Optical range finder for 1.5-1 0-m distances

likka Kaisto, Juha Kostamovaara, Markku Manninen, and Risto Myllyla

A time of flight method is suggested for measuring distances in large manipulator range-finding applica-tions. Light pulses of a SH laser diode are used. Special attention was paid to reducing the pulse amplitude

variations in the transit time measurement. Preliminary results with a prototype have proved the methodto be highly usable in measuring position, orientation, and shape of the objects within a 1.5-10-m range.

The resolution of the distance measurement is ±1 mm with a measuring time of 10 msec.

1. Introduction

To support machine vision in future automation theneed for range information is most important. Therange information can be used for different purposes:to measure the position and orientation of objects in areal scene1 ; to measure the shape of objects2 ; to improveman-machine communications; and to get a real depthmap from a 2-D intensity image.4

As a means of supporting computer vision, four op-tical range-finding methods have been studied: stereodisparity; texture gradient; the triangulation-basedmeasurement technique; and the time of flight method. 1

Of these the first two are directly image-based tech-niques and as such computationally very expensive.The triangulation method is suitable for small distancesbut suffers from the missing part problem so that thereceiver cannot always see what the source beam is il-luminating. The time of flight method, based on de-tecting the phase shift between the transmitted andreference signals, is normally not fast enough and easilyleads to expensive construction. The other possibilityin time of flight measurement is to measure the transittime of light pulses.

The requirements for a range finder to support ma-chine vision vary slightly depending on the particularapplication. In this study the goal for designing therange finder was to improve the control of heavy com-puter-based machinery such as robotic manipulators.When the main goal of using range data is to improveman-machine communication the basic requirements

Markku Manninen is with Technical Research Centre of Finland,

Electronics Laboratory, P.O. Box 181, SF-90101 Oulu 10 Finland; the

other authors are with University of Oulu, Department of Electrical

Engineering, SF-90570 Oulu 57, Finland.Received 12 May 1983.0003-6935/83/203258-07$01.00/0.

© 1983 Optical Society of America.

can be imposed as follows: to avoid the missing partproblem; to get one range measurement every 10 msec;to measure the distances from 1 to 10 m with a posi-tional accuracy of ±2 cm and resolution of ±5 mm; thesize of the beam spot on the target must be at most 2 cm2

and to construct the mechanical apparatus in a lightcompact housing. From commercial range finders thedevices based on the time of flight method are at bestsuited for the purpose mentioned above. They have aresolution of from ±1 to ±10 cm (Refs. 5 and 6) de-pending on the measuring time (>0.1 sec) and range.

In this study the pulse time of flight method with adiode laser as the light source was chosen. The goal wasto decrease the measuring time to 10 msec by using anefficient analog transit-time-to-amplitude converter,which allows flexibility to vary the measuring time.Even the transit time of single pulses can be measured.Special attention has been paid to the reduction of theeffect of pulse amplitude variations on the transit timemeasurement. Both the variation of the reflectance ofthe object and the distance between receiver and objectcause variations in the amplitude of the detected lightpulse. These variations induce timing walk on the stoppulse detection. To reduce this effect a constant frac-tion (CF) discriminator and a feedback controlledPIN-diode attenuator were combined. The purpose ofthe CF discriminator was to reduce the fast amplitudevariations. The larger and slower amplitude fluctua-tions were limited with the PIN-diode attenuator.

The construction of the range finder is presented.The device was tested in laboratory; results of electricaltests and distance and shape measurements are in-cluded.

II. Range Finder

The block diagram of the range finder is presentedin Fig. 1. It consists of three subsystems: the trans-mitter; the receiver; and the time interval measurementunit.

3258 APPLIED OPTICS / Vol. 22, No. 20 / 15 October 1983

I Time interval measurement unit I

Re ev r t PIN-diode

_ _ _ _ _ ITACIAverage

rlTas Pigtailed |!| vrg

Fig. 1. Electrical block diagram of the range finder.

A. Transmitter and ReceiverThe transmitter consists of a diode laser, a pulser, and

an oscillator. The SH-type diode laser emits 5-nsec(FWHM) light pulses at a wavelength of 900 nm and ata repetition rate of 10 kHz. The diode laser is drivenwith 20-A current pulses, which are generated by uti-lizing the avalanche effect in common small signaltransistors.

The start timing pulse is produced using optical de-tection of the transmitted light pulse, which enables oneto decrease the timing errors caused by the delaychanges in the diode laser. The optical feedback is re-alized by a pigtailed PIN photodiode. The currentsignal of the PIN photodiode is amplified with a 140-MHz transimpedance preamplifier, which was con-structed with discrete components.

The receiver feeds the stop channel of the time in-terval measuring unit and is composed of a silicon ava-lanche photodiode and a transimpedance preamplifier,which is constructed with discrete bipolar junctiontransistors. The transimpedance of the preamplifieris 3.9 k, and the bandwidth is 130 MHz (tr 3 nsec),respectively. The dynamic range of the preamplifierwas found to be -80 dB in normal in-house lighting.

B. Time Interval Measurement UnitThe time interval measurement unit consists of

timing discriminators and a time-to-amplitude con-verter (TAC). The timing discriminators produce ac-curate, logic level, start, and stop pulses for the con-verter. The TAC generates an averaged dc voltage,which is proportional to the time difference between thetiming pulses.

A leading edge (LE) discriminator was used to gen-erate the logic level start pulses because the transmittedpower of the laser diode varies only about ±10% of themean value. In stop pulse generation the discriminatormust be able to handle a wider dynamic range. Boththe distance between the object and the receiver and thereflectance of the subject cause amplitude variations inthe received light pulses.

The distance range of 1-11 m induces amplitudevariations of -1-10. The reflectance of passive objectsalso varies in the range of -1-10, so that the total am-plitude variation is 1-100. The stop timing discrim-inator consists of a feedback controlled PIN diode at-tenuator and a CF discriminator. The purpose of thePIN diode attenuator was to increase the effective dy-namic range of the CF discriminator, which was mea-

sured to be 1-10. The LE discriminator and the CFdiscriminator were constructed with emitter coupledlogic (ECL) comparators.

In the TAC a capacitor is discharged by a constantcurrent from a reference voltage proportionally to thetime difference between timing pulses. The voltagechange of the capacitor is amplified to a positive pulse;thus the converter produces a separate result for everymeasurement. Successive pulses are averaged with asample and hold module and a RC filter r -10 msec).The converter also includes a logic which prevents theconverter from functioning if the time difference be-tween the start and stop pulses exceeds the time rangeof the system, which is -100 nsec. So the output volt-age of the TAC is stable even if the stop pulse is even-tually missing.

C. Optical DesignThe goal of the design was to construct optics for a

range finder measuring distances from 1 to 10 m. Thevariation of the received optical power caused by thedistance range should be minimized, and a SNR of atleast 10:1 would be desirable.

In principle it is possible to use either coaxial orparallel optics. Coaxial optics does not reject themeasurement range, because the optical axes of thetransmitter and receiver are the same. The receivedoptical power is proportional to the inverse of the dis-tance squared. In the distance range of 1-10 m thevariation would be 100-1.

In parallel optics the optical axes are not the samecausing rapid degradation of the received optical powerin very short distances. On the other hand, when theobject is between the range finder and the intersectionpoint of the optical axes the dynamics of the receivedoptical power is reduced compared to the coaxial optics,if the very short distances are neglected.

In Fig. 5(b) it is shown how a transmitter beam (di-vergence, 5 mrad) and a receiver field of view (diver-gence, 12 mrad) can be adjusted to cover the wholemeasurement range of 1-10 m when parallel optics isused. The measured amplitude variations for suchoptics are presented in Fig. 5(a).

The main noise source of a wide field of view receiverwith an avalanche photodiode (APD) as a detector iscaused by background irradiance. The total receivedbackground noise power can be expressed as

PB= M X X Ar X A X T e-+-), (1)

where M is the irradiance (at 900-nm wavelength M =0.45 W/m2nm in normal atmospheric conditions withthe sun at its zenith, 10 mW/m2nm in extreme brightindoor lighting), is the beam divergence of the receiverin steradians (1.1 X 10-4 sr), A is the area of the re-ceiving aperture (1.8 X 10-3 2

), A\)X is the spectralbandwidth (500 nm without filter), T is the transmis-sion of the receiver optics, is the reflectivity of thetarget (0.1 in a worst case), and T is the atmospherictransmission (=1, the worst case).

Calculating the noise power Pb under bright indoorlighting gives a result of 100 nW, which corresponds to

15 October 1983 / Vol. 22, No. 20 / APPLIED OPTICS 3259

Fig. 2. Photograph of the range finder. The sight (top), receiver(middle), and transmitter (bottom) are mounted on the right side of

the base plate; the left side is for electronics.

a noise level of 28.5 pA/H compared with the noise The transmitter, receiver, time interval measurementlevel of 3.5 pA/\/fHz of the receiver preamplifier. If an unit, power supplies, and the sight by which theoptical filter (bandwidth, 20 nm) were to be used, the rangefinder can be aimed at the object are all in thenoise is limited to a level slightly above the preamplifier same case. The dimensions of the case are 200 X 200noise in indoor circumstances. In outdoor circum- X 150 mm. A photograph of the range finder is pre-stances the use of such a filter is a necessity. sented in Fig. 2.

To achieve an SNR of 10:1 the received signal powerP, should be at least 1 W in indoor circumstances. E- TestingThus the minimum optical power from the transmitter The linearity of the time interval measurement unitwith an illuminating area not larger than the object is was tested with pulse generators and a pulse heightgiven by analyzer. The other pulse generator fed pulses at a

Psr X R 2 constant rate to the start channel of the unit and thePT = A (2) other generator Poisson distributed pulses to the stop

r r t t channel of the unit, respectively. The separate outputwhen the atmospheric attenuation is neglected. In this pulses should then produce a uniform distribution fromexpression Tr (09) is the transmission of the transmitter which the linearity of the unit can be calculated. In theoptics, and R is the distance to be measured (10 m is the time range of 70 nsec (-11 m) the differential nonlin-worst case). Other parameters were previously defined- earity of the unit was less than ±2% (channel width, 100From Eq. (2) a transmitter minimum power of 2.2 W is psec), and the integral nonlinearity was less thanobtained. ±0.07%, respectively.

The walk error in the stop pulse generation wasD. Optical and Mechanical Realization measured by connecting adjustable attenuators between

Parallel optics were used. The receiver was fixed the receiver and the stop channel of the time intervalsolidly on the base plate, and the transmitter was fas- measurement unit. The range finder was aimed at atened to an optical mount by which the intersection solid object to get a constant pulse amplitude from thepoint of the optical axes could be adjusted. receiver. The effective distance measurement error is

A laser diode of 5-W peak power (M/A COM Laser- presented as a function of the attenuation in Fig. 3. In

diode LD-62) was chosen to be the light source of the the attenuation range of 0-20 dB the error is caused onlytransmitter. As the emitting area of the laser diode is by the PIN diode attenuator, which was adjusted to150 X 2 pim, a beam divergence of 5 mrad was achieved limit the input pulses to the level of -20 dB. In thewith a plano-convex lens having a focal length of 30 mm. attenuation range of 20-40 dB the walk error is causedWith a diameter of 18 mm the lens collects over 80% of by the CF discriminator. The walk error of the stop

the emitted power. The receiver optics consists of an channel is less than ±4 cm in the 40-dB dynamicaspheric condenser lense (f.l. = 40 mm, 0 = 50 mm) range.giving a divergence of 12.5 mrad, as the diameter of the The resolution of the range finder was measured tophotodetector is 0.5 mm (RCA C30902E). No correc- be -1 mm (FWHM) when averaged with the timetion of the color aberration is needed because the laser constant of 10 msec. The resolution of the single outputdiode emits nearly monochromatic light (AX - 3 nm). pulses was -3-4 mm (FWHM). The temperature

3260 APPLIED OPTICS / Vol. 22, No. 20 / 15 October 1983

coefficient of the range finder was -0.5 cm/C in thetemperature range of 15-251C.

The measured peak power of the laser diode was only1 W, because short driving pulses (5 nsec) were used.The divergence of the transmitted light pulse and thefield of view of the receiver were measured to be 4(HPBW) and 12 mrad. The size of the laser spot at adistance of 10 m was 3 X 40 mm (HPBW).

Ill. System PerformanceThe performance of the designed range finder was

tested against some of the most important requirementsof supporting machine vision: object modeling; rec-ognition; and inspection. The goal of the laboratorytests was to find the positional accuracy in the mea-surement range of 1-10 m and the sensitivity for vari-ations in reflectance and shape of the target. It was alsoimportant to illustrate the capability of the range finderto measure the profile of objects with different kinds ofcontour.

All the tests were made in a laboratory environment.The measured range was read via a digital dc voltmeterand the reference data by using a line tape which wasequipped with a mechanism that reduced the posi-tioning error.

A. Positional Accuracy for Various TargetsThe first task was to find the positional accuracy of

the range finder at a 1-10-m distance and also to studythe effect caused by variations of target reflectance onthe observed range data. A large variety of objects,including a burned brick, a planed piece of wood, a pieceof pine, birch, and spruce logs, a piece of dark stone, awhite paper roller, and a piece of asbestos plate, weremeasured over the whole range with 50-cm steps.

A straight line was adjusted to the observed data ofthe burned brick. In Fig. 4 three curves which presentthe deviations from the adjusted line are depicted. Thecurves are drawn for the data of the planed wood, darkstone, and burned brick. The curve drawn from therange data of the dark stone deviates mostly from theadjusted line to the direction representing too longdistances, and the curve of the planed wood deviatesmostly to the opposite direction. The observed rangedata of all the other samples were between these twoextreme curves.

The reflectance was determined for the sampleobjects measuring the amplitudes of the received lightpulses using an oscilloscope connected at the output ofthe receiver.

The observed amplitude data for some of the sampleobjects are drawn in Fig. 5 as a function of distance.The intensities for the planed wood are higher than thatof magnesium oxide, because the reflectance of theplaned wood is not totally diffuse. The reflectance ofthe dark stone is much weaker than for any otherobjects. The roughness of the surface of the stone alsoaffects the intensity data because reflectance is notperfectly diffuse. The measured amplitudes of thesample objects not shown in the picture were as follows:

5/

5MI

-232

0-1-2-3-4

The CFD gets constantamplitude pulses

. . ....

`/[cm)Attenuation of thePIN-diode is in its

1minimum . -4

.~ ~ ~ ~~~. a...-F 3 ... I -2

I ~~~~~~~01~~~~~~~~~~~~~~-

-2-3-4

. .

0 -6 -12 -20 -26 -32 -40Pulse amplitude dB)

Fig. 3. Effect of variations of the pulse amplitude on the rangedata.

7

6

5

4

3

2

0-1

-2

-3

1 2 3 4 5 6 7

1 2 3 4 5 6 7

~PLANED O7 \ ~BRICK Ra)-7

DARK STONE / 6

32

0

-D3

8 9 10 11 S/Im

Fig. 4. Curves of three sample objects representing the deviationsfrom the fitted straight line to the range data of the brick as a function

of the measured interval.

(a)

(b)0 1 2 3 4 5 6 7 8 9 10 11

'1]Fig. 5. Amplitude variations of the received light pulses caused bythe 1-11-m distance range for different materials (a) and schematic

diagram of the range finder optics (b).

15 October 1983 / Vol. 22, No. 20 / APPLIED OPTICS 3261

1 2 3 4 5 6 7 8 9 10 11

QPLANED WOOD V1 1 1

500 BRICK 500300 , DARK STONE 300200 A_ ®()MgO 200

(5)ASBESTOS

55-1130 2 345789101sm

X - @

a 9 in 11

3in 30 400 420 4.40

355 3.60 365 3.70 ^5

m/m11,0 . . . | | ' | ' ' ' I ' ................ 1.010, (i) PLANED WOOD 08

06 (2) ASBESTOS 060.6 0.4

02 ti / \-02

-0.6 .6-06-0.8 . . -08-1.0 .. I . . . -1,0

3.55 3.60 365 370 S4mI

t-m range interval with 5-cm step size (a) and in the 3.5-3.7-m interval with 1-cm step size (b).

the piece of spruce log was very similar to the asbestosplate; the birch was between brick and magnesiumoxide; and the white paper roller was similar to mag-nesium oxide.

Figures 4 and 5 show how the reflectivity of the targetaffects the range data. When the intensity of the re-ceived laser pulse is very low (-30 to -40 dB in Fig. 3),the measurement error increases sharply as a functionof the measured interval.

It is possible to reduce the error caused by reflectancevariation in two ways: (1) by adjusting the calibrationlines separately to the observed data for each target and(2) by adjusting the transmitted laser power to be highenough for each application. When separate calibra-tions were used the maximum error in the case of theplaned wood was reduced to less than ±1 cm.

The results of this experiment show that the designedrangefinder measures the distances from 1.5 to 10 mwith an accuracy of ±1 cm for an object with constantreflectance and with an accuracy of ±3 cm even if thereflectance of the measured objects varies in the ratioof 1-10.

B. Positional Accuracy for Short Intervals

The accuracy of the range finder for short intervalsdetermines how well the shape of the objects can bemeasured. The positional accuracy for short intervalswas studied in two range intervals. The other intervalwas between 3 and 4 m and the other between 6 and 7m. The accuracy was studied by shifting the targetsstep by step inside these intervals. Two step sizes wereused: 1 and 5 cm. Flat surface objects (a planed piece

of board, asbestos plate) were used as targets to mini-mize the possible error in accuracy due to the local shapevariations.

The results of this experiment show that with thelonger step size (5 cm) the positional accuracy within1 m was ±1 mm, and when using the shorter step size (1cm) the accuracy within 20 cm was ±0.5 mm. Thus therelative error in the former case is 0.1% and in the lattercase 0.25% Sample curves from the observed data areshown in Fig. 6. In addition to resolution there are twoimportant factors which must be taken into accountwhen evaluating the results. The surfaces of the sampletargets are not totally flat, and they have small localvariations in their shape. These variations affect therange data because the illuminating laser beam moveson the surface when the target is shifted along themeasured interval. Second, the positional accuracy ofthe reference data does have small variations whichaffect range results.

C. Effect of Shape and Orientation of the Target onthe Range Data

In the shape measurements it is important that thevariations of the angle between the illuminating beamand surface normal do not affect the observed rangedata. Besides, the location of the beam spot on thesurface must be exactly determined and the size of thespot restricted. The size of the spot of the designedrange finder is 1.2 cm2 (3 X 40 mm) at a distance of 10m.

The goal was to find how the read range data varieswhen objects with different shapes are measured at aconstant average distance. The experiments were madeat two discrete distances in the first and latter half of

3262 APPLIED OPTICS / Vol. 22, No. 20 / 15 October 1983

1,5

1.0

0.5

(a) o-0,5

-1.0

-1.5

ASBESTOS PLATE

"I[mm)1.5

1.0

0.5

0

-0.5

-1.0

-1.5

(b)

Fig. 6. Positional accuracy in the 3.5-4.-

... .... 1-1-1 .......

3,60 UO 4JDO 4.20 4.40 Mml

o WOODEN PLANES( OBSERVED DATA

LASER-BEAM

M* Z=,

RANGE mmi 3600

TRANSITION mii

* 200

.180

-160

-140

120

100

3700

so

60

.20

Fig. 7. Observed range data for the convex and concave corner onthe target surface when the target was moved over the laser beam step

by step (step size, 1 cm).

( CYLINDER SURFACE TRANSITION mm]( OBSERVED DATA IN A HORIZONTAL 2 . 10

DIRECTION l -160© OBSERVED DATA IN A VERTICAL 140

DIRECTION 140

1' 120

LASER- BEAM 100

80

60

40

-~ 20

RANGE mm! 6300 6400

Fig. 8. Observed range data for cylirder target when it was movedover the laser beam in the vertical and horizontal directions.

PLANED

OBSERVED DATA

TRANSITION[ mm]

400

300

200

100

RANGE [mm] 3400 3500 3600Fig. 9. Profile measurement for the target with source and birch logs

fastened on a wooden plate.

the total range. Four different kinds of experimentwere done: the angle between the illuminating beamand a plane surface was varied; objects with sharpconvex and concave corners were shifted over the illu-minating beam; and also objects with cylinder and ballsurfaces were shifted over the source beam.

1. Angle Variations Between Illuminating Beamand Plane Target

The experiments to find the effect of the angle vari-ations between the source beam and the surface normalof the target were done with two sample objects: a plateof planed piece of wood and a plate covered with whitecardboard. The measurements took place at two dis-crete intervals. The angle was varied by turning theplane target around the intersecting point of the illu-minating beam and the surface. This turning was donearound both the horizontal and vertical axis.

The results of this experiment show that in the caseof planed wood the angle variation has a small effect onthe observed range data. In the worst case the observederror was 2 cm at a distance of 6.3 m. This error tookplace when the target plate was turned 750 from theperpendicular position. In the case of cardboard therewas no noticeable effect on the observed data at eithermeasured distance. The results can be explained byreflectivity: the cardboard has diffuse reflectance, butthe reflectance of the planed wood is not totally dif-fused, causing greater pulse amplitude variations.

2. Target with Sharp CornerBecause of the finite dimensions of the spot on the

target surface it was desirable to find out how a sharpcorner on the surface of a target affects the range mea-surements. For this purpose a target was constructedwhich had two planed board faces perpendicular to eachother. The target was shifted over the illuminatingbeam at two discrete distances (3.6 and 6.3 m) so thatboth the convex and concave corners were in turn illu-minated.

The effect of the sharp corner on the range data canbe seen in Fig. 7, where observed range points are de-picted as a function of the shifted distance in the verticaldirection. The range finder integrates the illuminatedarea on the target surface and thus measures the dis-tance shorter or longer at the corner point dependingon what type the corner is-concave or convex. Theerror due to that phenomenon depends on the sharpnessof the corner and was in our experiments smaller than1 cm.

3. Target with Cylinder SurfaceHow a cylinder surface of a target affects the range

measurements was tested with a white paper rollerwhose reflectivity is almost diffuse and the reflectancewas very similar to magnesium oxide (see Fig. 5). Thetarget was shifted over the illuminating beam both inthe horizontal and vertical directions. The results arein Fig. 8.

15 October 1983 / Vol. 22, No. 20 / APPLIED OPTICS 3263

< - < loll l | l l l l l

PLANED

LASER- BEAMI=77Z:47~ =

RANGE mm! 3500 3550 3600

Fig. 10. Profile measurement for the brick lying on a wooden

plate.

From Fig. 8 it can be seen that the cylinder surfacehas such a small effect on the measurement that thediameter of the cylinder can be quite correctly ap-proximated from the observed data (possible errorsmaller than 1 cm), but the shape of a small cylindersuch as in our experiment is more difficult to evaluate.There is also a slight difference in determining the di-ameter depending on which direction the movementwas made. This difference is caused by the profile ofthe source beam.

4. Target with a Ball SurfaceThe ball surface which is curved in two directions was

the third typical surface shape, whose effect on rangemeasurements was studied. The target was a plasticball covered with a thin white paper to make the re-flection diffuse. The ball was moved over the illumi-nating beam also at two discrete distances (3.6 and 6.3m) step by step (step size, 1 cm) in a horizontal direc-tion.

As in the cylinder case the diameter of the ball can bequite correctly determined from the data. At a distanceof 3.6 m the shape of the ball is slightly twisted, but ata distance of 6.3 m the observed range data follow quitewell the contour of the ball. The possible error in de-termining the diameter of the ball from observed datais <1 cm at both measured distances.

D. Profile Measurement for a Couple of Wood Logsand for a Burned Brick

Large scale computer-controlled manipulators canbe applied to manipulate different kinds of heavy ma-terial, such as large metal parts, wood logs, and bricks.

The designed range finder was tested for measuring theprofile of two adjoining wood logs connected on a planedwood plate and the profile of a burned brick lying on thesame type of plate.

Two wood logs, spruce (130-mm diam) and birch(-i59-mm diam), were fastened on a wooden plate veryclose to each other. This target was moved over theilluminating source beam at two discrete distances.The test arrangements with the observed range data aredrawn in Fig. 9 at a distance of 3.6 m. From this figureit can be easily seen how the profiles of each wood logare separated and can be even almost correctly deter-mined.

For testing the brick it was mounted on a woodenplate, which was moved over the source beam perpen-dicular to the beam. The arrangements for this testwith the observed data are shown in Fig. 10. The heightof the brick observed by the range data is smaller thanthe height of the actual brick. This is due to the planedbackground which had better reflectivity than the brick,and this difference in reflectivity affects range data.Also the edges of the observed brick are twisted, but thedimensions of the brick can be quite correctly deter-mined.

IV. Conclusions

In the case of large manipulators a distance mea-surement within the range of -10 m is needed. Thisstudy shows that the time of flight of the light pulsemethod is very well suited for this purpose. By usinga high pulse repetition rate, averaging, and an efficienttime-to-amplitude converter, the information of theposition and orientation of the objects can be extractedfrom continuous distance information. By taking intoaccount the special features in different applications,the performance of the device can be optimized. Thetests in the laboratory environment show that subcen-timeter resolution can be achieved.

References1. R. A. Jarvis, IEEE Comput. COM-15, 8 (1982).2. G. B. Porter III and J. L. Mundy, Proc. Soc. Photo-Opt. Instrum.

Eng. 336, 67 (1982).3. T. Hasegawa, IEEE Trans. Syst. Man Cybern. SMC-12, 250

(1982).4. J. M. Tenenbaum, H. D. Barrow, and R. C. Bolles, "Prospects for

Industrial Vision," in Computer Vision and Sensor-based Robots(Plenum, New York, 1979), pp. 239-259.

5. D. E. Smith, Hewlett-Packard J. 31, No. 6, 3 (June 1980).

6. G. Kompa, "Laser-Entfernungsmesser hoher Genauigkeit fur denindustriellen Einsatz," in Conference Proceedings, Laser 79Opto-Electronics, Munich, 2 July 1979.

The authors would like to thank R. Ahola for helpfuldiscussions in developing the range finder and J. Hon-kala concerning the construction of the device.

The work reported in this paper was supported pri-marily by the Ministery of Trade and Industry.

3264 APPLIED OPTICS / Vol. 22, No. 20 / 15 October 1983