Case Study: Split-Field Order Blocking Filters for Hyperspectral Imaging

Challenge

The customer’s line-scan hyperspectral camera separates incoming light by wavelength across a detector array and needed an order-blocking filter to prevent higher diffraction orders from aliasing into the measurement band. This filter needed to do something that a standard optical filter couldn’t. It needed to split the field of view into two zones with different spectral properties on a single substrate.

One half required a longpass coating transmitting 1700 to 2400nm while blocking shorter wavelengths. The other half required an antireflection coating passing 1000 to 1300nm. The substrate was a non-standard round-with-chord shape, and the transition line between zones had to land within 100 microns of nominal position including a flat-to-center dimension held to plus or minus 25 microns and the transition zone itself under 200 microns wide. These tightly toleranced features demanded a precision solution.

Solution

Meeting the tight tolerances required two advances working together. The first was precision substrate cutting via CNC milling to configure substrates to custom geometries that enabled consistent, repeatable jig placement. The second was the development of custom mechanical masking tooling, in conjunction with an outside tooling partner to hold each part firmly in its pocket and eliminate any movement that would affect the transition zone. Together, these developments enabled a coating boundary to within 200 microns using mechanical masking alone, without photolithography.

For this project, Chroma coated a full plate with an antireflection layer, cut and chamfered the parts to the round-with-chord geometry, measured every part for conformance, seated each in its custom jig insert, applied the longpass coating to the unmasked region, and measured transition line placement and zone width on the finished parts.

Implementation

1. Apply the precision antireflection coating to a full plate

2. Cut and chamfer parts into the round-with-chord geometry on CNC milling equipment

3. Seat each part in a custom jig insert and apply the longpass coating to the unmasked region

4. Measure transition line placement and zone width on the finished parts

Results

» Held the transition line within 100 microns of nominal position, with the flat-to-center dimension held to +/- 25 microns

» Achieved a transition zone width under 200 microns, far tighter than a standard pocket jig’s shadow zone

» Delivered split-field order-blocking filters meeting the hyperspectral system’s spectral and dimensional requirements

Conclusion

Requests like this used to go to partner companies, since standard tooling couldn’t hold the tolerances required. Precision CNC milling and custom tooling, developed together, now enable Chroma to hold a coating boundary on a substrate with unique geometry to within a couple hundred microns.