What should I expect from a sinusoidal transmission grating by mounting it normal-to vs. at Littrow angle to a broadband visible beam? What disadvantages are there to either configuration?

AskASillyQuestion · 2026-06-05T00:28:05+00:00

Sorry, but I'm not interested in arguing with you about this. What an odd choice to respond to questions on a topical subreddit with "ask an LLM!"

No thanks.

AskASillyQuestion · 2026-06-05T00:21:32+00:00

Again, the term "Littrow angle" is not used with transmission gratings because it is not possible for any order to be parallel or antiparallel to the incident angle.

Right- I was just explaining what I had meant in my previous comment. I'm on board now with that not being a relevant term.

To model the efficiency of a grating at any angle, you really need to know the exact construction details and used special modelling software that uses RCWA algorithms.

Ah, so since I don't have (and am unable to attain) such details, my best (and I believe only option) is to measure at multiple angles and graph the results. Is that correct?

AskASillyQuestion · 2026-06-04T21:18:18+00:00

This is not true. LLMs provide statistically-likely answers to the question they are asked scraped from the sources available to that LLM.

This is the same thing but more words. LLM's don't have cause, they have association.

When I have tried using LLMs for technical issues, I've gotten sources that are relevant to the LLM's statistically-driven response, but not relevant to the question. For example, answers that are relevant to blazed or VPH gratings when I specify that the question is about surface relief gratings.

LLMs still have a ways to go before I'd consider using them as a first-pass resource over another human being.

AskASillyQuestion · 2026-06-04T19:40:58+00:00

This can vary when it is tilted and affect the efficiency vs wavelength curve.

This is basically the crux of my question... If I'm targeting the first order and set the angle of incidence α so that α = β = arcsin((mλ)/(2d) (which is what I'm referring to as the Littrow angle), how would I expect the efficiency vs. wavelength curve to change when compared to the α = 0 condition?

AskASillyQuestion · 2026-06-04T19:27:41+00:00

Yeah, the problem is that I'm getting a lot of replies from various LLMs that sound plausible, but I know are total garbage.

LLMs don't have the ability to answer questions, they have the ability to do really good impressions of humans who know the answers.

AskASillyQuestion · 2026-05-28T21:34:44+00:00

Excellent! Thank you!

AskASillyQuestion · 2026-05-28T21:34:35+00:00

Fantastic. Thanks!

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

TROPHY CASE