The 5% Rule for Filter Edges

Why "Cuts at 650nm" Doesn't Tell the Whole Story

Edges Are Slopes, Not Cliffs

People talk about filter edges like they're guillotines: "It cuts at 650 nm." In reality, edges are slopes, not cliffs. So the only honest question is: "Cuts to what level?"

Here's the important detail that often gets skipped: the cut-on/cut-off wavelength is typically defined using a 5% absolute transmission criterion. That single sentence can prevent a lot of painful misunderstandings between optics, lighting, and mechanical teams.

Why 5% Matters More Than You Think

Five percent leakage isn't "basically zero" when the unwanted light source is huge.

Consider what you're up against: Daylight is bright—orders of magnitude brighter than your controlled illumination might be. Shiny fixtures can specularly blast the lens. Reflections from machinery, floors, and walls can overwhelm your intended signal.

If you assumed a different criterion (like 50%), you might think the filter blocks more than it actually does at the wavelengths that matter. That assumption can turn into image noise and unstable contrast on the production floor.

The Practical Habit

Always look at the full transmission curve. Don't rely on a single number on the spec sheet. Confirm what criterion the manufacturer uses for "cut-on/cut-off." Transmission is usually presented in filter diagrams (often on a log scale) precisely so you can see how much gets blocked across the entire wavelength range.

Check out-of-band blocking. Is it sufficient for your ambient environment? This is where optical density (OD) specs become helpful. An OD of 3 means 0.1% transmission; OD 4 means 0.01%. Know what you need to reject.

Remember stacking rules. If you're combining filters, optical densities add and filter factors multiply. Your exposure and illumination budget has to survive the whole stack.

The Bottom Line

Filter edges are slopes, not cliffs. The 5% transmission criterion defines "the edge," but 5% leakage isn't zero—especially when ambient light is orders of magnitude brighter than your illumination. Always examine the full transmission curve. Engineers don't get bonus points for optimism; they get bonus points for repeatability.