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Other Filter Specifications The majority of filters produced by SpectroFilm are photometric filters where only the isolation of certain spectra is required. Generally the physical characteristics are 80/50 scratch dig and 5-10 waves an inch across the face of the filter. Imaging filters for use with a variety of detectors may require the addition of many other optical specifications and often changes in the methods used to make the filters. Flatness, parallelism, pin holes, and surface quality are some of the additional specifications. With the exception of pin holes (which reduce the overall depth of blocking in some photometric applications), these specifications are not required for photometric filters. The manner in which coatings are deposited on substrates can affect the quality of an imaging system. In fast systems, F6 and faster, filters can generally be photometric type filters. In systems f10 and slower the technical requirements for filters increases to its highest, in that filter coatings are either deposited on one substrate or are physically located as close as possible to one another. One of the trade offs in an imaging system filter is transmission. Due to the number of layers required to make narrow band filters, coatings on single substrates require that metal blocking be used. This type of blocking achieves deeper levels of blocking out-of-band with fewer layers than dielectric blocking. It also serves to limit in-band transmission. Dielectric blocking allows a higher transmission but due to the number of layers to make the filter, should be deposited on separate substrates. This separation of coatings may cause multiple internal reflections which may degrade performance of the system. In general, if 1/4 wave flatness is required of a filter after it is finished, the elements must first be assembled and the external surfaces polished to specification. As mentioned before,temperature degrades the laminates holding a filter together and to achieve a "hard" AR coating requires temperatures in excess of 300 °C. The trade off is that only a "soft" AR coating can now be deposited. Newer cold AR coating technologies are being developed, primarily ion packing, which yield equally hard AR coatings. When coatings are deposited, the atoms of the materials tend to stack themselves in microscopic columns. Pin holes are tiny voids in the coating columns that allow unfiltered light to pass. All coatings if inspected under high enough magnification will give indication of pinholes. Specification of "no visible pinholes" implies under rear illumination with a bright light source that no pin holes will be observable to the unaided eye. The level of pinhole examination and filter rejection may increase the cost of filters. Ultra High Discrimination Filters SpectroFilm has worked with a number of research groups world wide in the development of filters that provide high transmissions, very deep out-of-band attenuation levels and highly defined passband shapes for applications where the desired output signal is weak and easily buried by other undesirable spectral signals. The six or more cavity filters are commonly used in fluorometric analysis instrumentation such as flow cytometers, fluorescent microscopes and other instruments designed to excite a sample and read the resultant Stokes shifted emission characteristics. Commonly, the difference between the key excitation wavelength and the Stokes shifted emission wavelength is relatively small and impossible to discriminate using the standard 3 cavity filters. This lack of discrimination is due to the relative slowness with which the a 3 cavity filter passes from maximum transmission to maximum attenuation with relation to 6 or more cavity filters which exhibit virtually square passband shapes. This allows the two filters to be placed spectrally close to one another while providing the lowest level of cross-talk between the channels. Typically, filters with 6 or more cavities exhibit greater than 50 percent transmission in the visible, with combined out-of-band blocking in excess of 1Oe-8 (0.000001% transmission )between the passbands. This level can be further enhanced in some instances with the introduction of a high performance. 1 \| 2 \| 3 \| 4 \| 5 \| 6 \| 7 \| 8 \| 9 \| Next page – 10
	©2008 SpectroFilm. All rights reserved. (978) 670-7192 • 34 Dunham Road, Billerica, MA 01821 SpectroFilm is the leading supplier of standard and custom OEM laser diode and optical passband filters.

Other Filter Specifications

The majority of filters produced by SpectroFilm are photometric filters where only the isolation of certain spectra is required. Generally the physical characteristics are 80/50 scratch dig and 5-10 waves an inch across the face of the filter. Imaging filters for use with a variety of detectors may require the addition of many other optical specifications and often changes in the methods used to make the filters.

Flatness, parallelism, pin holes, and surface quality are some of the additional specifications. With the exception of pin holes (which reduce the overall depth of blocking in some photometric applications), these specifications are not required for photometric filters.

The manner in which coatings are deposited on substrates can affect the quality of an imaging system. In fast systems, F6 and faster, filters can generally be photometric type filters.

In systems f10 and slower the technical requirements for filters increases to its highest, in that filter coatings are either deposited on one substrate or are physically located as close as possible to one another. One of the trade offs in an imaging system filter is transmission. Due to the number of layers required to make narrow band filters, coatings on single substrates require that metal blocking be used. This type of blocking achieves deeper levels of blocking out-of-band with fewer layers than dielectric blocking. It also serves to limit in-band transmission.

Dielectric blocking allows a higher transmission but due to the number of layers to make the filter, should be deposited on separate substrates. This separation of coatings may cause multiple internal reflections which may degrade performance of the system.

In general, if 1/4 wave flatness is required of a filter after it is finished, the elements must first be assembled and the external surfaces polished to specification. As mentioned before,temperature degrades the laminates holding a filter together and to achieve a "hard" AR coating requires temperatures in excess of 300 °C. The trade off is that only a "soft" AR coating can now be deposited.

Newer cold AR coating technologies are being developed, primarily ion packing, which yield equally hard AR coatings.

When coatings are deposited, the atoms of the materials tend to stack themselves in microscopic columns. Pin holes are tiny voids in the coating columns that allow unfiltered light to pass. All coatings if inspected under high enough magnification will give indication of pinholes. Specification of "no visible pinholes" implies under rear illumination with a bright light source that no pin holes will be observable to the unaided eye. The level of pinhole examination and filter rejection may increase the cost of filters.

Ultra High Discrimination Filters

SpectroFilm has worked with a number of research groups world wide in the development of filters that provide high transmissions, very deep out-of-band attenuation levels and highly defined passband shapes for applications where the desired output signal is weak and easily buried by other undesirable spectral signals. The six or more cavity filters are commonly used in fluorometric analysis instrumentation such as flow cytometers, fluorescent microscopes and other instruments designed to excite a sample and read the resultant Stokes shifted emission characteristics.

Commonly, the difference between the key excitation wavelength and the Stokes shifted emission wavelength is relatively small and impossible to discriminate using the standard 3 cavity filters. This lack of discrimination is due to the relative slowness with which the a 3 cavity filter passes from maximum transmission to maximum attenuation with relation to 6 or more cavity filters which exhibit virtually square passband shapes. This allows the two filters to be placed spectrally close to one another while providing the lowest level of cross-talk between the channels.

Typically, filters with 6 or more cavities exhibit greater than 50 percent transmission in the visible, with combined out-of-band blocking in excess of 1Oe-8 (0.000001% transmission )between the passbands. This level can be further enhanced in some instances with the introduction of a high performance.

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