Filters and Filter Wheels

Technology:

  When used in conjunction with appropriate detectors, filters form basic wavelength selective detection systems. A filter spectrometer has the advantages of simplicity, high signal to noise ratio, low cost and high throughput. A rotatable filter wheel allows multiple filters to be mounted and sequentially selected into the light path.

Operation:

  The combination of filters in the light path, that have characteristic transmission curves, generates variable pass bands. When equipped with stepping motors and computer interfaces, the filter wheels can be automated to perform programmed sequences. Applications of filter wheels have been found in atomic spectrometry, environmental monitoring, illuminators, laser spectroscopy, and so on.

Considersation:

• Center Wavelength:

     The arithmetic mean of the pass band expressed in nanometers.

• Percent Transmission:

     The amount of power received by the detector compared to the total power available. The traditional formula is %T = I/I₀ x (100), where I₀, is the incident power and I is the transmitted power.

• Half Bandwidth:

     The width of the pass band in nanometers at the half-power points of the pass band. It is often expressed as full width at half maximum (FWHM).

• Out-of-Band Rejection (Blocking):

     The amount of energy, outside the filter pass band, reaching the detector. It is often expressed as an absolute level, such as 10^-4, meaning there are no transmission peaks outside the pass band exceeding 0.0001 T or 0.01%T. The rejection range in nanometers must accompany this specification.

• Band Pass Shape:

     Pass band shapes can vary from triangular to nearly square. The number of cavities involved determines the overall shape.