When the Shape Is Part of the Problem: Odd-Shaped Optical Filters and Coatings

For years, requests like these came to Chroma and we passed on them. The limitation was never technical. We understood what it would take to cut a substrate to an unusual geometry and coat it precisely, but the demand did not justify the investment, so we referred the few customers who needed it to partner companies. That changed as our reputation for high-performing filters carried us into new markets, including semiconductor, satellite communications, and hyperspectral imaging, and into longer wavelengths in the SWIR and MWIR. Serving that customer base properly meant owning the capability ourselves, so we built it before a single production order justified the cost.

Why Shape Matters in Optical Instruments

Most optical filters are specified with shape as an afterthought. The coating is the hard part; the glass is just a substrate. But in practice, instrument design often dictates a specific form factor that a standard round or rectangular filter won’t satisfy.

Some instruments use rectangular optics, in which a standard round filter wastes space or causes vignetting. Others use detectors that are themselves non-rectangular, requiring filters to follow the same outline. In hyperspectral imaging systems, a filter may need to cover only part of a detector and can be split into two functional zones, each with different spectral properties. Various instruments, including machine vision systems and miniaturized spectrometers, all present their own geometric demands.

Common odd shapes that appear in practice include:

• Cut corners — rectangular substrates with one or more corners removed, allowing the filter to sit in a housing with a locating flat or to avoid mechanical obstruction

• Round with a chord — a circular filter with one straight edge (flat), used when rotational orientation needs to be fixed or when space is limited on one side

• Ovals — elongated shapes that match non-square detector areas or slit geometries

• Asymmetric polygons — irregular multi-sided shapes that conform to specific instrument cavities

• Machined features — notches, cutouts, or through-holes that allow the filter to clear a post or interact with another component

The Limits of Standard Tooling

Our engineers noticed how often requests for odd-shaped parts came in and we turned them away. The reasons were consistent: we could not meet the dimensional tolerances required to coat the parts reliably, and we had no way to index to a non-standard reference feature during coating. Standard pocket jigs create a shadow zone, which is an uncoated or poorly coated region near the edge of the part, which can span 500 to 2,000 microns, depending on jig condition and how well the substrate sits in its pocket. For most applications, that is acceptable. For split-field patterning or tightly toleranced coatings on oddly shaped substrates, it isn’t.

Early R&D explored masking materials to define coating zones on parts with non-standard geometry. Results were usable on some of the transition zone widths, but low yields and inconsistent widths kept the method short of what most quote requests required.

The types of shapes requested tell the story of where the demand comes from: small parts with cut corners for miniaturized imaging systems; large beamsplitters with cut corners and center features for ophthalmology instruments; ovals for endoscopic or wearable optics; and irregular shapes for industrial machine vision housings. Across those requests, the underlying need was the same: a coated optical filter that matched a specific physical form.

Building the Capability

The path to reliably manufacturing odd-shaped parts involved two separate advances that turned out to be mutually dependent.

The first was precision substrate cutting. Chroma uses CNC end mills to cut substrates into custom geometries. Unlike a coring-and-dicing two-step process, CNC machining can execute complex outlines in a single operation and hold tolerances tight enough to enable consistent, repeatable jig placement. For a part that needs to sit in a precision pocket with a flat that must align within 25 microns of nominal, the cutting process has to be at least as precise as the coating jig. Bringing that operation in house made the tolerance achievable.

A Worked Example: Split-Field Order-Blocking Filters for Hyperspectral Imaging

The value of this capability became clear on a project involving a hyperspectral camera manufacturer. Their instruments use a line-scan imaging architecture, in which lenses, a slit, and dispersive elements (gratings and prisms) separate incoming light by wavelength across a detector array. A critical requirement in such systems is blocking higher diffraction orders, light at wavelengths that would alias into the measurement band, using order-blocking filters, or OBFs.

The OBF for this application needed to do something unusual: split the field of view into two. One half of the detector required a longpass coating, transmitting wavelengths from 1700 to 2400 nm while blocking shorter wavelengths. The other half required only an antireflection coating, passing wavelengths from 1000 to 1300 nm. The filter substrate was round with a chord cut — a flat edge — and the transition line between the two coating zones had to fall within 100 microns of its nominal position, with the flat-to-center dimension held to ±25 microns.

The transition zone width itself had to be under 200 microns. To put that in context: a standard pocket jig in good condition might produce a shadow zone of 500 microns. The material masking process Chroma previously used produced shadow zones of approximately 250 microns.

The process Chroma developed for this project ran in sequence: coat a plate with an antireflection coating on both sides, cut and chamfer the parts on the CNC Machine tools, measure 100% of parts for dimensional conformance, seat each part into the precision jig insert, apply the longpass coating to the unmasked region, and measure transition line placement and zone width. The result was a transition zone well within 200 microns.