DOLPi - RasPi Polarization Camera

Reader Paul Wallace contacted me to tell me about the DOLPi electro-optic polarization camera that he built for his iPhone. His ingenious solution makes use of the iPhone's flashlight to calibrate and synchronize the control of the polarization analyzer (hacked from a welder's mask as described in the DOLPi whitepaper).

The drive voltage at which quasi-45 degree rotation of polarization occurs varies over time, so Paul used a phototransistor to detect light from the iPhone's flashlight that passes through a 45-degree polarizer on its way through the liquid-crystal panel. By varying the drive voltage across the liquid crystal panel, a minimum in transmitted light occurs when the 45 degree polarizers cross. As shown in the following picture, this level can be detected and used to set the corresponding 45 degree voltage reference:

LCP transmission to 45-degree polarized light as a function of voltage in Paul Wallace's iPhone implementation of a DOLPi polarimetric camera

The iPhone’s flashlight is also used to synchronize the change in polarization with the image taking. The iPhone blinks the flashlight after every three images have been taken. The device uses this flash to reset the 0,45,90 sequence to a known state.

Paul provides details of the Arduino-controlled circuit, its firmware, and the iPhone app in his webpage: http://ynformatics.com/2016/iphone-polarisation-camera/

Thanks Paul for sharing, and kudos for your very ingenious solution!

Arduino-controlled circuit for the iPhone-based DOLPi Polarimetric camera developed by Paul Wallace

The final version of the whitepaper is now available at: http://www.diyphysics.com/wp-content/uploads/2015/10/DOLPi_Polarimetric_Camera_D_Prutchi_2015_v5.pdf

Copies of all project files are available for download from Dropbox and Github

All DOLPi Project files for the productized versions are now available on Dropbox. The project's whitepaper and finalist video are also included.

CLICK HERE to be taken to the Dropbox file

What a marathon weekend! I'm almost done though...

I just posted the finalist video for DOLPi:

This weekend I was able to complete the construction and test of the DOLPi-UI imager. It is capable of viewing and polarimetric imaging in the IR-VISIBLE-UV bands. A FLIR Lepton module extends its imaging (but not polarimetric) capabilities down to the longwave IR ("thermal IR"). Raspberry Pi is able to simultaneously present the thermal IR image while acquiring images in the near-IR, visible, and near-UV ranges.

Had a very productive weekend for DOLPi-UI (Universal Imager). The main Chassis is assembled and all electronics and software are working. I added a FLIR Lepton module to extend the imaging range (although not polarimetry) to the longwave IR ("thermal" infrared).

Still have to complete the assembly with the new bandpass filter wheel and light shield that are *still* being 3D-printed.

I have a few surprises for the final submission!

HAD just announced the 2015 HAD Prize Finalists, and DOLPi is one of them! Thanks HAD!

Pushing along for the final submission, parts for the final product-ready prototypes of DOLPi and DOLPi-MECH are now being 3D printed. Here is a DOLPi-MECH filter wheel being printed by an Ultimaker 2:

Although the circuit of shown so far as the liquid crystal panel's AC driver works well, I’m not too happy with the intrinsic non-linearity of the FET. Because of this, today I designed and tested an alternative, a bit more complex, but I believe more elegant design. In the circuit shown below, the LCP drive amplitude tracks linearly with the DAC’s output.

The non-inverting amplifier built around U1 approximately doubles the output of the DAC. This voltage is then presented to an H-bridge implemented by the analog switches in U2. The H-bridge produces a continuous biphasic train at a frequency given by the oscillator built around U3A and U3B. U4 doubles the +5V coming from the Raspberry Pi to power U1 and U2. The LCP is connected between the legs of the H-bridge.

An updated version of the project's whitepaper is available at: http://www.diyphysics.com/wp-content/uploads/2015/10/DOLPi_Polarimetric_Camera_D_Prutchi_2015_v4.pdf

V.3 of the DOLPi whitepaper is now available at: http://www.diyphysics.com/wp-content/uploads/2015/09/DOLPi_Polarimetric_Camera_D_Prutchi_2015_v3.pdf