Bandpass filters transmit light only within a defined spectral band ranging from less than one to many nanometers wide. They are used in a wide variety of applications where spectral isolation is required. Bandpass filters are preferable to monochromators because of their higher transmission and better signal-to-noise.
- UV Bandpass
- VIS Bandpass
- IR Bandpass
For custom bandpass filters use our Build-a-Filter request form.
Bandpass filters are defined by three critical features (see Figure 1):
- Center Wavelength (CWL)—the wavelength at the center of the passband
- Full Width at Half Maximum (FWHM)—the bandwidth at 50% of the maximum
- Peak Transmission (T)—the wavelength of maximum transmission
Bandpass Filter Types
Single Cavity (SC)
Very narrow bandwidths between 0.10nm and 0.25nm, from the ultraviolet through the near infrared spectral regions (450nm– 850nm).
Two-cavity designs with bandwidth (FWHM) typically between 0.2nm and 8.0nm in the spectral region between 350nm and 2500nm. At CWL greater than 900nm, the minimum FWHM is O.5nm. NB filters have steeper transition and greater attenuation of energy just outside the passband than SC filters.
Three-cavity designs with FWHM between 0.4nm and 50nm in the ultraviolet to near-infrared spectral range (185nm–2,500nm).
Four- and five-cavity designs with FWHM greater than 30nm and up to several hundred nanometers in the UV to far IR spectral range (350nm–11,000nm).
Greater than 3,500 degrees phase thickness from numerous interfering cavities (10 or more), resulting in a very rectangular bandshape, steep edges, and extremely deep blocking, exceeding Optical Density (OD) 6 outside the passband. A typical DF filter in the visible region will have an average transmission of more than 75% and will attain OD 5 within one FWHM from the CWL. DF filters offer high performance, with FWHM similar to BP filters but with much greater attenuation outside the passband.
Raman Discriminating (RDF)
Similar to DF designs but custom engineered for applications requiring signal-to-noise of ³ 8 orders of magnitude (108) over a limited spectral range.
A longpass and shortpass coating are assembled to create a bandpass filter. ALPHA technology produces slope factors 10–50 times steeper than industry standards, with edge steepness to the following values at a 5-decade slope factor, defined as the slope between 50% and .001%T (or OD 5): 1% Epsilon; 3% Gamma; and 5% Beta. Filters are defined by cut-on and cut-off edge location, which is more important than center wavelength when determining performance. ALPHA edge tolerance of ±3nm produces a more accurate specification for filters with FWHM >15nm than a CWL tolerance of ±20%.
Dual-Band and Multi-Band (DB and MB)
Used for applications demanding the simultaneous observation of more than one wavelength while maintaining deep attenuation between the passbands. Transmission typically exceeds 70%.
SC and NB Filters
These single line resolving filters can be used in a tuning or scanning mode, where the center wavelength is controlled precisely by adjusting the angle of incidence. Many of the factors which are insignificant in the operation of interference filters with large FWHM/CWL ratiosÑsuch as light collimation, angle of incidence, operating temperature, and filter ageÑare critical in the use of SC and NB filters.
DF, RDF, ABP, and 3rd Filters
These rectangular bandshape filters are required for resolving energy bands and are widely used in applications such as fluorescence detection where high-intensity excitation light must be attenuated in order to detect low-intensity emission light.
Passband Shape and Near Out-of-Band Attenuation
The CWL and FWHM of a bandpass filter are determined by the materials used and their refractive index, as well as the number of layers within each Fabry-Perot cavity. The passband shape and the degree of attenuation outside the passband are determined primarily by the phase thickness of the dielectric coating, which is usually a function of the number of Fabry-Perot cavities. As a result, it is possible to have two bandpass filters with exactly the same specified CWL and FWHM, but with very different passband shapes and out-of-band attenuation levels. Increasing the number of cavities creates a more rectangular passband shape with a steeper transition to higher levels of attenuation outside the passband.
Figures 2a and 2b present the theoretical spectral curves of nine different filters, all with a CWL at 500nm and FWHM of 15nm but ranging in number of cavities from 1 to 10, and for three ALPHA edge slopes. Figure 3 presents relative bandwidths at increasing levels of attenuation for filters ranging in number of cavities from 1 to 10, as well as for the three ALPHA edge designs.
|1 Cavity||2 Cavity||3 Cavity||4 Cavity||5 Cavity||6 Cavity||10 Cavity||ß||γ||ε|
Figure 3. Bandwidth and Slope Multipliers
For 1–10 cavity designs, the bandwidth multiplier factors will give the bandwidth at the indicated level of transmission (or optical density) when multiplied by the nominal FWHM. For ALPHA designs, the slope factors will give the wavelength at the indicated level of transmission (or optical density) when multiplied/divided by the nominal cut-on/cut-off. Note that values are theoretical and may vary by 10% in practice.
Standard Specifications: Bandpass Filters
The following specifications apply to all Omega Optical bandpass filters unless other custom specifications are requested.
|CWL Tolerance||±20% of FWHM|
|NB||+20%, -0% of FWHM|
|SC||Peak wavelength rather than center wavelength is specified. To meet tolerance the specified peak must be located between the actual 80% cut-on wavelength and the actual peak wavelength|
|ALPHA & 3rd||Tolerance ±3nm for cut-on and cut-off edges, for bandwidth >15nm produces a more accurate|
|FWHM Tolerance||±20% of FWHM; SC filters, ±0.05nm|
|Angle of Incidence||0°|
|Temperature of Measured Performance||20°C|
|Operating Temperature Range||-60° to +80°C|
|Humidity Resistance||Per Mil-STD-810E, Method 507.3 Procedure I|
|Coating Substrates||Optical quality glass|
|Surface Quality||80/50 scratch/dig per Mil-0-13830A|
|Outside Dimension Tolerance||+0, -0.25 mm (+0, -0.01Ó)|
|Minimum Clear Aperture||4mm less than nominal outside dimension|
Spectrophotometric trace of the passband with a resolution of 0.1nm. Spectrophotometric curves provided with NB and SC filters have a resolution of 0.05nm and are calibrated to atomic emission lines. Transmission accuracy on all spectrophotometric traces is ±1%.