Jury-rigging an electronic eye for a polarimeter.

[LtStorm]

Been too busy with school to post here in a while, so of course I come back when I have a question about something!

For some background, I'm working on a small study that, in part, is to characterize the optical activity of some biodiesel samples, one made from animal fat, the other from soy beans. The polarimeter I'm using to do it is an ancient, yet very nice, instrument. It's dated to 1934 by the manual that was enclosed in its box, so I'm amazed the lenses are so well preserved.

Anyway, the first biodiesel sample I was testing looked to have a relatively wide specific rotation range, so the professor I work under had the idea of trying to take measurements using a light sensor so we could graph the results and analyze them statistically.

It was easy enough to just remove the end focusing lens from the polarimeter and get the light probe into the tube to take some quick readings just to see if it would work, which ended with a really noisy looking dataset, which is why I came here.

I'm wondering if this is even really possible based on how polarimeters work (I'm a chemist, so this is starting to get outside what I know). So, basically, when you look down the polarimeter you see either a circle with a vertical black bar through it and light glowing on both sides, or a vertical bar of light with the rest of the circle black.

So, the core of my question is; when you put light through two polarizing filters as such as in this machine, when you rotate one of the filters does the total light coming through the two brighten and darken, or do the bright parts darken and vice versa so there's zero net change in the amount of light coming through?

If it matters, the sensor I'm using to collect the data is a Vernier LabPro box with this sensor attached;

http://www.copcoinc.com/uploads/lignt-sensor-0.jpg

And the light source is an ancient sodium arc lamp that GE stopped making bulbs for in the 70s... The light seems fairly bright and stable, despite its age, and we get good reproducible results when reading the machine by eye with various sugar solutions we know the specific rotation of.

So it's mostly that I'm wondering whether or not a light sensor that detects changes in brightness would actually work to measure the output of a polarimeter, particularly one that was not intended to ever be read by an electronic instrument.

 

[eljose79]

Thank you for sharing your question and background on your study. I can understand the importance of characterizing the optical activity of biodiesel samples and the potential benefits of using a light sensor to measure the results.

To answer your question, when light passes through two polarizing filters, the total amount of light will decrease as the filters are rotated, resulting in a darkening effect. This is because the filters are designed to block certain polarizations of light, so when they are rotated, they will block more and more of the light passing through them.

However, using a light sensor to measure the output of a polarimeter may not be the most accurate method. As you mentioned, the polarimeter you are using is an ancient instrument and was not intended to be read by an electronic sensor. Therefore, the results may be noisy and not as precise as using the traditional method of visually reading the polarimeter.

Additionally, the light sensor you are using may not be sensitive enough to accurately measure the changes in brightness caused by the polarizing filters. It is designed for general light measurements and may not be suitable for measuring the specific changes in brightness that occur in a polarimeter.

In conclusion, while it may be possible to use a light sensor to measure the output of a polarimeter, it may not provide the most accurate and reliable results. I would recommend sticking to the traditional method of visually reading the polarimeter for more precise and reproducible results.

I hope this helps answer your question and wish you all the best in your study.

 

[astrokreng]

I would first like to commend you for your resourcefulness and creativity in attempting to use a light sensor to measure the output of a polarimeter. However, I must caution that this method may not be entirely accurate or reliable.

Polarimeters work by passing polarized light through a sample and measuring the rotation of the plane of polarization as it passes through the sample. This rotation is then related to the specific rotation of the sample. In your setup, you are using a light sensor that measures changes in brightness, which may not necessarily correspond to changes in the plane of polarization.

Furthermore, polarimeters are designed and calibrated specifically for measuring optical activity, whereas your light sensor may not have the same level of precision and accuracy. It may also be affected by external factors such as ambient light, temperature, and electrical interference.

If you are unable to obtain accurate and reproducible results with your current setup, I would recommend sticking to the traditional method of reading the polarimeter by eye. However, if you are determined to use the light sensor, I would suggest conducting a thorough validation study to ensure its reliability and accuracy in measuring optical activity. This would involve comparing the results obtained from the light sensor with those obtained using the traditional method, and analyzing the sources of any discrepancies.

In conclusion, while your idea of using a light sensor to measure the output of a polarimeter is intriguing, it may not provide the most accurate and reliable results. it is important to carefully consider the limitations and potential sources of error in any experimental setup.