Specifying Custom Optical Filters: A Buyer's Working Guide

Start from the light budget, not the catalog

A filter spec is a light-budget decision: what wavelengths must reach the detector, what must be blocked, at what angle, in what environment. Write it in one paragraph: 'block 532 nm pump light, pass 850 nm, uncoated back surface, 25 mm square, 500 pieces' is a better starting document than a copied catalog page, because a coating engineer can design to a light budget and cannot design to a catalog number of someone else's part.

The parameters a quote will ask for

Center wavelength and half-band width for bandpass; edge wavelengths for longpass/shortpass; OD depth of blocking; transmission band ripple; substrate (BK7, fused silica, others); size, thickness and clear aperture; angle of incidence; surface quality and scratch-dig; environmental testing if the part goes outdoors or into thermal cycling. Missing any one of these turns the quote into a guess.

What data should come back

A coating spectrum measured on the production lot (transmission/reflection curves), interferometric surface figure, dimensional inspection, and — for demanding programs — coating-adhesion and environmental test results per standard. Components without measurement data are parts you will test yourself, at your cost, every lot.

Cost levers that don't cut quality

Standard substrates are cheaper than exotic ones when performance allows; larger batches share coating-campaign cost; tolerance bands priced explicitly let you pay for what you need instead of inheriting an over-specified default. A good component supplier proposes these levers in the quote itself.