of the pixel numbers within these bins over time generates a curve which, not surprisingly, resembles the curves of the force and blood volume results. Critical to the automation of the nail press was the development of an algorithm that could be applied in the analysis of all of the data. Such an algorithm was developed and incorporated into both the Image Analysis and Data Analysis VIs. The typical image, force and blood volume data all revealed steep slopes during the nail press; therefore, the data were analyzed by taking step derivatives along the
192 Image Acquisition Processing with LabVIEW FIGURE 7.37 Changes during a nail press. FIGURE 7.38 Representative force and blood volume data. course of the press. Plotting the absolute values of these results reveals the areas of maximal change as defined by sharp peaks above baseline. The times corresponding to these peaks are calculated, and represent the times of maximal change during the nail press. The final results following the application of the step differentiation algorithm to the image data, and the force and blood volume data respectively, are shown in Figure 7.40. 7.5.3 SUMMARY Using results gained from the developed instrumentation, it is possible to more accurately determine the times, which parallel the application of force and the return of blood flow and color to the finger. As demonstrated, the accuracy of the result is limited by the resolution of the acquisition system; therefore increasing the image acquisition rate above 1 Hz will certainly provide more precise determinations. This automated system provides a valuable method for the evaluation of blood flow in vibration-exposed worker populations.
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IMAGE PROCESSING SERIES Series Edit
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Foreword The introduction of LabVIE
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FIGURE 1 Vision 6.1 FIGURE 2 Compan
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Acknowledgments My gratitude goes o
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Dedication To Don “old fella” P
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Contents Chapter 1 Image Types and
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Chapter 4 Displaying Images........
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Chapter 7 Image Analysis...........
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1 Image Types and File Management F
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Image Types and File Management 3 F
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Image Types and File Management 5 A
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Image Types and File Management 7 C
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Image Types and File Management 11
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Image Types and File Management 13
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2 Setting Up You should never under
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Setting Up 17 FIGURE 2.3 (a) Interl
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Setting Up 19 Linear Aray Light Sou
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Setting Up 21 FIGURE 2.7 National I
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Setting Up 23 2.1.3.2 Control and I
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Setting Up 25 FIGURE 2.11 X-Ray ima
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Setting Up 27 Apple Computer techno
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Setting Up 29 FIGURE 2.17 An inexpe
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Setting Up 31 Although this would b
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Setting Up 33 134 -115 \ Contrast =
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Setting Up 35 resolution printer wi
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Setting Up 37 C A B C Traditional T
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Setting Up 39 Camera Light Source O
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Setting Up 41 Camera Light Source (
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Setting Up 43 FIGURE 2.37 (a) An ob
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Setting Up 45 FIGURE 2.41 (a) PCB p
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Page 164 and 165: 6 Morphology When applied to vision
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Page 180 and 181: Morphology 157 6.5 CASE STUDY: FIND
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Page 242 and 243: Machine Vision 219 TABLE 8.1 “IMA
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Page 254 and 255: Glossary 231 binary display device
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Image Acquisitionand Proces

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