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Figure 7. Relative passband shape and near band attenuation of filters with multiple cavities in the visible. Note the square passband shape of the four cavity filter versus the two cavity filter design. As the number of cavities increase, the top of the passband will exhibit a certain amount of "ripple" or waviness. When the shape of the passband is important or when a certain amount of transmission is required across a number of wavelengths, the minimum transmission at the 70-90% of maximum transmission BW can be specified to limit waviness. Likewise, certain applications require that a filter exhibit a certain bandwidth at a point between .01-10% of maximum transmission. In certain instances, specifications of additional points on a passband may add to the cost of the filter depending upon the resulting complexity. Transmission This is the actual amount of light passing through the filter expressed as a percentage of the light incident upon the filter at a particular wavelength. For the purpose of this discussion we refer to the desired in-band transmission as opposed to out-of-band transmission. The transmission of a particular filter depends on the application in which it is used. Some maybe as high as 95% while others may be as low as 1%. Examples follow: Figure 8. One instrument manufacturer might wish to reduce the amount of costly computer control for their instrument and increase speed by specifying filters that inversely match the spectral response of their silicon detector. Filters of this type create an artificial base line transmission from which all readings could be matched without the instrument recalibrating (resetting the base line) after every change in wavelength. Filters towards blue wavelengths tend to be wider and have high transmission. As the wavelengths move toward the red, they are increasingly attenuated as the spectral sensitivity of the detector and lamp output increases. An astronomer using a Charge Coupled Device (CCD) array would need average blocking out-of-band and as much transmission as possible in-band. By doing this, they would be able to capture every available photon from a distant source thereby reducing instrument rental costs to the lowest possible. Increasingly, manufacturers find that transmission becomes secondary to the need for high signal-to-noise ratios. By providing deeper levels of out-of-band blocking, low-level in-band signals can be isolated from nearby signals. This is particularly true for fluorescence applications where the excitation wavelength and the insertion power of a laser or xenon light source may be thousands to millions of times stronger than the emission of the sample (sometimes measured in picowatts) and spectrally very close to one another. Please see the section on Ultra High Discrimination Filters. 1 \| 2 \| 3 \| 4 \| 5 \| 6 \| Next page – 7 \| 8 \| 9 \| 10
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Figure 7. Relative passband shape and near band attenuation of filters with multiple cavities in the visible. Note the square passband shape of the four cavity filter versus the two cavity filter design.

As the number of cavities increase, the top of the passband will exhibit a certain amount of "ripple" or waviness. When the shape of the passband is important or when a certain amount of transmission is required across a number of wavelengths, the minimum transmission at the 70-90% of maximum transmission BW can be specified to limit waviness.

Likewise, certain applications require that a filter exhibit a certain bandwidth at a point between .01-10% of maximum transmission. In certain instances, specifications of additional points on a passband may add to the cost of the filter depending upon the resulting complexity.

Transmission

This is the actual amount of light passing through the filter expressed as a percentage of the light incident upon the filter at a particular wavelength. For the purpose of this discussion we refer to the desired in-band transmission as opposed to out-of-band transmission.

The transmission of a particular filter depends on the application in which it is used. Some maybe as high as 95% while others may be as low as 1%. Examples follow:

Figure 8.

One instrument manufacturer might wish to reduce the amount of costly computer control for their instrument and increase speed by specifying filters that inversely match the spectral response of their silicon detector. Filters of this type create an artificial base line transmission from which all readings could be matched without the instrument recalibrating (resetting the base line) after every change in wavelength. Filters towards blue wavelengths tend to be wider and have high transmission. As the wavelengths move toward the red, they are increasingly attenuated as the spectral sensitivity of the detector and lamp output increases.

An astronomer using a Charge Coupled Device (CCD) array would need average blocking out-of-band and as much transmission as possible in-band. By doing this, they would be able to capture every available photon from a distant source thereby reducing instrument rental costs to the lowest possible.

Increasingly, manufacturers find that transmission becomes secondary to the need for high signal-to-noise ratios. By providing deeper levels of out-of-band blocking, low-level in-band signals can be isolated from nearby signals. This is particularly true for fluorescence applications where the excitation wavelength and the insertion power of a laser or xenon light source may be thousands to millions of times stronger than the emission of the sample (sometimes measured in picowatts) and spectrally very close to one another. Please see the section on Ultra High Discrimination Filters.

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