Patent DE-10040889-A1 (DE)
Title: Measurement of optical fields, e.g. above and below open waters, using spectral radiometer, is accompanied by correction of thermally-generated signal components
Inventor: BOCHTER KURT
IPC: G01J3/28
Filed: 2000-08-18
Abstract: To correct thermally-generated signal components of a photosensor in the spectral radiometer before and after measurement of light (or darkness) intensity, signals are received over equal time intervals. The mean value of the readings is subtracted from the brightness or darkness measurement.  Preferred Features: Measurements are transmitted from the spectrometer slit aperture by spacer-separated optical conductors. Spectral strips are produced in the focal plane, separated by dark gaps caused by the spacers. A CCD detector is employed. To correct for optical cross-interference and scattered light, the signal profile in dark regions adjacent to spectral strips (measurement channel) is measured and used to determine a spectral profile of signal patterns to be subtracted. Amplitude of the signal pattern is defined in spectral strip wavelength intervals, in which cross-interference and scattered signals are detected, but not external light. With similar signal profiles light signals received by various light measurement units are used for correction of the cross-interference in dark regions at upper edges, lower edges and the central region of the CCD sensor. For radiometric calibration of the multi-beam spectrometer using a number of light measurement sensors, a reference standard is used. A calibration correction is carried out from actual measurements and post-calibration measurements, producing a ratio Rk/RN. The correction strongly smoothes out additional noise. Variable optical attenuators are employed to standardize signal strengths detected by differing detectors, and are used in conjunction with a reference standard to match the signals. Optical fields may be measured in diverse open waters, above and below the surface. Signals from the upper (in air) sensors are matched by variable attenuators in the optical path, with the signals from underwater sensors. Calibration of attenuated spectra are made from transmission characteristics of the attenuation sections. For standardization of attenuated spectra an additional light measurement unit with its own detection unit is employed. This receives an incident light intensity in a narrow wavelength interval. Its signal is used for automatic control of the attenuators. For automatic adjustment of an illumination interval at given measurement depth during reception of a time-integrated signal, an additional, flashing measurement unit above to the water surface, with its own detector unit, is employed. To determine absorption coefficients of an optical field with the help of spectral radiometers, a net radiation/beam intensity (E(zo), E(zu)) and a scalar radiation intensity (E(z0), E(zu)) are determined simultaneously in two planes, an upper plane zo and a lower plane zu. From the values E(zo), E(zu) the attenuation coefficient K of the net radiation intensity is calculated as K(zm) =  DELTA z<-1> . ln(E(zo)/E(zu)). zm is the center of the zo and zu planes and  DELTA z is their separation. An additional expression in the form of the Gershun equation is provided, for solution. Net radiation intensities and scalar radiation intensities are determined successively with two light measurement units having a specially-designed angular sensitivity profile, exhibiting a blend of cosine- and 2 pi -reception characteristics in two planes (zo, zu).