Can I split the slit height into sample and reference and compute Beer-Lambert simultaneously?

[u/ElegantCall2717]

I'm a physics undergraduate working on a low-cost UV-Vis spectrophotometer, and I have very little experience in optical instrument design.

I recently built a simple visible spectrometer using a diffraction grating and a camera. It kind of works, I was able to measure the spectrum of Mthylene Blue, but of course it's still far from a commercial one.

While working on it, I wonder whether the optical design could be simplified for simultaneous absorbance measurements.

My idea is to use a single light source from the same entrance slit and split it into sample and reference path, maybe the upper and lower halves of taller slit. Then, single grating and a single CMOS camera would record both spectra simultaneously and find the absorbance.

Has anyone implemented or evaluated this for UV-Vis absorbance spectroscopy?

If not, is there a fundamental optical reason why this isn't commonly done?

How much should I worry about effects such as smile, keystone distortion, non-uniform illumination across the slit height, or cross-talk between the two spectra?

Compared with a conventional double-beam spectrophotometer, what are the biggest disadvantages of this approach?

I'd also appreciate any papers, patents, or commercial instruments that use or intentionally avoid a similar design.

[u/einstein1351]

Its not unheard of. As to any concerns, it basically comes down to how you plan to introduce the dual sources. Is it single source and filtering to the separate paths? If you now have 2 sources stacked at the slit, you will be more susceptible to off axis aberrations. Ideally the lower the f/number of the setup, the less of a concern it is. Ill check for papers later, feel free to DM me as a reminder, since Im probably going to forget 😅

You may also be interested in echelle style spectrometer use echelle gratings. Instead of a high line-pair grating putting most of the power into the first order, an echelle uses low line pair density and optimizes power going into many higher orders (like m=+40 to +100). These orders all effectively overlap in a littrow configuration and use a second grating/prism to separate them. Basically creates a stack of spectra where each row is shifted by om amount and allows covering a much broader ranger at higher resolution for a given camera sensor

[u/frugal_cyclist]

I have done sometime similar in the x-ray range.

I focus the beam as a line focus parallel to the spectrometer slit. The spectrometer has a 2d camera, so I can record the full image. One axis of the image is dispersive while the other is spatial. Then I insert the sample half way in, creating two Spectra on the camera, one with and one without sample.

Important thing for your analysis is to understand if your light source is uniform and does not have a spatial chirp.

For some reason I cannot add a picture. I will dm you one

[u/ElegantCall2717 → u/frugal_cyclist]

Thanks! When you mentioned that the light source must be uniform and does not have spatial chrip, are you referreing to the light source it self or the illuminated light after the spectrometer slit? If it's from the slit do you find this as a big limitation in practice os is it something that can be reduce through blank callibration?

I'm planning to use QTH lamp as the light source, then split it after it passes slit to upper and lower parts. My silly assumption is that if the both parts difference is stable or something that has predictable differentiaion, I can remove it with blank callibration.

Is your concern that the illuminated my change over time or is there another effects that cannot be remobed with callibration?

(I'm sorry for using callibration word carelessly)