Polarisation Flight (Vertical vs Horizontal)

Tuesday 13 November 2007

Null Hypothesis

The null hypothesis is that “Imaging of vertically polarized photons at critical angles makes no improvement in contrast between seagrass beds and surrounding substrate.” The aim of the experiment is to disprove the null hypothesis.

An abstract outlining the background for the experiment can be found here: Seeing seagrasses sidewards: marine angiosperms and the Stokes’ polarization parameters

The imagery captured using a camera with a vertically polarizing filter is the variable. The imagery captured using a camera with a horizontally polarizing filter is the control.

The Experiment

Engineering mounting took place on Sunday 11 November 2007 at the aircraft hanger at Murray Bridge aerodrome. Two Canon EOS 5D cameras (borrowed from Rowena’s close range photogrammetry study) were mounted on a bracket in the aircraft at an angle of 30 degrees forward from nadir. The chip orientation was portrait, so as to maximize the forward view (from near-nadir to beyond the Brewster angle of 53 degrees).

Figure 1 - Aircraft used for polarization flight

Figure 2 – Two Canon EOS 5D cameras and mounting bracket

Figure 3 - Interior view of camera port and camera bracket (with cameras mounted)

Figure 4 - Camera mounted 30 degrees forward from nadir

Spatial Scientific Technologies Pty Ltd undertook the mounting and image capture flight - Spatial Scientific Technologies.

On the day of the flight (Tuesday 13 November 2007) the cameras were mounted on the bracket in the aircraft. One polarization filter was adjusted to be horizontal on one camera, and the other adjusted to vertical on the other. A test picture was taken on the ground at 9:43 am.

In all of the following image pairs the horizontal polarization (H) images are on the left and the vertical polarization (V) images on the right. Note that the airframe is visible at the forward edge of the H images.

Figure 5 – Frames 0518(H) and 4499(V) – initial test photographs

Conditions were optimal, with clear skies and calm conditions. Ocean turbidity was expected to be minimal due to a least ten days since the last rainfall event.

Figure 6 MODIS satellite image showing conditions one hour after flight (image courtesy Australian Centre for Remote Sensing). Box shows location of map below.

The flight was from the coast eastwards from Point Malcolm (Semaphore South/ West Lakes) to a point seven nautical miles off-shore. Then from that location tracking to Brighton jetty, then the reverse course (a reverse figure 7 shape) back to Point Malcolm. An opportunistic run of photography was also captured over the Adelaide CBD.

Figure 7 Map showing approximate flight lines (map courtesy Geoscience Australia)

The image capture started at 10:21am and finished at 10:52am Australian Central Daylight Time (GMT +10hr 30min). Unfortunately the GPS failed, so no positional information was captured.

Initial Results

The data was supplied on DVD on Friday 16 November in .CR2, .TIF and .JPEG formats. The following is based on the associated full resolution 8-bit compressed jpeg images.

Initial inspection showed little to no difference between the H and V images when the sun is behind the aircraft (Run 1, west to the top):

Figure 8 Frames 0524(H) and 4505(V) - crossing the coast at Point Malcolm

Figure 9 Frames 0535(H) and 4516(V) - abandoned sewerage sludge outfall

Enhancing and balancing the green band of the tiff images shows similar subsurface information in the lower two-thirds of co-temporal horizontal and vertical polarisation frames. The top third shows little information in the horizontal polarisation frames, whereas the vertical polarisation frames show significant detail around the Brewster angle.

Videos of the horizontal and vertical polarised frames from above the abandoned sludge outfall to the deep water seagrass line show this effect.

However differences are apparent when directly viewing the sun reflection (Run 4, east to the top).

Figure 10 Frames 0754(H) and 4735(V) - approaching coast with full sun reflection

Enlargements of the just-submerged Point Malcolm breakwater at around the Brewster angle (top part of enlargements) show marked differences (Thanks to Ken for pointing out the solar aspect factor). Note the removal of nearly all wave action, including breaking waves, and sun glint. The breakwater is barely visible in the left enlargement, yet is clearly visible in the right enlargement. Dark areas further off-shore are remnant seagrass beds (posidonia spp.) and detrital matter. Note also on the enlargement of frame 4735 the return of sun glint away from the Brewster angle (towards the bottom of the enlargement).

Figure 11 Enlargements of frames 0754(H) and 4735(V) showing the Point Malcolm breakwater.

Beacon poles

Figure 12 Point Malcolm breakwater from shore (between two beacon poles)

Further differences are apparent in inland waters, especially where there is turbulence and/or turbidity.

For example the channels joining West Lakes and the Port River show significant removal of sky reflectance and sun glint, even though the surface is turbulent (Run 4):

Figure 13 Frames 0761(H) and 4742(V) – overview

The tide is going out so the water is flowing from right to left (south to north).

Figure 14 Enlargement of frames 0763(H) and 4742(V) at Port Adelaide

Likewise Torrens Lake (usually turbid) in the Adelaide parklands shows significant differences. Note the lack of reflectance of the white fountain in the lake near the Torrens Weir in the V image, whereas the shadow is visible in both enlargements (Run 5, south-east at top):

Figure 15 Frames 0763(H) and 4744(V) – overview

Figure 16 Enlargement of frame 0763(H) and frame 4744(V) at the Torrens weir in the Adelaide parklands

Figure 17 Fountain in Torrens Lake

Where to from here

Initial inspection of the data shows that I have disproven the original null hypothesis – at least at certain sun aspects. Run 4, directly towards the sun, showed sun glint, and wave action in the horizontally polarising filtered image sufficient to mask most subsurface detail. The vertically polarising filtered imagery removed the glint, even from breaking waves. Therefore imaging of vertically polarized photons at critical angles makes a great improvement in contrast between seagrass beds and surrounding substrate at certain geometries compared to imaging of horizontally polarized photons. (Italics indicate experimental constraints on the results).

Further analysis is required to explain why the sea floor is visible in both sets of imagery in Run 1. One hypothesis is that depolarization occurs during the water and atmospheric path length (post-initial reflectance), resulting in equal amounts of orthogonal polarized light reaching the cameras.

Another hypothesis is that the polarisation properties of photons in back reflection is different to forward reflection, at least for sun glint.

Sky reflectance is polarised at 90 degrees from the sun, which would influence the surface reflectance polarisation reaching the cameras.

Another hypothesis is that the waters were so clear that minimal (quantum theory suggests 4%) surface reflectance was occurring; therefore the images should be 96% similar. Conditions were too good on the day!

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