The IR-Cut Filter Focus Trap

Why Switching to IR Lighting Can Throw Off Your Focus

The Hidden Problem

IR lighting feels like a simple swap. Change the LEDs, maybe add an IR-pass filter, and you should have stable contrast for your machine vision application. But there's a sneaky mechanical and optical gotcha that catches many engineers off guard.

Here's what most people don't realize: changing filters alters your optical path length. This shifts your focus and working distance in ways that are very real on a production machine. We're not talking about subtle image-color changes—we're talking about build-impacting distance shifts.

Why Does This Happen?

A light filter isn't optically "nothing." It behaves like a plane-parallel glass plate that lengthens the optical path, pushing your focus and working distance around.

The rule of thumb: With typical optical glass (n≈1.5), placing a filter in front of the lens increases working distance by roughly one-third of the glass thickness. Placing a filter in front of the sensor increases working distance by roughly the glass thickness divided by three times the magnification squared.

Depending on where your filter sits and your imaging scale, a "small" thickness change can turn into a surprisingly large mechanical shift.

Practical Mitigation Strategies

Design for filter swaps. Build your mount so focus has enough travel to absorb a filter change. Treat "filter stack thickness" as a controlled mechanical parameter—not an afterthought.

Use a thickness-matched clear window. If you remove an IR-cut filter for IR illumination, consider replacing it with a clear window of similar thickness. This keeps the optical path close to what the lens expects.

Validate focus at your actual IR wavelength. Many lenses shift focus with wavelength due to chromatic effects. Don't validate focus under room lights if you'll be running in near-IR.

Keep filters square to the optical axis. Tilt can introduce aberrations and even a lateral axis offset—another real-world consequence of "just adding glass."

The Bottom Line

Swapping or removing filters affects more than image color. Plan for the focus and working distance shift by treating filter stack thickness as a controlled design parameter. Engineers don't get bonus points for optimism—they get bonus points for repeatability.