Optical measurement system for determination of an object's profile or thickness

An optical measurement system for determination of a profile or thickness of an object includes first and second optical heads directing first and second light beams, respectively on first and second points on the surface of the object. Photo-sensors are included respectively in the first and second optical heads for receiving reflected lights from said first and second points and providing first and second outputs which varies in proportion to perpendicular distances from a reference plane to said first and second points on the object's surface. The first and second outputs are transmitted selectively to a single processing circuit through a switch. The processing circuit operates to process the first and second outputs in sequence to measure by triangulation the perpendicular distance of the first and second points from the reference plane and to analyze a surface or thickness of the object based upon thus measured perpendicular distances. With the use of the single processing circuit, the first and second outputs can be processed in the identical conditions to enable reliable determination of the perpendicular distances of the first and second points from the reference plane and therefore accurate analysis of the surface profile or the thickness of the object.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is directed to an optical measurement system for 
determination of an object's profile or thickness, and more particularly 
to such an optical measurement system using two optical heads directing 
individual light beams to different points on the object's surface to 
measure distances of these points from a reference plane by triangulation 
for analyzing the surface profile or the thickness of the object based 
upon the measured distances of the two points on the object's surface. 
2. Description of the Prior Art 
In order to obtain a depth or height of a step on the surface of an object 
or thickness of an object by optical triangulation measurement, it has 
been proposed to use a pair of optical heads disposed to direct individual 
light beams to different points on the object's surface for measuring the 
positions of these points. The distances of these points are processed by 
triangulation and are analyzed to determine the object profile. For 
example, when the two heads are disposed to measure the positions of the 
points spaced along the object's surface for measuring individual 
perpendicular distances to the surface from a reference plane, the 
difference of the measured distances gives the height or depth of a step 
existing between these two points. On the other hand, when the optical 
heads are disposed on the opposite of the object to measure like 
perpendicular distances of the positions of two points on the opposite 
surfaces of the object from a reference plane selected to be within the 
thickness of the object, the addition of the measured distances gives a 
thickness of the object at these points. 
In such optical measurement systems, the optical head is normally designed 
to have a photo-sensor which receives the light beam reflected on a point 
on the object's surface and provides an output which varies in proportion 
to the perpendicular distance of the point from a reference plane selected 
to be generally parallel to the object's surface. The output from the head 
is processed in an associated signal processing circuit so as to determine 
a true distance of the point from the reference plane. In this connection, 
when the two heads are connected to the individual signal processing 
circuits, there is a potential problem that the distances measured in 
these separate processing circuits may include individual deviations or 
discrepancies due to inherent variations in the circuits, for example, 
deviations in the temperature characteristics of certain elements 
consisting the circuits. Since these discrepancies are inherent to the 
individual circuits, they are difficult to be compensated for in obtaining 
the step in the object's Surface and the thickness of the object. Thus, no 
reliable analysis is not expected in this system having two optical heads 
connected respectively to the individual processing circuits. 
SUMMARY OF THE INVENTION 
The above problem has been eliminated in the present invention which 
provides an improved optical measurement system for determination of a 
profile or thickness of an object. The system includes a pair of first and 
second optical heads and a single processing circuit which is responsible 
for processing the outputs from the first and second optical heads for 
surface or thickness measurement of a target object. The first and second 
optical heads include individual light projectors directing first and 
second light beams respectively to first and second points on the object's 
surface and include individual photo-sensors receiving the correspondingly 
reflected lights from the first and second points and providing first and 
second outputs, respectively which vary in proportion to a perpendicular 
distance from a reference plane to the first and second points on the 
object's surface. A switch is included in the system to selectively 
connect the first and second outputs from the first and second optical 
heads to the single processing circuit. The processing circuit operates to 
process the first and second outputs in sequence to measure by 
triangulation the perpendicular distance of the first and second points 
from the reference plane and to analyze a surface profile or thickness of 
the object based upon thus measured perpendicular distances. With the use 
of the single processing circuity to commonly process the first and second 
outputs from the first and second optical heads, the positions or 
perpendicular distances of the first and second points can be obtained 
through the identical triangulation processing, which gives reliable 
measurements for the perpendicular distances of the first and second 
points, thereby assuring correspondingly reliable determination of the 
surface profile or thickness of the object based upon thus measured 
perpendicular distances. 
Accordingly, it is a primary object of the present invention to provide an 
improved optical measurement system which is capable of assuring reliable 
determination of the surface profile or thickness of the object. 
In a preferred embodiment, the first and second optical heads are 
controlled to project the first and second light beams as pulse modulated 
ones in sequence such that only one of the first and second light beams is 
directed to the object surface at a time. With this result, it is readily 
possible to avoid any interference between the first and second light 
beams, in addition to well discriminate the light beam from the optical 
heads from a background illumination. Thus, more reliable determination 
can be achieved without suffering from interference between the light 
beams from the first and second optical heads and from the background 
illumination, which is therefore another object of the present invention. 
In another preferred embodiment, the first and second optical heads are 
controlled to project first and second light beams which are pulse 
modulated to have different oscillating frequencies from each other. With 
this scheme, it is also possible to avoid interference between the light 
beams of the first and second optical heads as well as from the background 
illumination, yet without requiring a sequence control of directing the 
light beams in sequence from the first and second optical heads, which is 
therefore a further object of the present invention. 
The processing circuit includes a calibrator which compensates for 
variations in the perpendicular distances measured respectively with 
respect to the first and second outputs when directing the first and 
second light beams to the first and second points selected on an optical 
flat plane parallel to the reference plane. Thus, possible misalignment 
between the first and second optical heads can be readily compensated for 
to thereby improve measurement reliability, which is therefore a still 
further object of the present invention. 
Preferably, each of the modulated first and second light beams is 
configured to have high and low levels alternating to each other so that 
the corresponding one of the first and second optical heads produces high 
and low level values with respect to each of the first and second outputs. 
Thus obtained high and low level values are processed in the processing 
circuit to obtain a difference therebetween. The difference is used in the 
processing circuit as a true value for each of the first and second 
outputs to measure the perpendicular distance of each of the first and 
second points. In this scheme, therefore, the system can successfully 
cancel any errors resulting from background illumination as well as from 
variations in the characteristics of the elements forming the processing 
circuity. 
It is therefore another object of the present invention to provide an 
improved optical measurement system which is capable of assuring reliable 
measurement substantially free from being influenced from the background 
illumination and characteristic variations in the elements forming the 
processing circuit. 
The processing circuit is also configured to invalidate the measurement of 
the first and second distances when the high level value exceeds a 
predetermined maximum level or the low level value falls below a 
predetermined minimum level. That is, when the high and low level values 
are out of a workable range given to the processing circuit, the system 
itself can acknowledge erroneous measurement and disregard the measured 
results for retaining reliable measurement, which is therefore a still 
more object of the present invention. 
These and still other objects and advantageous features of the present 
invention will become more apparent from the following description of the 
preferred embodiments when taken in conjunction with the attached drawings 
.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
First Embodiment 
FIGS. 1 To FIG. 4 
Referring now to FIG. 1, there is shown an optical measurement system in 
accordance with a first embodiment of the present invention. The system 
includes two optical heads, namely, first optical head 10 and second 
optical head 20, and a processing circuit 40 connected through a switch 31 
to the first and second optical heads 10 and 20. The first and second 
optical heads 10 and 20 are provided to measure two positions on an 
object's surface for determination of the object's surface. In FIG. 1, the 
system is adapted to determine a depth D of a step in the object surface 
in such a manner as to measure the positions or perpendicular distances 
D.sub.1 and D.sub.2 of two spaced points from a reference plane PLref by 
the individual optical heads 10 and 20 and obtain the difference between 
D.sub.1 and D.sub.2 as the depth D of the step (D=D.sub.1 -D.sub.2). Such 
measurement or determination of the step's depth is usually made with the 
object running in along the length of the step or in the direction 
perpendicular to the sheet relative to the first and second optical heads 
10 and 20 which are normally fixed. The lateral distance between the 
optical heads 10 and 20 are suitably selected to measure the positions of 
the object's surface on both side of the step. In this connection, the 
heads 10 and 20 are supported on a suitable frame (5) and are held movable 
relative to each other in order to adjust the lateral distance. Further, 
the heads 10 and 20 are movable toward and away from the object surface 
for fine position adjustment. The system can be also adapted to determine 
the thickness of the object with the first and second optical heads 10 and 
20 disposed on the opposite sides of the object, as shown in FIG. 4. In 
this case, the reference plane PL.sub.ref is selected within the thickness 
of the object such that the first and second optical heads 10 and 20 can 
measure the distances D.sub.1 and D.sub.2 of the points on the opposite 
surfaces from the reference plane PL.sub.ref. Whereby, the thickness T can 
be determined by addition of D.sub.1 and D.sub.2 (T=D.sub.1 +D.sub.2). In 
either application, the depth of the steps and the thickness of the object 
can be continuously determined so as to check variations thereof along one 
dimension of the object. 
The first and second optical heads 10 and 20 are of the identical 
configuration and each comprises a laser generating element 11,21 driven 
by a common oscillator 50 through an amplifier 12,22 to generate a laser 
beam which is directed through a projector lens 13,23 to an object's 
surface, a semiconductor position sensing device [PSD] 14,24 receiving a 
laser beam reflected from the object surface through a collector lens 
15,25. As shown in FIG. 2, PSD 14,24 has an elongated light receiving 
surface with a pair of terminals T.sub.1 and T.sub.2 at the opposite 
lengthwise ends thereof and is characterized to develop currents I.sub.1 
and I.sub.2 at the terminals T.sub.1 and T.sub.2 of which values vary 
depending upon a point of receiving the light. These currents I.sub.1 and 
I.sub.2 are fed through individual amplifiers 16.sub.1 and 16.sub.2, 
26.sub.1 and 26.sub.2 as outputs of the first and second optical heads 10 
and 20. The center of PSD is aligned with an optical axis of the collector 
lens 15,25 so that a displacement .DELTA.X of the receiving point from the 
center can be obtained from the following relation: 
##EQU1## 
wherein L is an effective length of the light receiving surface of PSD. 
Taking into account for somewhat non-linear characteristic of PSD, the 
displacement can be obtained more precisely by the following relation: 
##EQU2## 
wherein k is a constant selected to compensate for the non linearity. 
Turning back to FIG. 1, each of the optical heads 10 and 20 is disposed 
with an optical axis of the projector lens 13,23 perpendicular to the 
object surface and with an optical axis of the collector lens 15,25 
inclined at an angle of .theta. with respect to the perpendicular axis. 
The center of the collector lens 13,23 is spaced along the perpendicular 
axis by a fixed distance Rc from a reference point or the point at which 
the two optical axes cross at an angle .theta.. The reference points of 
the two optical heads 10 and 20 therefore define the reference plane 
PL.sub.ref perpendicular to the axis of the projecting laser beam and is 
parallel to a general plane of the object surface. The PSD 14,24 in each 
head is disposed with its photo-sensitive surface perpendicular to the 
optical axis of the collector lens 15,25 and spaced from the center of the 
lens by a focal length f thereof. When the laser beam from the optical 
head is reflected at a point on the object's surface spaced from the 
reference point or plane PL.sub.ref by a distance d along the 
perpendicular optical axis, the reflected laser beam will impinge on the 
surface of PSD at a point offset from the longitudinal center of PSD by a 
displacement of .DELTA.X. From this geometrical relationship, the 
positioning of the object surface can be effected by triangulation through 
the following equation: 
##EQU3## 
Therefore, combining this equation (2) with the above equation (1) can 
gives the distance d of the points on the object surface from the 
reference plane PL.sub.ref by incorporating the outputs I.sub.1 and 
I.sub.2 of the PSD. Such arithmetic operations are made at the processing 
circuit 40. 
The laser beam generated at the first and second optical heads 10 and 20 is 
modulated by a pulse signal from the oscillator 50 such that each optical 
head receives modulated laser beam reflected from the object's surface and 
issues corresponding modulated outputs I.sub.1 and I.sub.2 therefrom. The 
outputs I.sub.1 and I.sub.2 are demodulated in the processing circuit 40 
to give corresponding values which are free from background illumination 
on the object surface and therefore give reliable data for measurement of 
the distance D.sub.1 and D.sub.2. The pulse signal from the oscillator 50 
is controlled by a switch controller 30 to be fed through a switch 32 
alternately to the first and second optical heads 10 and 20 such that the 
optical heads 10 and 20 are enabled alternately. The switch controller 30 
also controls the switch 31 for feeding the outputs from the first and 
second optical heads 10 and 20 alternately to the processing circuit 40. 
The processing circuit 40, oscillator 50, switch controller 30, and 
switches 31 and 32 are assembled into a housing (not shown) and the 
optical heads 10 and 20 are connected to the housing through individual 
cables leading to the switches 31 and 32. 
The processing circuit 40 comprises a pair of amplifiers 41.sub.1, 
41.sub.2, demodulators 42.sub.1, 42.sub.2, and analog-to-digital [A/D] 
converter 43.sub.1, 43.sub.2, in addition to a CPU 44, a digital-to-analog 
[D/A] converter 45, and memories 46 and 47. The outputs I.sub.1 and 
I.sub.2 from either of the optical heads 10 or 20 are amplified and 
converted at the amplifiers 41.sub.1, 41.sub.2 into corresponding voltages 
which are then demodulated at 42.sub.1, 42.sub.2 to provide analog signals 
indicative of the position of the object surface free from being 
influenced by the background illumination. The analog signals are 
converted at the A/D converters 43.sub.1, 43.sub.2 into digital signals 
for arithmetic operation at the CPU 44 to measure the perpendicular 
distances D.sub.1 and D.sub.2 of the object surface form the reference 
plane PL.sub.ref in the manner as discussed hereinbefore. CPU 44 provides 
an output indicative of the measurement result which is converted at D/A 
converter 45 into analog signal for analog indication at an exterior 
display or processing at another device connected to the processing 
circuit. The above operation is illustrated in FIG. 3, composed of FIGS. 
3A to 3G, in which the outputs I.sub.1 and I.sub.2 of the first optical 
head 10 is shown as a position current I.sub.A while the I.sub.1 and 
I.sub.2 of the second optical head 20 is shown as a like position current 
I.sub.B (FIG. 3B). The current I.sub.A and I.sub.B are sequentially 
processed into digital signals S.sub.A0 and S.sub.B0, S.sub.A1 and 
S.sub.B1 . . . . (FIG. 3C) in synchronism with a switch control signal 
SW.sub.c (FIG. 3A) which effects the changeover of the switches 31 and 32 
to alternately activate the optical heads 10 and 20 and process the output 
from the corresponding one of the heads 10 and 20. Digital signals 
S.sub.A0, S.sub.A1, . . . and S.sub.B0, S.sub.B1, . . . are stored 
respectively into memories 46 and 47 (FIGS. 3D and 3E), and are then 
processed at CPU 44 to provide outputs or measurement results D.sub.0, 
D.sub.1, D.sub.2, . . . (FIG. 3F). The CPU's outputs are thereafter 
converted at D/A converter into continuous analog signals (FIG. 3G). In 
this manner, the measurement is made continuously while the processing 
circuit 40 receives the outputs of the first and second optical heads 10 
and 20, alternately. 
The CPU 45 is programmed to enable a calibration which compensate for any 
variations in characteristics of the electrical components used in the 
processing circuit 40 as well as possible misalignment between the two 
optical heads 10 and 20. That is, the calibration is made by the use of an 
optical flat plane to measure two points on the optical flat plane such 
that the system gives zero difference between the measured distances 
D.sub.1 and D.sub.2 by incorporating an offset value which is stored in 
one of the memories 46 and 47 or another memory and is subsequently 
utilized for providing a correct measurement. 
Second Embodiment 
FIG. 5 
FIG. 5 illustrates a like optical measurement system in accordance with a 
second embodiment of the present invention which is identical in 
configuration and operation to those of the first embodiment except that a 
pair of first and second oscillators 51 and 52 are included in the system 
for generating at the first and second optical heads 10A and 20A modulate 
laser beams of different oscillating frequencies in order to avoid 
interference between the laser beams from the first and second optical 
heads. In this embodiment, the first and second optical heads 10A and 20A 
are kept activated continuously as opposed to the first embodiment where 
they are alternately activated. With the inclusion of the two oscillators 
51 and 52, a switch controller 30A is configured to operate demodulators 
42.sub.1 A and 42.sub.2 A at the corresponding frequencies by means of a 
switch 33, in addition to alternately transmitting the outputs from the 
first and second optical heads 10A and 20A to a processing unit 40A 
through a switch 31A. Like parts or elements are designated by like 
numerals with a suffix letter of "A". 
Third Embodiment 
FIGS. 6 To 9 
FIG. 6 illustrates a like optical measurement system in accordance with a 
third embodiment of the present invention which is identical in 
configuration and operation to the first embodiment except that a pair of 
sample-and-hold (S/H) circuits 48.sub.1 and 48.sub.2 is included in a 
processing circuit 40B in place of the demodulators in the first 
embodiment. Also included in the processing circuit 40B in association 
with the S/H circuits is a timing circuit 55 which generates timed pulses 
LP, SH, and CN based upon the oscillating frequency of the oscillator 50B. 
Pulses LP are fed alternately to the first and second optical heads 10B 
and 20B through a switch 32B to generate the pulse modulated laser beam of 
a given frequency, as shown in FIG. 7A. The resulting laser beams are each 
characterized to have high and low levels such that each of the first and 
second optical heads 10B and 20B provides correspondingly high and low 
level values for each of the outputs I.sub.1 and I.sub.2 from the opposite 
ends of PSD 14B within one cycle T of the pulses D.sub.1. Pulses SH are 
fed to the S/H circuits 48.sub.1 and 48.sub.2 in order to sample and hold 
the high level value for each of the outputs I.sub.1 and I.sub.2 within a 
half cycle (T/2) of the laser beam generating pulses LP and subsequently 
sample and hold the low level value for the same within the other half 
cycle of the pulses LP, as shown in FIG. 7B. In this manner, the high 
level and low level values within one pulse of the reflected laser beam 
from the object's surface to the PSD are taken and then converted at 
individual A/D converters 43.sub.1 B and 43.sub.2 B into corresponding 
high and low digital values V.sub.H and V.sub.L under the control of the 
pulses CN fed to the A/D converters as control pulses, as shown in FIG. 
7C. Thus obtained digital values are processed at the CPU 44B to provide a 
difference value V.sub.def between the high and low level values 
(V.sub.def =V.sub.H -V.sub.L) within one cycle of the laser beam received 
at the PSD for each of the outputs I.sub.1 and I.sub.2 from each one of 
the optical heads 10B and 20B. This subtraction can therefore cancel any 
variations in characteristics of the electrical components in the circuit 
as well as the background illumination because of that such variations 
will appear equally in the high and low level values and can be therefore 
eliminated in the difference value (V.sub.def =V.sub.H -V.sub.L). This is 
more apparent when considering the followings with reference to FIGS. 8A 
and 8B which illustrate exemplarily the waveform of the laser beam 
received at the PSD and the waveform in solid line of the output of the 
amplifiers 41.sub.1 B or 41.sub.2 B. The output of the amplifier gives 
high and low level values V.sub.H and V.sub.L which includes true values 
V.sub.HT and V.sub.LT (shown in dotted waveform in FIG. 8B) plus the 
variations var of the same extent, as expressed in the below. 
EQU V.sub.H =V.sub.HT +var 
EQU V.sub.L =V.sub.LT +var 
Therefore, subtraction of V.sub.H and V.sub.L will cancel the variations 
and results in the difference between the true values (V.sub.HT 
-V.sub.LT). In this manner, the system can extract from the outputs 
I.sub.1 and I.sub.2 of the PSD of each optical head reliable and true data 
indicative of the position of the object surface and is therefore capable 
of reliably measuring the distances of the individual points on the 
object's surface from the reference plane PL.sub.ref for accurate 
determination of the depth D of the step in the object surface. 
The system additionally includes error-free capability of invalidating the 
measurement when the high level value V.sub.H or low level value V.sub.L 
goes beyond an allowable range R of the A/D converter 43.sub.1 B or 
43.sub.2 B. For example, as shown in FIG. 9A and 9B, when the level of the 
laser beam received at the PSD increases remarkably, as shown in FIG. 9A, 
due to, for example, an abrupt increase in reflectance of the object's 
surface, an overshooting will occur in the output of the S/H circuit such 
that the A/D converter 43.sub.1 B receives abnormally increased high level 
output beyond the allowable range R for a while, as indicated by a time 
interval of T.sub.1 in FIG. 9B, until the correct level output is reached. 
During that interval, A/D converter will generate a maximum level value 
although it does not actually indicate the intensity of the laser beam 
received at the PSD and would therefore result in erroneous measurement at 
CPU. However, such erroneous measurement can be avoided by the above 
arrangement which is applied to both of the outputs I.sub.1 and I.sub.2 of 
the PSD for each of the optical heads 10B and 20B. 
Fourth embodiment 
FIGS. 10 And 11 
FIG. 10 illustrates a like optical measurement system in accordance with a 
fourth embodiment of the present invention which is basically identical to 
the first embodiment except that a supervising section 60 is included in a 
processing circuit 40C for checking whether first and second optical heads 
10C and 20C are correctly coupled to corresponding connectors C.sub.1 and 
C.sub.2 of the processing circuit 40C. Like elements and components are 
designated by like numerals with a suffix letter of "C". The connectors 
C.sub.1 and C.sub.2 are provided on a housing incorporating the processing 
circuit 40C together with the supervising section 60, an oscillator 50C, a 
switch controller 30C, and the switch 31C. For reason of that there may be 
some variation in output characteristic of the separate heads 10C and 20C, 
the calibration is made in the system to compensate for that variation in 
order to give consistent measurement. That is, the processing circuit 40C 
is given suitable compensation at the calibration which is stored in the 
memory and is utilized in measurement of the individual distances of the 
points on the object's surface from the outputs of the separate heads 10C 
and 20C. Such compensation can be effective provided that the two optical 
heads 10C and 20C are correctly coupled to the connectors C.sub.1 and 
C.sub.2 of the processing circuit 40C. The correct connection is checked 
at the supervising section 60 for providing consistent measurements. As 
shown in FIG. 11, the supervising section 60 comprises a pair of first and 
second comparators 61 and 62 and a switch 63 which is controlled by CPU of 
the processing circuit to transmit in sequence different voltage signals 
V.sub.1 and V.sub.2 for the individual heads 10C and 20C through the 
connectors C.sub.1 and C.sub.2. The voltage signals V.sub.1 and V.sub.2 
are issued from address signal generators 17.sub.1 and 17.sub.2, 
respectively included in the optical heads 10C and 20C. In this instance, 
the voltage signal V.sub.1 for the first head 10C is obtained from a fixed 
voltage V through a resistor R.sub.1, while the voltage signal V.sub.2 is 
obtained directly from the fixed voltage V. A resistor R2 is connected 
commonly to (+) inputs of the comparators 61 and 62 so that the first and 
second comparators 61 and 62 receive at their (+) inputs the voltage 
V.sub.in =V.times.R.sub.2 /(R.sub.1 +R.sub.2) and V.sub.in =V, 
respectively when the first and second optical heads 10C and 20C are 
coupled correctly to the connectors C.sub.1 and C.sub.2, respectively. The 
first and second comparators 61 and 62 have reference voltages V.sub.ref1 
and V.sub.ref2 (V.sub.ref1 &lt;V.sub.ref2) so that the first comparator 61 
outputs a H-level signal when the input voltage V.sub.in exceeds the 
reference voltage V.sub.ref1 and outputs a L-level signal otherwise, and 
the second comparator 62 outputs a H-level signal when the input voltage 
V.sub.in exceeds the reference voltage V.sub.ref2 and a L-level signal 
otherwise. The outputs of the first and second comparators 61 and 62 are 
fed to the CPU of the processing circuit 40C where they are analyzed to 
judge whether the heads 10C and 20C are correctly coupled to the 
associated connectors C.sub.1 and C.sub.2. When, for example, the first 
comparator 61 outputs the H-level signal as indicative of that the either 
of the head 10C or 20C is coupled to the connector C.sub.1, then the CPU 
checks whether the output of the second comparator 62 and judges that the 
heads 10C and 20C are correctly coupled to the associated terminals 
C.sub.1 and C.sub.2, respectively when the second comparator 62 outputs 
the H-level signal, and that the heads 10C and 20C are mis-coupled to the 
terminals C.sub.1 and C.sub.2, respectively or even no connection is made 
to the terminal C.sub.2 when the second comparator 62 outputs the L-level 
signal. Further, when the first comparator 61 outputs the L-level signal, 
the CPU acknowledges that at least the terminal C.sub.1 is not connected 
to any one of the heads. When the non-connection or mis-connection is 
judged, the system responds to disable the measurement and produces a 
warning signal urging the user to reconnect the heads to the correct 
terminals. Instead of using the voltage signals differentiated by the use 
of the resistor R.sub.1, it is equally possible to provide coded signals 
from the individual heads so that the processing circuit can acknowledge 
the heads by analysis of the coded signals. 
It should be noted that the present invention should not be limited to the 
two-head measurement system and may includes three or more optical heads 
for analyzing the object's surface in view of three or more points on the 
object's surface. In such modification also, the single processing circuit 
is responsible for measurement of the individual distances or positions of 
these points from the outputs of the individual optical heads. 
______________________________________ 
LIST OF REFERENCE NUMERALS 
______________________________________ 
10 first optical head 
11 laser generating element 
12 amplifier 
13 projector lens 
14 PSD 
15 collector lens 
16 amplifier 
17 address signal generator 
20 second optical head 
30 switch controller 
31 switch 
32 switch 
33 switch 
40 processing circuit 
41 amplifier 
42 demodulator 
43 A/D converter 
44 CPU 
45 D/A converter 
46 memory 
47 memory 
48 sample-and-hold circuit 
50 oscillator 
51 first oscillator 
52 second oscillator 
55 timing circuit 
60 supervising section 
61 comparator 
62 comparator 
63 switch 
______________________________________