Method of measuring the distance of a target and apparatus for its performance

The distance of a target object is determined from the time of travel of a measuring light pulse which is emitted by a transmitter toward the target object, reflected thereby and received by a receiver. In timed relation with the instant of generation of the measuring light pulse, a start signal for beginning a measuring signal transit time measurement is generated, and, on receipt of the reflected measuring light pulse, a stop signal is generated for terminating this time measurement. A completely independent reference light pulse is generated and forwarded along a reference light path establishing a predetermined time of travel from the transmitter to the receiver, and the respective reference signal transit time is measured which contains the same undesirable additional time spans contained in the measured transit time of the measuring signal. The reference signal transit time measurement value is subtracted from the measuring signal transit time value to eliminate the additional time spans and to obtain a high precision measurement value difference to which the predetermined time of travel of the reference light pulse is added to obtain the genuine time of travel of the measuring light pulse.

FIELD OF THE INVENTION 
The invention relates to a distance measurement method measuring the 
distance of a target as a function of the time of travel of a measuring 
light pulse from a transmitter to the target and from the target to a 
receiver. 
BACKGROUND OF THE INVENTION 
In distance measuring methods measuring the time of travel of a measuring 
light pulse, it is difficult to correlate exactly in time the instant at 
which the time measurement is started with the instant, at which the 
measuring light pulse is emitted and, after its reflection at the target 
object, to correlate exactly in time the instant at which the time 
measurement is stopped, with the instant at which the measuring light 
pulse is received. Such exact correlation in time, however, is necessary 
to obtain a time measurement value, which, with great accuracy, 
corresponds to the time of travel of the measuring light pulse and thus 
makes possible an exact distance determination. 
Particularly when targets within a great distance measurement range of, for 
example, 0 to 100 kilometers and more are to be measured at an accuracy of 
up to .+-.1 millimeter, extra-ordinarily great difficulties result, since 
especially for short distance targets, the delay and signal processing 
times which arise at the transmitter side between the trigger signal 
triggering the transmitter to generate a measuring light pulse and the 
actual emission of the light pulse as well as on the receiver side between 
the arrival of the reflected measuring light pulse at the receiver, and 
the generation of the associated stop signal lie in the same order of 
magnitude as or even exceed the actual "genuine" time of travel of the 
measuring light pulse. 
If no appropriate measures are taken these delay and signal processing 
times fluctuate due to temperature changes and drift phenomena dependent 
on aging. These changes of delay and signal processing times can have such 
a strong effect on the measurement result that it becomes impossible to 
attain the aforementioned measurement accuracy. 
The problem of establishing on the receiver side an exact correlation in 
time between the instant of reception of the reflected measuring light 
pulse and the instant of generation of the stop signal for the time 
measurement can be regarded as solved in principle by the circuit 
arrangement disclosed in DE-OS No. 26 34 627. 
However, it is more difficult on the transmitter side to establish an exact 
correlation between the instant at which the transmitter responds to the 
trigger signal and a start signal since this response time or delay can be 
subject to strong changes. Therefore the trigger signal is not readily 
usable as a start signal for the time measurement. 
To overcome this problem, it is for example known from the U.S. Pat. No. 
3,652,161 to branch off a part of each measuring light pulse emitted by 
the transmitter, for example by means of a partially permeable mirror, and 
to use this branched off part as a reference light pulse which, via a 
reference light path, is forwarded to a photo-detector translating it into 
a start signal, correlated with the instant of transmission of the 
measuring light pulse. Two different receivers are used, the one receiving 
the measuring light pulses and the other one receiving the reference light 
pulses. If no further measures are taken, difficulties result from the 
different response behaviour and the different fluctuation phenomena of 
the signal processing times of the two receivers and of the two receiving 
channels connected thereto. If, on the other hand, measuring light pulse 
and reference light pulse are forwarded to one and the same receiver, the 
measurement of small distances becomes difficult, since the single 
receiving channel as well as the time measuring device can process without 
difficulties only signals which have a certain minimum spacing. Although 
it is possible with appropriate circuit effort to build time measuring 
device which can measure the time spacing of signals following one another 
practically as closely as desired, the amplifier and signal generating 
circuit of the receiving channel have a certain recovery time, i.e. a 
certain minimum time must elapse after the reception of a light pulse 
before a new pulse can be processed unobjectionably. This causes a lower 
limit for the smallest measurable distances, which limit can be made very 
small but can not be brought to zero. 
An objective of the invention is to provide a method and an apparatus of 
the foregoing kind which is simple, reliable and operates with great 
accuracy over a wide measurement range, the lower limit of which is 
practically zero. 
SUMMARY OF THE INVENTION 
The invention bases on the recognition that, for the attainment of a high 
measurement accuracy, it is neither necessary to produce an exactly known 
correlation in time, reproducible exactly over long periods, between the 
start signal for the time measuring device and the instant of emission of 
the measuring light pulse nor between the instant of reception of the 
reflected measuring light pulse and the stop signal for the time measuring 
device. Rather, a completely different concept is used according to the 
invention, in which it is no longer attempted to detect the "pure" time of 
travel of a measuring light pulse, i.e. the time between the actual 
generation of the measuring light pulse at the light emitting surface of 
the transmitter and the actual receipt of the measuring light pulse at the 
photo-sensitive surface of the receiver. Instead thereof, according to the 
method of the invention at least two light pulses are separately generated 
one after the other, the one for use as measuring light pulse being 
forwarded from the transmitter to the target object and from there back to 
the receiver, and the other for use as reference light pulse being 
forwarded via a reference light path which extends inside the instrument 
from the transmitter to the receiver and establishes a predetermined time 
of travel. For each of these at least two independent light pulses a 
complete signal transit time measurement is performed and the resulting at 
least two measurement values are used to determine one single target 
distance value. 
In both cases, the signal, the transit time of which is measured, is 
present for certain parts of this transit time in electrical form and for 
other parts of this transit time in the form of a light pulse. In other 
words, the measuring signal transmit time measurement value T.sub.M 
obtained for the at least one measuring light pulse comprises three 
components: A first time span t.sub.1 elapsing between the instant, at 
which the measuring signal transit time measurement for the measuring 
light pulse is started, and the instant, at which the measuring light 
pulse is actually generated by the transmitter, (this time span t.sub.1 
has a negative sign when the last named instant lies before the first 
named); furthermore the actual time of travel t.sub.M which the measuring 
light pulse needs for traversing the measurement route, and a third time 
span t.sub.3 elapsing between the arrival of the measuring light pulse at 
the receiver and the termination of the measuring signal transit time 
measurement. This can be expressed by the following equation: 
EQU T.sub.M =t.sub.1 +t.sub.M +t.sub.3, (1) 
wherein t.sub.M is the actually interesting value since it is proportional 
to the distance to be measured. 
According to the invention, it is not required for the attainment of a high 
measurement accuracy exactly to know the time spans t.sub.1 and t.sub.3 
and/or to stabilize them in particular manner against fluctuations or 
drift phenomena. It suffices rather to take care that both these time 
spans t'.sub.1 and t'.sub.3 likewise enter into the signal transit time 
measurement value T.sub.R of the reference light pulse so that for the 
reference light pulse, there applies 
EQU T.sub.R =t'.sub.1 +t.sub.R +t'.sub.3 ( 2) 
t'.sub.1 being the time span between the instant at which the respective 
reference signal transit time measurement is started and the instant at 
which the reference light pulse is actually generated, t.sub.R being the 
time of travel of the reference light pulse through the reference light 
path, and t'.sub.3 being the time span between the receipt of the 
reference light pulse and the actual termination of the time measurement. 
Additionally, both signal transit time measurements have to be performed 
within such a short period of time that the differences t.sub.1 -t'.sub.1 
and t.sub.3 -t'.sub.3 due to fluctuations become negligibly small or can 
be appropriately reduced through a simple interpolation or mean value 
formation from several measurement values for T.sub.M and T.sub.R. 
In the first case, i.e. when the measurement of the measuring signal 
transit time T.sub.M over the measurement route and the measurement of the 
reference signal transit time T.sub.R over the reference route can be 
performed within a very short period of time, for example within 50 
microseconds to 100 microseconds and/or when no extremely high demands are 
made on the measurement accuracy, t.sub.1 can be considered to be exactly 
equal to t'.sub.1 and t.sub.3 can be considered to be exactly equal to 
t'.sub.3 :t.sub.1 =t'.sub.1 and t.sub.3 =t'.sub.3. Then the above equation 
(2) can be solved for t.sub.1 +t.sub.2 and inserted into equation (1), 
from which one obtains, through resolution of the thus obtained equation, 
the magnitude t.sub.M : 
EQU t.sub.M =T.sub.M -(T.sub.R -t.sub.R); (3) 
t.sub.M thus results through a simple arithmetic operation from two very 
accurately measurable time distances T.sub.M and T.sub.R and an instrument 
constant t.sub.R. 
In all the cases, in which the assumption of t.sub.1 =t'.sub.1 and t.sub.3 
=t'.sub.3 is not permissible, because either switching-over is done more 
slowly and/or an extremely high measurement accuracy is aimed at, 
according to the invention each measuring signal transit time measurement 
over the measurement route can be included between two reference signal 
transit time measurements over the reference route. For both the latter 
signal transit time measurements, one then obtains 
EQU T.sub.R1 =t.sub.11 +t.sub.R +t.sub.31 ( 4) 
and 
EQU T.sub.R2 +t.sub.12 +t.sub.R +t.sub.32 ( 5) 
wherein the second suffices are to indicate that t.sub.1 and t.sub.3 and 
thereby also T.sub.R1 and T.sub.R2 have changed by reason of fluctuation 
phenomena in the period of time, which lies between both these 
measurements and into which also falls the measurement of the measuring 
signal transit time T.sub.M over the measurement route. 
Since it is always possible to determine both the reference signal transit 
time values T.sub.R1 and T.sub.R2 so quickly one behind the other that 
meantime changes can be regarded as linear to a sufficient approximation, 
a simple interpolation suffices to be able to perform a usable correction 
of the measuring signal transit time value T.sub.M obtained in this time 
interval. If one assumes that the measurement of T.sub.M takes place 
exactly in the time centre between both the measurements of T.sub.R1 or 
T.sub.R2, then there applies 
##EQU1## 
and in analogy to equation (3), one obtains 
##EQU2## 
T.sub.M, T.sub.R1 and T.sub.R2 again being measurement values of high 
accuracy and t.sub.R being the constant predetermined time of travel 
established by the reference light path. 
A further possibility according to the invention for the attainment of a 
very high measurement accuracy with simultaneous lowering of the speed at 
which switching-over is done between measuring signal transit time 
measurements over the measurement route and reference signal transit time 
measurements over the reference route, consists in performing several 
measuring signal measurements over the measurement route and several 
reference signal measurements over the reference route alternately, i.e. 
intercalated one among the other, and to take the mean values from each 
group of the thus obtained measurement values T.sub.M or T.sub.R and to 
insert these mean values into the above equations (3) and (7), 
respectively. In this case however, it is presupposed that all changes are 
either sufficiently slow or linear. 
Furthermore it is necessary that t.sub.R can indeed be considered as an 
instrument constant invariable over long spaces of time. This means that 
the length of the reference light path inside the measuring apparatus must 
be invariable. This can be attained most simply by directing the reference 
light pulses from the light path change-over switch substantially by way 
of a light fibre conductor to the receiver; if necessary an optical damper 
can be arranged in this light path. Although the length of such a light 
fibre conductor is dependent on temperature, it is possible continuously 
to measure the temperature in the interior of the instrument and to 
perform a correction of t.sub.R with the aid of this measurement value. 
Preferably, the length of the light fibre conductor substantially forming 
the reference light path is however chosen to be so short that its change 
in length even for great temperature fluctuations lies below the 
measurement accuracy aimed at. 
If one uses the mean values of several signal transit time measurements 
over the measurement and the reference route, respectively, for the 
calculation of a single distance measurement value, it is advantageous for 
obtaining a very high measurement accuracy to dispose in time the trigger 
signals for the transmitter so that the alternating measuring light pulses 
and reference light pulses arrive at the receiver as exactly periodically 
as possible. This is obtained by generating a first measuring light pulse, 
which serves for an approximate distance determination and by using the 
corresponding signal transit time measurement value to shift the instants 
at which trigger signals for measuring light pulses are generated relative 
to the instants at which trigger signals for the reference light pulses 
are generated so that measuring light pulses and reference light pulses 
arrive periodically at the receiver in spite of the different lengths of 
the measuring light path and the reference light path. 
This periodical operation of the receiver side of a distance measuring 
instrument is particularly advantageous for the reason that the analog 
circuits, which further process the light pulses in the receiving channel, 
like all analog circuits have the property that the magnitude of the error 
which is superimposed by them on the measurement signal to be processed 
depends on the time spacing between two successive signal processing 
operations of like kind. Through the periodicity of the drive, it is 
ensured according to the invention that these errors enter with the same 
magnitude into the signal transit time measurement values of the measuring 
light pulses as well as of the associated reference light pulses and thus 
cancel out in a subtracting step which is performed to obtain a distance 
value. 
Beyond that, a time base signal generally finds use in the time measuring 
device of a distance measuring instument under discussion here, which time 
base signal comes into use at the most diverse places and is therefore 
unavoidably present on practically all conductors as an albeit very small, 
periodically fluctuating interference singal. 
If one now operates the arrangement not at a desired periodicity, but at a 
sequence frequency, which amounts to an integral multiple of the frequency 
of the time base signal, then also the interference voltages caused by the 
time base signal enter into the analog signals at the same amplitude and 
therefore again drop out during the succeeding subtraction. 
As already mentioned, the method according to the invention requires that 
the times t.sub.1 and t.sub.3, in which the respective signal is present 
in electrical form and which contain the response times required for 
transforming the electrical signal into an optical signal as well as the 
optical signal into an electrical form, enter in like manner into the 
signal transit time measurement values T.sub.M or T.sub.R for measuring 
and reference light pulses belonging together. 
To meet this condition, two suitable ways are provided according to the 
invention in dependence on the kind of the respectively used transmitter. 
In the one kind of transmitter, the typical representative of which is a 
Laser diode, the reaction time, which passes between the feeding of a 
trigger signal to the transmitter and the generation of a light pulse, is 
reproducible with sufficiently good accuracy at least for the short 
periods of time, which are needed for the performance of the signal 
transit time measurements contributing to one distance measurement value, 
when one takes care of a suitable, constant supply voltage of the Laser 
diode. 
This reproduceability of the reaction time of the transmitter makes it 
possible to use the trigger signal generated by the trigger generator as 
start signal or as start-preparing signal for the time measurement circuit 
for the measuring light pulses as well as also for the reference light 
pulses, which trigger signal is conducted to the transmitter preferably 
later by a predetermined delay .tau. than to the time measuring circuit. 
In this case, in which the time measurement is started before the 
generation of the light pulse, the time span t.sub.1 appearing in the 
above equations is positive and comprises substantially the additional 
delay time .tau. and the responsive delay of the transmitter. 
Both delays are completely uncritical in respect of their medium term 
constancy and long term stability, since it can be presumed that during 
the short period of time needed for the performance of the measurements 
for a distance measurement value, both change either not at all or at most 
linearly, wherein the latter can be taken into consideration by an 
appropriate interpolation. 
A substantial advantage of the additional, electronically generated delay 
.tau. is that it can be chosen to be so great that the spacing between 
start signal and stop signal even in the case of the measurement of very 
small distances is always sufficiently great in order also to be able to 
be measured readily with a simply constructed time measuring circuit. 
A further advantage of the delay member provided according to the invention 
is to be seen in that the start of each signal transit time measurement, 
which makes necessary to detect exactly in time an edge of a generally 
pulse-shaped signal, takes place in a period of time, in which the 
transmitter has not yet received the trigger signal. The transmitter 
reacts to this trigger signal by the generation of a very rapid and 
comparatively great current pulse through the transmitting diode so that 
very strong interference signals are generated, which would make it 
extraordinarily difficult exactly in time to detect a start signal, 
falling into this space of time, for the signal transit time measurement. 
If each signal transit time measurement is started by the trigger signal 
which is not correlated with the time base signal, it is necessary to 
perform for each signal transit time measurement a complete three-part 
time measurement. In this case it is useful to measure the time distance, 
which the trigger or start signal has from a defined subsequent pulse edge 
of the time base signal by an analog measuring circuit, to obtain a first 
precision time measurement value, and to measure by means of the same 
analog measuring circuit the time distance which the respective stop 
signal generated after the arrival of the light pulse at the receiver for 
stopping the time measurement has from a defined second subsequent pulse 
edge of the time base signal, to obtain a second precision time 
measurement value, and to count as coarse measurement value the pulses of 
the time base signal which occur between both the just mentioned pulse 
edges. The signal transit time can then be calculated from these three 
measurement values by adding the first precision time measurement value to 
the coarse measurement value and by subtracting the second precision time 
measurement value from the thereby obtained sum. Consequently, alltogether 
four precision measurements value formations and two coarse measurement 
value formations are required for obtaining the signal transit time 
difference of a measurement light pulse and a reference light pulse. 
However, in this embodiment an increased effort in terms of measurement 
technique is required and a disadvantage results from the fact that the 
analog measuring device and the entire measuring channel can not be 
operated at constant frequency, since the times of travel of the measuring 
light pulses will normally be different from those of the reference light 
pulses. This aperiodic operation of the measuring channel can however, as 
already mentioned, have the consequence of worsening the accuracy of the 
thus obtained measurement results. 
To avoid these problems, it is advantageous to generate the trigger signals 
in synchronism with the time base signal. In this case, the trigger signal 
no longer serves as actual start signal of the signal transit time 
measurement, but only as preparatory signal for an in any case required 
coarse time measurement, which takes place through counting of the pulse 
edges of the time base signal and begins with a pulse edge of the time 
base signal, which stands in defined correlation with the trigger signal, 
for example with the fifth edge following the trigger signal. With the 
omission of the above described respective first precision measurement 
value formation for a distance measurement value, it thus suffices to 
determine only the coarse measurement value for the transit time of at 
least one measuring light pulse and, in case this can not be considered as 
instrument constant, to determine the coarse measurement value for the 
transit time of a reference light pulse as well as the "second" precision 
measurement values for the transit times of both light pulses in order to 
obtain therefrom the sought transit time difference with the accuracy, 
with which the period of the time base signal is constant and known. 
Through the omission of the first precision measurement value formations, 
it is possible through an appropriate choice of the instants, at which the 
trigger signals are generated, to obtain the light pulses in strict 
periodicity at the receiver in the above described manner. 
An other kind of transmitter, the typical representative of which is a gas 
or solid-state Laser, thereagainst has the property that its reaction time 
can fluctuate to an appreciable degree even when the light pulses 
concerned are generated very rapidly one behind the other. Consequently a 
start of the transit time measurement by the trigger signal would lead to 
intolerable measurement errors. Therefore, within the scope of the 
invention, the start of the signal transit time measurement for 
transmitters of that kind is effected for each measuring light pulse as 
well as also for each reference light pulse by means of the output signals 
of an auxiliary receiver, to which a split off part of the respective 
light pulse is forwarded via an auxiliary light path inside the 
instrument. On receipt of each split off light pulse part the auxiliary 
receiver generates an output signal, which is used as start signal for the 
respective signal transit time measurement. 
In this case, in which the signal transit time measurement is started after 
the generation of the respective light pulse, the time span t.sub.1 is 
negative and contains the transit time of the split off light pulse part 
from the transmitter to the auxiliary receiver as well as the reaction 
time of the auxiliary receiver. These parts of t.sub.1 are however again 
constant or at least variable linearly for the short periods of time to be 
considered so that the above equations can again find use. It is decisive 
that here, too, all possible delays enter in like manner at least into the 
mean values of the the signal transit time of measurement light pulses and 
reference light pulses and therefore are eliminated by the corresponding 
subtraction operation. 
A synchronisation between the trigger signals and the time base signal of 
the time measuring circuit is here not sensible because of the fluctating 
reaction behaviour of the transmitter. For this reason, a three-part time 
measurement resulting in two precision time measurement values and one 
coarse measurement value must be performed for each signal transit time 
measurement for a measuring light pulse, whilst only the precision time 
measurement values need be determined for the reference light pulses, when 
the respective coarse time measurement value can be considered as 
instrument constant. Altogether, a somewhat smaller absolute measurement 
accuracy results in this case than for the transmitters of the first kind, 
which is however more than compensated for in respect of the relative 
accuracy by the substantially higher power and thereby appreciable 
enlarged maximum range of these transmitters of the second kind. 
In order also to be able to measure distances as short as desired and in 
particular to be able to use a short reference light path, in the case of 
a version of a distance measuring instrument according to the invention 
operating with an auxiliary receiver, a time meausring device is 
preferably used, which in respect of the precision time measurement value 
formation processes the start signal and the stop signal for the signal 
transit time measurement each in a respective analog measuring channel. A 
time measuring device of that kind is described for example in the DE-OS 
No. 28 42 450. 
This affords the advantage that the trigger pulses can again be so 
controlled in time that the light pulses arrive periodically at each of 
the receivers so that the errors introduced by the analog signal 
processing channels are kept small. Although the fluctuating reaction 
behaviour of the transmitter does not permit a highly accurate 
periodicity, the deviations in the case of the anyway limited maximum 
light pulse sequence frequency of transmitters of that kind remain in the 
range of a few tenths of a percent or less so that a substantial 
improvement in the measurement accuracy can be attained for the reasons 
already mentioned above through the timing control of the trigger signals 
according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
The embodiment, represented in FIG. 1, of a distance measuring instrument, 
according to the invention and operating on the principle of the signal 
transit time measurement, comprises a transmitter 1, which comprises a 
Laser diode and a circuit arrangement, which supplies the Laser diode with 
energy and consists substantially of a "slowly" chargeable energy store in 
the form of a capacitance and a controllable electronic switch which 
serves to discharge the energy collected in the energy store rapidly 
through the transmitting diode for the generation of a Laser light pulse. 
The drive of this switch takes place through a trigger generator 3, the 
output signal of which at the same time serves as start or start-preparing 
signal for the respective signal transit time measurement since the 
response behaviour of the Laser diode is well reproducible at least within 
short periods of time. Connected between the trigger generaor 3 and the 
transmitter 1 is a delay member 2, which on the one hand takes care that, 
even when measuring a very short distance, the stop signal for the transit 
time measurement has a sufficiently large time spacing from the start 
signal so that both these signals can be processed readily one after the 
other by one and the same time measuring channel, and which on the other 
hand has the effect that the signal transit time measurement is started 
before that instant and thereby free of interference, in which the 
transmitter reacts to the trigger signal by the generation of a very rapid 
and comparatively great current pulse through the transmitting diode, 
during which very strong interference signals are generated, which would 
make it extraordinarily difficult exactly in time to detect a start signal 
for the signal transit time measurement delivered exactly sumultaneously 
with or shortly after the light pulse generation by the transmitter 1. 
The light pulses generated by the transmitter 1 are fed through a 
transmitter light path 7 to an optical switching and damping unit 8, which 
contains a light path change-over switch 10, which according to its 
setting directs a light pulse issuing from the transmitter light path 7 
either into a transmitter measurement light path 15 or into a reference 
light path 11 inside the instrument. 
The light path change-over switch 10 can be formed by a mechanically 
movable mirror device or the like. Also a stationary beam splitter can be 
used, in each of the two outgoing light paths of which a respective 
damping member is arranged which is variable between a very high 
(preperably infinitely high) and a very small damping, wherein both the 
damping members are then controlled in opposite sense to achive the 
switch-over function. 
The light pulses fed into the transmitter measurement light path 15 by the 
light path change-over switch 10 in its measurement light setting are 
conducted further to an optical transmitting system 16, which is 
illustrated simplified as single lens and emits the light pulses to the 
target object 17, the distance of which shall be measured. 
That part of each light pulse, which is reflected by the target object 17, 
is fed by an optical receiving system 18, which is likewise illustrated 
schematically as single lens in FIG. 1, through a receiving measurement 
light path 19, a variable optical damper 20 contained in the optical 
switching and damping unit 8 and a receiving light path 22, which contains 
an interference filter 21, to a receiver 23, which for example as 
light-electrical transducer comprises a photo-diode with an amplifier and 
signal generating circuit which is connected therebehind and serves to 
feed a signal generated by it on the reception of a light pulse as stop 
signal through a line 24 to a time measuring device 25 in order to 
terminate the signal transit time measurement performed by this time 
measuring device for the light pulse concerned. This signal transit time 
measurement was started previsously, as mentioned above, in dependence on 
the trigger signal which was delivered by the trigger generator 3 and 
which is feedable through the line 27 to the time measuring device 25. 
For the case that the output signals of the trigger generator 3 shall be 
synchronised with a time base signal generated in the time measuring 
device 25, a line 29 is provided, through which appropriate signals can be 
transmitted from the time measuring device 25 to the trigger generator 3. 
The time measurement results obtained by the time measuring device 25 are 
conducted through lines 28 to a central run-down control, arithmetic and 
evaluating unit 30, which on the one hand determines the corrected 
distance measurement values from these transit time measurement values and 
causes them to be indicated and which on the other hand controls the 
functional courses in the entire measuring instrument. Preferably, this 
run-down control, arithmetic and evaluating unit 30 can comprise a 
microprocessor. 
The light pulses fed into the reference light path 11 by the light path 
change-over switch 10 in its reference light setting pass through an 
optical damper 33, which in a given case is controllable, and are 
conducted at a branching point 25 into the part 22 of the receiving light 
path leading to the receiver 23. The reference light pulses running over 
this path from the transmitter 1 to the receiver 23 are subject to the 
same delay and signal processing times at the transmitter side in respect 
to the trigger signal and at the receiver side up to the generation of the 
stop signal for the time measuring device 25 as the measurement light 
pulses, from which they differ substantially only in respect of the length 
of the light path traversed between the light path change-over switch 10 
and the branching point 35. Since the length of this reference light path 
11 and thereby also the time required for its traverse, i.e. the time of 
travel established by it, is very accurately known, the above mentioned 
delay and signal processing times, which form part of each measuring 
signal transit time value, can be eliminated by subtracting a reference 
signal transit time measurement value therefrom. 
In principle, the optical switching and damping unit 8 can be built up with 
any desired known switching and damping devices in the above described 
manner. A particularly advantageous build-up of such a switching and 
damping unit 8 is however described in the simultaneously filed German 
Patent Application No. P 32 19 452.8. 
A signal exchange between the optical switching and damping unit 8 and the 
run-down control, arithmetic and evaluating unit 30 takes place through 
the line 39, which can be constructed to be multi-core or bidirectional so 
that data concerning the instantaneous setting of the light path 
change-over switch 10 can be transmitted through the line 40 to the 
run-down control, arithmetic and evaluating circuit 30 and command signals 
adapted to the respective operational state can be delivered by the 
run-down control, arithmetic and evaluating unit 30 through the lines 41 
and 42 to the optical damper 20 or 33. 
For the determination of the required damping of the light pulses and for 
adaptation of the amplitudes of the reference light pulses to that of the 
associated measurement light pulses, the receiver 23 comprises comparators 
which transmit their signals through the line 43 to the central run-down 
control, arithmetic and evaluating unit 30 for further processing. 
In this embodiment, the time measuring circuit 25 as well as the receiver 
23 can be designed fully single-channelled, i.e. all analog precision time 
measurements are performed by one and the same analog measuring device, 
the signal processing times of which inclusive of the fluctuations and 
drift phenomena cancel out by reason of the subtraction operation between 
measurement light and reference light signal transit time measurement 
values obtained rapidly one behind the other. A time measuring device of 
this kind preferably coming into use here is disclosed in German Patent 
Application No. P 32 15 847.5 (corresponding to U.S. Pat. No. 4,569,599) 
and is illustrated together with a central run-down control, arithmetic 
and evaluating unit 30 and a trigger generator 3 in FIG. 6. 
According to FIG. 6 the time measuring device comprises a quartz-controlled 
oscillator 101 which delivers a time base signal at a frequency of, for 
example, 15 MegaHertz. This time base signal is forwarded along a line 102 
to all circuit parts which shall in any manner generate or transmit 
signals synchronised with this time base signal. 
The stop pulses, generated by the receiver 23 (FIG. 1) upon receipt of 
measuring light pulses as well as of reference light pulses are each fed 
through the input E to the time measurement device according to FIG. 6. 
From there, they get directly to a controllable delay arrangement 105, on 
the output line 106 of which these individual signals appear, in 
depencence on a signal delivered by the central run-down control, 
arithmetic and evaluating unit 30 thorugh line 107 to the controllable 
delay arrangement 105, with a delay O..tau..sub.O, i.e. undelayed, or with 
a delay which is an integral multiple of the unit delay .tau..sub.O. 
The output signals of the delay arrangement 105, which, in the following 
are supposed to be rectangular pulse signals, are used as a trigger signal 
for D-flip-flop 108 to the clock input of which they are forwarded on line 
106. This flip-flop 108 can be reset by the output signal of an OR-gate 
109 forwarding either a master resetting pulse delivered by the central 
run-down control, arithmetic and evaluating unit 30 or a resetting pulse 
delivered from a sample-and-hold control 110. 
Flip-flop 108 is a D-flip-flop the data input of which constantly lies at a 
positive voltage, i.e. at a logic 1. Therefore, the Q-output of the 
previously reset flip-flop 108 is set from a logic 0 to a logic 1 by the 
rising edge of each individual signal supplied on line 106. The rising 
edge, which is generated in that case at the Q-output of D-flip-flop 108 
gets on the one hand through an OR-gate 112 to the start input of a 
time-to-amplitude converter circuit 114, which thereupon starts with the 
measurement operation, and to a trigger input of the sample-and-hold 
control 110 as well as on the other hand to the trigger input of a first 
counting circuit 115, which at its second input receives the time base 
signal through line 102 and, after being triggered by the Q-output signal 
of the flip-flop 108 counts the rising edges of the time base signal and 
delivers in synchronism with the fourth counted edge an output signal via 
an OR-gate 116 to the stop input of the time-to-amplitude converter 
circuit 114, which hereby ceases its measurement operation. That means, 
that the time-to-amplitude converter circuit 114 measures the time 
distance between a stop signal generated by the electro-optical receiver 
23 upon receipt of a light pulse and a defined subsequent, in this case 
the fourth subsequent pulse of the time base signal in order to obtain a 
precision time measurement value for the respective transit time 
measurement. The output voltage of the time-to-amplitude converter circuit 
114, which represents this precision time measurement value is retained 
until it has been taken over under the control of the sample-and-hold 
control 110 into a sample-and-hold circuit 117 serving as analog 
intermediate store. For this purpose, the sample-and-hold circuit 117 is 
supplied by the control 110 through line 118 with an appropriate control 
signal. 
When this signal transfer is completed, then the sample-and-hold control 
110 through line 119 gives off a resetting signal, by which the 
time-to-amplitude converter circuit 114 and, as already mentioned above, 
the D-flip-flop 108 are reset into their respective initial state, in 
which they are ready for the processing of the next individual stop signal 
coming in through input E. 
The analog measurement result present at the output of the sample-and-hold 
circuit 117 gets through line 120 to an analog-to-digital converter 122, 
which under the control of the central run-down control, arithmetic and 
evaluating unit 30 translates it into a digital form and gives this 
digital value off to the central run-down control, arithmetic and 
evaluating unit 30 for further processing through bidirectional line 123, 
on which it also receives its control commands. The central run-down 
control, arithmetic and evaluating unit 30 processes the measurement 
results thus received and the calibration measurement values obtained in a 
similar manner as required and calculates the measurement result actually 
of interest, which can be displayed by a display unit 125. 
The D-flop-flop 126, which is represented in the right lower region of FIG. 
6 as being the main part of trigger generator 3 has a D-input to which a 
logic 1 is supplied at given instants as a command signal from the central 
run-down control, arithmetic and evaluating unit 30 through line 44. 
This D-flip-flop 126 represents the simplest form of a synchronising 
circuit which, each time an appropriate command signal has been given off 
through line 44 by the central run-down control, arithmetic and evaluating 
unit 30 gives off a trigger signal having an exactly defined phase 
relation to the time base signal for the transmitter 1, illustrated in 
FIG. 1, and which thus generates in a strictly phase-rigid correlation 
with the time base signal the individual light pulses, which later, after 
having travelled through a measuring light path or through a reference 
light path are received by receiver 23 the corresponding output signals of 
which appear at the input E. For this purpose, the time base signal of 
oscillator 101 is supplied to the clock input of D-flip-flop 126 through 
the appropriate part 29 (see also FIG. 1) of line 102. This has the 
consequence that the D-flip-flop 126, which was previously reset through 
an OR-gate 128 either with the aid of the master resetting pulse generated 
by the central run-down control, arithmetic and evaluating unit 30 or with 
the aid of the trigger signal appearing at its own Q-output and delayed 
through the delay member 129 for the attainment of a certain minimum pulse 
width, it set through the first rising edge of the time base signal 
following on the command signal supplied through line 44 so that the 
desired trigger signal for the transmitter appears at the Q-output of 
flip-flop 126 in synchronism with the time base signal. 
The command signals, which are given off by the central run-down control, 
arithmetic and evaluating unit 30 for the generation of a respective 
synchronised trigger signal, are so generated that they have sufficiently 
large time distances. In response to these trigger signals the transmitter 
generates a first group of n, i.e. for example eight individual light 
pulses, and then a second group of n, i.e. again eight light pulses. The 
information whether the light pulses of the first group or the light 
pulses of the second group are to be used as measuring light pulses or as 
reference light pulses or vice versa is supplied to the optical switching 
and damping unit 8 (see FIG. 1) from the central run-down control, 
arithmetic and evaluating unit 30 along line 39. 
It is presupposed that each of the such generated light pulse signals, on 
the way from the transmitter 1 to the input E needs a certain transit 
time, the magnitude of which is unknown but the same for each of the n 
measuring light pulses. Correspondingly each of the so generated n 
reference light pulse signals, on the way from the transmitter 1 to the 
input E needs a certain transit time the magnitude of which is generally 
different from that of the measuring light pulse signals. 
Not only the time distances of the delayed individual measuring and 
reference light pulse signals from a defined succeeding edge of the time 
base signal but also substantially larger time distances, namely the whole 
time of travel of the individual measuring and reference light pulse 
signals, are to be measured. The corresponding measurement values can be 
obtained with the aid of a main counter 132, which receives the time base 
signal of the oscillator 101 through line 102 and counts the relevant 
(i.e. either the rising or the falling) edges thereof. The counting start 
of the main counter 132 is initiated by the synchronisation flip-flop 126 
through line 27. The count result, which represents a coarse time 
measurement value for the respective transit time measurement, can be 
transferred from the main counter 132 through bidirectional lines 133 into 
the central run-down control, arithmetic and evaluating unit, which adds 
this coarse time measurement value to said precision time measurement 
value in order to obtain a signal transit time measurement value. The 
counting operation of the main counter 132 for each individual measuring 
and reference light pulse signal is terminated by the output signal of the 
OR-gate 116, which indeed also stops the time-to-amplitude converter 
circuit 114. Beyond that, this output signal of the OR-gate 116 is 
conducted to the central run-down control, arithmetic and evaluating unit 
30 in order to indicate to this unit that the count result can now be 
taken over from the main counter 132 on lines 133. 
In order that the time-to-amplitude converter circuit 114 can always be 
calibrated again with the aid of the time base signal, a start-stop 
control 135 is provided which can perform two different types of 
calibration measurement according to whether it receives a trigger signal 
at either its input E1 or its input E2 from the central run-down control, 
arithmetic and evaluating unit 30. 
Being triggered at input E1, it delivers exactly simultaneously at its 
start output and at its stop output a respective signal, both signals 
being synchronised with the time base signal supplied through line 102 to 
the pulse input of the start-stop control 135. The one of these two 
signals is fed through OR-gate 112 to the start input of the 
time-to-amplitude converter circuit 114, which thereupon starts to 
operate, while the other is applied to the trigger input of a second 
counting circuit 137, which at its pulse input likewise receives the time 
base signal of oscillator 101. This second counting circuit 137 thereupon 
counts the following relevant, for example the rising, edges of the time 
base signal and provides in synchronism with the third of these edges a 
stop signal which is fed through a delay member 138 to OR-gate 116, from 
where it gets as stop signal to the time-to-amplitude converter circuit 
114, to counter 132 and to the central run-down control, arithmetic and 
evaluating unit 30. By reason of this triggering through the input E1 of 
the start-stop control 135, the time-to-amplitude converter circuit 114 
thus operates for a space of time which is somewhat longer than three 
periods of the time base signal and thus delivers a first calibration 
measurement value suitable for the determination of a straight calibration 
line, since the period length of the time base signal delivered by the 
quartz-controlled oscillator 101 is after all known with very great 
accuracy and constant. 
When the central run-down control, arithmetic and evaluating unit 30 
applies a trigger signal to the input E2 of the start-stop control 135, 
then this again at both its outputs generates a start signal and a stop 
signal, respectively, which however this time are displaced one relative 
to the other in time by exactly one period length of the time base signal. 
They are fed in the same manner as described above to the 
time-to-amplitude converter circuit 114, which for this second kind of 
calibration measurement operates for a period of time, which is somewhat 
longer than four period lengths of the time base signal. Thereby, one 
obtains a second, very exact calibration value which together with the 
previously described calibration value is suitable for the definition of a 
straight calibration line, with the aid of which the actual characteristic 
curve of the time-to-amplitude converter circuit can be approximated. 
In cases (see especially FIGS. 2 and 3) in which the trigger signal is not 
to be synchronised with the time base signal the lower part 29 of line 102 
can be omitted and line 44 from central run-down control, arithmetic and 
evaluating unit 30 can be connected to a set input (not shown in FIG. 6) 
of flip-flop 126. In these cases two precision time measurement values 
have to be obtained for each transit time measurement, the first of which 
represents the time distance between the corresponding start signal, and, 
for example, the fourth subsequent pulse of the time base signal whereas 
the second one represents the time distance between the corresponding stop 
signal and the fourth pulse of the time base signal following thereupon. 
The distance measuring instrument illustrated in FIG. 1 operates in the 
manner that the light path change-over switch 10 periodically changes 
between measurement light setting and reference light setting. The central 
run-down control, arithmetic and evaluating unit 30 in time dependence on 
the switching state of the light path change-over switch 10, on which it 
obtains data by means of the lines 39 and 40, controls the trigger 
generator 3 through line 44 so that it causes the transmitter 1 at the 
right instants in time to generate light pulses which by reason of the 
respectively instantaneous setting of the light path change-over switch 10 
are used either as measurement or as reference light pulses and are 
forwarded to the receiver 23 with an appropriate damping. When measuring a 
new target object 17, the correct damping value is at first determined by 
a series of trial measurements and, as soon as it is fixed, preferably a 
plurality or series of alternating measuring and reference light pulses is 
generated so rapidly that neither the distance of the target object nor 
the signal delay times entering into the measurement values change within 
this time. From the signal transit time measurement values thus obtained, 
mean or average values are taken and the sought distance measurement value 
is calculated therefrom. 
The embodiment of a distance measuring instrument according to the 
invention, represented in FIG. 2, corresponds in many parts, which are 
also all designated by the same reference numerals, with the embodiment 
shown in FIG. 1. 
The most substantial difference is that, in the embodiment of FIG. 2, as 
transmitter 1 a Laser is used which on the one hand by reason of its 
substantially higher power permits the measurement of substantially 
greater distances (of more than 10 kilometers up to over 100 kilometers). 
On the other hand however the response delay of this Laser to the trigger 
signal, other than in the case of the Laser diode, can not be regarded as 
constant even over very short spaces of time. Rather, changes in the 
response time of up to several microseconds can occur from light pulse to 
light pulse even with very rapid light pulse sequence in such a Laser. 
Consequently somewhat different signal transit times must be established 
and measured, when one wants to attain a high measurement accuracy. For 
this purpose the output signal of the trigger generator 3 is no longer 
used as start signal for the time measuring device 25. Instead thereof in 
the transmitter light path 7 a beam splitter is provided which is formed 
by a partially permeable mirror 45, which is inclined at 45.degree. to the 
optical axis of the transmitter light path 7 and which allows the greatest 
part, for example 99% of each light pulse emitted by the transmitter 1, to 
pass through rectilinearly to the optical switching and damping unit 8 and 
deflects only the remaining small part at an angle of 90.degree. and feeds 
it into an auxiliary light path 46, which extends inside the instrument 
and which leads through a damping filter 47 and an interference filter 48 
to an auxiliary receiver 50. 
This auxiliary receiver 50 differs from the main receiver 23, which 
receives the reflected measurement light pulses or the reference light 
pulses, substantially in that it has no expensive avalanche diode as 
photoelectric transducer, but a substantially cheaper PIN diode which does 
not need a high voltage supply and the sensitivity of which, smaller by 
comparison with an avalanche diode, is fully sufficient, since a 
relatively great brightness stands at disposal through the auxiliary light 
path 46. 
For the remainder, the auxiliary receiver 50 likewise contains amplifier 
and processing circuits which permit an output signal to be generated, 
which is correlated exactly in time with the arrival of a light pulse and 
which is conducted as start signal through line 52 to the time measuring 
device 25. 
Preferably used as time measuring device 25 in this case is an arrangement, 
in which the precision measurement values for the start signals, here no 
longer correlated with the time base signal, are determined by a different 
analog measuring device than the precision measurement values for the stop 
signals. A time measuring arrangement, of that kind and making possible 
very exact measurement results, is for example disclosed in DE-OS No. 28 
42 450. 
Through the use of two separate analog measuring channels, the use of a 
short reference light path 11 as well as the measurement of distances as 
short as desired is made possible, since no time concurrence can occur 
between start signals and stop signals. 
Although this embodiment of a distance measuring instrument according to 
the invention has in its optical part as well as in its time measuring 
device 25 two "channels" separated one from the other, namely the 
measurement route 7, 15, 17, 19 and 22 or the reference route 7, 11 and 22 
on the one hand and the auxiliary light path 46 on the other hand as well 
as both the analog measuring devices for the formation of both the 
precision measurement values each belonging to a respective signal transit 
time measurement, its manner of function and accuracy of measurement is 
that of a single-channel arrangement. 
In a genuine two-channel arrangement, as it is for example described in 
DE-OS No. 27 23 835, a reference light pulse part is branched off from 
each measurement light pulse and conducted to an individual receiver, the 
output signal of which serves for the start of a transit time measurement 
which is terminated by the output signal of another receiver which 
receives the measurement light pulse reflected by the target object. Since 
start signal and stop signal for the transit time measurement can follow 
one another very rapidly in the case of short measured distances, the 
required precision time measurement is each time performed with an 
individual analog measuring device. Only one single kind of light pulse 
transit time measurements takes place here, namely the transit time 
measurements for the measurement light pulses, and the respectively 
resulting measurement value contains, apart from the sought distance, also 
the difference of the reaction and signal processing times of both the 
channels; this difference is subject to temporal fluctuations and drift 
phenomena. For the elimination of this error source, the so-called zero 
deviation of both the channels is measured, i.e. the oscillatory circuits 
connected behind both the receiving photo-diodes are for example 
electrically triggered exactly at the same time, and the then resulting 
transit time difference is measured and subtracted from the previously 
obtained measurement light pulse transit time value. In that case, the 
strongly temperature-dependent response delays of the photo-diodes are 
however not detected. 
According to the invention, however, two kinds of signal transit time 
measurements, namely measuring signal transit time measurements over the 
measurment route 7, 15, 17, 19 and 22 and reference signal transit time 
measurements over the reference route 7, 11 and 22 are performed ona after 
the other also in the second embodiment, wherein both the optical 
"channels" as well as also the two analog measuring circuits so come into 
use for each of these signal transit time measurements that all arising 
delay and signal processing times enter in like manner into the resulting 
signal transit time measurement values. Due to the fact that measurements 
follow one behind the other so rapidly that the just named "parasitic" 
times either do not change at all or at most linearly, they drop out 
completely in the subtracting and averaging operations performed for the 
determination of a single distance measurement value. This second 
embodiment thus represents a quasi single-channel system, in which 
particularly the reproducibility of the response behaviour of the 
transmitter plays no part and which in its function and accuracy of 
measurement largely corresponds to the pure single-channel system of the 
embodiment according to FIG. 1. 
In a third embodiment illustrated in FIG. 3, the light paths are no longer 
illustrated as light fibre conductors, but are symbolized merely by a 
respective central ray. This embodiment comprises, apart from the main 
transmitter 1 serving for the generation of the measurement light pulses, 
an auxiliary transmitter 55, which can possess a substantially lower 
power, since it serves only for the generation of the reference light 
pulses. The trigger generator 3, according to command signals forwarded to 
it through the line 44, triggers both the transmitters 1 and 55 
alternately through the lines 56 and 57, respectively, so that one time 
the main transmitter 1 generates a measurement light pulse which is 
forwarded via the measurement route 7, 15, 17, 19 and 22 to the main 
receiver 23 and one time the auxiliary transmitter 55 generates a 
reference light pulse which is forwarded via the reference route 58, 11 
and 22 to the main receiver 23. 
A correlation of the trigger signals with a time base signal of the time 
measuring device 25 does not take place and a Laser with strongly 
fluctuating response behaviour can be used particularly as main 
transmitter 1. 
A signal transit time measurement over the measurement route 7, 15, 17, 19 
and 22 takes place in the manner that the main transmitter 1 by reason of 
an appropriate trigger signal generates a measurement light pulse, the 
main part of which rectilinearly traverses a first beam splitter 60, for 
example a partially permeable mirror, arranged in the output light path 7 
of the main transmitter 1 in order to be emitted towards the target 
object, while a small part of the measurement light pulse is fed by the 
first beam splitter 60 into a first auxiliary light path 61 and gets 
through a totally reflecting deflecting mirror 62, a partially permeable 
coupling-in mirror 63, a light path section 64, a controllable damping 
device 65 and an interference filter 48 to the auxiliary receiver 50. 
The damping device 65 is so controlled by the run-down control, arithmetic 
and evaluating unit 30 through a line 66 that the amplitude or brightness 
of the light pulses getting to the auxiliary receiver 50 is always about 
equally great. 
On reception of the part branched off from the measurement light pulse, the 
auxiliary receiver 50 generates an output signal which is conducted as 
start signal through the line 67 to the time measuring device 25 for the 
signal transit time measurement under discussion. This start signal is not 
correlated in any manner with the time base signal of the time measuring 
device 25. Therefore the time distance of this start signal from a 
subsequent defined pulse edge of the time base signal must be determined 
by means of an analog measuring circuit to obtain a first precision time 
measurement value, and beginning with said defined pulse edge a counting 
of the periods of the time base signal has to be started, in order to 
obtain a coarse measurement value. 
This measuring signal transit time measurement over the measurement route 
7, 15, 17, 19 and 22 is terminated by the output signal generated by the 
main receiver 23 and forwarded through line 24 to the time measuring 
device 25 when it receives the measurement light pulse coming back from 
the target object 17. Since this stop signal can follow very rapidly on 
the start signal in the case of short target distance, the time distance 
of the stop signal from a subsequent defined pulse edge of the time base 
signal is measured by means of a second analog measuring circuit contained 
in the time measuring device 25 in order to obtain a second precision 
measurement value for this signal transit time measurement. 
Only the reaction times of both the receivers 23 and 50 as well as the 
difference of the signal processing times in both the analog measuring 
circuits of the time measuring device 25, but not the reaction time of the 
main transmitter 1, enter as parasitic time values into this measuring 
signal transit time measurement. 
The same applies to each reference signal transit time measurement over the 
reference route 58, 11 and 22, for which a light pulse is emitted by the 
auxiliary transmitter 55, the main part of which rectilinearly traverses a 
second beam splitter 70 in order to be directed into the reference light 
path 11, which comprises a totally reflecting deflecting mirror 71 and a 
controllable damping device 33, and is united by means of a partially 
permeable coupling-in mirror 72 with the receiving light path 22 leading 
to the main receiver 23. 
The smaller part, coupled out by the second beam splitter 70, of each 
reference light pulse is fed into a second auxiliary light path 73, which 
is united by means of the partially permeable couling-in mirror 63 with 
the light path section 64 leading to the auxiliary receiver 50. In this 
reference signal transit time measurement, too, the output signal of the 
auxiliary receiver 50 serves as start signal and the output signal of the 
main receiver as stop signal for the time measuring device 25, which again 
undertakes a three-part time measurement, providing two precision time 
measurement values and a coarse time measurement value, wherein, as 
already mentioned, the resulting total measurement value is independent of 
the reaction time of the auxiliary transmitter 55 and for the remainder 
contains the same parasitic magnitudes as the previously obtained total 
measurement value of the measuring signal transit time measurement over 
the measurement route. The parasitic magnitudes can be eliminated 
completely according to the invention by subtracting the reference signal 
transit time value from the measuring signal transit time value or by 
subtracting a mean value which is taken from several such reference signal 
transit time values from a mean value which is taken from several such 
measuring signal transit time values. In any case the condition has to be 
met that all the measurements are performed so rapidly one after the other 
that no changes in these parasitic time spans arise in the period of time 
concerned. 
This condition is fulfilled particularly easily in this embodiment, since 
the use of two separate transmitters 1 and 55 and the omission of 
mechanically movable light path change-over switches permits a 
particularly high light pulse sequence frequency in the order of magnitude 
of 20 kiloHertz. 
If one triggers the auxiliary transmitter 55 more frequently than the main 
transmitter 1, a largely periodic operation of the auxiliary receiver 50 
and the main receiver 23 as well as of both the analog measuring circuits 
contained in the time measuring device 25 can be attained, which leads to 
the already mentioned substantial increase in the accuracy of measurement. 
The embodiment illustrated in FIG. 4 is constructed almost identically with 
the variant according to FIG. 3. It differs from that mainly in its manner 
of operation, which is made possible through a different drive of the time 
measuring device 25. 
Synchronising signals are hereagain delivered by the time measuring device 
25 through the line 29 to the trigger generator 3, so that the trigger 
signals thereof are strictly synchronised with the time base signal. Since 
only Laser diodes with a response behaviour, which is well reproducible 
over the short term, come into use here as transmitters 1 and 55, the 
signal transit time measurements over the measurement route 7, 15, 17, 19 
and 22 as well as also over the reference route 58, 11 and 22 can be 
started in dependence on the associated trigger signal solely with the 
coarse measurement value formation while dispensing with the first 
precision measurement value formation described above in conjunction with 
FIG. 3, which is symbolized in FIG. 4 by the trigger lines 56 and 57 being 
connected with the start input of the time measuring device 25. 
The respective signal transit time measurement is then terminated by the 
output signal of the main receiver 23, wherein a precision measurement 
value formation again takes place here. 
However, the reaction times of the main transmitter 1 and of the auxiliary 
transmitter 55 also enter into the thus obtained signal transit time 
values so that an elimination of all parasitic magnitudes is not 
completely possible through a simple subtraction of a reference signal 
transit time measurement value or a corresponding average value from a 
measuring signal transit time measurement value or a corresponding average 
value. 
In order to attain this object, two further signal transit time 
measurements are performed here over both the auxiliary light paths 7, 61, 
64 and 58, 73, 64. These first and second auxiliary signal transit time 
measurements are carried out as two-part time measurements just as was 
described above with respect to the measurement and the reference route. 
Amongst other things, the reaction times of the main transmitter 1 and of 
the auxiliary transmitter 55 again enter into the difference of both the 
auxiliary signal transit time measurements under discussion here so that 
all parasitic times can be eliminated by subtracting the difference of the 
auxiliary signal transit time measurement values from the difference 
between the measuring and the reference signal transit time measurement 
values. 
In order to be able to decouple the signal transit time measurements over 
the measurement and reference routes from those over the first and second 
auxiliary light paths 61 and 63, respectively, controllable damping 
devices 20, 33 and 65 are used. When a signal transit time measurement is 
for example to take place over the measurement route 7, 15, 17, 19 and 22, 
then the first auxiliary light path 61, which is coupled through the 
static beam splitter 60 with the measurement route, is made ineffective 
thereby, that the damping device 65 is switched to minimum light 
permeability, while the controllable damping device 60 also serving for 
dynamic control can be used for blocking the measurement light path. 
Instead of with the aid of the damping devices 20, 33 and 65, the same 
effect can also be obtained through an appropriate alternate blocking of 
the lines 24 and 67. 
At least four two-part time measurements are thus performed in this variant 
for the formation of a distance measurement value, which though it reduces 
the measurement speed, however offers the advantage that the time 
measuring device need comprise only one single analog measuring circuit 
which moreover can be operated strictly periodically just as both the 
receivers 23 and 50. 
The embodiment illustrated in FIG. 5 differs from all preceding ones above 
all in that its reference light path 11, which is branched off from the 
measurement route 7, 15, 17, 19 and 22 with the aid of a beam splitter 77, 
ends not at the main receiver 23, but at the auxiliary receiver 50. 
For the performance of a measuring signal transit time measurement over the 
measurement route 7, 15, 17, 19 and 22, a light pulse is generated in this 
embodiment at first by the auxiliary transmitter 55 in reaction on a 
trigger signal synchronised through the line 29 with the time base signal. 
The so generated light pulse is forwarded via transmitter light path 78 
from the auxiliary transmitter 55 to a second beam splitter 79 which is 
traversed by the main part of this light pulse. This main part, for use as 
first auxiliary light pulse is directed into a first auxiliary light path 
80 inside the instrument, comprising a totally reflecting deflecting 
mirror 81 and a controllable damping unit 65. By means of a partially 
permeable coupling-in mirror 82 the first auxiliary light pulse is further 
directed into the light path 22 leading to the main receiver 23. The 
output signal generated on the reception of this first auxiliary light 
pulse by the main receiver 23 is simultaneously conducted through the 
lines 83 and 84 to the start input and the stop input of the time 
measuring device 25; it is however suppressed by a not illustrated 
electronic switch behind the stop input of the time measuring device 25 so 
that it is used only as start signal. 
Hereby, a three-part signal transit time measurement over the measurement 
route is initiated, i.e. the time distance of this start signal from a 
following defined pulse edge of the time base signal is determined by an 
analog circuit and, simultaneously with this pulse edge, a counting of the 
pulses of the time base signal is started. 
The small part, coupled out by the beam splitter 79, of the light pulse 
generated by the auxiliary transmitter 55 is, for use as second auxilary 
light pulse, fed into a second auxiliary light path 85 inside the 
instrument and gets through a partially permeable coupling-in mirror 86 
into the light path 87, which leads to the auxiliary receiver 50 and 
contains a controllable damping device 33. The output signal generated 
thereupon by the auxiliary receiver 50 is conducted through the lines 88 
and 89 simultaneously to the stop input and to the start input of the time 
measuring device 25, however, in this case, remains ineffective, because 
the inputs are blocked by the above mentioned electronic switches. It is 
expressly pointed out here that the provision of a static beam splitter 
79, in conjunction with the described operation of the electronic switches 
of the time measuring device 25, results in an operation corresponding to 
that of a light path change-over switch. 
A defined number of periods of the time base signal after the above named 
trigger signal for the auxiliary transmitter 55, the trigger generator 3 
generates a trigger signal, likewise synchronised with the time base 
signal, for the main transmitter 1, the light pulse of which gets through 
the transmitter light path 7 and is split into two parts by a partially 
permeable mirror 77. The main part of this light pulse is, for use as 
measuring light pulse, directed into the measurement route 15, 17, 19 and 
22 and to the main receiver 23, the output signal of which becomes 
effective through the line 83 as stop signal for the time measuring device 
25. The second precision time measurement now required can be performed 
even for very small distances by the same analog measuring circuit as the 
first precision time measurement since the time distance of both the 
trigger signals, which according to the invention is an integral multiple 
of a period of the time base signal, can be chosen to be sufficiently 
large. 
Apart from this known time distance, only the response behaviour of both 
the transmitters 1 and 55 enter as parasitic magnitudes into the measuring 
signal transit time measurement over the measurement route, while the 
response behaviour of the mian receiver 23 as well as the signal 
processing time of the analog measuring circuit and the time measuring 
device 25 each enter twice with opposite sign and thus cancel each other 
out in the case of a sufficiently short time distance between start signal 
and stop signal. 
The small part, coupled out by the beam splitter 77, of the light pulse 
generated by the main transmitter 1 is, for use as reference light pulse, 
fed into reference light path 11 and gets, after reflection by a totally 
reflecting deflection mirror toward auxiliary receiver 50, the 
corresponding output signal of which, however, at the just described 
measuring signal transit time measurement is suppressed by the above 
mentioned electronic switches of the time measuring device 25. 
To eliminate the response delays of the two transmitters 1 and 55 a 
reference signal transit time measurement over the reference route is 
performed thereafter. For this purpose the auxiliary transmitter is at 
first again triggered correlated strictly with the time base signal. Of 
the light pulse emitted in that case, by an appropriate control of the 
electronic switch of the time measuring device 25, now however only that 
small part fed by the beam splitter 79 into the second auxiliary light 
path 85 becomes effective, which causes the auxiliary receiver 50 to 
generate a start signal through the line 89, while the line 88 is blocked. 
A three-part reference signal transit time measurement starts, which is 
terminated by a light pulse, which is triggered later likewise after a 
defined number of periods of the time base signal and which gets from the 
main transmitter 1 through the transmitter light path 7, the beam splitter 
77, the reference light path 11, the coupling-in mirror 86 and the light 
path 87 likewise to the auxiliary receiver 50 and causes this to generate 
a stop signal, while all other lines 89, 83 and 84 to the time measuring 
device 25 are blocked. 
The thus obtained reference signal transit time value over the reference 
route 7, 11 and 87 again contains only the response behaviour of both the 
transmitters 1 and 55 as parasitic time magnitudes, while the response 
behaviour of the auxiliary receiver 50 and the signal processing time of 
the analog measuring circuit each again eliminate themselves. 
The influence of the transmitters can also be eliminated by a subtraction 
of the reference signal transit time value from the measuring signal 
transit time value, when the above repeatedly named condition of a rapid 
measurement sequence is fulfilled. 
The particular advantage of this variant is, as already mentioned, that the 
time measuring device need have only one single analog measuring circuit, 
thus genuinely operating single-channelled, which assures a particularly 
good accuracy of measurement.