Optical Filters for Forensic and Security Imaging

Microprinting: Visible Only With the Right Band

In the background of the note is a repeating "100" pattern, printed small enough to blend into the surrounding linework. Under normal light, it barely registers.

Filtered through an MV470/20 narrow blue vision filter, the same microprinting becomes sharp and legible. Switch to an MV635/20 narrow red vision filter, and it nearly disappears again. Same ink, same note, two 20 nm bandpass filters centered 165 nm apart give two very different results.

Figure 1: The same microprinting under no filter, an MV470/20, and an MV635/20.

Color-Shifting Ink: A Hidden Second Layer

The bell in the inkwell is best known for shifting from copper to green when the note is tilted. Under standard light, that color shift is the only effect most people ever notice.

Filtered through an MV785/40 near-infrared vision filter, the bell develops a sharp, clean outline that isn't visible under normal light. The filter enables imaging a wavelength range that isolates a feature that would ordinarily be indistinguishable from the surrounding area.

Figure 2: The same security feature imaged without filter and with the aid of an MV785/40 bandpass filter.

Why Wavelength Matters

Inks and substrates used in currency, passports, and other secure documents are formulated to behave differently across the spectrum on purpose. Isolating the right wavelength with the help of an optical filter is what turns a hidden feature on these items into a visible one.

This principle is utilized across a range of applications including:

• Document Examination: Verifying security features that are only visible under specific wavelengths.

• Currency and Printing Inspection: Confirming ink and substrate behavior at the wavelengths those materials were designed around.

• Forensic Imaging: Isolating spectral detail that standard photography cannot capture.

• Machine Vision Inspection: Automating the same wavelength-specific detection at production speed.

Partner with Chroma for Your Imaging Application

Wavelength-specific imaging shows up in more than currency. Whether you're inspecting product on a machine vision line, screening documents for counterfeits, or authenticating artwork, the challenge is the same: isolating the exact band of light a feature depends on.

Our engineering team can help you find that band, whether it's a catalog filter like the ones shown here or a custom design built around your application.