Non-contact sensor, system and method with particular utility for measurement of road profile

Electro-optical apparatus, system and method for measuring distance to a relatively moving surface, such as distance to a road surface from a vehicle frame of reference passing thereover. The apparatus includes a light transmitter for projecting a rectangular beam vertically downwardly onto the road surface. A rotating scanner includes a circumferential array of facets for sequentially receiving the image diffusely reflected from the road surface and reflecting such image through a rectangular reticle onto a photodetector. Reference beams are sequentially reflected by the road image-reflecting scanner facets onto a reference detector. Distance to the road surface is then determined as a function of the angle of incidence of the road image onto the scanner by comparing the time of incidence of the road image to the times of occurrence of the reference reflections on the reference detector.

The present invention relates to distance measuring apparatus and methods, 
and more specifically to non-contact methods and systems for measuring 
surface profile. Yet more specifically, the invention relates to an 
apparatus, system and method for measuring the distance from a vehicle 
frame of reference to a road surface beneath the vehicle as the vehicle is 
driven over the road surface at normal traffic speeds. 
BACKGROUND AND OBJECTS OF THE INVENTION 
As applied specifically to measurement of road surface profile, a variety 
of transducers or sensors have heretofore been proposed for measuring the 
distance between the road surface and a vehicle frame of reference driven 
thereover. For example, Spangler et al U.S. Pat. No. 3,266,302 employs a 
potentiometer carried by a trailer and responsive to movement of the 
trailer suspension system with respect to the trailer frame as the trailer 
is drawn over the road surface in question. This distance signal is 
combined in Spangler et al with the twice-integrated output of an 
accelerometer carried by the trailer for providing an overall measurement 
of road profile. Non-contact sensors, such as ultrasonic sensors, have 
been proposed for replacement of the potentiometer in the basic Spangler 
et al system, but have not appreciably improved the reliability and 
accuracy of the frame-surface distance measurement. 
One important object of the present invention, therefore, is to provide a 
non-contact sensing apparatus, system and method which finds particular 
utility in the measurement of road profile, and which is adapted for 
improved reliability and accuracy as compared with comparable systems and 
methods of the prior art. 
A more general object of the invention is to provide non-contact distance 
measuring apparatus, systems and/or methods which embody improved economy, 
reliability and accuracy in assembly and use. In furtherance of the 
foregoing, a more specific object of the invention is to provide an 
improved electro-optical scanner and system for measuring the profile of a 
relatively moving surface. 
SUMMARY OF THE INVENTION 
Briefly stated, the apparatus in accordance with the invention comprises a 
light source and projection system for focusing a rectangular light beam 
onto a road surface beneath a moving vehicle. An optical receiver is 
spaced from the light source on the vehicle and receives an image of the 
rectangular beam diffusely reflected from the road surface. The optical 
receiver is coupled to electronics for effectively measuring the distance 
between the vehicle frame of reference and the road surface as a function 
of angle of incidence of the reflected beam onto the receiver. 
The optical receiver includes a rotating scanner comprising a plurality of 
plane reflective surfaces mounted in a circumferential array around the 
scanner axis of rotation. As the scanner rotates, each reflective surface 
in turn deflects the road image through a reticle onto a photodetector. 
Thus, the angle of incidence of the reflected road image may be 
effectively determined as a function of the angle of rotation of the 
scanner at the moment at which the road image is reflected onto the 
photodetector. 
To affect the latter determination, the optical receiver further includes a 
reference system comprising an optical transmitter and photodetector 
disposed in respective fixed positions on opposite sides of the scanner 
plane of rotation. The reference transmitter projects a pair of 
rectangular beams onto each facet of the scanner in turn as the scanner 
rotates. Each scanner facet reflects the reference beams successively 
through a reticle onto the reference photodetector. The reference beams so 
reflected establish a measurement window corresponding to respective 
angular positions of the scanner and within which the road image is 
received. The distance between the vehicle frame of reference and the road 
surface is then determined for each reflected road image as a function of 
the time-position of the detected road image within the measurement 
window. Most preferably, the reference beams are directed onto the same 
scanner facet as will reflect the road image within the corresponding 
measurement window. 
In accordance with a particularly important feature of the apparatus of the 
invention previously described, the reticle through which the road image 
is reflected and focused onto the corresponding photodetector is 
dimensioned to be identical to or congruent with the focused and reflected 
road image at the effective midpoint of the measurement window. Likewise, 
the reticle through which the reference beams are reflected and focused 
onto the reference photodetector is dimensioned to be congruent with the 
incoming reference beams. The nominal photodetector output for each beam 
therefore is the convolution integral of each rectangular beam image and 
corresponding reticle, having the waveform of an isosceles triangle. Peak 
detectors are employed for reliable detection of the times of occurrence 
of each waveform. 
The system of the invention includes process circuitry for receiving 
signals from the photodetectors indicative of the road and reference 
image, determining the temporal relationship therebetween--i.e. the 
time-position of the road image within the reference measurement 
window--and calculating distance to the road surface. The distance 
measurement may be fed to a storage device such as strip chart recorder or 
an electronic memory strobed or advanced by a signal from a wheel 
transducer or the like as a function of vehicle travel. Alternatively, the 
distance between the vehicle frame of reference and the road surface may 
be employed for real time determination of road profile as the signal 
"W-Y" in the system disclosed, for example, in the above-referenced 
Spangler et al patent.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings, and particularly FIGS. 1-7, a presently 
preferred embodiment of the apparatus 10 in accordance with the invention 
illustrated therein comprises a base plate 12 (FIGS. 1 and 2) which is 
adapted to span an opening 14 in a vehicle frame 16 and to be rigidly 
affixed thereto (by means not shown) so that the vehicle frame will 
effectively constitute a reference (hereinafter termed the "frame of 
reference") in connection with which all measurements are taken. The 
vehicle of which frame 16 is illustrated fragmentarily in FIG. 1 may 
comprise a separate towed vehicle or, more preferably, a special purpose 
truck or van in the floor of which opening 14 is formed so as to adapt the 
van for measuring road profile in accordance with the invention. Although 
the invention will be described in connection with a presently preferred 
application thereof in the field of road surface profile measurement, it 
will be appreciated that the principles of the invention in their broadest 
aspects may find application in other fields where it is desired to 
measure the distance to or profile of a surface which moves relative to 
another frame of reference. 
A first light source 18 (FIG. 1) is mounted on base plate 12 and includes 
an incandescent lamp 20 carried within a downwardly directed concave 
reflector 22. A plate 24 (FIGS. 1 and 5) is mounted beneath reflector 22 
by the clamp 26 and screw 27, and has an elongated central rectangular 
opening or slot 28. A focusing lens assembly 30 is mounted beneath slotted 
plate 24 coaxially with reflector 22 for projecting that portion of the 
light beam received from lamp 20 and slot 28 through a glass plate 32 and 
an opening 34 in base plate 12 onto the road surface. Lamp 20, reflector 
22, slotted plate 24, lens assembly 30 and glass plate 32 are all mounted 
in fixed position on a rack 36 and affixed as a subassembly to base plate 
12. Lens assembly 30 is adjustable in the usual manner for focusing the 
rectangular road-illuminating beam onto the road surface as will be 
described. Lamp 20 is connected to a source of energizing power by means 
not shown. 
An optical receiver 38 (FIGS. 1 and 2) is carried by base plate 12 within a 
sealed enclosure 40. Receiver 38 includes an optical scanner 42 which 
comprises a wheel 44 on which is mounted a peripheral and circumferential 
array of plane reflective surfaces or mirror facets 46. Wheel 44 is 
rotated continuously in operation about its central axis 48 by the motor 
drive mechanism 50. Scanner 42 is mounted to base plate 12 by the bracket 
51 so as to locate scanner axis of rotation 48 in fixed position 
(following initial set-up) spaced from the axis of the road-illuminating 
beam, perpendicular to the direction of such spacing and parallel to the 
longitudinal dimension of slot 28. In a preferred embodiment of the 
invention, scanner 42, including wheel 44 and motor 50, comprises a model 
1816 POLYSCAN Scanner Assembly marketed by Scanco Inc. of South Norwalk, 
Conn., having 16 facets or reflecting surfaces 46 mounted in a 
circumferential array around the wheel periphery, with the plane of each 
reflecting facet being tangential to wheel axis 48. The scanner wheel 
rotates at 1800 RPM. 
A glass plate 66 (FIG. 1) spans an opening 64 in baseplate 12 for admitting 
a portion of the illuminating beam diffusely reflected by the road 
surface. An optical receiver assembly 60 (FIGS. 1, 2 and 4) is mounted by 
a bracket 62 with the receiver axis lying in the central plane of rotation 
of scanner 42 for receiving successive road image beams as reflected in 
turn by scanner facets 46. As best seen in FIG. 4 (and schematically in 
FIG. 8), receiver assembly 60 includes a photodetector 68 mounted on an 
insulating sleeve 70 which is held within a detector housing 72 by the set 
screw 73. A first focusing lens 74 is captured by sleeve 70 against an 
opposing shoulder 75 within housing 72. A second focusing lens 76 is 
adjustably carried by a sleeve 77 within the threaded bore 71 of housing 
72 and is held therein in adjusted position by the threaded jam ring 79. 
A mask or reticle 78 (FIGS. 4 and 6, and schematically in FIG. 8) is 
positioned between lens 74 and photodetector 68, and comprises an opaque 
disc 80 having a rectangular transparent region 82 extending diametrically 
thereacross. The longitudinal dimension of open region 82, which may also 
be termed a "slot" in the optical sense, is parallel to the longitudinal 
dimension of slot 28 in plate 24 (FIG. 1). Preferably, reticle 78 
comprises a photolithographically reproduced mask adhered to the face of 
detector 68. Reticle "slot" 82 is dimensioned to be congruent with the 
road image at the focal distance of lens assembly 30 (FIG. 1) as reflected 
by scanner facets 46 and focused by lenses 74,76. Detector 68, which is 
shown schematically in FIG. 13, preferably comprises a silicon diffused 
guard ring photodiode. 
A reference optical system 83 (FIGS. 1-3 and 7, and schematically in FIGS. 
9-10) is carried within enclosure 40 by bracket 62. Reference system 83 
comprises an optical transmitter assembly 84 and a receiver assembly 86 
disposed in fixed complementary positions on opposite sides of the plane 
of rotation of scanner 42 (as best seen in FIGS. 2 and 9) and with 
respective axes in reference plane 103 (FIGS. 8 and 10). Transmitter 
assembly 84 comprises a lamp 88 (FIG. 3) mounted by a jam nut 89 within 
the threaded end of a hollow housing 90. A reference mask 96 is held 
within housing 90 by the set screw 91 and cooperates with a sleeve 93 to 
capture a first focusing lens 92 against an opposing internal housing 
shoulder. A second focusing lens 94 is adjustably carried by a sleeve 95 
within the threaded bore 97 and is held therein in adjusted position by 
the threaded jam ring 99. Power is applied to the various motor and optic 
elements, and signals are fed therefrom to external electronics (FIG. 12) 
through the array of connectors 101 in FIG. 2. 
Mask 96 comprises an opaque disc 98 (FIG. 7 and schematically in FIGS. 
9-10) having a pair of transparent rectangular regions 100,102 extending 
transversely thereacross at opposite angles psi (FIG. 7) with respect to 
the disc diameter 104. Disc 98 is photolithographically produced and 
adhered onto a cylindrical glass substrate 105 (FIG. 3). The purpose of 
the paired slots 100,102 and the angles psi will be discussed hereinafter. 
Reference receiver assembly 86 is identical to road image receiver 
assembly 60 previously discussed. The reference receiver photodetector, 
mask and focusing lenses are illustrated schematically at 114,116,118 in 
FIG. 9. The axes of the reference receiver and transmitter assemblies, the 
longitudinal dimension of the transparent region in mask 118 and the 
diameter 104 of mask 96 are all coplanar at 103 (FIGS. 8 and 10) with 
scanner axis 48. 
Operation of the optical portion of the apparatus to the extent thus far 
described, as well as additional structural details thereof, will be 
discussed in connection with FIGS. 8-11 of the drawings. In FIG. 8, which 
is a schematic diagram of the road imaging optics, and in FIGS. 9-10, 
which are schematic diagrams of the reference optics, the schematic 
representations of optical elements previously described are identified by 
correspondingly identical reference numerals. Turning first to FIG. 8, 
lamp 20 and reflector 22 cooperate with slot 28 and lens assembly 30 to 
project a road illuminating beam on the axis 110 vertically downwardly 
from the vehicle frame of reference onto the road surface. As will be 
described hereinafter, the road image and reference optics cooperate to 
measure distance between the frame of reference, which is a horizontal 
line or plane 103 (FIG. 8) passing through the scanner axis 48, and the 
road surface between maximum and minimum limits H.sub.1 and H.sub.2. 
Preferably, illumination lens assembly 30, which may comprise a 50 mm 
Vivitar camera lens having an aperture adjustable from f16 to f2.0, is 
adjusted to focus the illumination at a distance H.sub.F midway between 
limits H.sub.1,H.sub.2. The illumination pattern at the road surface is a 
rectangle, having a longer dimension laterally of the direction of vehicle 
travel 52 (FIGS. 1, 5 and 8) and a shorter dimension in the direction of 
travel. 
A portion of the illumination beam is diffusely reflected by the road 
surface toward scanner 42, and is thence reflected in turn by each scanner 
facet 46 onto detector 68 as previously described. Preferably, the 
longitudinal distance L between beam axis 110 and the image-reflecting 
facet at the point of perpendicularity with the frame of reference, i.e. 
point D, is adjusted so that the angle of incidence of a road image from 
focal distance H.sub.F to point D is 45.degree.. 
In the arrangement of FIG. 8, an unknown distance H to be measured between 
the frame of reference and the road surface is given by the equation: 
##EQU1## 
where L is the distance from axis 110 to point D, R is the radius of 
scanner 42 measured perpendicular to the planes of facets 46, and theta-R 
is the angle of such radius at the image-reflecting facet to reference 
plane 103 at the time of incidence of the road image onto photodetector 
68. L and R are constants, so that the distance H to the road surface may 
be measured as a function of angle of incidence by determining the angle 
theta-R of the reflecting scanner facet at the time of incidence. 
Turning to FIGS. 9 and 10, reference optical system 83 cooperates with the 
facets of scanner 42, specifically the particular facet 46 which is moving 
into position for road image reflection, to define a measurement window 
within which the road image signal is received. More specifically, two 
rectangular reference beams are projected from lamp 88 through reticles or 
"slots" 100,102 and lenses 92,94 onto each facet in turn as scanner 42 
rotates. With scanner 42 rotating in the direction 43 (FIGS. 1 and 10), 
the rectangular reference beam through slot 102 will be incident on 
reference detector 114 when the reflecting facet 46 is at an angle theta-1 
with reference plane 103, which angle corresponds to the maximum 
measurable height H.sub.1 (FIG. 8). The rectangular reference beam through 
slot 100 is next incident on detector 114 when the reflecting facet 46 is 
at angle theta-2 (phantom in FIG. 10) to plane 103, which angle 
corresponds to the minimum height H.sub.2 (FIG. 8). 
As previously described, the reticle in mask 118 at reference detector 114 
is horizontal. To insure that the rectangular reference beams incident 
thereon are also horizontal, and thereby achieve the convolution 
integration feature previously described, the slots 100,102 in mask 96 
must be angulated with respect to the horizontal. The angles psi (FIG. 7) 
of each slot 100,102 on opposite sides of the horizontal diameter 104 are 
given by the equation: 
##EQU2## 
where theta is either theta-1 or theta-2 (which are equal to each other in 
this configuration) for the desired maximum and minimum heights 
respectively (FIG. 8), and alpha is given by the equation: 
EQU .alpha.=tan.sup.-1 (1-2 sin .theta. sin .theta.). (3) 
Thus, two reference signals are successively incident on reference detector 
114 as each scanner facet 46 passes the reference optics. Because the 
image of each reference slot 100,102 seen at detector reticle 118 is 
congruent therewith, passing of the reflected image across the detector 
reticle performs a convolution integration operation which yields a 
waveform at the detector output in the form of an isosceles triangle. 
These successive reference signals are illustrated at theta-1 and theta-2 
in FIG. 11, and are separated in time by an amount corresponding to the 
available measurement window. The road image theta-R (FIG. 11) is received 
at detector 68 (FIG. 8) during this time. Theta-R, the unknown in equation 
(1), is given by the equation: 
##EQU3## 
where theta-1 and theta-2 are constants as previously described, 
t(.theta..sub.2 -.theta..sub.1) is the time interval between the theta-1 
and theta 2 reference pulses, and t(.theta..sub.R -.theta..sub.1) is the 
time interval between the first reference pulse and the road image. These 
time intervals are illustrated in FIG. 11. 
FIG. 12 illustrates a presently preferred embodiment of an electronic 
system 119 for measuring road surface distance H per equation (1) as a 
function of time intervals t(.theta..sub.2 -.theta..sub.1) and 
t(.theta..sub.R -.theta..sub.1), and FIGS. 13-15 illustrate various 
details of system 119. Road image detector 68 is connected in FIG. 12 
through a preamplifier 120 (to be described in detail in connection with 
FIG. 13) and a peak detector 122 to provide an input A to a sequence 
controller or sequencer 140 upon occurrence of the peak or tip of the road 
image theta-R (FIGS. 11 and 12) signal. Likewise, reference detector 114 
is connected through a preamplifier 142 and a peak detector 144 to provide 
an input T to sequencer 140 at the peak of each of the theta-1 and theta-2 
reference signals. Sequencer 140 receives a clock input from an 8.7 MHz 
oscillator 148 which, in the optics configuration thus far described, 
provides a resolution of 0.0015 inches. 
As will be described in greater detail hereinafter, sequencer 140 operates 
to provide an output r to reset a pair of digital counters 150,152. 
Sequencer 140 thereafter provides a clocked output a to the counting input 
of counter 150 from the time of occurrence of the theta-1 signal to the 
theta-R signal. Likewise, sequencer 140 provides a clocked output b to 
counter 152 between the theta-1 and theta-2 signals. Thus, upon occurrence 
of the theta-2 signal, counters 150,152 have counts therein respectively 
indicative of t(.theta..sub.R -.theta..sub.1) and t(.theta..sub.2 
-.theta..sub.1). These counts are loaded into respective latches 154,156 
under control of a latch signal 1 from sequencer 140. The latch outputs 
are connected through respective digital-to-analog converters 158,160 to 
an analog divider 162 which provides an output indicative of the 
time-fraction in equation (4). A microprocessor 164 receives the output of 
divider 162, signals indicative of constants .theta..sub.1, .theta..sub.2, 
L and R, and computes H per equations (4) and (1). The road distance 
signal H is then fed to a suitable storage device 166, such as a memory 
device or a strip chart recorder, which receives a strobe input signal 
from a suitable vehicle position transducer 168, such as a code disc at a 
selected vehicle wheel. 
Referring now to FIG. 13, preamplifier 120 shown in block form in FIG. 12 
comprises an operational amplifier 170 connected as shown to road image 
photodetector 68 so as to receive therefrom an electrical input signal as 
a function of total light incident thereon. As previously described, an 
important feature of the invention lies in the provision of a reticle at 
the detector which is congruent with the road image focused thereon. Thus, 
as the rectangular road image scans the rectangular detector reticle as 
scanner 42 rotates, the output of detector 68 and amplifier 120 takes the 
form of an isosceles triangle illustrated at theta-R. Reference 
preamplifier 142 (FIG. 12) is identical to preamplifier 120 (FIGS. 12 and 
13). 
FIG. 14 illustrates details of peak detector 122 shown in block form in 
FIG. 12, and also illustrates voltage/time waveforms that appear at 
various points of the detector circuitry. Peak detector 122 includes a 
zero crossing detector 123 which comprises a differentiator 124 receiving 
an input from preamplifier 120, and a gate 126 which directs the output of 
differentiator 124 to sequencer 140 (FIG. 12). Gate 126 receives an 
enabling input from a threshold detector 128 which is connected to 
preamplifier 120 by the amplifier 130. 
In operation of peak detector 122, the output of differentiator 124 
switches from a low to a high state as the slope of the triangular input 
theta-R switches from positive to negative, i.e. at the time Z of the 
signal peak. To avoid ambiguity, the differentiator output is gated to 
sequencer 140 (FIG. 12) only when threshold detector 128 indicates 
occurrence of a detector output. That is, detector 128 receives a minimum 
threshold signal, corresponding to voltage level 136, from a factory-set 
variable resistor 134, and enables operation of gate 126 only during the 
time X.sub.1 to X.sub.2 when the theta-R signal exceeds this threshold. 
Thus, the peak detector output to sequencer 140 is normally high, switches 
low at time X.sub.1 when the theta-R input exceeds threshold 136 and the 
output of differentiator 124 is still low, and then again switches high at 
time Z, the latter being detected by sequencer 140. At time X.sub.2 when 
the theta-R input declines below threshold 136, the peak detector output 
is disenabled and remains high. Reference peak detector 144 (FIG. 12) is 
identical to detector 122 (FIGS. 12 and 14). 
A particularly important advantage of the optical convolution and pulse 
detection technique in accordance with the invention will be appreciated 
with reference to FIG. 14. More specifically, it may be expected that less 
than optimum road surface quality will normally be such that the intensity 
of the reflected road image seen by road signal detector 68, and the 
electrical output therefrom, may be less than the optimum sharp triangular 
waveform, and thus may assume the degraded quality illustrated in phantom 
in FIG. 14. However, zero crossing detector 123 cooperates with threshold 
detector 130 in a manner illustrated in phantom identical to that 
previously described so as to place the one-going output at time Z from 
pulse detector 114 at the same time under degraded condition as would be 
the case with a preamplified detector input signal of optimum nominal 
triangular configuration. 
The operating control sequence of sequencer 140 (FIG. 12) will be best 
understood with reference to the sequencer state diagram of FIG. 15. In 
particular, starting in State 0, the a, b, r and 1 outputs to counters 
150,152 and latches 154,156 are at a zero or non-enabling state. Upon 
receipt of a T input from pulse detector 144, indicating receipt of either 
a theta-1 or a theta-2 reference signal, the sequencer advances to State 1 
wherein the clocked a and b outputs are fed to counters 150,152 to 
initiate the respective counting operations therein. The r and 1 outputs 
remain low or off. Receipt of an A input from pulse detector 122, 
indicative of receipt of a road image theta-R signal, switches sequencer 
140 from State 1 to State 2 wherein the a output to counter 150 is 
terminated for ending the counting operation therein. The b output to 
counter 152 is maintained high so as to continue the latter counting 
operation. Receipt of a second T input to the sequencer, indicative of the 
receipt of the theta-2 reference signal at detector 114, switches the 
sequencer to State 3 wherein the b output goes low and the counting 
operation in counter 152 is therefore terminated. 
After an automatic delay in State 4, the sequencer advances to State 5 
wherein the 1 output to latches 154,156 is switched on so as to store 
therein the counts in counters 150,152 respectively. These counts are fed 
to divider 162 and microprocessor 164 as previously described. In the 
meantime, the sequencer is automatically cycled to State 6 wherein the r 
output is fed to counters 150,152 for resetting the respective counters in 
preparation for the next operating sequence, and the sequencer is returned 
to State 0. 
It may occur, particularly during initial startup, that a theta-R or A 
input to sequencer 140, indicative of receipt of a road image, will occur 
while the sequencer is in State 0 before receipt of a reference or T 
input. In such an event, the sequencer is stepped from State 0 to State 7 
wherein all outputs are held low or off awaiting receipt of the first T 
input from the reference pulse detector. The first such T input following 
an A input will be indicative of receipt of the theta-2 reference signal, 
and will advance the sequencer to State 6 wherein the counters are reset, 
and then to State 0 wherein all outputs are off awaiting receipt of the 
next or theta-1 indicating T input signal. It is also possible following 
receipt of such first T input signal and advancement of the sequencer to 
State 1 that the next input will also be a T or reference input. Such an 
occurrence may take place, for example, where the road signal is lost in a 
crack or pavement expansion joint, or during initial startup where the 
first signal received is the theta-2 indicating T signal. In either event, 
the sequencer is automatically advanced to State 6 wherein the counters 
are reset, and thence to State 0 awaiting the next or theta-1 indicating T 
input signal. 
It will thus be appreciated that there has been disclosed a distance 
measuring apparatus, system and method which finds particular utility in 
the environment of road profilometry, and which otherwise fully satisfies 
all of the objects and aims previously set forth. However, many 
alternatives, modifications and variations are contemplated. In a 
presently preferred embodiment of the invention, the transmission optics 
are adapted to focus the road illuminating beam vertically downwardly to 
illuminate a patch of road surface six inches long laterally of the 
direction of travel and 0.1 inch long in the direction of travel at focal 
height H.sub.F (FIG. 8). The six-inch lateral dimension approximates the 
width of a typical passenger car tire tread. The 0.1 inch dimension 
permits measurement of small variations in the direction of travel, such 
as cracks or expansion joints. The apparatus measures average distance to 
the illuminated rectangle, and the six-inch width insures that the beam 
will not be lost or yield a false reading due to a pebble hole or the 
like. 
It is also possible that the illumination beam could be other than 
vertically directed onto the road surface. Such a modification could be 
advantageous under some circumstances, particularly where the angle of 
incidence is equal to the angle of reflection from the road, which would 
provide for maximum reflection to the scanner. However, this technique 
would complicate calculations because the illuminated road patch would 
move both horizontally and vertically with height, and is not preferred. 
In a preferred embodiment of the invention, focal height H.sub.F is 
eighteen inches, and heights H.sub.1 and H.sub.2 are twenty-one and 
fifteen inches respectively. Although the road image is out of focus at 
the maximum and minimum measurable heights, the ability of the peak 
detector arrangement (FIG. 14) to tolerate substantial degredation in road 
image provides reliable operation. The 45.degree. angle of incidence at 
scanner 42 from focal height H.sub.F is preferred because sensitivity 
given by the equation: 
##EQU4## 
is maximum at this angle. Other nominal or focal angles of incidence may 
be used. 
It will be recognized that reference source reticle 96 (FIGS. 9 and 10) and 
reference detector reticle 118 are interchangeable. Likewise, it will be 
appreciated that the reference reticle angle psi (FIG. 7) depends upon 
theta-1 and theta-2, which in turn depend upon desired maximum and minimum 
distances H.sub.1 and H.sub.2. In the above-described preferred 
embodiment, psi is equal to 4.4.degree.. As a modification to FIG. 12, it 
is contemplated that latches 154,156 may be connected directly to 
microprocessor 164 and thereby eliminate any requirement for D/A 
converters 158,160, divider 162 and the requisite A/D converter at the 
input of microprocessor 164.