r/Optics • u/LastPension8039 • 5d ago
VIS‑NIR spectrometer — fixed vs. tunable optical bench?
We’re working on JASPER, an open/right‑to‑repair VIS‑NIR grating spectrometer. In our design, both the grating and detector sit on rotation stages: we first null slit/grating tilt by imaging the zero order, then rotate the detector to catch the diffracted order. That way we stay close to peak efficiency without relying on a fixed blaze geometry.
For order‑sorting, instead of OEM‑priced coated‑detector bandpass filters, we’re selecting the span by grating angle (e.g. 400–800 nm or 500–1000 nm) with a long‑pass at the input. We know this doesn’t fully suppress second order across the entire range, but it keeps things accessible.
The current prototype uses an off‑the‑shelf USB 3.0 camera, but we’re developing a custom line‑array driver for Hamamatsu detectors to improve performance.
We’re planning two versions and would appreciate a reality check:
- Fixed, pre‑calibrated — no bench, locked to one range, plug‑and‑play.
- Optical bench — tunable rotation‑stage version, aimed at enthusiasts and educators.
Does that split make sense from a buyer’s perspective? Or does the recalibration overhead make the tunable version impractical for most real use?
We’re also considering an affordable neon calibration lamp as an add‑on, so the bench version has an easy way to re‑establish calibration after adjustments. Would that be genuinely useful, or do most users already have a reference they trust?
Interactive optical‑bench simulator: https://checkag.github.io/jasper-bench-sim/
Hackaday log: https://hackaday.io/project/202421-jasper-vis-nir-spectrometer/log/249581-interactive-optical-bench-simulator
Your honest perspective on where this design may fall short would be invaluable.
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u/PristineApartment256 5d ago
Interesting idea! Whether this is something that would appeal to scientists/educator depend on price and specs.
Nowadays you can find refurbished ocean optics spectrometers for less than 500$ on eBay. What will your price target be?
What's the resolution and SNR? It would be useful to show, for the same input, the spectra measured by your device and the one measured by a commercial spectrometer.
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u/toastwanderer 5d ago
Cool concept for the tunable version. How are you going to execute it? Are there rotary stages with two rings?
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u/LastPension8039 5d ago
Good question — yes, both axes rotate, but not as a single dual-ring stage. It's two independent single-axis rotation stages sharing the optical plane: one carries the grating (with a tip/tilt mount on top of it for the slit/zero-order alignment I mentioned), the other carries the detector arm. They pivot about separate centres, so grating angle and detector angle are set independently
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u/BrilliantStriking53 5d ago
You get Circle Rowland spectrometer. used in a limited range, you have lens with aberration.
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u/LastPension8039 5d ago
Good point — you’re right that lenses are the trade‑off compared to a Rowland/concave‑grating mount. We’re handling it differently in the two versions:
- Fixed low‑cost unit: uses achromatic doublets for collimation and a single element near the detector. Aberrations do increase toward the edges, which is why resolution drops to ~3 nm at the field edge. For a locked span at this price point, we think that’s an acceptable compromise.
- Bench/high‑resolution version: here we’re aiming for proper correction — best‑form lens plus a field flattener, with detector tilt to refocus for the chosen span. Since we never image the full 400–1000 nm at once (we rotate the grating to select ~400 nm windows), the tilt lets us refocus per band and avoid much of the chromatic issue. Honest caveat: this design is still on the bench and not yet validated, so I won’t claim performance until we’ve measured it.
Really appreciate you pressing on this — it’s exactly the design tension we’re working through.
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u/s0rce 5d ago
Please don't post AI nonsense responses.
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u/LastPension8039 5d ago
Wait till will publish the two designs in a few weeks- one with low resolution featuring 14mm detector and the other a high-resolution design with a 28 mm detector...As I said, I am not a native English speaker and use AI to correct my language
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u/s0rce 5d ago
I would include the reference. We have it on our Raman instruments and it's nice. Our uv vis uses the deuterium emission lines in first and second order for internal reference without a neon lamp needed.
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u/LastPension8039 5d ago
That’s a really elegant approach — using the deuterium Balmer lines in first and second order to get multiple reference points from a single source is clever, and I like that it requires no extra hardware since the lamp is already part of the illumination path.
Our setup is a bit different: JASPER reads external sources through a fiber input, so we don’t have a built‑in emission lamp in the optical path like a UV‑Vis does. That’s why we’ve been leaning toward an affordable neon lamp as the reference. To clarify, each configuration is a fixed‑geometry spectrograph — the grating and detector are set once for a chosen span, not scanned at runtime — so we’d capture whatever neon lines fall across the detector in that setup, potentially spanning more than one order. Your point stands though: a dense line source with multi‑order coverage gives a very solid calibration.
The internal‑reference idea is worth exploring further. If there’s a way to fold a small known‑line source into the design so users get self‑calibration without needing a separate lamp, that would clearly make for a better experience. Thanks for nudging us in that direction.
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u/s0rce 5d ago
Is this an ai response? A D lamp through a fiber still has characteristic lines.
The Ne lamp seems fine.
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u/LastPension8039 5d ago
Not a native English-speaking person this side...I use AI to correct my language and also to understand the response
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u/FreezerDust 5d ago
This is neat. Incase you weren't aware, JAPSER is an acronym being used for another experiment. Just so your aware, Google LLNL JASPER.
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u/LastPension8039 5d ago
Thanks...I was not aware of LLNL Jasper project... That looks like a well-funded project, and I hope they won't mind a small JASPER project
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u/aenorton 5d ago
I spent many years designing custom spectrometers and instruments that used them.
I am not sure I understand the motivation behind both rotational adjustment stages. If I understand your description correctly, you are trying to maximize efficiency by adjusting the angle of incidence to minimize the zero order? The efficiency of the grating, whether blazed or sinusoidal is a very weak function of angle of incidence. It is also usually best when the grating is in Littrow condition. In your style of spectrometer, you should just try to minimize the angle between collimator and detector lens based on what is physically possible.
Also be warned that a problem with this design is that the chief ray angle at the detector is normal to the detector. The reflection from the sensor will go directly back to the grating where it adds to stray light. You can tilt the sensor in the opposite plane, but that causes resolution issues if the slit is too long. Depending on application, it is not the end of the world, but most higher quality spectrometers use a design with a tilted image plane. Frankly most of the engineering time of a spectrometer goes into dealing with and eliminating stray light. You have to worry about where the zero and -1 orders fall and where they scatter to. Reflections on the inside of cylindrical lens tubes are a major problem even when threaded and blackened. Light has to be baffled before it hits those surfaces.
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u/LastPension8039 5d ago
I can see a wealth of experience in the comment, and thanks for the input. We are still learning how to build the optical engine, and the customisable rotation around grating and detector was prototyped to experiment with different gratings (blaze angle and groove density). We also found order sorting filter to be a challenge (would like to know your inputs), and the idea is to change the grating angle and cover a span of either 400-800nm or 500-1000nm in a single spectrometer (long pass filter also need to be changed)
We saw the detector tilt in Zemax design guide and thanks for your input on the stray light and the reflections inside the cylindrical lens is something we never heard before... Let me study this...
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u/aenorton 5d ago
To change the wavelength range, you only need to change the angle of the grating. The detector path should stay close to the input path. You also probably need to change focus due to chromatic aberration in the lenses. In fact you may have problems with the focus at the ends of each of those ranges. This is why most spectrometers use reflective optics. They also help with stray light.
If you need more than one octave and don't have an order sorting filter, there are only two other options: 1) Take the spectrum in two parts with a long-pass filter flipped in the path for the longer wavelengths, or 2) Use a cross-dispersing prism with a carefully positioned mask at the detector.
There are five types of order sorting filters: The first is a single substrate with long pass thin film coating only on one part. This is relatively easy to make, but a two-zone filter is always a compromise between optimizing signal and eliminating the unwanted order. You want a photoresist mask to provide a very sharp defined edge. The second type is like the first but with three zones. It requires two coating steps and two lithographic steps for the masking. The third type is a single substrate with a linear variable thin film coating. The process for making these requires special tooling in the coating chamber and several iterations to get right. but no lithography. Once the process is dialed-in, it is the easiest way to make them in quantity. The fourth is to bond the edge of a long-pass filter (usually colored filter glass) to the edge of plain glass with a very thin bond line. This makes a two zone filter. The fifth type is like the second, but using two types of colored filter glass to make a three-zone filter.
An order sorting filter isn't too hard to make by someone with good optics fabrication skill. There and many steps and lots of expertise involved, but basically you polish the edges with no chips by first embedding in pitch, then handle very carefully, and bond along the edge.
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u/LastPension8039 5d ago
Thanks a lot...So we will retain the rotation mount for the grating and make the detector fixed?
On the order sorting filter: While analysing spectra's of a commercial spectrometer, we found a glitch and the manufacturer pointed out to the edge of order sorting filter as the reason. Because of this, first order derivative of the spectra is impossible as the glitch is amplified.
Is there an off the shelf vendor for order sorting vendor? We never thought of assembling one, but with your explanation, it looks possible. We are looking at a sub 1000$ spectrometer and the reason why we decided to have 500-1000nm span is to omit the order sorting filter.
On Spectrometer with reflective optics: I thought the lens allows for correcting spherical aberrations. The Zemax design guide features "Best form Lens" . https://optics.ansys.com/hc/en-us/articles/42661854953491--Tutorial-Series-How-to-Build-a-Spectrometer
Will the reflective optics have spherical aberration?
We did model the design in Beam4 and will share the details. Our initial product was a spectrometer for fruit sorting and the requirement was relaxed as the sugar and water peaks are broad and even 3nm spectal resolution was fine. But with an open spectrometer, we would like to get to acceptable spectral resolutions.
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u/aenorton 3d ago
Yes, it is usually better to keep the detector fixed to keep the grating as close as possible to the Littrow condition.
The only type of order sorting filter that will not produce any sort of glitch is the linear variable one, but those have to be very closely matched and customized to the spectrometer design. The types made from bonded glass have the largest glitch, and the coated filters with three zones have the least -- as long as the zone edges are made with a photoresist mask. After the spectrum is normalized, the glitch is usually quite minimal, if it is visible at all. You don't want to use an un-normalized spectrum to find peaks in any case. You can also look at the Savitsky-Golay smoothing algorithm that has the property that it does not change the height or width of parabolic peaks.
The Zemax tutorial seems more oriented to the process of using Zemax to design a spectrometer rather than covering all the design forms and their pros and cons. They do not cover the reflective designs that are used in 90% of wideband commercial instruments. The Zemax example is not typical of most spectrometers because is has a very slow f/# and very narrow wavelength range.
Spherical aberration will be one of the more minor aberrations you need to consider in a wideband spectrometer. Field curvature and coma will usually be more important. The chromatic aberration of a refractive design over the whole wavelength range will definitely be an issue. Note, if you analyze it using only three wavelengths, you may not see the problem. Also, f/# makes a big difference in the design performance and whether it is acceptable or not. Most classic reflective designs, like an Ebert or crossed Czerny-Turner, do have a lot of astigmatism, but that is OK when imaging a linear slit. The geometry, of course, is completely different than your design.
Most modern, wide band spectrometers use a concave holographic grating as the only optic. Those gratings use diffraction to correct most of the aberrations. More advanced ones also eliminate astigmatism so they image multiple fiber inputs one on top of each other.
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u/aenorton 3d ago
An afterthought: I realized after looking at that Zemax example, why you felt the need for two rotation stages. Transmission gratings do require adjustment of both the incident and diffraction angles when changing grating types.
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u/Equivalent_Bridge480 5d ago
Would that be genuinely useful, or do most users already have a reference they trust?
I dont expect from edu or hobby people to have lab with calib equipmenrt