Hey folks, if you’ve ever dabbled in spectroscopy—whether you’re calibrating a benchtop elemental analyzer, tweaking a hyperspectral imaging setup for agricultural scanning, or troubleshooting a high-resolution fluorescence spectrometer—you’ve probably run into flat-field concave holographic gratings (FFCHGs). They’re the unsung workhorses of most modern spectroscopic systems, right up there with light sources and detectors. But here’s a question I get at least a dozen times a week from new customers and seasoned spectroscopists alike: What’s the actual spectral range of a typical FFCHG? And why does that even matter for what I’m building? Flat-Field Concave Holographic Grating
I started selling these gratings back in 2012, straight out of a lab where my partner and I were prototyping custom ones for our own academic research. Back then, we thought a “typical” FFCHG was like… a one-size-fits-all for UV-Vis-NIR, turns out that was totally wrong. Over the last 11 years, I’ve worked with everyone from grad students building portable water quality testers to aerospace engineers calibrating satellite sensors, and I can tell you this: spectral range isn’t just a number on a spec sheet—it’s the difference between a system that works for its intended use and one that’s full of gaps you have to patch with extra filters or mirrors.
Let’s break this down, no fancy jargon, I promise. First, a quick refresher for anyone who might be new here: a concave holographic grating is a diffraction grating carved directly into a curved mirror, so it does two jobs at once—dispersing light by wavelength (that’s what gratings do) and focusing it (that’s what the concave part does). The “flat-field” part means the focal plane is flat, not curved, so your detector (which is almost always a flat linear array, like a CMOS or CCD) can sit right there without weird optical aberrations that blur the edges of your spectrum. That’s why FFCHGs are so popular now—they cut down on the number of parts in your system, which means less alignment, less cost, and more reliable performance.
Now, back to that spectral range question. When a customer first calls, they’ll say something like “I need a grating for my system, what range do you have?” And I always have to follow up with “What’s your application? What’s the shortest wavelength you care about, and the longest?” Because “typical” is super dependent on what you’re actually using it for. But if we’re talking about a standard off-the-shelf FFCHG—not the custom ones we build for clients with super specific needs—there’s a pretty solid baseline that most folks fall back on.
Let’s start with the core: most off-the-shelf FFCHGs we stock cover the deep UV (DUV) through the near-IR (NIR), roughly 200 nm to 1100 nm. Wait, that’s a big range, I know, but let’s split that up to make it make sense. The deep UV end at 200 nm is where things get interesting— that’s the range for things like detecting ozone in water, or measuring nucleic acids in biological samples. We don’t usually go lower than 200 nm off-the-shelf, because below that, you start running into absorption from air and the coating materials on the grating. You can get custom ones that go down to 120 nm, but those are more for vacuum UV setups, not the standard lab or field systems most people are building.
On the upper end, 1100 nm is the near-IR, where you’re looking at things like glucose monitoring in medical devices, or chlorophyll absorption in plants for hyperspectral imaging. We used to cap that at 900 nm a few years back, but better mirror coatings (aluminum with a protective overcoat, or gold for more aggressive NIR work) pushed that up. The off-the-shelf sweet spot for most general-purpose FFCHGs is 200 to 1100 nm. But wait— is that the only “typical” range? No, of course not. A lot of our customers don’t need that full range. For example, people building portable UV-Vis spectrometers for testing pool water only care about 200 to 800 nm, so we make those too, and they’re way more affordable than the full-range ones. Then there are NIR-focused systems, like those for paint analysis in forensics, which run from 700 nm to 1700 nm. That’s another common “typical” range, just a narrower slice for a specific use case.
Here’s the thing that trips people up: spectral range isn’t just about how far apart the shortest and longest wavelength are. It’s also about how the grating performs in that range. Like, a 200-1100 nm grating isn’t equally efficient across the whole thing. Our standard 200-1100 nm FFCHGs have around 80% diffraction efficiency from 250 nm (deep UV) all the way up to 900 nm (near-IR), and it drops a little after that—down to about 65% at 1100 nm. If you’re building a system that needs consistent sensitivity across the full range, that matters way more than just having coverage. We’ve had customers buy a cheap off-the-shelf grating that claimed 200-1100 nm, only to find out it’s only 50% efficient above 950 nm, so their detector can’t pick up the signal. That’s why buying from a supplier that actually tests their gratings (we test every single one before shipping, just fyi) is non-negotiable.
Now, why is the “flat-field” design so important for spectral range here? Let’s say you have a regular concave grating, not flat-field. The focal plane is curved, so the spectrum you get on your detector will be stretched and blurry at the edges. If you have a 200-1100 nm range, the long wavelengths (near-IR) will be focused further away from the short wavelengths (UV) on a curved plane, so your detector array—which is flat—can only capture a small part of the range at once. Flat-field fixes that: all wavelengths in your chosen range focus evenly on a single flat plane, so you can get the full spectrum across your entire detector in one shot. That’s a huge win for speed, especially for things like hyperspectral imaging where you need to capture hundreds of wavelengths in milliseconds.
Wait, let’s get into some edge cases because I know you’re thinking “what if I need mid-IR?”. Mid-IR (roughly 2500 nm to 25,000 nm) is a whole other ballgame for gratings, right? Most of our mid-IR gratings are not holographic—they’re ruled, because holographic gratings have trouble with the longer wavelengths there. There are some custom holographic ones for mid-IR, but they’re not “typical” by any stretch. The typical FFCHG, for 90% of the work we do, is still that 200-1100 nm range for UV-Vis-NIR.
Another common question: how do blazed gratings factor into this? Blazing is when you shape the grooves of the grating so that most of the light goes to a specific order, which boosts efficiency. A typical FFCHG is blazed for the first order, which is where most spectroscopy happens. If you blaze for a specific wavelength, that peak efficiency is tight, but a general-purpose one is blazed for a middle wavelength—say, 500 nm—and the efficiency drops off on either side, which is why you see that 80% from 250 to 900 nm I mentioned earlier. That’s exactly what you want for a typical system that doesn’t have a single target wavelength.
Let me share a real customer example to make this concrete. Last year, we worked with a small startup building a portable spectrometer for detecting microplastics in river water. They came to us saying they needed to cover 220 nm to 850 nm—they didn’t care about anything above that, because their target was the absorbance of polymer particles in the UV-Vis range. We sold them our standard 200-900 nm FFCHG, and they swapped out their old detector for one that cut off at 900 nm, which saved them a bunch of money. They told us later that it worked way better than the ruled grating they’d used before, because the flat field let their linear array capture every microplastic signal across the whole range, no blurriness at the edges. That’s exactly the sweet spot for a typical FFCHG.
Another example: a customer in the semiconductor industry, testing photoresist layers on wafers. They needed high resolution in the deep UV, so they went with a custom FFCHG that covered 190 nm to 400 nm, with very high efficiency below 250 nm. That’s a niche range, not typical, but it goes to show that “typical” is just the baseline we keep in stock for most use cases.
Wait, I should also clear up a common misconception people have about spectral range and groove density. Groove density (that’s how many lines are etched per millimeter) goes hand in hand with range. A lower groove density (like 300 lines/mm) covers a wider spectral range, but lower resolution. A higher groove density (1200 lines/mm) has higher resolution, but a narrower range. Our typical 200-1100 nm FFCHGs come in 300, 600, and 1200 lines/mm, so customers can pick what fits their need. That’s why spec sheets will often list both range and groove density—they’re not separate things. If you see a grating that’s 1200 lines/mm, don’t be surprised if its range is only 300 nm to 900 nm, because higher resolution = narrower range.
Now, let’s talk about what affects the spectral range of an FFCHG, just so you know it’s not random. The two big factors are the grating design (the curvature, the groove spacing, the blaze angle) and the coating on the grating surface. Coatings are huge—aluminum is the standard for UV-Vis, works great from 200 nm up to 1100 nm, but it’s not great above that. Gold coatings are better for near-IR, above 900 nm, so if you need that extra range, we swap out the coating. There’s also silver, which has higher efficiency in the visible range, but it’s more prone to scratching, so we only use it for specific applications.
Another thing: the size of the grating. We make FFCHGs in sizes from 10 mm to 50 mm in diameter. The larger the grating, the wider the spectral range it can typically cover, because there’s more area for the light to disperse. A small 10 mm grating might only cover 200 to 800 nm, while a 50 mm one hits that 200 to 1100 nm mark. That makes sense when you think about it—more surface area means more grooves to work with across longer and shorter wavelengths.
I’ve seen a lot of new suppliers online marketing “universal” FFCHGs that cover 120 nm to 2500 nm, and let me tell you, those are a scam. The physics just doesn’t work that way. Holographic gratings have limits—air absorption below 200 nm, coating absorption above 1100 nm, aberrations that get worse the wider you go. A real typical FFCHG, built with actual tested materials and design, is going to be the 200-1100 nm UV-Vis-NIR range. If you need something narrower, we have those, if you need something wider, it’s custom, but the generic “typical” one is that core range.
Let me wrap this up so it’s not just a bunch of specs. The short answer to “what’s the spectral range of a typical FFCHG?” is 200 nm to 1100 nm, covering deep UV through near-IR, with enough efficiency for nearly all standard spectroscopy applications. But it’s also important to note that this range can shift depending on your needs—narrower for high resolution, or extended for near-IR work— that’s why we don’t just ship one grating and call it a day.
If you’re working on a project right now and trying to figure out what grating you need, whether it’s a portable spectrometer for water testing, a lab setup for biological research, a hyperspectral imager for agriculture, or something else entirely, don’t guess at the spectral range. We’ve been doing this for over a decade, and we can help you pick the right FFCHG for your exact use case, no jargon, no hidden fees, no overpriced “universal” junk. Just send us a note about your application, your desired range, and your budget, and we’ll walk you through the options.
Broadband Infrared Grating References
- Hutley, M. C. (1982). Diffraction Gratings. Academic Press.
- Loewen, E. G., & Popov, E. (1997). Diffraction Gratings and Applications. CRC Press.
- Palmer, C., & Loewen, E. (2005). Diffraction Grating Handbook (6th ed.). Newport Corporation.
- Wang, B., et al. (2018). Flat-field concave holographic gratings for portable spectroscopy applications. Optical Engineering, 57(10), 105101.
Jilin Juyao Technology Co., Ltd.
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