Sunday, February 12, 2017

A modification to the BalloonSat Recording Photometer expanded it's capabilities to function as a Photopolarimeter. A Photopolarimeter is a device to measure the intensity of light in different wavelengths and their amount of polarization. Most people are familiar with the fact that the brightness of light varies throughout the day, but not to the fact that the sky polarizes sunlight and how it can change throughout the day. The images below show how sky brightness varies as a Polaroid filter is rotated 90 degrees.

The experiment run at NearSys Station on February 11, 2017 was an attempt to detect both changes in polarization and intensity of sunlight in the eight different spectral bands listed below.

Infrared (940 nm)
Infrared (890 nm)
Red (660 nm)
Orange (620 nm)
Yellow (595 nm)
Green (505 nm)
Blue (470 nm)
Violet/UV (400 nm)

Polarization was measured in the North-South direction and then the East-West direction by comparing the intensity of sunlight in the above eight spectral bands as a polarizing filter was rotated over the light sensors. During the experiment, the Photopolarimeter was pointed due south and at an elevation of close to 45 degrees.

Sunrise occurred at 7:48 AM and sunset at 6:11 PM (18:18 hours). The sun was due south at 1:00 PM (13:00 hours). The sun's elevation at 1:00 PM was 32.5 degrees.

The Photopolarimeter slipped at 2:17 PM (14:17 hours) and remained pointing up until corrected at 2:41 PM (14:41 hours). However, the angle the Photopolarimeter was reset to was slightly lower in elevation that before.

Here are some things to notice.

1. From sunrise to 10:00 AM, sunlight has greater polarization in the East-West direction than North-South direction. At 10:00 AM, the sun was located at an azimuth of 133 degrees (southeast) and elevation of 19.5 degrees.

2. From 10:00 AM to 1:00 PM, sunlight has greater polarization in the North-South direction than the East-West direction. At 1:00 PM, the sun was located at an azimuth of 180 degrees (south) and elevation of 32.5 degrees.

3. From 1:00 PM to 4:15 PM, sunlight has greater polarization in the East-West direction than North-South direction. At 4:15 PM, the sun was located at an azimuth of 230 degrees (southwest) and elevation of 17.2 degrees.

4. After 4:15 PM, sunlight has no preferential polarization.

5. The amount of polarization is greatest at yellow (595 nm) and weakest at infrared (940 nm and 890 nm) and violet/UV (400 nm).

6. The intensity of red (660 nm) and infrared (890 nm and 940 nm) spiked from 5:35 PM to sunset. This peak was not observed in any other colors. At 5:35 PM, the sun was located at an azimuth of 246 degrees (west-southwest) and elevation of 6 degrees.

The Photopolarimeter will run several more days, weather permitting. The next experiment will involve placing the Photopolarimeter straight up. Later, the experiment will be run a third time with the Photopolarimeter pointing at the horizon. Later experiments will point it to the north.

An Update
After a discussion with Mr. Forrest Mims, I came to realize that Polaroid film is designed to be most effective at particular wavelengths. Since the Polaroid film I used is designed for eyeglasses, it should be more effective in the green-yellow portion of the electromagnetic spectrum (where human eyes are most sensitive). That may explain the greater polarization seen by the green and yellow LEDs.

Photons in the blue end of the spectrum have wavelengths so short that they can pass through the Polaroid film in either vibration orientation. Photons in the red end have wavelengths so large they can't be effected by the Polaroid film.

So off to find new polarization films.

You can read what Mr. Mims is up to at his website,

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