What is a Polarimeter ?
A polarimeter is an optical instrument that measures the angle through which the plane of polarization of plane polarized light is rotated when that light passes though a sample. A sample that causes the rotation of the plane of polarization is said to be “optically active”. An example of an optically active sample is a solution of a substance in which the molecules possess a “chiral centre”. Such molecules can exist in two non-superimposable mirror-image forms (optical isomers, or enantiomers - see the image below right) - each of which rotates plane polarized light in opposite directions. In such a case the two forms are each “optically active” and the sign of the optical rotation can be used to distinguish between the two isomers.
Many samples exhibit this property – common examples include amino acids (except glycine), carbohydrates such as sucrose, glucose and fructose, vitamin C (ascorbic acid) and many components of essential oils (lavender oil – (±) limonene, peppermint oil – menthol, spearmint oil - carvone).
In a “racemic” mixture containing exactly equal amounts of two optical isomers the optical rotation vanishes – thus polarimetric measurements are important for characterising the enantiomeric purity of a sample. Measurements of this type are extremely important, for example, in drug development for the pharmaceutical industry.
The PicoPol polarimeter leverages the impressive feature set of the RPi PICO in a compact, low cost instrument capable of research grade performance that is assembled using 3D-printed components. The fully automatic instrument reports measurement results either stand-alone to an organic light emitting diode (OLED) display panel or alternatively, via a PC connection to a LabVIEW graphical user interface (GUI).
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In the I4C-POL, 1 is a yellow LED and 3 is a fixed polarizer mounted inside the source assembly, described below. Items 7 and 8 are housed in a detector assembly - again see below for details.
Instrumentation - How the PicoPol Works ...
Plane-polarized LED Source
The light source in this compact polarimeter is a high-brightness yellow light-emitting diode (LED) producing light with a peak wavelength near 590 nm, closely matching the D-line sodium (Na) emission that is commonly employed in more costly instruments. The LED’s output then passes through a fixed disk of polarizer sheet material, resulting in a plane polarized source beam.
The images above show the source printed circuit board (PCB) whose LED is mounted centrally into a 3D-printed source housing that encapsulates both the source PCB and the polarizer disk. The LED brightness is adjustable via the small trimpot (hover over using the loupe) and located just to the right of the centre of the PCB.
Analysing Polarizer/Detector
At the analyser end of the instrument, a polarizer disk is affixed to the shaft of a stepper motor that spins this disk at a constant rate. Located on the PCB behind the analysing polarizer (and just below its black hex retaining nut) is an optical detector. This optical detector registers a cosine squared waveform as the analyser disk spins.
The instrument is controlled by a Raspberry Pi PICO micro-controller. The PICO is housed in a separate enclosure (see later), with a USB-A to mini-USB cable making the connection to the analyser PCB.
Many traditional educational polarimeters use an optical null method, where the analyzer is rotated until the detected light intensity is minimized. While simple, this approach relies on visually identifying the minimum signal and can introduce operator-dependent errors.
In PicoPOL the analyzer rotates automatically while the instrument records a dense set of detector measurements over a full rotation of the analysing polarizer. These waveforms are averaged over multiple rotations, after which curve fitting is used to determine the precise position of the intensity minimum.
By analysing the complete waveform rather than relying on a single null point, the instrument determines optical rotation more robustly and repeatably. A new optical rotation reading becomes available every 5 seconds.
Operating Modes
The instrument can be operated in one of two ways. For fully stand-alone operation, user interaction is via an IR remote keypad, with the results being displayed on a 2.8" OLED panel. We call this option the PicoPol-OLED.
The PicoPol-OLED interface PCB is shown below - this connects to the polarimeter's optical bench via the USB Type A Connector visible near the upper left corner.
Alternatively, if data logging is required (for example, in a kinetic study) the instrument can be connected to a PC - the results are then displayed in a LabVIEW front panel. This option (denoted the PicoPol-PC) has an interface PCB containing just a Raspberry Pi PICO and a USB-A connector - see the bottom of the PicoPol-PC interface page for further details.
PicoPOL is supplied with a 590 nm yellow source LED, but some experiments are reported on the Results page that employ source LED’s having different colours - I.e. source PCB's can be assembled with other LED peak wavelengths. Contact I4C for further details should this be of interest,
The OLED controller interface PCB, (PicoPol-OLED option) with the RPi PICO module in the middle. A 2.8” OLED display plugs into this PCB via the 14-way and 4-way headers visible at the left and right edges of the PCB. In conjunction with an IR remote keypad, the OLED panel provides a graphical user interface giving the user full control of the PicoPol polarimeter.
Data Displays
When connected to a PC, the acquired waveforms are uploaded to a LabVIEW vi that serves as a phase measurement system, curve fitting the received cosine squared waveform to extract an initial phase value Φb when a non-optically active “blank” is placed between the two polarizers. The phase is then re-measured with a sample in place (Φs) ; from the phase difference Φs - Φb the sample’s optical rotation can be determined. The polarimeter vi allows optical rotation values to be logged versus time, affording the possibility for kinetic studies.
When operating in stand-alone mode, a custom floating point multiple linear regression algorithm running on the RPi PICO performs the curve fitting, processing each summation-averaged 8000-point waveform directly. Additionally, an IR receiver also interfaced to the PICO allows user interaction via an IR remote keypad to activate/deactivate the stepper motor, to zero the instrument, and to view either the acquired waveforms, the waveform fitting results or the optical rotation readings during an experiment. To streamline this process, the two cores on the RPi PICO work co-operatively.
The LabVIEW analysis screen during a short experiment. The instrument is initially zeroed with a blank and then a cell containing R-(-)-carvone is measured. A new cell containing S-(+)-carvone is then inserted and finally the original blank solution re-measured. While this LabVIEW vi shows the model fitting results, a simpler version of the vi is also provided and can be seen on this page.
The OLED Analysis Screen after pressing Remote key 1 = "Fit parameters"
The PicoPol Polarimeter Optical Bench
The PicoPol Polarimeter - with the source end at right and detector/analyser at left. The 10 cm polarimeter cell is supported by two cradles. The instrument shown above is controlled by a RaspberryPi PICO on a separate OLED controller PCB (-OLED option), or alternatively via the lower cost -PC option.
The source and detector housings described earlier are mounted on a laser-cut acrylic baseplate along with two carrier blocks to support a 10 cm path length polarimeter cell. The fully assembled polarimeter optical bench is shown in the photo above.
Development of the PicoPol polarimeter has cut fabrication costs to an absolute minimum while achieving the best possible performance. Leveraging the use of a low-cost stepper motor control module, an inexpensive optical detector and the RPi PICO’s many powerful on-board features has been key to achieving this goal.
