Optical Bandpass Filters Technical Information

Bandpass optical filters are defined by four 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 transmission;
  • Peak Transmission (T)—the wavelength of maximum transmission.
  • The blocking range—the spectral region in which the filter does not transmit
bandpass optical filter
Figure 1: Multi-Cavity Passband Coating

An optical bandpass filter that was made by depositing alternating layers of zinc sulfide and cryolite on a glass substrate according to a 3-cavity Fabry-Perot interferometric design. The bandpass filter’s CWL is located at 576.1nm; its FWHM is 9.6nm.

Optical Bandpass Filter definitions

bandpass optical filter Formula

Optical Bandpass Filter Types

A number of different filter designs are available which tend to reflect the sharpness or steepness of the cut-on curve and or the level of transmission and blocking required. Here is a brief description of the available designs;

Single Cavity Filter (SC Filter)
Very narrow bandwidths between 0.10nm and 0.25nm, from the ultraviolet through the near infrared spectral regions (450nm– 850nm).

Optical Narrow Bandpass Filter (NB Filter)
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. Narrow Optical Bandpass Filters (NB filters) have steeper transition and greater attenuation of energy just outside the passband than Single Cavity Filters  (SC filters).

Bandpass Filter (BP Filter)
Three-cavity designs with FWHM between 0.4nm and 50nm in the ultraviolet to near-infrared spectral range (185nm–2,500nm).

Wide-Band Filter (WB)
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).

Discriminating Filter (DF)
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 Optical Bandpass filters (BP filters) but with much greater attenuation outside the
passband.

Raman Discriminating Filter(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.

ALPHA Filter (ABP)
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%.

3rd Millennium Filter (3rd)
Manufactured using an ALPHA longpass and ALPHA shortpass coating on fused silica substrates to create a bandpass with edge steepness and edge placement characteristic of ALPHA filters. These coatings are then assembled in a patented, air-spaced, hermetic assembly which eliminates auto-fluorescence, improves transmitted wavefront, and lengthens filter life.

Dual-Band and Multi-Band Filter (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%.

Additional Information

Single Cavity Filters and Optical Narrow Bandpass Filters (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 single cavity filter and narrow Optical bandpass filter (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 an optical 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.

optical bandpass filter graph
Figure 2a. ALPHA Bandpass Designs. The theoretical spectral curves of the three ALPHA bandpass designs with a
CWL at 500nm and a FWHM of 15nm. Edge slope is a critical feature.
Fabry-Perot Bandpass Designs
Figure 2b. Fabry-Perot Bandpass Designs. The theoretical spectral curves of six different bandpass filter designs with a CWL at 500nm and a FWHM of 15nm. The range from 2-cavity to 10-cavity coatings illustrates the differences in bandshape and attenuation characteristics.

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

 

T

 

SC

 

NB

 

BP

 

WB

 

DF

 

DF

 

ALPHA

 

 

 

1 Cavity

 

2 Cavity

 

3 Cavity

 

4 Cavity

 

5 Cavity

 

6 Cavity

 

10 Cavity

 

ß

 

?

 

e

 

80% T

 

0.43

 

0.65

 

0.83

 

0.92

 

0.96

 

0.96

 

0.96

 

 

 

 

 

 

 

50% T

 

1.00

 

1.00

 

1.00

 

1.00

 

1.00

 

1.00

 

1.00

 

1.00

 

1.00

 

1.00

 

10-1

 

3.01

 

1.70

 

1.04

 

1.20

 

1.10

 

1.10

 

1.01

 

.003

 

.002

 

.001

 

10-2

 

10.00

 

3.20

 

2.00

 

1.50

 

1.40

 

1.20

 

1.04

 

.009

 

.006

 

.003

 

10-3

 

 

 

5.60

 

3.00

 

2.10

 

1.60

 

1.10

 

1.09

 

.018

 

.010

 

.004

 

10-4

 

 

 

11.00

 

4.50

 

3.70

 

2.00

 

1.70

 

1.15

 

.029

 

.016

 

.006

 

10-5

 

 

 

 

 

 

 

 

 

 

 

1.80

 

1.23

 

.410

 

.022

 

.009

 

10-6

 

 

 

 

 

 

 

 

 

 

 

2.20

 

1.33

 

.056

 

.030

 

.012

 

10-7

 

 

 

 

 

 

 

 

 

 

 

2.74

 

1.46

 

.075

 

.038

 

.015

 

10-8

 

 

 

 

 

 

 

 

 

 

 

3.60

 

1.65

 

.102

 

.047

 

.019

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

§ 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

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 specification than CWL ±20% tolerance

FWHM Tolerance

±20% of FWHM; SC filters, ±0.05nm

Angle of Incidence

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

Maximum Thickness

10mm

Documentation. 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%.

Minimum peak transmission, out-of-band attenuation range, and attenuation degree are highly variable parameters of filter design.

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