TCD1304 Spectrometer - Part 5

Mar 26

In this video, I show you the recent upgrades regarding my TCD1304-based spectrometer. The updates involve a better fitting algorithm, a better PC software and some demonstration with different light sources.

Improvements

The main update is that I replaced the linear equation with a 3rd order polynomial for calibration. This has a noticeable impact on the performance of the spectroscope.

In the figure next to the text I show both curves fitted over the same four points. I derived those points using a compact fluorescent tube measured by my spectrometer and a reference spectrum on Wikipedia.

We can see that even in the middle, at around 1500 pixels, the difference between the two curves is 10 nm. This further increases towards the pixels above 2500. At 3500, the difference is 33 nm! As a reference, the whole visible light range is around 300 nm (roughly 400-700 nm), so this error is more than 10% of this range…

Actually, this error was “perfectly” demonstrated in my previous video where I calibrated the spectrometer using a linear fitting and then I measured a red laser’s spectrum. The wavelength was 678 nm instead of 650 nm… Also, for a green laser, the wavelength was 528 nm instead of 532 nm.

By using the polynomial fitting, I got 532 nm for the green laser and 653 nm for the red laser. Both are really close to their nominal wavelength.

I also implemented a “baseline" subtraction”. This is a fairly simple but useful feature. When there’s no light falling on the CCD, the output signal is nonzero. I can store these nonzero values in an array for each pixel and then subtract from the subsequent measurements. This results in an aesthetically better spectrum.

Linear and polynomial calibration curves.

Below you can see three different designs with the TCD1304 Linear CCD that might interest you. The first two are recent designs. The third from 2022 is specifically made for the spectrometer I have. I bought the spectrometer body without the lid and the CCD from this seller on eBay.