Specifying a Custom Bandpass Filter

Most custom-filter failures are decided before the drawing is drawn. A buyer finds a catalog filter "close enough" to the wavelength, orders hundreds, and discovers at system test that the pass band slid, the blocking leaked the pump laser, or the coating fogged in the field enclosure. The five numbers below are the whole argument.

1. Center wavelength — at the angle you actually use

Bandpass centers blue-shift with angle of incidence: an 850 nm design at 45 degrees runs visibly short. Catalog parts are quoted at normal incidence. If your cone or your fold sits off-axis, the part must be designed at that AOI, not shifted by hope.

2. Half-band — matched to the emitter, not the datasheet

High-power 850 nm LEDs drift 10-30 nm with junction temperature. A 10 nm narrowband filter that looked precise on paper clips the signal when the LED heats. Match the half-band to the emitter's real working envelope; narrow only when a laser line or a crowded spectrum forces it.

3. Blocking — name the enemy and its wavelength

"OD4 blocking" means nothing without a band: blocking of UV-to-red while passing 850, or blocking of sunlight while an active 940 nm illuminator runs? Write the blocking spec against the actual light source in the field of view, then hold the factory to the measured curve.

4. Environment decides the coating, not the price

An outdoor sensor sees cleaning cycles, humidity and temperature swings. Hard ion-assisted coatings survive them; soft indoor coatings do not, and will tell you with a haze film within a season. Specify the environment and let the coating process follow — a $0.40 delta that prevents a field return is the cheapest insurance in optics.

5. The measured curve, per lot

The drawing box means nothing if nobody measures inside it. Require the production spectrophotometer curve — CWL, half-band, peak transmission and blocking marked against acceptance — shipped with every lot. A factory that owns the instrument does this without flinching; one that doesn't, will explain why it can't.

The one-sentence shortcut

If drawing-making isn't your business, write the light budget as a sentence: "Pass 850 nm LED, reject sun and pump laser, 45 degrees, outdoor wash-down enclosures, 500 pieces a month." A coating factory that designs for a living will return the five numbers for you — with the tradeoffs stated. That reply, not the part, is the reason to buy from the coater.