EP0966694B1 - Process and device for determining bi-directional reflectance distribution - Google Patents

Description

The invention relates to a device and a method for determining the bidirectional reflectance distribution.

By means of the bidirectional reflectance distribution function (BRDF) e.g. Conclusions about the health status of forest areas and others Floor areas pulled or other climate-relevant aerosol parameters derived become.

The bidirectional reflectance distribution function depends on the wavelength of the examined light and the radiance of the incident unreflected Radiation. These in turn depend on the azimuth and zenith angles the position of the sun or the observation azimuth and the observation time angle. The expression bidirectional therefore indicates that the function not only from the zenith and azimuth of the observation point, but also from Zenith and azimuth of the light source (sun) is dependent.

Data is used to determine the bidirectional reflectance distribution function used by satellites (e.g. NOAA 6/7) or measurements with spectro or Radiometers made. Such a spectrophotometer is such. B. from the Brochure "SP1A" from Dr. Schulz & Partner, by means of which the Global radiation is measured. The spectrophotometer is attached to one Rotary device attached to the pivoting of the spectrophotometer two axes allowed in all directions. To from the measured radiance To determine the reflectance factor is usually the radiance above one Reference surface ("white disk") determined. This is preferably a spectral plate with a well-defined reflectivity that is independent on the direction of the incoming and outgoing radiation. Disadvantageous the known device is its lack of resolution. When measuring in a filter position and increments of 1 ° in the azimuth and zenith direction are required the device 18 hours for a measurement since every two seconds can be driven and measured. With a step size of 5 ° in azimuth and The zenith direction can be measured every three seconds. The series of measurements thus takes one hour and five minutes. Because the bidirectional Reflectance distribution function from the azimuth and zenith angle of the position of the sun is dependent, the measurement must be carried out very quickly in order to ensure an almost constant position of the sun. To do this when measuring To ensure the entire half-space must be in the known Process either the field of view and / or the The step size for the azimuth and zenith angles can be chosen to be relatively large. In the Usually you use an opening angle and a step size of 5 ° to 15 ° and then measures in correspondingly many settings, some of which are made manually the entire lower half space. The result is one bidirectional reflectance distribution function with a resolution of 5 ° up to 15 ° angle step forward. Another disadvantage of the known device is the lack of accuracy of the determined reference, since the constancy of the Reflectivity is not fully guaranteed, both at the different Points of the spectral plate as well as related to the dependency from the line of sight.

From the two specialist articles MAGNER THOMAS J: "Moderate-resolution imaging spectrometer-tilt baseline concept "1991, EARTH AND ATMOSPHERIC REMOTE SENSING; ORLANDO, FL, USA APR 2-4 1991, PROC SPIE INT SOC OPT ENG; PROCEEDINGS OF SPIE - THE INTERNATIONAL SOCIETY FOR OPTICAL ENGINEERING 1991 PUBL BY INT SOC FOR OPTICAL ENGINEERING, BELLINGHAM, WA, USA, PAGE (S) 272-285; and KOVALICK W M ET AL: "Data processing and calibration of the Advanced Solid-State Array Spectroradiometer ", IGARSS'94. INTERNATIONAL GEOSCIENCE AND REMOTE SENSING SYMPOSIUM, SURFACE AND ATMOSPHERIC REMOTE SENSING: TECHNOLOGIES, DATA ANALYSIS AND INTERPRETATION (CAT.NO.94CH3378-7), PROCEEDINGS OF IGARSS'94 - 1994 IEEE INTERNATIONAL GEOSCIENCE AND REMOTE, ISBN 0-7803-1497-2,1994, NEW YORK, NY, USA, IEEE, USA, PAGE (S) 1652 -1654 VOL.3. are each BRDF measurements using CCD cameras from a satellite or airplane known.

From the technical article KARNER KONRAD F ET AL: 'Image based measurrment system for anisotropic reflection "August 26, 1996, PROCEEDINGS OF THE 1996 17TH ANNUAL CONFERENCE AND EXHIBITION OF THE EUROPEAN ASSOCIATION FOR COMPUTER GRAPHICS, EUROGRAPHICS'96; POI TIERS, FR AUG 26-30 1996, COMPUT GRAPHICS FORUM; COMPUTER GRAPHICS FORUM; GRAPHICS-VIRTUAL REALITY-GRAPHICS HIGHWAYS SEP 1996 BLACKWELL SCIENTIFIC PUBLISHERS, OXFORD, ENGL; PAGE (S) 119-128; a BRDF soil survey with a CCD camera known. A single picture is taken with the one under a fixed one Angle of the CCD camera aligned to the surface to be measured. The one with this The achievable accuracies of the BRDF measurement are low and for many use cases are not sufficient.

From the article KARNER K F: "Using images to estimnate reflectance function, WSCG 96th FOURTH INTERNATIONAL CONFERENCE IN CENTRAL EUROPE ON COMPUTER GRAPHICS AND VISUALIZATION 96, IN CO-OPERATION WITH IFIP WORKING GROUP 5.10 ON COMPUTER GRAPHICS AND VIRTUAL WORLDS. CONFERENCE PROCEEDINGS, PROCEEDINGS OF WSCG 96: FOURTH INTERNA, ISBN 80-7082-238-4,1996, ZIP codes, CZECH REPUBLIC, UNIV. WEST BOHEMIA, CZECH REPUBLIC, PAGE (S) 133 - 140 VOL. 1 is a BRDF soil measurement with a CCD camera known, the measurement at different angles of the radiation source is carried out. This is done again using a single shot the CCD camera recorded the half space, so that it CCD sensor is a CCD matrix, which is the achievable resolution for the BRDF restricted.

The invention is therefore based on the technical problem of a device and a method for determining the bidirectional reflectance distribution create, with an improved resolution of the bidirectional reflectance distribution function is achievable.

The solution to the technical problem results from the features of the claims 1 and 4. By the design of the optical detector device as a CCD line camera can each simultaneously, according to the Opening angle of the camera, a segment of the surface to be scanned measured and a horizontal adjustment of the detector device for It is not necessary to record individual measuring points. This allows the measurement surface faster by a factor of 2000, so that the errors are due to a change in the position of the sun is negligible. The length of time for a series of measurements including polarization measurement is approx. 65 seconds with a resolution of up to 0.5 °. Such wide-angle CCD line cameras have long been known from aerospace technology. An exemplary Description of such a camera is the technical article "wide-angle stereo camera WAOSS concept and mode of operation, Sandau et al .; picture & sound, 9/10, 1992, pp. 224 ff. Can be seen on the here regarding the training of the camera explicit reference is made. The procedure assumes that after Tilt of the CCD camera by 90 ° - 0.5 IFOV the pixels of the CCD lines as the zenith angle can be interpreted, the measurement at different azimuth angles but by rotating the tilted camera around a vertical axis can be realized so that a parallactic mounting of azimuth and Zenith distance as is unnecessary in the prior art. In addition, it does not apply the requirement of a white disk, which is due to the state of the art not exactly Lambertian properties is another source of error. Further advantageous embodiments of the invention result from the subclaims.

For the detection and suppression of the self-polarization of the CCD line camera Measurement errors caused can include at least one further reference measurement be performed at certain points on the surface be measured in a different CCD line position. Because the polarization the CCD line camera is known can by means of the two Measurement data of the measurement errors can be deducted.

By quickly acquiring all measuring points, you can determine the reference without a spectral plate and the reference directly through Rotation of the rotating device by 180 ° around the horizontal axis and repetition the measurement in the cardinal direction.

Fig. 1

eine perspektivische Darstellung der Vorrichtung bei Erfassung der Meßdaten,

Fig. 2

eine perspektivische Darstellung der Vorrichtung bei Erfassung der Referenz und

Fig 3

eine perspektivische Darstellung der Vorrichtung bei Erfassung einer weiteren Referenz zur Kompensation der Meßfehler aufgrund der Eigenpolarisation der CCD-Zeilen-Kamera.

The invention is explained in more detail below on the basis of a preferred exemplary embodiment. The figures show:

Fig. 1

1 shows a perspective view of the device when the measurement data are recorded,

Fig. 2

a perspective view of the device upon detection of the reference and

Fig. 3

a perspective view of the device upon detection of a further reference to compensate for the measurement errors due to the self-polarization of the CCD line camera.

The device for determining the bidirectional reflectance distribution comprises a CCD line camera 1 and a rotating device 2 on which the CCD line camera 1 is mounted. By means of the rotating device 2 is the CCD line camera 1 both about a vertical axis 3 and about a horizontal axis 4 swiveling. Known CCD line cameras usually have three CCD lines with a line width of 5184 pixels. These lines are for the purpose the stereo image processing arranged so that the middle line is vertical look down and the two others look 25 ° forwards and backwards, where only to determine the bidirectional reflectance distribution middle CCD line is used. To suppress stray light can be used a slit-shaped diffuse protection is arranged in the camera optics. to Detection of the respective radiation density of a surface to be examined 5, the CCD line camera 1 is aligned so that one end of the middle CCD line looks perpendicular to surface 5 and thereby an imaginary Circle center 6 of the surface 5 defined. The opposite The end of the CCD line is thus directed to a point 7 off-nadir. At a The opening angle of 80 ° is the optical axis of the CCD line camera 1 to the surface 5 at an angle of 40 °. By means of a simultaneous recording the segment shown in dashed lines is included. Subsequently is the CCD line camera 1 by a certain angle vertical axis 3 rotated and another segment added. This The process is repeated until the CCD line camera 1 is rotated through 360 ° was and thus measured a circle 8 of the surface 5. The Gradual vertical rotation can be done either manually or automatically by means of a suitable programmable control.

To determine the bidirectional reflectance, the detected radiation density has to be determined with a reference quantity corresponding to the incident radiation be compared. For this purpose, the CCD line camera 1 according to FIG horizontal axis 4 rotated by 180 ° and the incident radiation density again recorded in segments. This means that for each point on the surface 5 both the incident as well as the reflected radiation density are known, so that from it the resulting bidirectional reflectance distribution function of the surface 5 can be derived.

Due to the large opening angle and the optical components the CCD line camera 1 or the camera optics have a certain intrinsic polarization on. The intrinsic polarization of the camera optics is in the area of optical Axis almost zero and increases towards both ends of the CCD line. Both Previously known CCD line cameras 1 can use the self-polarization Margins up to 20%. The polarization of the incident, from the earth's surface Depending on the background, reflected light can be up to 30% red Light and up to 60% in blue light. Thus, the decreasing Wavelength increasing measurement errors due to the polarization up to to be 6% or 12%. To determine and suppress these measurement errors due to the intrinsic polarization of the CCD line cameras 1, according to FIG another reference measurement can be made. For this, e.g. the CCD line camera 1 aligned such that the optical axis of the CCD line camera 1 is directed to the off-nadir point 7 of the first measurement, ie the points where the largest self-polarization of the CCD line camera 1 in the previous measurement occurred. Since in the area of the optical axis Eigenpolarisation is zero, the measurement error is due to polarization for the off-nadir point 7 in the reference measurement zero. By means of a comparison between The two measured values can thus be based on the degree of polarization of the Surface 5 reflected radiation can be inferred. Because the polarization and their distribution over the CCD line a fixed, determinable Device size, the measurement error can be due to the polarization for everyone Points of a segment and thus the entire surface 5 are eliminated become.

Claims (4)

1. Method of determining the bidirectional reflectance distribution by means of a CCD line-scanning camera (1), which is disposed on a rotating apparatus (2), the rotating apparatus (2) being configured to be pivotable about a vertical axis (3) and a horizontal axis (4), said method including the following method steps:

   a) aligning the central CCD line with the surface (5) in such a manner that one end of the CCD line looks perpendicularly relative to the surface (5) and thereby defines an imaginary centre of a circle (6) of the surface (5),

   b) simultaneously detecting the optical radiance of a segment of the surface (5),

   c) storing the detected radiance according to method step b),

   d) rotating the CCD line-scanning camera (1) about the vertical axis (3) through a specific angle and repeating the method steps b), c) and d) until the CCD line-scanning camera (1) has rotated about the vertical axis (3) through 360°,

   e) displacing the optical axis of the CCD line-scanning camera (1) by means of the horizontal axis (4) of the rotating apparatus (2) through 180°,

   f) segmentwisely detecting and storing the incident radiation,

   g) rotating the CCD line-scanning camera (1) about the vertical axis (3) through a specific angle and repeating the method steps f) and g) until the CCD line-scanning camera (1) has pivoted through 360°.
2. Method according to claim 1, wherein the determination of the reference according to the method steps e) to g) is effected prior to and/or subsequent to the detection of the optical radiance of the surface (5).
3. Method according to claim 1 or 2, wherein, in order to determine the inherent polarisation of the CCD line-scanning camera (1), the optical axis is directed towards the off-nadir point (7), and the optical radiance is detected and stored.
4. Apparatus for accomplishing the method according to one of the preceding claims, said apparatus including an optical detector, which is in the form of a CCD line-scanning camera (1), and a rotating apparatus (2), which pivots the CCD line-scanning camera (1) and is configured to be pivotable about a vertical axis (3) and a horizontal axis (4).

Images