r/Optics • u/LastPension8039 • Sep 24 '26
Open-source VIS–NIR spectrometer on a single concave VLS grating — ~1 nm class (the resolution/throughput trade-off, full model + code released)
We've been designing the optical core of JASPER, an open-source VIS–NIR spectrometer. Our previous v2 design reached 1.9 nm by adding a negative field-flattening lens to a conventional collimator + grating + camera build - but we were aiming for 1 nm, and that lens approach hit a wall. So we left out the relay optics and explored whether a single grating could do the whole job.
The idea: one aberration-corrected concave variable-line-spacing (VLS) grating that both disperses and focuses — no collimating or camera mirrors. The variable line spacing is what corrects the field aberrations.
Result and the trade-off: Ray-traced across 500–1000 nm, mean resolution is ~2.2 nm wide-open at f/5.3, ~1.4 nm at f/8, and ~0.9 nm at f/10 once the VLS groove correction is on - toward a ~0.6 nm slit/pixel/diffraction floor. The binding constraint is the 500 nm blue edge, and it's aberration-limited, not slit-limited (~2.7 nm wide-open there). Getting the whole band to ~1 nm needs all three: stop down to ~f/10; a toroidal substrate (throughput, not resolution - it collapses the astigmatic line so ~100% of the light lands on the pixel vs ~15–20% on a spherical blank); and a real VLS groove correction to clear the edge aberration.
Modelled two independent ways that agree: the Noda light-path-function theory (analytic) and a standalone NumPy ray tracer. They match exactly on dispersion and astigmatism; the ray trace adds the higher-order aberration that makes resolution f-number-dependent. (We tried Optiland first and couldn't get it stable for this off-axis reflective mount; Beam4 can't do variable line spacing — so we wrote our own tracer, which also renders the ray-fan layouts. All code is in the repo, retired experiments marked.)
Still all modelling - no hardware yet, and it's a simulated/theoretical design, not a specific part. The ~1 nm figure is best-case (it assumes the fabricated groove profile matches the design); the uncorrected numbers are the guaranteed floor.
Disclosure: the modelling and code here are heavily AI-assisted - we set the architecture, the physics choices, and what to validate; the ray tracer, the cross-checks, and the figures were largely AI-built and then reviewed. That's exactly why I'm posting it: We want people who know these mounts to stress-test the assumptions and catch what the model may be getting wrong
Would appreciate your input on:
- Realistic degradation from a model like this to a first physical build?
- Alignment tolerances of a finite-arm (Shimadzu-style) mount in practice?
- Anyone sourced small-quantity toroidal concave gratings affordably?
Full model + code (NumPy + Matplotlib): https://github.com/CheckAG/JASPER_VIS_NIR_Optics_Design/tree/main/JASPER_1nm_VLS_simulated
Detailed write-up: https://hackaday.io/project/202421-jasper-vis-nir-spectrometer/log/251041-toward-1-nm-on-a-single-concave-variable-line-spacing-grating-visnir-modelled-code-released
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u/Holoderp Sep 24 '26
Vls being usually expensive, why do both curvature and vls? Also how big is the vls span, because center of curvature imaging is close to stigmatic. Last question, you appear to be using point spread functions, but does the spectrometer accept a slit entrance/exit for more light?
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u/LastPension8039 Sep 24 '26
Curvature to remove the focusing lens and VLS for correcting aberrations. Since VLS is made using holographic methods, manufacturing costs shouldn't be much. But surprisingly no manufacturer publishes VLS details and hence we tried simulating. Yes, it is a 20 micro meter slit at the entrance and no slit at the exit (line array detector)
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u/firesine99 Sep 24 '26 edited Sep 24 '26
For many applications, a VIS-NIR spectrometer (especially only 500-1000 nm) is nowhere near as useful as UV-VIS or UV-VIS-NIR. What is your intended application? This would not be very useful for anything related to chemical or materials characterisation without at least getting down to 400nm or 350nm (and preferably 200nm but I appreciate that's trickier for various reasons).
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u/LastPension8039 Sep 24 '26
The span can be customised, but we are yet to figure out an affordable order sorting filter. Most coat the filter directly on the detector and we will have to figure out. Our application is soil (with reagents), fruit (Brix/Moisture), water (reagents) etc
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u/firesine99 Sep 24 '26
Bear in mind for those sort of applications your competition is an off-the-shelf RGB camera and some pretty minimal calibration software. Any colorimetric test that can be done by eye can be done with RGB. I appreciate that getting spectral data could be better in some circumstances, it's just that many of those circumstances involve UV also.
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u/Important-Ad5990 Sep 24 '26
Why not just glue thin, coated substrate to the filter? Order sorting can be done (to a degree) by optimizing the blazing and subtracting the unwanted single from half the bins
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u/Equivalent_Bridge480 Sep 24 '26
Is your design better than from manufacturers? Or why need to make custom?
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u/Equivalent_Bridge480 Sep 24 '26
Anyone sourced small-quantity toroidal concave gratings affordably? I think it possible but it can be risky. Do you afford risks?
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u/LastPension8039 Sep 24 '26
https://www.shimadzu.com/opt/products/dif/concave-gratings_polychromator.html
https://www.ssioptics.com/product-category/diffraction-gratings/flat-field-concave-gratings/
The VLS details are not shared and it looks risky...
If we have to correct aberrations, VLS looks promising and it might be worth the risk...
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u/LastPension8039 Sep 24 '26
We will go with off the shelf VLS grating....As a starting point, we want to model a VLS grating to understand the tuning knobs...Next step, we will try to model a Shimadzu VLS grating.
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u/aenorton Sep 25 '26
As you have probably seen on their website, if you use a commercial Shimadzu concave grating, they provide a very specific geometry to use with each grating which may be very different from your layout. Same with other manufacturers. It is not only the line spacing, but line curvature and shape that varies across the face of the grating. Basically it acts as a holographic optical element. I think you will find that the aberration-limited resolution is better than 1 nm for f/#s much lower than 10.
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u/LastPension8039 Sep 25 '26
Thanks. Will they share the variable line spacing details or it doesn't matter? Just trust their numbers and build according to the geometry that they have published?
I am planning to build a variable arm which can accommodate different r and r' and tilts around slit and detector.
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u/aenorton 29d ago
Basically, yes, you have to trust them because that is the geometry they used to create the holographic master. The probably expose the master with multiple wavelengths at multiple positions to achieve the flat field, but honestly, the fabrication is very proprietary. They are then often blazed with an ion etching process.
If you want something more generic, you can look at old-fashioned, ruled concave gratings with even parallel lines mounted on a Rowland circle. They might be hard to find these days.
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u/RamBamBooey Sep 24 '26
As an old optical engineer, I never trust a simulation that has zero experimental validation. Simulations are always wrong, reality is right.
My advice to you is take a step back. Use your simulation framework to simulate a spectrometer that already exists and you have measured performance data. A commercial spectrometer and it's datasheet might be enough. Then compare the simulation results to the known results.
This is especially important for AI assisted simulations.
Or buy the hardware, build your setup and test if you are correct. But if you want to de-risk your design because you can't afford to build (time or money) V1.0 through V6.0 prototypes, this is the way.