Reading Filter Specs Like an Engineer

Understanding Cut-On, Cut-Off, HPP, HBW, and Center Wavelength

Why "Close Enough" Doesn't Work

Filter spec sheets look deceptively simple: a few wavelengths, a bandwidth number, maybe a curve. But in machine vision, "close enough" optics becomes "mysteriously unstable" performance pretty quickly.

If you misunderstand where the passband truly starts, how bandwidth is defined, or what "center wavelength" actually means, you can end up with a filter that technically matches the part number you wanted—and still leaks ambient light, starves your illumination, or shifts your focus.

The Core Vocabulary

Transmission (T) is simply the ratio of transmitted light energy to incoming light energy. If 70% of the light gets through, T = 0.70 or 70%.

Cut-on and cut-off wavelength (λc) describe where an edge filter transitions between low and high transmission (or vice versa). The key detail: the 5% absolute transmission point is typically used as the criterion for that "edge."

Half-power points (HPP) are the wavelengths where transmission equals half the peak transmission. So if your filter's peak is 70%, the HPP are at 35%.

Center wavelength (λ0) for bandpass filters is calculated as 2×λ1×λ2÷(λ1+λ2) using the two HPP points. Important: this is not the arithmetic average. We'll dig deeper into why this matters in a separate article.

Half-bandwidth (HBW) is the width between the two half-power points. This tells you how selective the filter is.

Translating Specs into System Consequences

Peak transmission tells you how much light budget you'll lose—and whether you'll need more illumination power or longer exposure to compensate.

Bandwidth tells you how selective the filter is. But here's the tradeoff: selectivity often costs transmission. Narrower interference filters tend to have greater passband loss. Very narrow bandwidths can dramatically reduce transmitted light.

Stacking behavior matters when you use multiple filters. Transmission losses compound—multiply filter factors or add optical densities. That's exactly how you should budget exposure and illumination when filters are in series.

The Bottom Line

Learn the vocabulary so you can translate specs into system consequences. Peak transmission affects your light budget. Bandwidth affects selectivity (and often costs you transmission). Stacking compounds losses. Reading specs like an engineer means thinking through what each number does to your actual imaging performance.