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.
Full Width at Half Maximum (FWHM):
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.
Why do CWL and FWHM matter?
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.
Example:
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 |
Summary:
- 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.
Frequently Asked Questions
https://www.kupooptics.com/en/blogs/q-a/cwl_fwhm
What is CWL (center wavelength) in an optical bandpass filter?
CWL is the center wavelength of the filter's pass band — the wavelength of peak transmission, or the midpoint between the T50% wavelengths on each side. CWL is the primary specification for matching a bandpass filter to a specific light source or signal wavelength.
What is FWHM (Full Width Half Maximum) and how is it measured?
FWHM is the width of the bandpass filter's transmission peak measured at 50% of peak transmission. It quantifies the filter's spectral bandwidth. To measure: find peak transmission, identify the two wavelengths where transmission = 50% of peak, and calculate the difference.
How do I choose CWL and FWHM for my application?
Set CWL to match your signal source wavelength (e.g., 905 nm for LiDAR, 488 nm for laser line). Choose FWHM based on: signal spectral width (FWHM must exceed source linewidth), ambient rejection needs (narrower FWHM = better rejection), and temperature shift budget (leave margin for thermal drift).
What tolerances should I specify for CWL and FWHM?
Typical production tolerances: CWL ±2–5 nm for standard filters, ±1 nm for precision. FWHM ±10–20% for standard, ±5% for precision. Tighter tolerances increase cost significantly. Always define tolerances at the measurement conditions (temperature, AOI) matching your application.
