SIMPLE RED-BEAM ATTENUATION METER FOR SEA TRUTH MEASUREMENTS

Transcription

1 SIMPLE RED-BEAM ATTENUATION METER FOR SEA TRUTH MEASUREMENTS Vladiir I. Haltrin Naval Research Laboratory, Ocean Sciences Branch, Code 7331 Stennis Space Center, MS , USA. e-ail: Michael E. Lee, and Oleg V. Martynov Optical Oceanography Laboratory, Marine Hydrophysical Institute of Ukrainian National Acadey of Science 2 Kapitanskaya St., Sevastopol, Criea, , Ukraine. e-ail: <ocean@hi2.sebastopol.ua> ABSTRACT Red bea attenuation is useful in analyzing profiles of reotely sensed hydrooptical paraeters and for accurate resolution of the vertical structure of chlorophyll and suspended particles. Hence, the developent of a probing red bea attenuation eter has practical interest. Auto-colliating optical scheatics with a cat s eye have been used in the eter developed. The instruent is intended for application both as an independent device and as a odule for oceanographic systes. High accuracy with inial size and power consuption are realized by using electro-optical seiconductors. The dual bea principle of stabilization with negative optical feedback for the light eitting diode (LED) is applied. The exponential shape of LED odulated ipulses causes the instruent output to be proportional to bea attenuation. By placing the optical attenuator into the reference bea one can copensate for pure water absorption and easure the volue scattering coefficient. The optical base of the cat s eye can be varied fro 50 c on down. The device is not sensitive to abient light. 1.0 INTRODUCTION Unlike the apparent optical properties of sea water, inherent optical properties are steady characteristics of a scattering ediu, independent of both the abient light conditions and the spectral changes due to light penetration into deep sea water. Knowledge of the inherent optical properties substantially iproves the quality of the physical interpretation of under-satellite easureents and perits to iprove ethods for the solution to inverse probles of arine optics. Therefore, besides copulsory light field easureents, the SeaWiFS Pre-launch Science Working Group has recoended additional easureents of inherent optical properties. At the Marine Hydrophysical Institute of Ukrainian Acadey of Sciences and at Naval Research Laboratory, Stennis Space Center, MS, USA ethods and instruents for the easureent of bea attenuation are the ost advanced. For a long tie such instruents were used in perforing field hydrooptical investigations. The choice of the bea attenuation coefficient as an additional paraeter which is essential in carrying out under-satellite optical easureents is not accidental. Because the attenuation coefficient is the su of the absorption and scattering coefficients, it contains inforation about the features of these paraeters in various waters. The ost iportant features, investigated to the present tie, are as follows: 1. In the bea attenuation spectru, absorption bands fro suspended particulate atter * Presented at the Second, San Francisco, California, June II-272

2 becoe barely distinguishable due to copensating changes in scattering. This was shown in theoretical accounts by Morel and Bricaud (1981) with the use of Mie scattering theory to biological particles. The shape of the absorption spectru of yellow substance is reproduced in the bea attenuation spectru practically without distortions and is described by an exponential dependence (Jerlov, 1961): ~ exp λ, (1) 2. In the red band the absorption both by particulate atter and by yellow substance is negligible. Detailed investigation of these features required the developent of new instruents as well as the iproveent of existing instruents for bea attenuation easureents. When perforing under-satellite experients, these instruents will allow not only to easure vertical profiles of bea attenuation and light scattering coefficients, but they also will essentially copleent conventional biocheical observation with detailed data of their stratification, and will also provide an opportunity to take saples at optial depths. 2. METHODS For a long period of tie the transparency of upper layers was evaluated using the Secchi disk depth. Despite the extree siplicity of these easureents, their application is liited due to the intense influence of abient light and surface conditions on the easured data. Besides, the transparency evaluation of sea water by eans of Secchi disk depth is not applicable to cases when it is iportant to know the vertical distribution of the trasparency. As a paraeter ore fully describing the light attenuation, it is convenient to use the bea attenuation of sea water. A nuber of bea attenuation eters (or transparency eters) which use various optical and photo-electric layouts have been developed (Zhiltsov and Nikolaev, 1980; Lee, 1980; Vinokurov, Migulya, Prikhach and Shishkin, 1988; Anonyous, 1989). Most of the are based on a ethod which consists of easuring the bea attenuation coefficient c averaged over a water path l defined by the instruent optical base: F = F exp cl, (2) 0 where F 0 and F are the light fluxes before and after traversing the water path. The easureent accuracy of the bea attenuation coefficient by this ethod is defined ainly by the instability of the eleents in the photo-electric circuits and by such geoetrical paraeters as the light source colliation angle, the photodetector acceptance angle and the length of the optical base. Because of the instability of the eleents in the photo-electric circuits, the error could be diinished by the choice of an appropriate photo-electric layout. Other factors have different influences on the accuracy of bea attenuation easureents in natural waters with various turbidity and they require specific attention in each particular case. The calculations of optial light source colliation angle and photodetector acceptance angle are ade by taking into account the given accuracy and the shape of the volue scattering function. In this case the selection of optial easureent conditions can be ade only with a change of optical base length. Because of the elongated shape of the sea water volue scattering functions the light source colliation and the photodetector acceptance angles should be extreely sall. This leads to a very stringent requireent to aintain optical alignent of the instruent which is frequently oved fro air to water and back. In the forer eters optical scheatics did not always fulfill these requireents; therefore a partial isalignent, forced by external factors, occurred. It is possible to explain the data discrepancy of different instruents by this fact. In forer eters the opportunity to change the optical base length is not stipulated for the sae reason and they are intended for easureents of bea attenuation of the scattering ediu only for a liited range of optical characteristics variations. II-273

3 3.0 A NEW PRINCIPLE FOR BEAM ATTENUATION MEASUREMENT Modern requireents iposed on instruents for easureent of absorption coefficient and bea attenuation are high, and to eet the it is necessary to develop a new design concept and to apply innovative decisions. The iniu instruent characteristics adopted by SeaWiFS Project Working Group for the easureent of inherent optical properties are shown in Table 1. Hence, for spacecraft sensor calibration and validation, it is necessary to develop a new bea attenuation design which would first have high resistance to the influence of various factors and, second, allow changes of the optical base length even in field conditions. One such realization (Lee, 1980) is offered by the Laboratory of Optical Oceanography of Marine Hydrophysical Institute and its ain feature is application of auto-colliating optical scheatics with a cat s eye as a reflecting eleent. With such optical scheatics a easured ediu fills the space between the illuinator and the cat s eye, which reflects the incident light precisely in the opposite direction, irrespective of pris orientation. In this auto-colliating optical scheatic the bea alignent is not changed when a slight deviation of pris position occurs. Therefore it is possible to soothly change the length of the optical base of the bea attenuation eter by siply oving the pris along the optical axis, as long as the instruent alignent is not affected. Hence, the opportunity arises to draatically expand the range of bea attenuation easureent. It is even possible to easure under optiu conditions with the sae relative error over a range of environents characterized by drastic changes in optical properties. The opportunity to easily change the optical base length over wide ranges allows to easure absolute values of the bea attenuation. It is enough to conduct easureents of the sae ediu for two various bases which are short enough to be regarded as single-scattering paths. In this case, the necessity to correct for the reflectance change of the glass surface situated in the water is eliinated. At the present tie, this correction is perfored in a theoretical anner for each particular optical design. Whereas the value of the correction considerably exceeds the bea attenuation of pure water, istakes in its deterination frequently are the ain reasons for the high additive coponent in the resulting error. This circustance was the ain obstacle to high easureent accuracy for bea attenuation absolute values. Another advantage of auto-colliating scheatics with a cat s eye as a reflecting eleent is that it does not appreciably respond to inute deviations of beas when it coes into contact with the easured ediu, and also to various deforations of optical details and of echanical units aking up the instruent optical base. Therefore, the occurrence of instruent isalignent Table 1. Instruent characteristics for easureent of inherent optical properties Instruent characteristics Spectral Resolution: 410, 443, 490, 520, 510 and 670 n Bandwidth: 10 n Accuracy: Precision for λ < 650 n: Precision for λ > 650 n: Stability width: Teperature: (centigrade) Sapling interval: > 4 saples -1 Source Colliation Angle: < 5 rad Detector Acceptance Angle: < 20 rad Depth Capability: 200 II-274

4 during operation is excluded, a situation which has iportant significance for easureents in field conditions, especially at large depths where water pressure is high. The auto-colliating optical scheatics with a cat s eye as a reflecting eleent was applied in a siple eter for easureent of sea water bea attenuation. It is intended for wide application, both as an independent instruent and as a coponent of various oceanological ounting: STD sounds, towed apparatus, buoy and oored stations. In such routine devices the ain requireent is to achieve high accuracy at inial diensions and power consuption. An optial cobination of these requireents can be reached if such a copact low consuption eter is realized on the basis of electro-optic seiconductor eleents as both light source and photoreceiver. Nevertheless, until recently, the use in hydrooptics of electro-optic seiconductors was constrained because of the low output power of the light-eitting diodes over the visible spectru, especially the blue-violet band, where the sensitivity of photo-diodes is also inial. Another reason for the liited application of electro-optic seiconductors is their significant teporal and teperature instability, hindering high accuracy of easureents. To avoid these liitations the developent of instruents for transparency easureent only in the red spectral band is eployed, using various electronic ways of stabilization of the properties of the light eitting diode (LED). A typical exaple of such a transparency eter using seiconductor eleents is a transissoeter presently anufactured by Sea Tech, Inc. The instruent is designed as a bea photoeter and suffers fro the typical liitation of this ethod of easureent. Besides, the optical scheatics applied in this eter are very sensitive to inute deforations in the illuinators and design eleents coposing its easuring base. 4.0 PRINCIPLE OF VOLUME SCATTERING COEFFICIENT MEASUREMENT The iportant advantage of the proposed two-bea principle for red bea attenuation easureent is that it perits easy access to easureent of the volue scattering coefficient b. In the use of one-bea eters this characteristic can only approxiately be calculated. The volue scattering coefficient is one of the ain hydrooptical characteristics. First, this variable is one of the paraeters in the radiation transfer equation and, hence, deterines the law of propagation of light in the sea; secondly, it appears to be the best way to evaluate the scattering property of sea water suspended particulate atter. Therefore it is possible by eans of volue scattering coefficient easureents to obtain inforation about sea water particulate atter content. Other existing ethods (filtration and weighing, study of particle size distribution by Coulter counters, special opto-electronic sensors and video caeras (Morel, 1983) are not ore accurate at the present tie and require expensive and coplex instruents (Ivanoff, 1978). Besides, knowledge of the volue scattering coefficient is necessary for calculations of yellow substance concentration fro reotely sensed data. The ost widespread ethods for the deterination of the volue scattering coefficient (Kopelevich, 1975) are : 1. ethod of the volue scattering function integration, 2. nepheloetric ethod, 3. ethod of direct easureents. The volue scattering function integration ethod uses the following equation: π b = 2π β ( θ )sin θdθ, (3) 0 where βθ is the volue scattering function. It should be taken into account that, owing to the elongated shape of the volue scattering function of sea water in the forward direction, fro 10 to 40 % of the scattered light can be found in the angular range fro 0 to 1 degree. As a rule, for cases when in situ easureents of the volue scattering function do not involve easureents II-275

5 over sall angles, the errors in deterining the volue scattering coefficient by these ethod is 20% or less. The nepheloetric ethod is based on the deterination of an epirical relation between the volue scattering function easured under one or several fixed angles and the volue scattering coefficient. The theoretical accounts ade by Kopelevich (1975) have shown that application of a single angle of 6 degrees leads to an error of 16 %, use of two angles ( 1 and 45 degrees ) decreases the error up to 15 %, while three angles ( 1, 6, 45 degrees ) give an error of 9 %. The known ethod of in situ easureents of the volue scattering coefficient offered by Jerlov (1961) has a shortcoing, because the ethod of calibration of a real instruent is not clear. The proposed direct ethod of easureent of the red volue scattering coefficient gives the opportunity to easily reach high accuracy of easureents and to circuvent the above liitations of the other ethods. As is well known, bea attenuation is the su of the absorption coefficient a and the scattering coefficient b: c = a+ b. (4) Taking into account the contribution fro various coponents it is possible to write: c = aw + ap + ay + bw + bp, (5) where the subscripts w, p, and y stand for pure water, suspended particles and yellow substance, respectively. The values of a w and b w are calculated theoretically and are known with high accuracy. The particle absorption coefficient is deterined by phytoplankton pigent absorption, and in the red band of the visible spectru does not exceed 2-4 % fro easured values of bea attenuation (Shifrin, 1988). As already entioned, the absorption by yellow substance decreases exponentially with increasing wavelength and becoes negligible in the red band. Hence Eqn. (5) can be accurately approxiated by: c = bp + const. (6) When perforing the two-bea easureent the constant in Eqn. (6) can easily be copensated by the insertion into the reference bea of a precisely calibrated optical attenuator. Thus, in this case the device for red bea attenuation easureent gives the possibility of directly easuring the particle volue scattering coefficient in the red band of the visible spectru. The value of the particle volue scattering coefficient is related to the total particle concentration C p by the equation: Cp = k 1 bp, (7) where k 1 is a factor of proportionality depending on the easured wavelength, total nuber of particles and particle size distribution. In spite of the fact that the total concentration and particle size distribution vary fro location to location, these changes have an insignificant effect on the value of the constant k 1. And with absolute calibration of the instruent it is possible to easure total particle concentration. 5.0 AUTO-COLLIMATING ELECTRO-OPTIC SCHEMATICS OF BEAM ATTENUATION MEASUREMENT In the red bea attenuation eter for in situ investigation the two-bea principle of easureent with negative feedback through an optical channel and auto colliating optical scheatics with a cat s eye as a reflecting eleent is applied. The structure eter scheatics is shown in Fig. 1. The ain eleents of the scheatics are divided into two channels: easuring and reference. II-276

6 The easuring channel contains a generator of exponential ipulses, a differential aplifier, LED, an aplifier and photodiode, an input diaphrag, irror, bea splitter, illuinator-objective and a cat s eye. The reference channel contains a reference voltage, coparator, an aplifier and photodiode, an optical attenuator, LED, irror and bea splitter. The bea attenuation eter works in the following anner. Light fro an LED is divided by the bea splitter into two beas. After the bea splitter the easuring bea is colliated by the illuinator-objective into a parallel bea and goes into the investigated ediu, where it traverses a distance up to a cat s eye, is reflected by it, passes across the ediu again in the opposite direction, and through the sae illuinator-objective coes back inwards into a heretic case. Inside the heretic case, the light is attenuated by water, reflected by the splitter and in turn by the irror, focused into the easuring diaphrag center and further drops on one of the photocathode of a two-eleent photodiode. The reference bea is fored fro the LED light which is reflected by the bea splitter before the output diaphrag. A irror reflects the reference bea and sends it through the optical attenuator to the second photo-cathode of a two-eleent photodiode. Figure 1. Red Bea Attenuation Meter Structure Scheatic II-277

7 While developing this optical scheatics, attention was focused to the question of reference and of easuring the utual influence of the beas. To separate these two beas all cases of potential interference were carefully considered, and optical shielding of beas by echanical screens is provided. The generator of exponential ipulses produces a continuous sequence of decreasing exponential ipulses U g with stable paraeters (aplitude U 0, frequency and a tie constant τ ). U U exp t/τ. (8) g = 0 These ipulses go to the differential aplifier input. A signal U ( t) fro the output of the easuring channel aplifier is connected to another input of this differential aplifier. The output signal of the differential aplifier controls the light intensity of the LED. When the aplification of the differential aplifier is very high the voltage to its inputs will be equal: On the other hand, U is provided by the LED light flux F LED : U () t = U () t. (9) g U = S F exp 2 cl, (10) LED where c is the bea attenuation coefficient, l is the instruent easuring base length, S is a factor of transforation of the light flux to a signal voltage of the photodiode. Fro Eqns. (9) and (10) we deduce the expression for the LED light flux: F LED = S U U g cl exp( cl) = exp S ( 2 2 ). (11) Fro Eqn. (11) the voltage, defined by F LED, at the output of the reference signal aplifier is: U S F S U g r = r LED = r exp( 2 cl), (12) S where S r is a factor of transforation of the light flux to the signal voltage of the photodiode. Since the electrical paraeters of the channels are identical we assue, that Sr = S. Then fro Eqns. (8) and (12) we have U U exp cl t/τ. (13) r = 0 2 In accordance with Fig. 2, for the oent at tie t p, when the voltages at the inputs of the coparator becoe equal E U ( t ), we can write: ( = 0 r p ) E 0 = U 0 exp 2cl+ t p /τ, (14) where E 0 is the reference voltage of the coparator and t p is the tie for the coparator action. We take the logarith of Eqn. (14) to obtain II-278

8 [ ] t cl ln U E τ, (15) = + p and finally we can write: t = 2 p cl τ + const. If the value of U 0 approaches E 0, then const will approach zero. Thus the output signal of the coparator is a sequence of ipulses with a duration proportional to the bea attenuation coefficient c. 6.0 CONCLUSION The proposed principle of easureent of bea attenuation and volue scattering coefficients are realized in a single sall-sized instruent with the help of odern electro-optical eleents. The applied auto-colliating optical scheatics with a cat s eye as a reflecting eleent has allowed to siplify the deterination of the correction for the reflectance change of glass surfaces situated in water. This circuit also perits easy changes in the wide liits of the instruent optical base length and to solve such a proble as the definition of the absolute value of the bea attenuation coefficient by easureents of the sae water saple at two fixed bases. The variable base of the instruent allows us to expand the range of bea attenuation easureents, so that easureents can always be perfored under optial conditions with an equal relative error. The two-bea principle of easureent of red bea attenuation perits a new direct ethod of easureent of the volue scattering coefficient. The proposed direct ethod of easureent of the red volue scattering coefficient also perits high accuracy of easureent and avoids the liitations of other ethods. The instruent is intended for application both as an independent device and as a odule for oceanographic systes. Figure 2. Explains the Work of Red Bea Attenuation Meter. II-279

9 7.0 ACKNOWLEDGMENTS This work is supported by Ukrainian National Agency of Marine Researches and Technology Thee: Instruent Building, section: Developent of Mooring Station of Module Design for Marine Ecological Investigations and by Russian Space Agency through the Institute of Radioelectronics of Russian Acadey of Sciences Project/Proposal Title: Russian-Aerican Project ALFA Developent of optical odels, algoriths and of data processing techniques of space color scanners for the ecological onitoring of arine environent. The authors also wish to thank continuing support at the Naval Research Laboratory through the Littoral Optical Environent Progra (LOE ) and Walton E. McBride III for useful help. This article represents NRL contribution NRL/PP/ REFERENCES Anonyous, Transissoeter Manual. Sea Tech, Inc., Corvallis, Oregon, USA, p. 22, A. Ivanoff, Introduction a l Oceanographie (in French), Proprietes Physiques et Chiiques des Eaux de Mer, Toe I-II, Libraire Vuibert, Paris, p.371, N. G. Jerlov, Optical easureents in the eastern North Atlantic, Medd. Oceanogr. Inst. Goteborg, Ser. B., No. 8, p. 40, N. G. Jerlov, Optical Oceanography, Elsevier Press, Asterda-London-New York, p. 231, O. V. Kopelevich, On the Deterination of the Scattering Coefficient of the Sea Water. (in Russian, abstract in English) In Optical Investigation in the Ocean and in the Atosphere above the Ocean, ed. K.S. Shifrin, Institute of Oceanology USSR Acadey of Sciences, Moscow, pp , M. E. Lee, Subersible auto-colliating transissoeter. (in Russian), In Optical Methods of Study of Ocean and Reservoirs, Inst. of Theral and Electrical Physics of Estonian Acadey of Sci., Tallinn, pp , A. Morel and A. Bricaud, Theoretical Results Concerning the Optics of Phytoplankton, with Special Reference to Reote Sensing Applications. In Oceanography fro Space, Part II, Elsevier press, London-New York, pp , J. L. Mueller and R. W. Austin, Ocean Optics Protocols for SeaWiFS Validation. In SeaWiFS Technical Report Series, Vol. 5, eds. S. B. Hooker and E. R. Firestone, NASA Goddard Space Flight Center, Greenbelt, Maryland, 44 p., K. S. Shifrin, Physical Optics of Ocean Water, Aerican Institute of Physics, New York, N.Y., p. 285, A. K. Vinogradov, ed. Modern Methods of Quantitative Valuation of Marine Plankton Distribution, (in Russian), Nauka, Moscow, pp , V. V. Vinokurov, V. V. Migulya, A. S. Prikhach and D. I. Shishkin, Module of Transissoeter-stratificator OPTRON-2. (in Russian), In Marine and Atospheric Optics, State Optical Institute, Leningrad, pp , A. A. Zhiltsov and V. P. Nikolaev, Transissoeter Stratificator. (in Russian), in Optical Methods of Study of Ocean and Reservoirs, Institute of Theral and Electrical Physics of Estonian Acadey of Sciences, Tallinn, pp , II-280