Searching for Sub-Visual Atmospheric Structures in the Daytime Sky

On sunny, warm days the sun heats the Earth’s surface and the air close to it. Periodically a parcel of air will rise from this area due to the warmed air being buoyant. This parcel is thought to rise in an elongated column of fairly large size such that several hundred tons of air are lofted skyward. In doing so many of the particles generated by Earth-bound processes (pollen, smoke, dust, pollution, water vapor, etc.)  are brought with it. These particles are commonly known as aerosols. If the column reaches an altitude where the contained water vapor condenses then a cumulus cloud will form.

It is known that aerosols have a large effect on polarization of light, up to 30% or so. My first experiment in photographing these columns of air was to take 2 sequential photos of the sky with a linear polarization filter set to 90 degrees apart. Then in accordance with the article excerpt shown below and using an image processing program (Image Magik) I calculated the degree of linear polarization (DOLP) of each pixel from the formula given in the article. The resulting pictures are interesting and strange but do not show the expected structures.

I encourage others to make their own attempt at this goal as I am really a novice at image processing. No doubt there are many other ways of looking at this problem and I welcome all comments, thoughts and ideas. Thanks!

Excerpt from the article “Digital All-Sky Polarization Imaging of Partly Cloudy Skies” from Nathan J. Pust and Joseph A. Shaw

“It is our feeling that unseen aerosols and possibly thin clouds in what has recently been called the “twilight zone” between a cloud and the clear sky are reducing the DOLP in what appears to be clear sky. We believe that this effect on the sky polarization is directly related to the recently described observations of enhanced optical depth near clouds. In partially cloudy skies, we see DOLP reductions in clear sky areas between clouds that appear to be caused by subvisual aerosols and/or clouds. (Even though clouds appear to have hard edges, they are in fact surrounded by thin clouds.) Furthermore, these DOLP reductions show up in the clear sky long before we can physically see clouds in the sky.”

To determine Degree of Linear Polarization (DOLP) in each pixel he uses this formula:
(Image1pixel value – Image2pixelvalue) / (Image1pixel value + Image2pixelvalue)

Then he normalizes and stretches the result so it fills the whole 8 bit range of 0 to 255 pixel brightness values.

Some of my resulting pictures:

Author: Deane Williams, Connecticut, USA

Colours in Citric Acid

Lately I experiment a lot with crystal growing and I knew that Citric Acid can refract the light, but when I saw this I was more than surprised. These flat crystals of about 1-2 cm in diameter (the larger ones) were grown between two glass plates, then put in front of a polarized light source and photographed with a polarization filter. It took a few tries to find a proper way to grow them flat enough for this purpose though. More Photos see here and here.

Author: Rolf Kohl, Germany

Plastic objects in polarized light

Very exiting to experiment with plastic in front of polarized light sources. This is a simple foil from a cracker box put in front of a TFT(!) Monitor. Somehow not every foil or plastic object produces these colours. I tried a cup from the hospital I brought home, the top of a little box for keeping minerals and this foil with success. Best results can be reached by switching the monitor in a single white surface, then put the item in front, darken the room and use a pole filter with the camera. Turning the objects slightly produces different ranges of colour. Once I observed this effect even in the daylight on a plastic windshield of a motor-cycle with a pole filter on my camera.

Here still a further picture of Windshield of a motorcycle 90 degrees away from the sun, viewed through a polfilter on a Canon Reflex Camera.

More pictures: 1 – 2 – 3 – 4 – 5

Author: Rolf Kohl

Polarized Light in Nature

My book “Polarized Light in Nature” is now online available.

The pdf can be downloaded from my site, www.guntherkonnen.com. Go to English/Articles and scroll down to the year of publication (1985). The size of the download is 24 Mb.

The Dutch version (1980) can also been downloaded from the site (18 Mb).

Direct access is also possible:
http://s3.amazonaws.com/gunther-konnen/documents/249/1985_Pol_Light_in_Nature_book.pdf?1317929665 (English version)
or
http://s3.amazonaws.com/gunther-konnen/documents/246/1980_Gepolariseerd_Licht_boek.pdf?1317928523 (Dutch version),
but these addresses may change in case my site migrates to another server.

Author: Günther Können, Netherlands

Polarization Colours in Airplane Window

Patrice Touhar took this photograph through the window of an airplane using a polarization filter and so made these polarization colours appear.

The colours are generated by the combination of three circumstances:

1. There is polarized light behind the window
2. The window is made of a double refracting matter
3. A polarization filter is used in front of the window

Under these circumstances, the light gets split up into two coherences of polarized components which travel through the window at different speeds. This causes a phase difference the amount of which depends from the wavelength. This means that the polarization gets transformed once into a linear polarization of a different direction than the origin for a certain colour, and into a circular polarization for another colour (Lissagiu interference of two waves). As the polarization is now dependent from the colour, the colours appear when a polarization filter is used, no matter if this filter is placed in front of the eye or in front of a camera.

These colours can be seen and photographed under the following circumstances:

1. There is polarized on one side of the window coming from the blue sky (Rayleigh scattering) and reflection from water and the scattering caused by clouds
2. The window is made of plastic. In this material there are mechanical tensions which cause a double refraction.
3. There is a polarization filter in front of the camera.

The last condition is not necessary if you look at the window from an acute angle. Then the light becomes already polarized by the refraction. In this case there is no filter necessary to see the colours. However, the colours you can see under this circumstance are fainter than the colours seen through the polarization filter because refraction polarizes only a part of the light.

You can also find another observation of this kind in this former thread.

Colours in the airplane window

While flying from Izmir to Ankara in Turkey on monday the 2nd of April 2007 at 10.15am Selen Ediger wanted to take an aerial shot of the mountains below with my canon ef 10-22mm lens and Hoya multi coated polarizing filter. With naked eye the colors were not visible but when she looked through the camera she saw that the land and the sky was covered with rainbow colors. The contrast and the saturation are adjusted a bit.

What’s the origin of this colours?