What this application demands
Biometric modules place the optical filter close to the sensor, in a short optical path with a wide cone of angles, and expose it to whatever lighting the user happens to be standing in. That combination — wide incidence angles plus uncontrolled ambient light — is what makes the filtering requirement harder than the wavelength alone suggests.
Face-recognition modules typically illuminate in the near-infrared, where the sun is bright but human faces are not, and the filter has to pass the illumination band while rejecting the ambient background in the same region. Iris scanners work at a closely defined wavelength and need the pass band to hold across the sensor's field of view.
Specifications we recommend
| Component | Typical specification | Why |
|---|---|---|
| 850 nm band-pass filters | Narrow pass band designed to hold across a wide angular range | Ambient rejection with uniform response across the sensor |
| IR cut-off filters | Visible-transmitting, infrared-blocking coating | Keeps infrared from contaminating the visible imaging channel |
| Cover windows | AR-coated, hardened glass sized to the module | Protects the sensor in a consumer device while minimising reflection |
| Blue glass and absorbing substrates | Absorptive elements combined with coated layers | Suppresses out-of-band light that a coating alone passes at steep angles |
Failure modes we design against
Angular shift is the defining problem. In a compact module the light reaching the filter is far from collimated, and a narrow-band coating optimised at normal incidence will show a blue-shifted effective band at the edge of the field. Combining a coated filter with an absorptive substrate is the standard remedy, because the absorption continues to reject out-of-band light even at angles where the interference layers have shifted.
Spectral leakage at steep angles is the second issue, and it produces exactly the failure that matters most: the system accepting ambient light it was designed to reject, reducing contrast between a real face and a photograph.
Frequently asked questions
Why combine an absorptive substrate with a coating?
Because interference coatings shift with angle and absorption does not. The combination holds rejection across a wide field where a coating alone would leak.
Can you supply filters matched to a specific sensor?
Yes. Send the sensor's spectral response and angular field and we will design the pass band around it.