GB2224175A

GB2224175A - Weather lidar

Info

Publication number: GB2224175A
Application number: GB8922245A
Authority: GB
Inventors: Christian Werner; Juergen Streicher; Wilhelm Krichbaumer; Hartmut Herrmann; Ulrich Oppel; Eberhard Gelbke; Christoph Muenkel; Uwe F W Berghaus
Original assignee: Impulsphysik GmbH
Current assignee: Impulsphysik GmbH
Priority date: 1988-10-05
Filing date: 1989-10-03
Publication date: 1990-04-25
Anticipated expiration: 2009-10-03
Also published as: GB8922245D0; DE3930272C2; GB2224175B; DE3930272A1

Description

2 2_ ',/-' 4 17 5 A _Lidar The invention relates to a lidar arranRement

Yhich can be used inter alia to measure atmospheric turbidities, to determine causes of turbidity of gases, - and to indicate a distance to a visibility obstacle in the atmosphere.

It is known to use a lidar to measure transmission, extinction and backscattering in the atmosphere, In order to detect gases or particles therein and determine their concentration and distance from the site of the lidar. Examples are the publications by V. E. Derr, "Estimation of the extinction coefficient of clouds from multiwavelength lidar backscatter measurements", Appl. Opt., Vol. 19, No. 14, pages 2310 - 2314 as regards Investigation of clouds, J. S. Randhawa et al, "Lidar observations during dusty infrared Test-l", Appl. Opt., Vol. 19, No. 14, pages 229'1'- 2297 as regards measurement of back-scattering and transmission on clouds from TNT explosions using a lidar equipped with a CO, or ruby laser, and D. K. Kreid, "Atmospheric visibility measurement by a modulated cw lidar", Appl. Opt. Vol. 15, No, 7, pages 1823 - 1831. DE-OS 23 52 972 and DE-OS 23 28 092 also relate to measurement of absorption and/or scattering of light In the atmosphere, The aforementioned publications describe a lidar which transmits a laser beam via a transmissirn lens system. The transmitted laser beam is absorbed and scattered in the atmosphere. The scattered light Is measured by a receiving lens system which has an aperture angle approximately equal to that of the transmitted beam. The extinction coefficient of the atmosphere and consequently the density of admixed gases and particles can be determined from the Intensity of' the scattered light, the known scattering cross-sections of various gases and particles and the spectral adsorption, since the laser wavelength Is known.

In, -another kind b-f known laser, two -11dar- beams of-.: neighbouring wavelength are used, '.one wavelength being - particular strongly absorbed or scattered 'by the gas to be deter-mined, whereas the adjacent wavelength is not 'appreciably 'absorbed or scattered by the gas. The "uninfluenced" back-scattered - laseri, radiation z Is then used as a reference for determining' the absorption of the gas in the atmosphere.

Since the Intensity of back-scattered light decreases with more then the fourth power of the distance of the scattering space from the 1 Ider, use Is made of electronic gete circuits which only evaluate a given distance range. Usually the lasers transmit short radiation pulses of the order of 10 nanoseconds, but modulated continuouslyoperating lasers are also used.

The known lidar embodiments are of use for measuring the extinction coefficient of the atmosphere In gener&l and for measuring the extinction of an admixed gas or particles. However they are not suitable for clearly and rapidly distinguishing between different kinds of atmospheric opacity, e.g. snow from fog, rain or a fixed obstacle to vision.

An object of the Invention Is to design a lIdar which clearly and rapidly distinguishes different kinds of atmospheric opacity, e.g. snow, fog and rain.

The present invention provides a lidar arrangement for measuring backscattered light received at two mutually separated reception regions and including a transmitter for transmitting a light beam at a given aperture or apex angle, a receiver having a first and a second reception device for the backscattered part of the transmitted light beam, the respective devices having reception optical systems designed such that their reception regions do not overlap, and an evaluation device for evaluating electrical signals of the first and second receiving device to determine the kind of turbidities in the atmosphere.

In a modification of this arrangement for determining the causes of turbdities or cloudings of gases, the light transmitter is designed for transmitting linearly polarized radiation, the receiver including a first receiving device having a first detector for detecting a part of backscattered radiation which propagates parallel to the transmitted rays and which is polarized parallel to the polarization direction of the transmitted rays, and a second detector for detecting rays backscattered parallel to the transmitted rays and being polarized perpendicularly to the polarization direction of the transmitted rays. The evaluation device determines from the ratio of the electrical output signals of the two detectors and causes of the turbidities. In another modification for use in motor vehicles to indicate distance of visibility obstacle, the transmitter transmits pulses of the radiation and the evaluation device includes means for evaluating time intervals between pulses received by one of the receiving devices.

--- L - 1 An-. embodiment, of, the lider.,- - according-' tw. the. Inventiory.. and the distance monitor according to th.c.. Invention will.- be. described hereinafter with reference to drawings, -in which:

Fig. 1 shows a diagram of a lidar according to the Invention', Fig. 2 shows an Idealized distance-corrected lidar signal from a well of fog; Fig. 3 shows an idealized non-distance-corrected lider signal from a well of fog; Fig. 4 shows an idealized distance-corrected lider signal from a well of snow; Fig. 5 shows an Idealized distance-corrected lidar signal from a wall of rain; Fig. 6 shows an Idealized non-d Is tan ce-corrected lidar signal from a solid obstacle to vision, and Fig. 7 Is a block circuit diagram of additional components for controlling the speed of a motor vehicle.

Fig. 1 shows a lidar 1 comprising a transmitter 3, a receiver 5 and an evaluating device 7. The transmitter 3 is a laser 9 which transmits pulsed linear-polarized radiation In the infrared wavelength region at a wavelength of 1.4 pm, so that Its radiation is not harmful to the eyes. The electric field vector of the linearpolarized radiation from laser 9 is In the plane of the drawing. The radiation Is widened by a transmission lens system 11 to form a beam 12 having an aperture angle of approximately 10 mrad.

r, - A well of fog 14 Is situated at a distance d in front of the lidar 1, and the beam 12 penetrates It and is partly reflected or backscattered on the mist droplets in a beam 15 in the opposite direction to the Incident beam. Another part of the Incident radiation 12 Is multiply scattered and returns to the lidar 1 outside the cone of radiation 12, mainly In a radiation cone 17 having a width of 10 to 30 mrad. For clarity, the angles of beams 12 and 15 and the radiation cone 17 are highly exaggerated In Fig. 1.

A first receiving device for detecting the beam 15 has a Cassegrain mirror arrangement 19 (a main mirror 20 perforated in the middle and a collecting mirror 22 disposed along the optical axis over the perforation 21), a shutter 23, an optical system 24 which converts the bundle of beams 15, 17 leaving shutter 23 into parallel beams and transmits only radiation coming from the spectrum of laser 3 (a narrow-band filter 26), a Wollaston prism 25 which divides each bundle of rays 15 and 17 into two respective bundles 15a/17a and 15b/17b, and respective focusing lenses 27a, 27b which focus the radiation 15a, 15b respectively on to a respective photodiode or detector 29a, 29b. The Wallaston prism 25 is disposed so that linear-polarIzed radiation from bundle 15, whose electric field vector Is parallel to the electric field vector of beam 12, I.e. In the plane of the drawing, is in the form of a beam 15a when it leaves the prism. Radiation from bundle 15 polarized at right angles thereto Is in the form of a beam 15b when it leaves the Wollaston prism 25.

A second receiving device Is for detecting the bundle of rays 17. Like the first receiving device, It contains the Cassegrain mirror arrangement 19, the shutter 23, the optical system 24 and the Wollaston prism 25, which divides the radiation In bundle 17 Into a linear-polarized beam 17a, whose electric field vector lies In the plane of the drawing, and a linear-polarized beam 17b, whose polarization plane is at right angles to the beam 17a, and also comprises respective focusing lenses 31a, 31b which focus beams 17a,

17b respectively. on to a photodic)de- or- -detector 3U, - 33b respectively. To avoid overloading Fig. r, only the- "beam,. -boundaries" 12, 15, 15a, 15b, 17, 17a and 17b are shown.- The electric signals generated by the photodiodes 29e, 29b,' 33e and 33b In dependence on the radiation Intensity I,, %, I, J- ' 11 75 and I,-, _L striking them are amplfied as previously described by respective amplifiers 34a, 34b, 34c and 34d, the amplification of which Is adjustable.

A small part of the beam 12 emitted by laser 9 is sent via a photoconductor 35 to a photodiode 36 connected to a timing circuit 38. An output 37e of the timing circuit 37 Is connected to a signal processing element 41 and each of four other outputs is connected to a respective one of the four amplifiers 34a, 34b, 34c and 34d. Four indicators 43e - 43d, labelled "solid obstacle", "fog", "snow" end "rain", are electrically connected to the signal-processing element 41. A fifth Indicator 44, likewise connected to the signel-processing element 41, Is for showing the distance d between the lider 1 end the wall 14 of fog.

The laser 9 transmits pulses at a repetition frequency of a few hundred Hertz and having a pulse width of a few tens of nanoseconds, the pulses being widened by the transmitting lens system 11 to form a beam 12 having an aperture angle of about 10 mrad. A wall of fog 14 Is situated at a distance d of 200 metres from the lidar 1. A part of each laser pulse travels via the photoconductor 35 to the photodiode 36, which starts the timing circuit 37. The timing circuit 37 Is designed so that 60 ns, after the first laser pulse Its outputs 37a 37e deliver a digital-coded item 1190.011 corresponding to a transit time of 60 ns for an outward and return Journey of 180 m, Le. a distance d of 90 m from an obstacle to vision, if any.

1 W; The digital-coded date sets the amplifiers 34a - 34d for about 10 ns to a sensitivity sufficient to amplify an electric signal from the photodiodes 29a, 29b, 33a or 33b affected, corresponding to backscattered radiation from an expected obstacle to visibility at a distance of 90 m. After about 1 millisecond (corresponding to the set repetition frequency of the laser pulses) and additional laser pulse Is transmitted and after 70 ns the timing circuit 37 delivers a digital-coded item '191.5" corresponding to an obstacle, If any, at a distance of 91.5 m from the lidar 1 to amplifiers 34a - 34d and the signal-processing element 41, etc.

The Intensity of the backscattered radiation 15 and 17 decreases with more than the fourth power of the distance, and consequently an electronic "window" Is used to divide the length into small spatial compartments and, after each transmitted laser pulse, to scan a different spatial compartment for radiation back-scattered from the Just- transmitted pulse. This Is a method of suppressing strong scattered radiation from the areas near the lidar 1 and adjusting the amplification of the received signals so that their Intensity Is corrected for distance. The distance of the spatial region transmitting the scattered radiation is determined by the time when the electronic "window" is switched on. The geometrical resolution of the distance is obtained from the opening time of the "window" (10 ns = 1.5 m).

Since the wall of fog 14 Is Initially at a distance of 200 m, the aforementioned measurement does not give a signal. It Is only af ter the timing signal 37, 1340- ns later, delivers the digital-coded Item "201" to the amplifiers 34a to 34d and to the signal processing element 41 that a signal, radiation from the multiply scattered radiation of beam 17, Is received by photodiodes 33a and 33b and by photodiode 29a, which receives only scattered, reflected radiation within the beam 15, which Is polarized In a plane parallel to the polarization plane of the laser radiation in beam 12. During the subsequent laser. pulsesw, during each,-, of'. whIch.---back-scattered radiation is received from c respective spatial compartment.-distant by a further 1.5 m, the prepared electric signal from the parallelpolarized back-scattered radiation _of beam 15 increases along the abscissa (Fig. 2a) up to a maximum value I,.,, and then decreases. In Fig. 2e this is shown by a logarithmic ordinate for the,signal 1,. (distance-corrected electric signal I,11 from diode 29a after its amplifier 34a). The distance-corrected signals I,,11 and I,1 from the light scattered by beam 17 and received by photodiodes 33a and 33b are plotted logarithmically in Figs. 2c and 2d against the linear distanced from the lidar 1. In Fig. 2b the d is tance- corrected scattered light I,,r, 1 with vertical polarization of the beam 15 Is plod logarithmically against the linear distance d. As before, the signals I,, and I, -,J- rise to a maximum value and then decrease. The signal 1,..J. does not reach an values above the background no ise.

Fig. 3, by way of illustration, shows the signal measured by the photodiode 29a but not corrected for distance, up to a distance of 500 m, when the wall of fog 14 is 200 m thick and at a distance d of 200 in.

If these measurements are repeated with a wall of snow and a wall of rain, the idealized results for snow are as in Figs. 4a to 4d and for rain in Figs. 5a to 5d. The diagrams are associated with the photodiodes 29a, 29b, 33a and 33b In similar manner to Figs. 2a to 2d. In order to distinguish the measured results, the measured distance-corrected idealized intensities I,sa 11 S.L I I, -,I I and L-71 are given an additional Index N for the measurement on the wall of fog, R for the wall of rain and S for the wall of snow.

Unexpectedly, measurement by the aforementioned process has yielded the following:

11 Z 9 - In the case of the well of fog 14, distance-corrected signals II S 11 NI 11 7 11 N and II7J-N respectively are obtained; signal II^.L N Is missing; In the case of a well of snow (not Mown) the respective distancecorrected signals are 11,.11 5, 1, 51, 1,711. and I,..L 5 and In the case of a well of rein (not shown) a distance-corrected s Igne 1 11.5 11, is rece ived; the s igne Is I,, 1,, I,, 11,, and 11 71 are negligible.

In the case of a solid obstacle to visibility (not shown), for which the signal from photodiode 29a, not corrected for distance, is shown in Fig. 6, i contrast to the previouslymentioned signals the photodiode 29a detects a signal 115 11 H having a pointed needle-like rise. A component 11r,_LH may a lso be present, depending on the nature of the surface (mirrorreflecting or diffuse).

The above criteria provide the signal-processing element 41 with a clear classification, as to whether the measured obstacle to visibility Is a well of fog, snow or rain or a solid obstacle. In the case of a wall of fog, snow or rain, the signal processing element 41 determines a respective distance value d for each measurement, from the digital-coded values transmitted by the timing circuit 37. The value d Is approximately In the middle of the signal gradients I,,11 NI IisA s and I1511R. In the case of a solid obstacle there Is no formation of an average, owing to the needle-like pointed gradient of the signal I,. A H. The aforementioned distance d Is given numerically In metres in Indicator 44. In addition, In accordance with the aforementioned evaluation, the respective Indicator 43a, 43b, 43c or 43d lights up, corresponding to c well of fog or snow or rain well or a solid obstacle to visibility.

1 1 1 1 __10 ---- If there so-lid -.,obsr.fecle.-insi.de%w tha.,.we.11' of fog, snow or'rein, the, curves shown in Figs. 2a, t2b, 4al.b., 56, 5b are overlaid by a -pointed' needle-like pulse which Is recognized as a;solid obstacle by the, signal- processing element 41. In this case the Indicator 43d for the solid obstacle lights up in addition to the indicators 43a, 43b or 43c for the fog or snow or rain well. The distance indicator 44 shows the distance d from the solid obstacle.

The aformentioned lidar 1 can be Incorporated In a motor vehicle (not shown), when the aforementioned measuring procedure Is carried out In the direction of travel. The indicators 43a, 43b, 43c, 43d for a well of fog or snow or rain or a solid obstacle respectively are an excellent help to the driver. If the date from the signal processing element 41 as shown In Fig. 7 are transferred to a data processing device 47 which determines the instenianeous speed of the vehicle via a tachometer 49, the Instantaneous speed of the vehicle can be controlled In optimum manner In dependence on obstacles to visibility as they occur, in that the data processing device 47 acts on the fuel supply 50 and consequently on the drive or brake system 51 of the vehicle In dependence on the date from the tachometer 49 and the signal-processing element 41.

The rain-wall indicator 43c is also activated if a vehicle in front splashes water on the road.

As the preceding measurements show, there Is not much difference between the results obtained by pho tod lod s 33a and 33b. Consequently one of the photodiodes 33a or 33b and its amplifier 34b or 34d can be omitted without reducing the accuracy of the results. Under certain environmental conditions, the evaluation Is more accurate If both diodes 33a and 33b are used.

Depending on the distance to be measured, different Initial and final values can be set in the timing circuit 37, and also the laser 9 can have other repetition frequencies and the resolution can be different from 10 ns corresponding to 1.5 m.

---12' - - -

Claims

CLAIMS:!:

1. A lidar comprising.-,- a, transmitter; for transmitting a beam having a preset aperture angle,-a receiver having a first and a second receiving device for back-scattered radiation from the transm-tted beam and each arranged so that the receiving areas thereof do not substantially overlap, and an evaluating device for evaluating the electric signals from the f4.rst and the second receiving device in accordance with the received back-scattered radiation, in order to determine the nature of opacity in the atmosphere.

2. A lidar according to claim 1, wherein t_ach receiving device includes a 19ns systemconstruced and arranged so that said receiving areas do not substantially overlap.

3. A lidar according to claim 2, characterised in that the transmitter is constructed so thatit transmits pulsed linear-polarized radiation in the infrared spectrum which is not harmful to the eyes.

4. A lidar according to claim 2 or claim 3, characterised in that the receiving lens system of the first receiving device is constructed so that substantially only the transmitted beam lies in its receiving area, and the receiving lens system of the second transmitting device is constructed so that the transmitted beam does not lie in its receiving area.

5. A lidar according to claim 4, characterised in that the receiving lens -system of the second receiving device is constructed so that its receiving area is disposed around the receiving area of the lens system of the first receiving device, and the second 1 receiving device has a first detector for detecting multiply scattered radiation from the beam.

6. A lidar according to claim 5, characterised in that the second receiving device has a second detector and a polarization-analyzing element is disposed in front of each detector in order to determine the intensity of radiation multiply back-scattered from the beam in accordance with its polarization parallel to and at right angles to the direction of polarization of the transmitted beam.

7. A lidar according to any of claims 1 to 6, characterised in that the first receiving device has a polarization-analyzing element,in front of a first and a second detector, in order to determine the intensity of radiation back-scattered from the beam in accordance with its polarization parallel and at right angles to the direction of polarization of the transmitted beam.

8. A lidar according to claim 7. characterised in that the evaluating device comprises a first indicator for a solid obstacle to vision, which indicates when the evaluating device of the first and/or second detector of the first receiving device receives a needle-shaped pointed pulse, a second indicator for fog as an obstacle to vision and indicating when the evaluating device receives from the second detector of the first receiving device receives a signal which is negligible compared with the signals from the first detector and receives a slowly rising and falling distance-corrected signal from the detector or the detectors of the second receiving device, a third indicator for rain or water splashed from the ground and constituting an obstacle to vision. and indicating when the evaluating device ---14--- - receIves - from.. the - second detector.. -of' -the.- -first receiving device receives,a-signal;negligiblL=,-compared"., with the signal from the first detector and 'A:r.eceives only a negligible slowly-rising and falling distance-corrected signal from the detector or-the detectors of the second receiving device, and a fourth indicator for snow constituting an obstacle to vision and indicating when the evaluating device receives from the first and second detector of the first receiving device a signal which is substantially constant during the time period and receives a slowly rising and falling distance-corrected signal from the detector or the detectors of the second receiving device.

9. A distance monitor for vehicles, comprising a lidar according to claim 8, characterised in that the evaluating device has a fifth indicator showing the distance of the obstacle to vision from the vehicle equipped with the monitor, the distance being determined from the transit time of the radiation pulses sent from the lidar to the obstacle and back, using the electric pulses from the first receiving device.

10. A distance monitor according to claim 9, characterised by an electronic data-processing device which determines a maximum speed from the distance and the nature of the obstacle allowing for the expected state of the road at the obstacle and/or acts on the vehicle drive and/or brake system to prevent the maximum speed being exceeded.

A lidar for determining the causes of opacity in gases, characterised in that the transmitter is designed for linear-polarized radiation and the receiver has a first receiving device 1.

- 15 comprising a first detector for radiation polarized parallel to the polarization direction of the transmitter and radiated back in the direction of transmission, and a second detector for radiation sent back in the transmission direction and at right angles to the polarization direction of the transmitter,, and an evaluating device for determining the ratio between the electric output signals of the two detectors.

12. A lidar according to claim 11, characterised in that the receiver has a second receiving device comprising a detector for back-scattered radiation from the transmitter from an area not containing the beam, the evaluating device also determining the ratio of the electric output signal 9f the detector of the second receiving device to the signal from the first and/or second detector of the first receiving device, and the first and second detector are designed so that they receive approximately only the radiation back-scattered within the transmitted beam.

13. A lidar substantially ashereinbefore described with reference to and as shown in Figures 1 to 6 of Figure 7 of the accompanying drawings.

Published 1990a;t7hePatent Office, StiLtoe House, 66171 High Holborn, LondonWClR4TP. Further copies maybe obtained fromThaPatentOffice. Sabn B=nch, fit, Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent, Con. 1/87