Notice that a just a slight wind and birds swimming through the water was enough here to disturb the reflections of this image. Even when the water is rough, using ultra-long exposures allows reflections to be rendered that were not visible to the naked eye Fig 8.
A polarising filter is a great tool to use if you find the sun is causing a harsh direct glare on parts of the water.
While polarisers are generally used to remove reflections from reflective surfaces, if you just rotate the filter slightly, it will reduce the glare but leave your reflection intact. However, you can use a long zoom for isolating a reflection from its busy surroundings such as in this image Fig Photocrowd is a contest platform for the best photo contests and photo awards around, with a global community of photographers of all levels and interests. Photocrowd Posted on February 23, All the intensities measured by the detector are normalized by this reference.
To minimize the sensitivity to the incident polarization state, it is desirable to illuminate the beam sampler as close to normal incidence as possible. For this reason, the voltage measured by the reference detector was found to increase linearly with incident power, but the coefficient is 5. The glintometer is a custom-built photopolarimeter based on a division-of-amplitude photopolarimeter DOAP design Azzam , A basic DOAP consists of a coated beam splitter and two polarizing beam splitters, so the incident beam is split into four different paths.
If properly constructed Azzam ; Azzam and De ; Savenkov , each path is a linearly independent projection of the Stokes vector. Thus, the incident-beam Stokes vector can be found at any instant by a linear transformation of the measurement vector. We chose to measure only the first three components of the Stokes vector both because this leads to a simpler design and because, with a linearly polarized incident beam, we expect no circular polarization in the reflected beam.
Figure 6 depicts the optical design of the glintometer; as with the source, the components are mounted within a cage system Fig. A nonpolarizing beam-splitter cube splits the incoming beam into two roughly equal parts. One is detected as is and is used to retrieve the scalar intensity, whereas the other travels to a polarizing beam splitter that transmits the component polarized in the plane of incidence and reflects that perpendicular to the same plane.
Both beam splitters are from Edmund Optics. A note of caution must be observed when indicating s and p components, which are always defined relative to a plane of incidence. Therefore, light that is parallel perpendicular to the plane of the page in Fig.
Three focusing lenses with short focal length These modules are compact because the photodiode and amplifier are integrated into a single 8-pin dual inline package DIP. This design avoids problems such as leakage current, noise pickup, and gain peaking due to stray capacitance. The photodiode modules are operated in photoconductive mode to enhance linearity and minimize dark current. The transimpedance amplifier amplifies the photocurrent and provides a voltage output linearly proportional to the incident intensity.
The sensitivity at wavelength nm is 0. Only the optical components are included in the movable unit on the rainbow, making it remarkably light and compact. The detector output is delivered via an extension cable to a separate electronics box. Since the instrument was custom built, it was deemed important to evaluate the performances of its individual components.
With the help of a Newport optical power meter, the energy of the beam was sampled at different positions along the optical path for s - and p -polarized incident light. When possible, recognizing the dependency of the reading on the angle of incidence on the probe, we mounted the power meter in a stable position and took the measurements before and after each optical component was inserted into the path.
A black cloth over the apparatus prevents room-light contamination. The nonpolarizing beam splitter showed little dependence of its transmittance on the incident state of polarization: Its reflectance exhibits twice as much variation, going from The polarizing beam splitter transmits p -polarized light with Each focusing lens exhibited a constant, polarization-independent transmittance of The total losses of the glintometer, with the beam splitters and the focusing lenses mounted, are given in Table 1.
For p -polarized incident light, This provides a ratio of 0. For s -polarized incident light, the percentages reaching the scalar-intensity and the s detectors are The total optical transmittance in this case is These values, determined as specified above from the readings of the power meter, are in agreement with those retrieved during the calibration [see section 3d 4 ].
Thus, the glintometer detects p -polarized incident light more efficiently better SNR than s -polarized light. This behavior is mainly due to the polarizing beam splitter element. Consider the intensity photodiode and its corresponding focusing lens. The photodiode has a square active area 2. Referring to Fig. An adjustable iris on the front of the glintometer baffle determines the FOV, controlling the cone of light reaching the detector as shown in Fig.
With the iris fully open, the intrinsic field of view cone A is protected against room-light contamination by the baffle.
Cone B shows the field of view with the iris partially closed. Table 2 gives theoretical values for the field of view and the diameter of the area intercepted on a flat water surface 1 m away as a function of the aperture diameter. Consider now the fields of view of the detectors for the polarization channels.
They receive light that has gone through two beam splitters. Their distances to the instrument aperture are the same by design. Thus, they have the same FOV, slightly different than that of the scalar-intensity detector because of the extra optical path which is equal to the distance between the centers of the two beam splitters.
A further complication for the p -polarization detector is that, to reach it, the rays must undergo two reflections. Because of the complex geometry, the field of view was determined experimentally.
The glintometer was mounted on a rotating table capable of arc-minute angular resolution and aligned with the source. The rotation takes place about an axis that goes through the focusing lens of the scalar-intensity detector, corresponding to the vertex of the cone defining the field of view see Fig.
The signal from each channel rises to a plateau and then falls as the incoming beam moves into and out of the field of view of the sensors. The field of view is the width of the plateau. The procedure is repeated for different diameters of the aperture stop. When no aperture stops are placed in front of the optics, the response of the photodiodes covers different angular ranges. The polarization channels have a larger width and are also found in a mirrored position relative to the curve of the scalar-intensity photodiode: this is believed to be the effect of the number of reflections the beam undergoes before reaching each photodiode and seems to indicate good mechanical alignment within the cage system holding the optics.
Given the chosen center of rotation, this procedure provides a measure of the FOV of the scalar-intensity photodiode only. Since this configuration does not correspond to the normal operational conditions, we use the iris to limit the aperture and have the signals from the three photodiodes rising simultaneously. In the situation where the iris is closed to a point where the cone matches the intrinsic field of view, the system is considered optimized in a throughput sense against stray light.
Closing the iris even further leads us closer to the paraxial approximation at the expense of a reduction of the field of view and a worse signal-to-noise ratio. The calibration of the glintometer loosely follows the procedures outlined by Azzam and Lopez and Krishnan The purpose is to determine the polarization state and the intensity and therefore the Stokes vector of a beam from the readings of the photodiodes.
To this end, light with known polarization states is selected on the source accurately aligned with the glintometer. The glintometer response is then interpolated to a characteristic instrumental function, which enables the determination of the parameters of any incident beam.
Errors are estimated with conventional techniques of error propagation Taylor This dependence on the square of sinusoidal functions relates to the Malus law Hecht , which describes the intensity transmitted by a linear polarizer. Since the system is designed to measure reflectances, the reference detector mounted on the source is optimized to respond to a different range of radiant energies than the glintometer.
Because of this low value, the effect of the polarization-dependent reflectance on the transmitted beam can be neglected. Since a power of 0. The response of the polarization channels as the source polarizer is rotated is proportional to the polarization components. Several sources of error have to be considered when estimating the uncertainty in the measured reflectance values.
Suspecting that the performance of the optical power meter was poorer than specified, we relied on repeatability tests to compute the total uncertainty rather than carrying out a rigorous error propagation analysis. This value determines the error bars of the measured reflectances see, e. The modular nature of the chassis-based system allows an easy expansion of the number of available channels.
Data can be simultaneously sampled with a frequency up to kilosamples per second, and logged with bit resolution into a computer. Before introducing waves, the apparatus was tested by reproducing the Fresnel equations from measurements of the specular reflection from the flat surface at a range of angles.
As seen in Fig. The error bars are assigned as discussed in section 4. Somewhat surprisingly, an extensive literature search did not reveal any other reported attempt to experimentally confirm the Fresnel equations over a flat water surface. We considered several wave states ranging from gravity to capillary waves. The wave amplitudes were bounded within values favorable to the detection of glints; for example, for source and detector in specular geometry the reflection of a crest or a trough falls into the field of view [see section 3d 2 ].
Gravity waves were produced with the hydraulic unit at a frequency of 1. With these values the unit shows a horizontal linear displacement of 2.
The resulting wave profile is close to sinusoidal with an average total wave height of 1. Three capillary wave states were also generated, corresponding to wind speeds of 1. Since two of the covering panels of the wave tank had to be removed to mount the rainbow structure, the wind field inside the tank could not be controlled as precisely as usual. The setup nonetheless worked surprisingly well and the horizontal wind fluctuated very little.
We image the surface in the area of the illuminated spot for approximately 60 s before and 60 s after each set of reflectance measurements, at a rate of 60 frames per second. The data collected with the imaging system are meant to provide the statistics of the wave slopes. Simultaneous detection of glints and surface images is avoided to eliminate the risk of damaging the charge coupled device CCD detector with the reflected laser beam.
Each image can be thought of as a 2D array of slope values. The resolution at the water surface is 0. The imaging system and its calibration have been described by Long and Klinke A Plexiglas calibration barge, with 11 slopes evenly distributed in the positive and negative domain, is floated across the field of view on the still water surface.
Connect and share knowledge within a single location that is structured and easy to search. Why does water reflect light? What is actually happening when light is reflected by water? We know why metals reflect light; water, however, is not metal, but it still reflects light and we can see our image reflected on calm water.
The most fundamental answer is that water reflects light because the wave impedance of water is different than the one of air and the electric and magnetic field must be continuous everywhere in space.
The important thing to note is that the wave impedance is the fixed ratio of the electric and magnetic field amplitude of the light wave and that the electric field and magnetic field must be continuous, i. This is a direct consequence from Maxwells Equations, the fundamental equations describing the propagation of light. If you think about the two requirements, you may think that this is contradictory and cannot be fulfilled simultaneously under all cases, e.
This is true only at the first sight and is the reason why there is a third beam. In a simplified view, the wave impedance of the reflected beam has a negative sign so that for the three beams incoming beam, transmitted beam and reflected beam the ratio of E and H field is the wave impedance of the relevant material and also E and H are continuous at the boundary.
If you think about it you will notice that this is only possible with three beams and not two. From this fundamental principle taking into account polarization of E and H all other laws dealing with reflection follow. Especially the Snell law with the refractive index. The fundamental principle predicts the amount of reflection as well as transmission and also the direction.
Also it predicts the Fresnel equations which specify the reflection and transmission for different polarization which cannot be derived with arguments involving only the refractive index. For metals the explanation also holds, the wave impedance is ideally zero for metals so that there is only a reflected beam.
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