Schott RG9 vs Hoya RM90: Choosing a Longpass Filter for IR Work

Quick answer

For most visible-to-NIR isolation work where you need a sharp, reliable cutoff in a compact 12.5 mm mount, the Schott RG9 is the straightforward pick. Choose the Hoya RM90 when you need a longer-wavelength IR transmitting filter and want the option of a larger 25 mm diameter for bigger beams or easier mounting. Both are 3 mm thick, so the real decision comes down to cutoff position, available diameter, and how much IR you need to pass.

What the RG9 and RM90 actually do

Both are longpass filters: they block shorter wavelengths and transmit longer ones. The Schott RG9 is a classic longpass glass that passes the near-infrared while suppressing visible light, making it useful for separating NIR signals from ambient visible background. The Hoya RM90 is an infrared transmitting filter with a longer-wavelength pass band, aimed at applications that need deeper IR transmission rather than just NIR isolation.

Because the two filters sit at different points on the spectrum, they are not direct substitutes. The RG9 is often used where you want to capture the near-IR edge; the RM90 is chosen when the visible and very-near-IR region must be strongly attenuated and the signal of interest lies further into the infrared.

Cutoff and transmission: the decision that matters most

The RG9's cutoff is positioned so that visible light is largely blocked while near-IR passes. The RM90's pass band begins further into the infrared, so it rejects more of the near-IR as well as the visible. If your detector or camera is sensitive across a broad range, the RM90 gives you a cleaner IR-only window; if you need to keep some near-IR response, the RG9 is the more permissive choice.

Neither filter is a narrow bandpass. They are longpass types, so they transmit everything above their cutoff, including far-IR if your detector reaches that far. That is an advantage when you want maximum IR throughput and a disadvantage when you need to exclude longer wavelengths. Match the filter's pass band to your source, detector, and any other filters in the stack.

Size and mounting: 12.5 mm vs 25 mm

The RG9 is listed at 12.5 mm diameter and 3 mm thickness. The Hoya RM90 is available in both 12.5 mm and 25 mm diameters, also 3 mm thick. That extra 25 mm option matters if you are working with a larger beam, a camera lens with a wider aperture, or a filter holder that needs a bigger clear aperture. A 12.5 mm filter can vignette or clip a wide beam, so check your beam diameter before ordering.

Both filters are stock optical items with worldwide shipping available. If you are building a compact setup around a small sensor or a fiber collimator, the 12.5 mm format keeps the footprint small. If you are mounting in front of a larger lens or need to cover a 20 mm beam, the 25 mm RM90 avoids the need for a step-down adapter.

How to choose without overspending

Start with the spectral requirement, then the size. If you need near-IR isolation and a 12.5 mm aperture is enough, the RG9 is the lower-cost route. If you need deeper IR rejection and a larger aperture, the 25 mm RM90 is the practical choice even though it costs more. Buying a larger filter than you need wastes money; buying a smaller one than your beam requires wastes the filter.

Also consider the rest of your optical stack. Adding a longpass filter in front of a silicon detector that already has an IR-cut filter may give you less benefit than expected. Conversely, using an RG9 where the RM90's longer cutoff is required will leave visible leakage that can swamp a weak IR signal. If you are unsure, order the 12.5 mm version of each to test the spectral match before committing to a larger size.

Trade-offs at a glance

• Cutoff position: RG9 passes near-IR and blocks visible; RM90 pushes the pass band further into the IR and rejects more near-IR. Choose RG9 for near-IR work, RM90 for deeper IR isolation.
• Available diameters: RG9 is listed at 12.5 mm only; RM90 comes in 12.5 mm and 25 mm. If your beam is wider than about 10 mm, the 25 mm RM90 is the safer fit.
• Cost and size: The 12.5 mm RG9 is the lower-cost entry point; the 25 mm RM90 costs more because of the larger substrate. Buy the size you need, not the size you might need later.
• Stack compatibility: Both are 3 mm thick longpass filters, so they can often share the same holder if the diameter matches. Order the same diameter for both if you plan to swap them in one mount.

Product facts

• Schott RG9: Schott RG9, 3 mm thick, 12.5 mm diameter, part numbers FRG92512 / FRG-92512 / FRG 92512. A compact 12.5 mm longpass filter that fits small mounts and fiber collimators; the 3 mm thickness is rigid enough for most lab fixtures.
• Hoya RM90: Hoya RM90, 3 mm thick, 12.5 mm diameter, part numbers HRM9012 / HRM-9012 / HRM 9012. The same small footprint as the RG9 but with a longer-wavelength IR pass band, so you can compare cutoff behavior without changing your mount.
• Hoya RM90: Hoya RM90, 3 mm thick, 25 mm diameter, part numbers HRM9025 / HRM-9025 / HRM 9025. The larger 25 mm option covers wider beams and bigger lenses, avoiding vignetting when your setup outgrows a 12.5 mm aperture.
• Hoya RM100: Hoya RM100, 3 mm thick, 12.5 mm diameter, part numbers HRM1012 / HRM-1012 / HRM 1012. A further-IR alternative in the same 12.5 mm format if the RM90 still passes more near-IR than your application can tolerate.

Which filter fits your work?

Best for

• Near-IR imaging or sensing where visible light must be blocked but near-IR signal is wanted.
• Compact setups with a 12.5 mm filter mount or small beam diameter.
• Users who want a lower-cost longpass filter to test an IR concept before scaling up.

Not for

• Applications that need strong rejection of near-IR as well as visible light — choose the RM90 or RM100 instead.
• Wide beams or large lenses that require a 25 mm clear aperture — the 12.5 mm RG9 will vignette.
• Situations where a narrow bandpass or a different spectral window is required; these are longpass filters, not bandpass filters.