More on Lakshmi Raju's Spectrophotometer
by Alane Lim
Hello, all! As you know, we're using the spectrophotometer of one 16-year-old Lakshmi Raju on our robot to help detect water on the moon.
You might be interested to see how it works, so I'm going to attempt at a very basic explanation here.
A spectrophotometer measures how much light is absorbed or reflected from a source. To be more precise in this type of measurement, you need to control what wavelengths of light you have. In our case, we're looking at the infrared range. Filters that only allow a very narrow range of wavelengths to pass through. The values used in this case were 940 nm and 1000 nm, +/- 9 nm wavelength.
Then, the humidity was controlled. Water vapor in the air can interact with the light and absorb it. Think a foggy day versus a sunny day.
After this, a lamp was used to shine light onto a convex lens through a path with a specified length. The light was "collimated" (i.e. the light waves were made parallel to each other) by a, well, collimating tube to make it much easier to select for wavelength. It also makes the process much more accurate. A jumble of wavelengths would have readings all over the place!
The convex lens then focused the light through one of the filters—again, getting rid of the unwanted wavelengths here—and onto a photodiode. A photodiode converts light intensity into voltage that people can measure.
And that's what she did. Lakshmi measured the voltage that resulted from the light photodiode with a typical multimeter and even made a Matlab program to read this.
In Chemistry and Physics, there's something called Beer's law, which relates the intensity of light transmitted and a solute (in this case, water). It also depends on path length of a single wavelength of light, so you see that everything was controlled for.
Turns out that there's a logarithmic relationship between the transmitted light and how much of the compound is in a sample. Using this relationship, the spectrophotometer was tested against a control wavelength, in high and low humidity—and it worked! Great job.