The Curve That Defines Image Performance

When building a vision system, selecting a machine vision optical filter that emulates the bell-shaped output (Gaussian transmission curve) of the illumination source's spectrum can directly influence image clarity, contrast and overall system performance.

The Bell-Shaped / Gaussian Curve

The spectral output of a single-color LED light source typically follows a Gaussian, or bell-shaped, curve. To maximize performance, the bandpass filter used in front of the camera should mimic this shape, aligning its center wavelength and bandwidth with the LED’s peak. Important notes:

• If the filter’s bandwidth is too broad, it can let in unwanted ambient light, reducing contrast.
• If the filter is too narrow, it can block part of the desired illumination, darkening the image and lowering signal strength.
• A well-matched Gaussian-shaped filter allows just the right amount of light through, preserving brightness while maintaining contrast.

Fluorescence Applications

In fluorescence imaging, it’s especially important to avoid passing regions of the spectrum where only a weak signal is emitted. Allowing light from these tails (combined with strong ambient or excitation sources) can overwhelm the system and bury the features you’re trying to detect. A Gaussian-aligned filter ensures the passband captures the strongest part of the emission spectrum while rejecting unnecessary background light.

Flat-Top vs. Gaussian Profiles

Some filters are designed with a flat-top transmission profile, marketed as providing “more even illumination.” While this may sound appealing, in applications, it can be an issue.

Flat-top filters often stretch their passband to cover the weaker tails of the LED emission curve. This means they admit significantly more ambient light relative to the minimal LED signal in those regions (what we want to avoid in a controlled vision setup). The result is less contrast and more noise in the captured image.

In contrast, a Gaussian-shaped passband mirrors the source more naturally, maximizing usable light and minimizing interference.

For applications where precision and repeatability matter, the curve of your filter makes all the difference.