Two quotations for the same IR filters part in aerospace and defense optics can differ by a factor of three, and the reason is rarely the material. This article breaks down the real cost drivers - tolerance, finishing operations, inspection level and batch size - and shows which specification changes save money and which only look as though they should.
IR Filters in Aerospace and Defense Optics
In aerospace and defense optics, that translates into a specific set of requirements rather than a general preference for quality. Aerospace optics are qualified against thermal vacuum, vibration, shock and radiation environments, and once qualified they are difficult to change, which makes documentation and reproducibility the dominant requirement. Weight is critical, coatings must survive without moisture uptake, and every part carries a certification trail.
What drives cost and lead time
Layer count is the main cost lever. A simple IR-cut stack is a routine chamber run; adding a broadband AR on the second surface, a hydrophobic top layer, or tightening the transition from 30 nm to 10 nm each adds chamber time, increases sensitivity to layer thickness error and reduces yield, so prices rise faster than the layer count suggests. Substrate grade and thickness matter less than expected, except for sapphire, which is materially more expensive in every respect. Size is the second lever: parts that fit comfortably inside the chamber's uniform zone cost far less per unit than parts that must be positioned at the edge, where thickness uniformity degrades. In Aerospace and Defense Optics, the cheapest real saving usually comes from relaxing the angle specification rather than from changing the material.
Substrate and material selection
The substrate decides transmission, thickness and durability. B270 and D263T borosilicate are the standard carriers for camera and sensor windows: clear, dimensionally stable, and thin enough to laminate or to fit behind a lens. Absorptive blue glass is used where the NIR must be removed without an angle-dependent coating, and it can also be combined with a dichroic layer to sharpen the edge. Sapphire enters when the window must resist abrasion - handheld devices and outdoor instruments - and its hardness also brings a wider transmission band, out to about 4.5 micrometers. In Aerospace and Defense Optics, the substrate is usually specified together with the cut-on wavelength, because the two interact: a dichroic edge deposited on a substrate with some intrinsic NIR absorption will read further blue than the same stack on clear glass.
Handling, cleaning and packaging
The dichroic surface is both the optical function and the most vulnerable part of the component. Handle by the edge, never wipe dry, and clean only with the approved process: neutral detergent, cascaded deionised water rinses, then an alcohol or nitrogen dry. Fingerprints etch a hard oxide layer over time and create a permanent localised transmission dip that cannot be polished out. Storage uses particle-free interleaves and never stacks two coated faces together. Because the coating is a stack of thin layers, it is also sensitive to mechanical impact at the edge, so a chamfer is standard. In Aerospace and Defense Optics, the assembly process deserves equal attention: adhesives that outgas, or a potting compound that cures with shrinkage, can both shift the spectrum of a well-made part.
The tolerances that actually matter
An IR filter is specified by its curve, so the tolerances that matter are spectral. A typical commercial part holds the cut-on wavelength to plus or minus five nanometers and the visible transmission to within a few percent of nominal; precision imaging parts tighten the wavelength to two nanometers and also constrain the slope of the transition. All of these numbers are only meaningful at a stated angle of incidence, and the acceptance angle belongs on the drawing next to the spectral tolerance. Mechanical tolerances follow the mounting: thickness to plus or minus 0.02 mm for a drop-in window, outline to plus or minus 0.05 mm, and a chamfer on every edge. In Aerospace and Defense Optics, the specification most often forgotten is the permitted variation of color across the aperture, which is what a user actually notices when a wide lens produces a green frame edge.
Coating and deposition considerations
Dichroic IR-cut designs run from roughly twenty to sixty layers of alternating high- and low-index oxides, with the layer thicknesses controlling both the position and the steepness of the edge. Ion-assisted deposition is the standard process because it produces dense layers that do not shift with humidity - an unassisted coating can move several nanometers after a week in a humid room. The residual reflection in the visible band has to be managed, since a strong reflection produces the green or magenta glint familiar on poorly coated camera windows; broadband AR layers are usually added on top of the dichroic stack to suppress it. For Aerospace and Defense Optics, also decide whether a hard, hydrophobic outer layer is required, because it changes both the durability and the price of the part.
Requirements specific to Aerospace and Defense Optics
In aerospace and defense optics, that translates into a specific set of requirements rather than a general preference for quality. Aerospace optics are qualified against thermal vacuum, vibration, shock and radiation environments, and once qualified they are difficult to change, which makes documentation and reproducibility the dominant requirement. Weight is critical, coatings must survive without moisture uptake, and every part carries a certification trail.
- Thermal vacuum and vibration survival
- Low weight with high specific stiffness
- Coating stability with no moisture shift
- Full first-article and process documentation
AS9100 quality systems, MIL specifications for coatings and surface quality, and customer-specific environmental qualification protocols.
Selection data at a glance
| Parameter | Typical value | Notes |
|---|---|---|
| IR-cut cut-on | 630-700 nm | Tuned to the sensor response |
| Visible transmission | T ≥ 90 %, R ≤ 1 % | Average 420-620 nm |
| NIR blocking | OD2 - OD4 (1 % - 0.01 %) | 700-1100 nm |
| Edge steepness | 10-30 nm transition | Dichroic designs |
| Angle of incidence | 0-30 deg specified | Passband shifts blue with angle |
| Wavelength tolerance | ± 5 nm standard, ± 2 nm precision | Measured at stated AOI |
| Substrate | B270, D263T, blue glass, sapphire | Absorptive option removes NIR without angle shift |
| Thickness | 0.3-3 mm | 0.55 - 1.1 mm typical for camera windows |
| Environmental | Humidity, salt spray, abrasion tested | MIL-C-48497 / MIL-PRF-13830B |
Frequently asked questions
What cut-on wavelength should I specify?
Most silicon sensors need the cut-on between 650 and 680 nm, before the sensor's NIR response climbs. If the application also has an 850 nm illuminator nearby, tighten the blocking to OD3 and confirm the edge position against the source wavelength.
Why does my image look green at the corners?
That is angular shift. The dichroic edge moves toward shorter wavelengths as the angle of incidence increases, so off-axis rays lose blue light and the frame edges shift color. Solutions are a wider transition, an absorptive element, or moving the filter closer to the stop.
Is absorptive blue glass better than a dichroic coating?
It depends on the acceptance angle and the budget. Blue glass is angle-independent, robust and cheap, but thicker and less controllable in cut-on. A dichroic coating gives a sharper, tuneable edge in a thinner part, at the cost of angle sensitivity.
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