What do CWL and FWHM mean on a bandpass filter?
Central Wavelength (CWL) and Full Width at Half Maximum (FWHM) are two key terms that describe how a bandpass filter works to pick out specific wavelengths of light.
Central Wavelength (CWL):CWL is the center point of the filter's transmission band—the main wavelength the filter is designed to "let through." For example, a bandpass filter with a CWL of 550 nm passes light most efficiently at 550 nm. Traditional coated filters reach their maximum transmission at the CWL, while hard-coated filters often have a flatter maximum across the band.
FWHM measures the width of the filter's transmission band, calculated as the wavelength span where transmission is at least 50% of the filter's maximum. If max transmission is 90%, the FWHM spans the range where the filter transmits 45% (half) or more.
- Narrowband filters (FWHM ≤ 10 nm): Used for laser clean-up and chemical detection.
- Mid-band filters (FWHM 25–50 nm): Common in machine vision.
- Broadband filters (FWHM > 50 nm): Typical for fluorescence microscopy.
CWL ensures your filter targets the right spectral region, while FWHM tells you how "selective" the filter is—whether it blocks out most wavelengths or lets a broad range through.
If a filter states CWL = 532 nm, FWHM = 10 nm, it will transmit light strongly centered at 532 nm, with transmission above 50% stretching from about 527 to 537 nm.
| Term | Meaning |
|---|---|
| CWL | Center of the filter's transmission band (main wavelength passed) |
| FWHM | Width of the band where transmission is ≥ 50% of max |
- CWL = "center" wavelength of highest transmission.
- FWHM = how wide the filter passes light at high efficiency.
- Together, they describe what part of the spectrum your filter targets and how selective it is.
