What are the standard manufacturing tolerances for CWL and FWHM?

In the world of optical filters—specifically bandpass filters—CWL (Center Wavelength) and FWHM (Full Width at Half Maximum) are the two most critical spectral parameters. Because thin-film deposition is an incredibly precise but still physical process, manufacturers provide "standard" tolerances that vary based on the filter's quality grade and the wavelength range.

1. Center Wavelength (CWL)

The CWL is the mathematical midpoint of the passband. Tolerances here are influenced by the total number of layers in the coating and the uniformity of the deposition across the substrate.

Note: CWL tolerances are often expressed as a percentage of the wavelength for IR filters (e.g., ±1%).

2. Full Width at Half Maximum ( FWHM)

FWHM defines the bandwidth of the filter at 50% of its maximum transmission. This is controlled by the design of the "cavities" in the thin-film stack.

• Standard Tolerance: Typically ±10% to ±20% of the nominal FWHM value.
  • Example: For a 10 nm FWHM filter, the actual result usually falls between 8 nm and 12 nm.
• Precision Tolerance: Can be tightened to ±5% or even ±2% for high-end applications, though this significantly increases the manufacturing cost and reduces yield.

3. Factors That Affect These Tolerances

Even if a filter is manufactured perfectly to spec, its effective CWL and FWHM can shift during use due to:

• Angle of Incidence (AOI): As the angle increases, the CWL shifts toward shorter wavelengths (blue shift).
• Temperature: Most coatings shift toward longer wavelengths (red shift) as temperature rises, typically at a rate of 0.01 nm/°C to 0.03 nm/°C.
• Operating Environment: High humidity can cause "shifter" filters (non-hard-coated) to swell, changing the spectral performance. Modern Hard-Coated (Ion Slung Deposition) filters are much more stable.

Comparison of Coating Technologies