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Assessment of the quality of reedbeds using holomicroscopic moiré
Assessment of the quality of reedbeds using holomicroscopic moir6 P.K. RASTOGI,
L. PFLUG,
P. HAINARD
Holographic interferometry is proposed to detect the change in the mechanical responses of reed stems growing in eutrophic, as compared with those in healthy, water bodies. The detection of the difference in behaviour could be a sensitive way to index the degree of eutrophication. An interferometric method based on holomicroscopic moire is developed and preliminary results obtained are presented. KEYWORDS: holomicroscopic moire, interferometry, ecology, reedbeds, eutrophication, environment
Introduction River and lake ecology management work requires an intensive interaction with the environment in which the flora and fauna flourishes to maintain the desired population of a particular species. Such an interaction is actively required if one were to reinvigorate the numerous reedbeds which have been under constant regression over the last few decades. To cite an example, the reedbed of Grangettes (Fig. l(a)), in the state of Vaud in Switzerland, which in 1942 was spread over 17 hectares occupied only 3 hectares in 1972. The progressive disappearance of these beds casts a shadow on the very survival of numerous aquatic bird or insect species. There are several species of acquatic birds which nidificate in a lacustrine ecosystem. There are species which choose the reedbeds to construct their nests (Fig. l(b)). They do so by piling up dry fragments of bulrush or reed mace and then give it a hollow shape to lay down their eggs. This type of nidification (nest building) behaviour is shown by birds such as great crested Grebes, little Grebes, Mallards, Coots, Water rails, Moorhens and certain Warblers like Sedge, Reed or Marsh Warblers. The destruction of reedbeds could thus irrevocably perturb their nidification behaviour and thereby put in danger the survival of these species in these zones. PKR and LP are in the Laboratory of Stress Analysis, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland. PH is at the Institute of Botany, University of Lausanne, CH-1015 Lausanne, Switzerland. Received 24 June 1992. Accepted 7 August 1992.
0030-3992/92/060341-03 Optics & Laser Technology Vol 24 No 6 1992
in-plane
displacements,
lake
One of the possible effects of the environmental disturbances on the reedbeds is the induction of changes in the growth behaviour of the reed plants, and these changes, in turn, can affect the latter’s strength. Although it has not been well established in literature, it seems probable that the weakening of the reed stems is caused by eutrophication. This hypothesis is based on the fact that a eutrophic water body favours the development of a stem by privileging the growth of tissues containing nutritive reserves at the expense of those reserves which serve to support the stem itself. This ultimately results in the decrease of the reed’s strength. For the same size attained, the strength of reed stems growing in clean waters is probably much higher as compared with those growing in eutrophic water bodies. The modification of the stem’s strength could thus be put in correlation with the growth rate of the reeds. Hence, it seems likely that the relative decrease of a reed stem’s strength in a sample reedbed with respect to a reed stem’s strength in a healthy reedbed could be used as a good indicator of the degree of eutrophication of the water body of the tested reedbed. Surely it should be possible to take measurements of reed strength with a micrometer and a force transducer? However, this type of measurement might lack the accuracy and sensitivity desired to detect the small different strength variations of the reed stems coming from normal and progressively eutrophic water bodies, respectively. In addition, making measurements with as sophisticated technology as holography is no longer as time consuming as it used to be owing to the development of rapid thermoplastic holographic cameras.
@ 1992 Butterworth-Heinemann
Ltd 341
Fig. 1 reedbed
(a) The reedbed of Grangettes (photo Eugene (photo
Claude
Binder).
(b)
A Reed
Warbler
feeding
her young
in a nest
constructed
in the
Vaucher)
Method With this perspective in mind, our attempt has been to develop a holographic method which would enable one to compare the mechanical responses of reed stems in clean water with those growing in eutrophic water bodies. The method is chosen because of its high sensitivity and ability to measure and visualize the whole-field deformations of an opaque object. However important the relative merits of holography in deformation analysis of diffuse objects, the application of holography to this problem seemed to be a difficult one at the outset. The main difficulty stems from the minute lateral size of the tubular walls (- 1 mm) making up the reed. In addition, the lateral cross-section of the reed shows fibrous structure composed of tubules extending longitudinally into the reed.
The reed stems are segmented into small parts. These parts are then mounted on a loading device. These segments are loaded under diametral compression. The loading increment is kept constant for the whole series of reeds to be tested. A holomicroscopic moirt technique is implemented to study the in-plane component of displacements across crosssections of reed segments. The schematic of the optical set-up is shown in Fig. 2. The object is illuminated by means of two coherent collimated beams contained in the x-y plane. The two beams make an equal angle 0 with respect to the surface normal. A microscopic objective is used to form a magnified image of the object. The choice of magnification is restricted by the tubular nature of the object and our desire to conserve the whole-field benefits of the method.
A holographic plate situated in the image plane of the object captures the wavefronts originating from the object due to the two beams. The in-plane displacements, in a direction containing the two beams, are visualized as a moiri: pattern between the interferograms due to each illumination beam. However, due to the fact that the two interference patterns are of variable pitches and localization, the corresponding moiri: may only be visible over a limited area of the object surface. This problem is resolved by adding an auxiliary set of fringes whose localization plane coincides with the object surface’,2. If u is the in-plane component of displacement along the x direction, the moirt: is given by
M : Mirrors L : Collimating lenses BS : Beam splitters SF : Compact spatial litters
Fig. 2
342
Optical set-up
1 TVmonitor TV camera
Video
-tape
where p is the moirt fringe number and 2 the wavelength of the laser beam. The in-plane component u of the displacement fields across the Optics & Laser Technology Vol 24 No 6 1992
Summary
Fig. 3 A typical fringe pattern across the cross-section of a reed (magnified view). The lateral size of the tubular wall corresponds to 0.7 mm
cross-section of reed segments are then observed in real-time on a TV screen. One such result is shown in Fig. 3. For an argon-ion laser emitting light at 514.5 nm and energy divided by the free-running energy for a given 8 = 40”, the sensitivity is given by 0.40 pm per moire fringe. A comparison of the mechanical responses, for reeds coming from the same reedbed, has shown that for segments cut from the reeds of the same size and having the same shape, the number of fringes appearing on their cross-sections are the same. This is a useful result as it shows that for a given waterbody; that is, for a given degree of eutrophication and under the conditions specified above, the mechanical responses of the reed segments are the same. The length of time from cutting to testing is a parameter that might affect the test. Hence care is taken to keep this lapse of time constant for all the reed stems being tested. Obviously, this study must be expanded to compare the mechanical behaviour of reed stems coming from different reedbeds.
From the point of view of the performance of the technique, phase shifting can be applied to holographic moire interferometry to obtain an accurate quantitative display of in-plane phase distribution3x4 over the tested surface. The results obtained also put in sharp evidence the adaptability of the technique to address objects of very small dimensions. On the other hand, from the point of view of the real utility of the technique, a successful use of this method in lacustrine management will depend upon the capability of the method to detect fairly minute changes in the mechanical responses of the reed stems coming from water bodies with different degrees of eutrophication. The high sensitivity of holographic interferometry gives way to an optimistic assessment of the technique and the hope that the data provided will be useful for lacustrine management decisions. Although it is too early to predict the relative efficiency of the technique with respect to the conventional methods for assessing the degree of eutrophication of reedbeds, the knowledge that the combined technology of pulse shifting and holomicroscopic moire interferometry can be successfully applied on objects of very small size will certainly be helpful in identifying new applications in fairly different fields and to which the technique could be applied with success.
References C.A., Rastogi, P.K., Jacquot, P., Narayanan, R. Holographic-moiri in real-time, Exp h4ech 22, (1982) 52-63 Rastogi, P.K., Spajer, M., Monwret, J. In-plane deformation measurement using holographic moire, Opt Lasers Eng 2, (1981) 799103 Rastogi, P.K., DenariC, E. Visualization of in-plane displacement fields by using phase-shifting holographic moire: application to crack detection and propagation, Appl Opt. 31, (1992) 2402-2404 Rastogi, P.K. Phase-shifting applied to four-wave holographic interferometers, Appl Opt 31, (1992) 1680-1681 Sciammarella,
Optics & Laser Technology
Optics & Laser Technology Vol 24 No 6 1992
343
L. PFLUG,
P. HAINARD
Holographic interferometry is proposed to detect the change in the mechanical responses of reed stems growing in eutrophic, as compared with those in healthy, water bodies. The detection of the difference in behaviour could be a sensitive way to index the degree of eutrophication. An interferometric method based on holomicroscopic moire is developed and preliminary results obtained are presented. KEYWORDS: holomicroscopic moire, interferometry, ecology, reedbeds, eutrophication, environment
Introduction River and lake ecology management work requires an intensive interaction with the environment in which the flora and fauna flourishes to maintain the desired population of a particular species. Such an interaction is actively required if one were to reinvigorate the numerous reedbeds which have been under constant regression over the last few decades. To cite an example, the reedbed of Grangettes (Fig. l(a)), in the state of Vaud in Switzerland, which in 1942 was spread over 17 hectares occupied only 3 hectares in 1972. The progressive disappearance of these beds casts a shadow on the very survival of numerous aquatic bird or insect species. There are several species of acquatic birds which nidificate in a lacustrine ecosystem. There are species which choose the reedbeds to construct their nests (Fig. l(b)). They do so by piling up dry fragments of bulrush or reed mace and then give it a hollow shape to lay down their eggs. This type of nidification (nest building) behaviour is shown by birds such as great crested Grebes, little Grebes, Mallards, Coots, Water rails, Moorhens and certain Warblers like Sedge, Reed or Marsh Warblers. The destruction of reedbeds could thus irrevocably perturb their nidification behaviour and thereby put in danger the survival of these species in these zones. PKR and LP are in the Laboratory of Stress Analysis, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland. PH is at the Institute of Botany, University of Lausanne, CH-1015 Lausanne, Switzerland. Received 24 June 1992. Accepted 7 August 1992.
0030-3992/92/060341-03 Optics & Laser Technology Vol 24 No 6 1992
in-plane
displacements,
lake
One of the possible effects of the environmental disturbances on the reedbeds is the induction of changes in the growth behaviour of the reed plants, and these changes, in turn, can affect the latter’s strength. Although it has not been well established in literature, it seems probable that the weakening of the reed stems is caused by eutrophication. This hypothesis is based on the fact that a eutrophic water body favours the development of a stem by privileging the growth of tissues containing nutritive reserves at the expense of those reserves which serve to support the stem itself. This ultimately results in the decrease of the reed’s strength. For the same size attained, the strength of reed stems growing in clean waters is probably much higher as compared with those growing in eutrophic water bodies. The modification of the stem’s strength could thus be put in correlation with the growth rate of the reeds. Hence, it seems likely that the relative decrease of a reed stem’s strength in a sample reedbed with respect to a reed stem’s strength in a healthy reedbed could be used as a good indicator of the degree of eutrophication of the water body of the tested reedbed. Surely it should be possible to take measurements of reed strength with a micrometer and a force transducer? However, this type of measurement might lack the accuracy and sensitivity desired to detect the small different strength variations of the reed stems coming from normal and progressively eutrophic water bodies, respectively. In addition, making measurements with as sophisticated technology as holography is no longer as time consuming as it used to be owing to the development of rapid thermoplastic holographic cameras.
@ 1992 Butterworth-Heinemann
Ltd 341
Fig. 1 reedbed
(a) The reedbed of Grangettes (photo Eugene (photo
Claude
Binder).
(b)
A Reed
Warbler
feeding
her young
in a nest
constructed
in the
Vaucher)
Method With this perspective in mind, our attempt has been to develop a holographic method which would enable one to compare the mechanical responses of reed stems in clean water with those growing in eutrophic water bodies. The method is chosen because of its high sensitivity and ability to measure and visualize the whole-field deformations of an opaque object. However important the relative merits of holography in deformation analysis of diffuse objects, the application of holography to this problem seemed to be a difficult one at the outset. The main difficulty stems from the minute lateral size of the tubular walls (- 1 mm) making up the reed. In addition, the lateral cross-section of the reed shows fibrous structure composed of tubules extending longitudinally into the reed.
The reed stems are segmented into small parts. These parts are then mounted on a loading device. These segments are loaded under diametral compression. The loading increment is kept constant for the whole series of reeds to be tested. A holomicroscopic moirt technique is implemented to study the in-plane component of displacements across crosssections of reed segments. The schematic of the optical set-up is shown in Fig. 2. The object is illuminated by means of two coherent collimated beams contained in the x-y plane. The two beams make an equal angle 0 with respect to the surface normal. A microscopic objective is used to form a magnified image of the object. The choice of magnification is restricted by the tubular nature of the object and our desire to conserve the whole-field benefits of the method.
A holographic plate situated in the image plane of the object captures the wavefronts originating from the object due to the two beams. The in-plane displacements, in a direction containing the two beams, are visualized as a moiri: pattern between the interferograms due to each illumination beam. However, due to the fact that the two interference patterns are of variable pitches and localization, the corresponding moiri: may only be visible over a limited area of the object surface. This problem is resolved by adding an auxiliary set of fringes whose localization plane coincides with the object surface’,2. If u is the in-plane component of displacement along the x direction, the moirt: is given by
M : Mirrors L : Collimating lenses BS : Beam splitters SF : Compact spatial litters
Fig. 2
342
Optical set-up
1 TVmonitor TV camera
Video
-tape
where p is the moirt fringe number and 2 the wavelength of the laser beam. The in-plane component u of the displacement fields across the Optics & Laser Technology Vol 24 No 6 1992
Summary
Fig. 3 A typical fringe pattern across the cross-section of a reed (magnified view). The lateral size of the tubular wall corresponds to 0.7 mm
cross-section of reed segments are then observed in real-time on a TV screen. One such result is shown in Fig. 3. For an argon-ion laser emitting light at 514.5 nm and energy divided by the free-running energy for a given 8 = 40”, the sensitivity is given by 0.40 pm per moire fringe. A comparison of the mechanical responses, for reeds coming from the same reedbed, has shown that for segments cut from the reeds of the same size and having the same shape, the number of fringes appearing on their cross-sections are the same. This is a useful result as it shows that for a given waterbody; that is, for a given degree of eutrophication and under the conditions specified above, the mechanical responses of the reed segments are the same. The length of time from cutting to testing is a parameter that might affect the test. Hence care is taken to keep this lapse of time constant for all the reed stems being tested. Obviously, this study must be expanded to compare the mechanical behaviour of reed stems coming from different reedbeds.
From the point of view of the performance of the technique, phase shifting can be applied to holographic moire interferometry to obtain an accurate quantitative display of in-plane phase distribution3x4 over the tested surface. The results obtained also put in sharp evidence the adaptability of the technique to address objects of very small dimensions. On the other hand, from the point of view of the real utility of the technique, a successful use of this method in lacustrine management will depend upon the capability of the method to detect fairly minute changes in the mechanical responses of the reed stems coming from water bodies with different degrees of eutrophication. The high sensitivity of holographic interferometry gives way to an optimistic assessment of the technique and the hope that the data provided will be useful for lacustrine management decisions. Although it is too early to predict the relative efficiency of the technique with respect to the conventional methods for assessing the degree of eutrophication of reedbeds, the knowledge that the combined technology of pulse shifting and holomicroscopic moire interferometry can be successfully applied on objects of very small size will certainly be helpful in identifying new applications in fairly different fields and to which the technique could be applied with success.
References C.A., Rastogi, P.K., Jacquot, P., Narayanan, R. Holographic-moiri in real-time, Exp h4ech 22, (1982) 52-63 Rastogi, P.K., Spajer, M., Monwret, J. In-plane deformation measurement using holographic moire, Opt Lasers Eng 2, (1981) 799103 Rastogi, P.K., DenariC, E. Visualization of in-plane displacement fields by using phase-shifting holographic moire: application to crack detection and propagation, Appl Opt. 31, (1992) 2402-2404 Rastogi, P.K. Phase-shifting applied to four-wave holographic interferometers, Appl Opt 31, (1992) 1680-1681 Sciammarella,
Optics & Laser Technology
Optics & Laser Technology Vol 24 No 6 1992
343