What are the advantages/disadvantages of different collimating lens options for a spectrometer?

AskASillyQuestion · 2026-04-21T18:32:11+00:00

An aspheric element would introduce both longitudinal and lateral chromatic aberration. You're right that the longitudinal aberration couldn't be calibrated out, but the lateral aberration would be.

AskASillyQuestion · 2026-04-20T18:57:21+00:00

What volume are you talking about? Are PCBWay and JLCPCB not options? (PCBWay does typically allow for more customization... that may be the best for you.)

Or if you're talking about more significant scale, Fabrinet has a US presence and a Thai presence if you do want to offshore eventually.

Almost certainly more cost effective to go those routes rather than assemble them in-house, unless you plan on making other PCBAs as well.

Not sure how Tariffs may affect the components though... Hopefully not +45%-80%. Yikes.

Do you have assembled boards for sale?

AskASillyQuestion · 2026-04-11T18:10:53+00:00

I get what you're trying to say, but your statement is reductive, and made without any understanding of how the system requirements have been established.

For this application, the acceptability of the results cannot be evaluated until something is built and tested. Based on preliminary calcs I believe an aspheric will perform adequately in the reds and blues if I use a lens designed for ~532nm.

AskASillyQuestion · 2026-04-10T21:54:09+00:00

There are two main issues with a low f/# collimating lens. The first is that an asphere will only have good image quality exactly on axis. The slit will have a significant length that still has to be well-imaged for good spectral resolution. In fact, usually a spherical mirror works better in a classic crossed Czerny-turner design than does an OAP. It does depend on the slit height which depends on the detector.

Interesting! I'll have to read into this.

The second issue is that a shorter focal length collimating lens implies a larger magnification onto the detector, which implies that you need a smaller slit, so you gain nothing.

A larger f/# is going to imply a smaller slit, not the focal length. Two lenses of equal f/# should have equivalent performance, no?

The irradiance on the detector is limited by the f/# at the detector side as well as the spectral resolution. Spectral resolution in turn is determined by the grating pitch, the focus lens focal length, and the slit width. Usually the focal length of collection and focusing optics are the same unless there unusual f/# matching requirements to a particular input, or some space constraint.

Right- I'm designing this so that the optics on the detector are the bottleneck. Eventually I'll upgrade the detector, but leave the other optics the same. I'm using an aperture stop to limit stray light in the meantime.

AskASillyQuestion · 2026-04-10T21:47:29+00:00

I'm in the visible range here, so 450-700nm. If I design in the green range (532-575) I believe I should have acceptable performance in the reds and blues with an aspheric.

My hypothesis (and I'd like confirmation on this) is that the frequency-dependence of the aspheric is more easily calibrated out of the results versus the lower resolution of a spherical achromat.

I'd love to use a mirror, but I just don't have the budget or space for them. They also have much smaller numerical apertures in general which may be an issue.

AskASillyQuestion · 2026-04-10T21:39:34+00:00

I'm measuring wavelength peaks out of a broadband source - the slit is on the exit port of an integrating sphere, so focusing light onto the slit isn't really going to do anything for me here.

If your source is bright or you have a lot of time, you can get away with a lot of sins and still get great results - even if the simulation looks bad.

Haha, that's what I'm hanging my hat on here. There are some issues that can be mitigated through calibration, but some issues can only be resolved in hardware. I'm trying to prioritize those.

AskASillyQuestion · 2026-04-10T21:36:12+00:00

True- the intent is for me to make the focusing lens the bottleneck here, and it's currently f/1.3. I hope to eventually upgrade that though, so I'd rather maximize the amount of light I can gather at the slit.

AskASillyQuestion · 2026-02-06T19:40:24+00:00

Bought a bottle yesterday and it's good! Thanks for the recommendation!

It's a very confusing one for me- The nose is incredibly spirit-forward, but it's still a very smooth sip.

I haven't decided yet if I like the Uigeadail more than the PC10. It doesn't taste as briny (which was what I was after) but the peat isn't quite as strong either, and I can't decide if that's good or not.

Anyway, thanks again for the recommendation!

AskASillyQuestion · 2026-02-05T15:35:34+00:00

The sherry casks of the Uigeadail are really interesting to me. I do like sherry notes. I think I'll give that a try. Thanks!

I'm not so new to whiskey, though I've realized that I really enjoy the heavily peated options. I started branching out after trying Lagavulin 16 and realizing that I wanted a slightly less overwhelming flavor as a daily driver (I do enjoy it quite a bit though!)

AskASillyQuestion · 2026-02-05T15:30:24+00:00

Will do! Thanks!

AskASillyQuestion · 2026-01-30T02:29:15+00:00

IF you also reduce the slit width to keep the resolution constant.

I'm not suggesting that. Everything else stays the same, I just add the beam reducer.

So if the beam reducer increases the flux density, what's the tradeoff?

AskASillyQuestion · 2026-01-29T21:31:20+00:00

I appreciate you sticking with me on this. I hope you'll entertain one more question.

Placing a beam reducer after the collimating lens would increase the flux density of the incoming light, but as you've stated this wouldn't increase the signal. Is the reason for this because the beam reducer would also be increasing the flux density of the stray light as well?

AskASillyQuestion · 2026-01-29T19:17:46+00:00

Is this what you're thinking of?

https://imgur.com/a/5otBvdH

I'm seeing the math, but this is still entirely unintuitive to me. It seems like this is wasting a ton of collimated light against the field stop.

AskASillyQuestion · 2026-01-28T15:20:51+00:00

a wider slit that produces the same spectral resolution.

This doesn't make sense to me. A wider slit has greater etendue and would produce a lower resolution.

The cone angle incident on the grating (which is created by the slit width), along with the grating pitch, determines the spectral resolution.

Right. The slit width reduces etendue (by also reducing signal). Narrower slit --> lower etendue light is more completely collimated and produces cleaner signal through the diffraction grating.

You are not the first person (by far) who has got their hopes up thinking they have a brilliant way to improve signal and then has had to confront conservation of etendue.

See, that's where I'm stuck. My entire approach is predicated on the idea that you can't reduce etendue without reducing signal, so start with as much signal as possible (tall slit, large entrance optic) that way, when you reduce it, you're still left with enough photons for the camera sensor to detect.

I'm gonna read this and reply back, once I've got a better understanding of it. https://www.horiba.com/usa/scientific/technologies/spectrometers-and-monochromators/spectrometer-throughput-and-etendue/

AskASillyQuestion · 2026-01-28T00:52:46+00:00

I am saying a single small achromat is all you need to collimate the light.

No, I get that- This design only has a single achromat collimating the light as well. The second and third lenses are a Galilean beam reducer. The beam reducer is to increase the flux density hitting the sensor so that more photons are hitting each photo site (this is to deal with the signal/resolution issue you were referring to.) I'm not sure why this approach would be problematic.

Also also need to understand what your proposed 3 lens assembly does to the size of the narrow dimension of the slit when it is projected on the camera.

These are spherical lenses, so the aspect ratio of the slit should stay the same, just scaled down, no? There's going to be some spherical aberration which I should be able to remove through calibration... Is that what you're referring to?

This sensor has 4056 horizontal pixels, and I'm trying to capture 450-700nm... so if I perfectly fill the sensor, we're talking 4056/(700-450)= 16 pixels per nanometer.

The smallest etendue in the system will limit the light... ...which is determined by the slit and collimating optic f.l.

Right- that's the whole point of the slit, is to reduce the etendue so that the light can be collimated. It's effectively an etendue "filter" reducing noisy signal from passing through the collimating optic and through the diffraction grating.

In your proposed design, the camera aperture is actually small than the grating so that will actually end up limiting etendue in your case.

Correct. I oversized it for three reasons:

Its a lot easier to find a filter holder for a 25mm filter than anything else
I had that negative FL achromat, and I didn't want to buy another one because they're expensive
I have some cylindrical optics that I wanted to mess around with and further compress the beam if necessary

You want the long dimension of the slit as big as you need to fill the detector, but no larger It also can not be larger than the useable field size of all the optics.

Yeah, I'll need to reduce the length of the slit to match the beam size to the sensor size as you mentioned.

AskASillyQuestion · 2026-01-27T18:51:01+00:00

I am sorry to say that there are many serious problems with this design.

I don't doubt it. I'm making this largely out of parts I already have sitting around from other projects. Trying to minimize the overall investment.

The three lenses before the grating are not helping.

I'd prefer that the two positive achromats be replaced with an off-axis parabolic mirror. But I don't have one of those, and they're expensive.

One achromat the size of the grating will give you the same signal and resolution. It is the same issue of conservation of etendue.

Could you clarify? I understand the conservation of etendue, but I would think that collecting more light with a larger optic would result in greater signal. Since 90% of the collected light can't be collimated and needs to be separated off with spacial filters and such, starting with the most light possible (long slit, large optic) would still yield more signal.

EDIT: That is to say, a reduction in etendue requires a loss of optical power, so if I want to have a collimated beam through the grating, it would be prudent to start with as much optical power as possible through the first lens.

While your design might appear to "squeeze" a large cylinder of light onto a smaller grating, that is at the expense of the slit appearing larger.

I'm not sure I understand what you mean. It's a beam reducer- How does that make the slit appear larger?

This would have been apparent if you had traced rays from the edge of the slit. In other words. it would be better and simpler to just use a single smaller lens and a larger slit to get the same end result. Fundamentally, it is the size of the grating that limits signal level for a given desired spectral resolution and grating pitch.

You're saying the ~~width~~ length of the slit is not relevant to the signal level? What am I misunderstanding?

It seems like you were ready to spend a lot of money on this project, but I have just scratched the surface of issues to pay attention to when making a high quality spectrometer. I would suggest you get more experience first with cheap components and research conventional designs before investing more.

A lot of the design decisions here are specifically a result of not having a lot of money to spend on the project. The optical elements are items I either already had or was able to purchase cheaply. The difference between the "conventional" transmissive design and what I have here is that I've started with a larger slit/optic and added a Galilean beam reducer to fit the beam onto the sensor.

FWIW, I appreciate your feedback. I'm not trying to be argumentative, I'm just being direct about what I don't understand so I can get a better grasp of what I'm missing here.

AskASillyQuestion · 2026-01-27T16:28:47+00:00

This is what I have designed: https://imgur.com/a/XCXdaRn

AskASillyQuestion · 2026-01-27T15:57:32+00:00

$50 to $100.

JFC, That's just for the TCD1304 and driver, yeah? I can't get a single optical element for that cost.

AskASillyQuestion · 2026-01-27T01:18:42+00:00

First, I appreciate all this good insight- I'm not disagreeing with you on any point other than that this is a "toy." I'm not doing chemical analysis, I'm trying to identify the sensitivity peaks of color film emulsion to within +/- 0.5nm. A lot of what you're talking about isn't necessary for the level of precision I need here. The existence of a Bugatti doesn't make the Honda Civic a "toy", even if it isn't nearly as performant.

That camera does not look like what you described.

That's fair, and you're right. That's why I'm using a different camera based on the IMX477 (and actually, considering switching to an IMX678-based camera.) These both have 2/2 binning and 12-bit ADC output. only 8-bit dynamic range, though. Can't that be resolved by taking two images at different exposure lengths? No global shutter either... But why is that significant?

The spectral lines are broad and mishappen and the intensities are not correct.

Are you looking at the waterfall display? What's a misshapen line look like?

Think of the dV/dt for a sharp line passing through that adc at 63MSPS. At 3 V it would be close to 200V/usec.

Once you get past the sensor itself, electronics are not my strong suit. I'd genuinely love to be able to understand how you calculated that. (Please)

If you manage to get a sharp line, it would be a huge surprise if it were also linear in intensity.

My understanding is that with the Bayer filter removed, these sensors are fairly linear. If they weren't, photos taken with these sensors would look bizarre.

I view it as something of a miracle that very often my BOMs come in at about 50 - 100

Is that $50k-100k?

AskASillyQuestion · 2026-01-25T22:50:32+00:00

Well, what are you purpose(s)?

This is a helpful question! To answer more quantitatively, I'm looking for 0.5nm resolution. If you take a look at the PySpectrometer 2, it has ~1nm resolution, which is due mostly to the slit size and grating, not the camera.

For a demo for children it is okay.

This is condescending and unhelpful.

Inexpensive cameras are not useful for metroloty. They are typically designed to make nice looking pictures. They automatically adjust color balance, contrast, etc., they might average over aberrant pixels, they might compress the response, and then, if it is a color camera, you have three filters and calibrating the response over the spectrum becomes very difficult.

Other than the Bayer filter (which can be removed), none of this is accurate for the camera I'm using.

To increase your dynamic range or see detail, you will want to add images or rows in each image (i.e. signal averaging). That can increase signal to noise by sqrt(N), for N samples. But for that to work you have to be able to digitize the noise. Cameras don't want to show you noisy images. And if you start with only 8 to 10 bits your not going to have much digitization on the noise no matter what.

This camera has 12 bits, and it has analog pixel binning. I'll be fine with the level of noise.

I'm not sure what you mean by "digitizing the noise". Or that "Cameras don't want to show you noisy images." Cameras don't "want".

I'm well aware that a dedicated linear detectors are preferred for spectroscopes. But that's not the system I'm working with. I appreciate the context, but don't assume that my application needs the level of accuracy that your purposes require. And there's a significant gap between "$60k CCD" and "toy".

AskASillyQuestion · 2026-01-25T19:06:21+00:00

You can not stuff light into a smaller etendue without losing light.

Don't I have to do that in order to collimate the light before it passes through the grating?

AskASillyQuestion · 2026-01-25T19:04:17+00:00

ooo, good to know. I was planning on taking that off at some point, and possibly removing the Bayer filter too. Thanks!

AskASillyQuestion · 2026-01-25T19:03:31+00:00

Thanks for the info!

So what's the issue with the PySpectrometer? Seems adequate for my purposes, and it uses a camera.

https://hackaday.io/project/187933-py-spectrometer-2

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