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DIGITAL OUTPUT CA MERAS ENHANCE IMAGE PROCESSING APPLICATIONS

Ash PrabalaSenior Project Engineer

Cohu, Inc., Electronics Division5755 Kearny Villa Road

San Diego, California 921 86-5623

INTRODUCTION

Video cameras designed for CCTV applicationsusually provide an analog composite video outputsignal. In most Digital Image Processingapplications, the analog video output from thecamera is converted into a digital format by anAnalog-to-Digital conversion process.

In a conventional CCD based camera, the CCDimage sensor generates a sampled analog signalwhich is processed to create a composite analogvideo output. At the input of an Image Processingsystem, the composite analog video from thecamera is digitized before any image processing isperformed. If the digitization is performed on thesampled analog output of the CCD, within thecamera, then the two-step data conversion processis replaced by a single-step process. In addition,the one-to-one correlation between charge sites on

the CCD imager and the pixels in frame memory ofthe image processor leads to greater precisionduring image processing.

This has led to the development of the Cohu 41 10series of cameras incorporating an eight-bit digital

video output. This paper deals with the benefits ofusing a digital output camera as well as some ofthe possible applications.

The following issues will be discussed:

Functional details of the Cohu 41 10 camera(See fig. 1,2, and 3)

Advantages of a Digital Camera based imageprocessing system.

Some applications of the Cohu 41 10 camera

W E 0 BOARD

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VERTICAL RESETMODE CONTROL INPUTS

Fig 1. Block Diagram of the Cohu Model 41 10 Digital Camera120

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Fig. 3. Cohu Model 41 10 Digital Camera

ADVANTAGES OF A DIGITAL CAMERA BASEDIMAGE PROCESSING SYSTEM:

@ A one-to-one correlation exists between the CCDimager charge sites and the resulting digital videodata. This allows a greater degree of spatialaccuracy than is possible with conventionalanalog-camera-based image processing systems.This is particularly important in measurementapplications.

@The use of the internal pixel clock (which iscorrelated to the serial clock of the imager) fordigitizing the video data leads to improved pixelsynchronization and virtually eliminates pixel jitter.This is particularly important in sub-pixelinterpolation applications.

0 In a conventional image processing system,low-level analog video signals are fed into the noisydigital environment of the image processor hostcomputer. Before the video signal is digitized, thevideo processing front end circuitry can injectenough noise into the signal to significantlydegrade the S/N ratio and thereby lower thenumber of effective bits. In other words, the lowersignificant bit(s) may be "lost" in noise. Thisdegrades the radiometric accuracy of the imagingsystem. The loss of the lower significant bits canseverely limit the performance of sub-pixelinterpolation algorithms.

APPLICATION (1): COHU 41 10 BASED OPTICALFOR ROUGH SURFACESNON-CONTACT INTERFEROMETRIC PROFILER

Information courtesy of WYKO Corporation,Tucson, Arizona

The WYKO Rough Surface Tester (RST) is anon-contact optical profiler that measures surfaceroughness or step heights in three dimensions(fig. 4). With the RST it is possible to examinesurfaces up to 100 pm (4 mil) peak to valley with aresolution of 3 nm. The R ST provides accurate andrepeatable measurements of surfaces that arecritical to the manufacturing processes in a varietyof products including paper, rolled steel, sheetmetal, laser engraved rollers, plastics, paintedsurfaces, diamond films, wafers and ceramics.

System components: The WYKO RST includes amicroscope, a Cohu 4110 digital camera, a videodisplay, an interferometric microscope objective, aPC compatible 486 based computer system withkeyboard and VGA monitor, calibration standards

and analysis software.

System description: The RST is a verticallyscanning interference microscope system thatoperates with one of several interchangeablemagnification objectives. Each objective containsan interferometer, consisting of a reference mirrorand beam splitter that produces interference fringeswhen light reflected off the reference mirrorcombines with light reflected off the sample. Whenshort coherence length white light is used, theseinterference fringes are present only over a veryshallow depth on the surface. The surface isprofiled by scanning vertically so that each point ont h e surface produces an interference signal andthen locating the exact vertical position where each

signal reaches its maximum amplitude.

The RST starts the measurement sequence byfocusing above the top of the surface being profiledand quickly scanning downward. At evenly spacedintervals during the scan, frames of interference

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COHU DIGITAL CAMERA 7

L c E IGITIZEDATA INTENSITY

BEAMSPLllTER

PZT TRANSDUCER

APERTURE STO

MICROSCOPEOBJECTIVE

MIRAUINTERFEROMETER

Fig. 4. The Cohu 41 10 camera provides true 8-bitgray scale resolution for the WY KO corporation

non-contact interferometric profiler for rough

data imaged by the Cohu 41 10 digital video cameraare captured and processed by high speed digitalsignal processing hardware. As the system scansdownward, an interference signal for each point onthe surface is formed. The system uses a series of

algorithms to precisely locate the peak of theinterference signal for each point of the surface.Each point is thus processed in parallel, allowingthe RST to determine surface height differencesquickly and with an extremely high degree ofaccuracy. The entire measurement and analysissequence takes only seconds.

The Cohu 41 10 digital camera is ideally suited forthis application which relies on its true 8 bit grayscale resolution and pixel stability. In the simplestterms, this application converts minute differencesin depth into differences in light intensities by theprocess of interferometry. What is needed toquantify these small differences in light intensities isa camera with superior gray scale characteristics. A

conventional analog camera based imaging systemwas found to lack the true 8 bit gray scaleresolution required for this application. The Cohu4110 camera satisfies the critical needs of thisapplication and plays a crucial role in achieving itsimpressive specifications.

The WYKO RST was a recepient of the 1993Photonics Circle of Excellence Award.

APPLICATION (2): COHU 4110 BASED DIGITALSTEREO IMAGING FOR IMAGE ACQUISITIONAT HIGHWAY SPEEDS.

Information courtesy of the Ohio State UniversityCenter for Map ping

Researchers at the Ohio State University Center forMapping have developed an Imaging Van that usestwo Cohu 41 10 digital cameras in a digital stereoimaging system.

In conjunction with a satellite based GlobalPositioning System (GPS) for accurate positionfixing (better than three meters anywhere in theworld), the data obtained from the imaging systemallows users of the database to make accurate

measurements of highway features. These can bebridge dimensions, overpass clearances, utilitypoles, etc. Perhaps the most interesting feature ofthis application is the simultaneous fusion ofseveral different state-of-the-art technologies.

The imaging system had to meet specifications sodemanding that conventional analog cameras werefound to be inadequate. The OSU researchersdecided to use Cohu digital CCD cameras to takeadvantage of their (1) higher spatial accuracy, dueto a one-to-one correlation between individual CCDelements and digital pixel data, (2) increased pixelstability, due to the digital quantization of sampledanalog signals and (3) true 8-bit performance,which is not compromised by the noisy digital

environment characterizing most digitalimage-processing systems.

From a system point of view, thesoftware-selectable electronic shutter and theability of the camera to accept asynchronousvertical reset pulses (while in the shutter mode)allows the acquisition of digital stereo images whilethe van is travelling at highway speeds.

To handle the large amounts of real-time, 8-bitparallel digital data generated by the camera-pair,each camera is connected to a Digital PortInterface (DPI) within a Recognition Concepts (RCI)TrapixP lus real-time image processing and storage

system. Images are stored on the Data-Store realtime disks or on an Exabyte tape drive.

The results obtained during calibration and testingof this imaging system are extremely encouraging.At a distance of 15 meters, co-ordinatedetermination by the centroid detection algorithm

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FRAGMENTSIZE

ANE PEAK FINDING+

XTRACTION + ORMALIZATION+AND FILING ALGORITHM DETERMINATION

IMAGECAPTURE

-ALLELEFREQUENCY

EPORT

GENERATION DETERMINATION

Fig. 5. Flow Chart of steps following image capture of the Ambis DNA typing system

(which can compute image positions to a tenth of apixel) is accurate to 5 centimeters. Since thisaccuracy is better than that of the GPS system, theOSU researchers believe that the Cohu 41 10 basedsystem is able to meet their specifications.

In addition to the Cohu 4110 cameras, manyconventional analog cameras and some veryexpensive digital cameras were tested. The Cohu41 10 was found to be the camera of choice basedon its ability to meet the specifications and satisfythe cost constraints.

APPLICATION (3): COHU 4110 BASED DNAFINGERPRINTING SYSTEM FOR CRIME LABS

Information courtesy of AMBIS Sysfems, SanDiego, California.

The field of DNA TYP ING (DNA Fingerprinting)addresses three broad applications. The first ofthese is associated with forensic science - linking asuspect to evidence at a crime scene through thematching of patterns produced through theelectrophoretic separations of specific genefragments. The second addresses a need inpaternity determination. Specifically, a child obtainsone of two copies of each gene from each parent.Usually, the genetic "fingerprints" of the mother andthe child are known. The individual components ofthe child's fingerprint that do not match those of themother may be attributable to the father. The thirdapplication for DNA Typing is genetic research.This addresses a broad variety of topics from thecharacterization of fragment distribution within andbetween races, to the engineering of larger andfatter beef cattle.

The common denominator in each of the areasdescribed above is the DNA "fingerprint". The DNAfingerprint is an artifact of the gathering of theessential information, namely the fragment sizesassociated with specific combinations of restrictionenzymes and hybridization probes. The process ofelectrophoretic separation displaces the fragmentsin inverse proportion to their molecular weights.This physical displacement is measured accuratelyby the imaging system.

Using the COHU 41 10 as a source of extremelyaccurate positional data, and a CORECO 500MSImage Processing board, a 512 X 512 X 16 bitmapped image file is captured on the system.

The steps following image capture are shown inFig. 5

The AMBIS DNA TYPING SYSTEM is aninnovative approach to solving a particularlychallenging problem in the field of genetic research.The Cohu 41 10 camera satisfies the critical needsof the highly accurate image acquisition systemwhich serves as the front end of this system.

CONCLUSION: As described above, the Cohu41 10 has been selected as the camera of choice inimaging applications requiring high spatial accuracyas well as the radiometric accuracy of true 8 bits.With the inclusion of a digital video interface byseveral image processing board manufacturers, itappears that the digital camera has gainedacceptance among the high end imaging systems.0

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