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Center Wavelength (CWL) This is the easiest of the specifications to set. It is defined as the mean wavelength between two relative 50% transmission points on the passband. It may be specified using any of the following: 5320.0 angstroms 532.0 nanometers 0.532 microns Filters are specified in angstroms or nanometers from the vacuum ultra violet to about 2.0 microns. Mid and far infrared filters are expressed in microns. Temperature, and the angle of light incident to the filter, heavily affects some filters. Tolerances should be designed to account for these variables within the operating range of the instrument. (See sections on Angle of Incidence and Operating Temperature) The narrower the bandwidth, the tighter this tolerance generally needs to be. Typically a CWL tolerance of 10-20% of the bandwidth is appropriate. In instances where tighter than normal tolerances are specified, the trade-offs may be cost and time. In some cases a greater importance is assigned to the out-of-band blocking of other lines or wavelengths with little restriction on CWL. This is often true for fluorescence applications where blocking is critical and the passbands can be moved slightly. Half Bandwidth (HBW) Also known as Half Power Bandwidth (HPBW) and Full Width at Half of Maximum transmission (FWHM). This is the interval between the relative 50 percent transmission points of the passband. Tolerances of 10 to 20 percent are typical although tighter tolerances are available and often dictate a higher cost. Narrow bandwidth filters are often used to isolate discrete spectral lines and by their complexity are more expensive than wider filters. If there is no unwanted radiation near the line of interest possible consideration should be given to using a wider filter for the job. Additionally, wider bandwidth filters exhibit higher transmissions. Passband Shape Passband shape is primarily a function of the number of cavities in the filter design. The greater the number of cavities, the more square the shape. As the number of cavities increase, the base of the passband narrows and the top will widen. Figure 6. 1 \| 2 \| 3 \| 4 \| Next page – 5 \| 6 \| 7 \| 8 \| 9 \| 10
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Center Wavelength (CWL)

This is the easiest of the specifications to set. It is defined as the mean wavelength between two relative 50% transmission points on the passband. It may be specified using any of the following:

5320.0 angstroms

532.0 nanometers

0.532 microns

Filters are specified in angstroms or nanometers from the vacuum ultra violet to about 2.0 microns. Mid and far infrared filters are expressed in microns.

Temperature, and the angle of light incident to the filter, heavily affects some filters. Tolerances should be designed to account for these variables within the operating range of the instrument. (See sections on Angle of Incidence and Operating Temperature)

The narrower the bandwidth, the tighter this tolerance generally needs to be. Typically a CWL tolerance of 10-20% of the bandwidth is appropriate. In instances where tighter than normal tolerances are specified, the trade-offs may be cost and time.

In some cases a greater importance is assigned to the out-of-band blocking of other lines or wavelengths with little restriction on CWL. This is often true for fluorescence applications where blocking is critical and the passbands can be moved slightly.

Half Bandwidth (HBW)

Also known as Half Power Bandwidth (HPBW) and Full Width at Half of Maximum transmission (FWHM). This is the interval between the relative 50 percent transmission points of the passband. Tolerances of 10 to 20 percent are typical although tighter tolerances are available and often dictate a higher cost.

Narrow bandwidth filters are often used to isolate discrete spectral lines and by their complexity are more expensive than wider filters. If there is no unwanted radiation near the line of interest possible consideration should be given to using a wider filter for the job. Additionally, wider bandwidth filters exhibit higher transmissions.

Passband Shape

Passband shape is primarily a function of the number of cavities in the filter design. The greater the number of cavities, the more square the shape. As the number of cavities increase, the base of the passband narrows and the top will widen.

Figure 6.

1 | 2 | 3 | 4 | Next page – 5 | 6 | 7 | 8 | 9 | 10

SpectroFilm is the leading supplier of standard and custom OEM laser diode and optical passband filters.