Choosing a Longpass Filter: Schott RG9, RG Series and Beyond
By UQG Optics | Published: 2026-10-15
Category: Buying Guides
A practical longpass filter buying guide: use the RG9 cut-on wavelength as your baseline, then match glass type, thickness and diameter to your setup.
Quick answer
Start with the cut-on wavelength you actually need, not the brand. If your work sits in the near-IR and you want a proven, stocked baseline, the Schott RG9 filter is the sensible default. Move to another RG series longpass only when your source, detector or blocking requirement clearly falls outside what RG9 handles.
Match the filter to the light you want to keep
A longpass filter transmits longer wavelengths and blocks shorter ones. The single number that decides whether it works in your setup is the cut-on wavelength, usually quoted at 50% transmission. Everything else — diameter, thickness, glass type — is secondary until that number is right.
Schott RG9 is a classic near-IR longpass glass. It is widely used to strip visible light while passing deeper near-IR, which is why it appears in imaging, sensing and laser-adjacent setups. If you are unsure where to begin, RG9 is a reasonable first candidate because it is well understood and commonly stocked.
Thickness and diameter are not cosmetic choices
Thickness affects how sharply the filter transitions from blocking to transmitting, and it also shifts where the cut-on sits slightly. A 2 mm filter and a 3 mm filter of the same glass are not drop-in equivalents. If your design was modelled around a specific thickness, order that thickness.
Diameter decides whether the filter fits your mount, cage or holder without adapters. Buying a larger disc and masking it down is possible but wastes aperture and can introduce stray reflections. Measure the clear aperture you need first, then choose the closest stocked size.
Check stock and part numbers before you commit
Optical filters are often specified by part number in drawings and purchase orders. UQG Optics lists Schott longpass filters with part numbers such as FRG92512 and FRG92525, where the digits encode the glass and the diameter. Quoting the full part number avoids the classic mistake of receiving the right glass in the wrong size.
Stock items ship faster and cost less than custom cuts. If a stocked diameter covers your aperture, take it. Reserve custom work for cases where thickness, shape or coating genuinely cannot be met by a catalogue part.
Decision rules for picking an RG series longpass
- If RG9 already covers your band: Stay with RG9 rather than chasing a near-equivalent glass. It is the most common near-IR longpass baseline and is stocked in more than one diameter. Default choice unless you have a specific reason to move.
- If your mount is 12.5 mm: Choose the 12.5 mm option rather than a 25 mm disc. Masking a larger filter reduces clear aperture and adds an unnecessary edge. Match diameter to the holder first.
- If your design specifies 3 mm: Do not substitute a 2 mm part of the same glass. The cut-on position and transition steepness shift with thickness. Thickness is a specification, not a preference.
- If you need a cut-on away from RG9: Move along the RG series or consider a different glass family such as N-WG320, which behaves differently and suits different sources. Change glass only when the wavelength demand changes.
Stocked Schott longpass options compared
- Schott RG9: Schott RG9, 3 mm thick, Ø 12.5 mm, part number FRG92512. A compact RG9 disc for small mounts and tight optical paths where a 25 mm filter would not fit.
- Schott RG9: Schott RG9, 3 mm thick, Ø 25 mm, part number FRG92525. The same RG9 glass and thickness in a 25 mm format, so you can match an existing 25 mm holder without adapters.
- Schott RG850: Schott RG850, 3 mm thick, Ø 12.5 mm, part number FRG85012. A different RG glass in the same small format, useful when your cut-on requirement sits away from RG9.
Who this approach suits
Best for
- Buyers who need a stocked near-IR longpass and want a proven default
- Setups where the mount diameter is already fixed at 12.5 mm or 25 mm
- Projects that specify Schott RG glass by part number
Not for
- Applications needing a cut-on well outside the RG9 range, where a different glass family is the better starting point
- Designs that require a custom thickness or shape not offered as a stock item
