Optical Filters

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Optical filters selectively allow certain wavelengths of light to pass freely while blocking other wavelengths. Shanghai Optics manufactures a wide range of optical filters for engineering, scientific, and photographic applications.

Our hard coated optical filters are dichroic filters. They are composed of a single substrate with a dense hard coating. These hard sputtered filters are expensive to produce, but they have excellent performance and are ideal for applications where precision is of paramount importance. We use cutting-edge IBS technology to give our dichroic filters uniform dielectric coatings, and no adhesives whatever are involved in manufacture.  This ensures a quality product with a very long filter life.   Since they have only one substrate covered with a thin film, Shanghai Optics dichroic filters are much thinner than traditional coated filters and offer much higher transmission.

Traditional coated optical filters consist of a complex assembly of absorbing material, metallic layers, and interference coatings. These are primarily used for analytical instrumentation and laboratory equipment, as their environmental stability is intrinsically limited.

Both of these types of filters are also known as interference filters. They are wavelength-selective through interference effects at the thin film boundary.

Our colored glass filters consist of a simple glass substrate with dye injected in the manufacturing process. The absorption and transmission can be controlled by the concentration of the dye used and by the thickness of the filter. These absorptive filters are highly stable and less expensive than other options. The optical properties are also more limited.

Filters can also be divided into three other categories: shortpass, longpass, and bandpass filters. Shortpass filters block wavelengths longer than a certain cut off while allowing short wavelengths of light to pass through unimpeded. These include hot mirrors, heat absorbing glass, and IR cutoff filters. Longpass filters block shorter wavelengths of light, and transmit longer wavelengths. Bandpass filters transmit a certain wavelength range (known as the pass band) and block or restrict all others.  The pass band may be narrow (2-10 nm) or broad (50-80 nm). Bandpass filters are more angle sensitive than other types, so you will want to pay extra attention to mounting and setup if you use a filter of this type.

Both shortpass and longpass filters are sometimes referred to as edge filters.

Types Of Optical Filters

Shanghai Optic’s high transmission OD4 notch filters have a broad transmission range, and feature an optical density of more than 4.0. These filters have a low cost design and are often selected as a more economical alternative to Rugate Notch filters.

Our OD6 bandpass filters feature greater than 93 percent transmission, and have excellent blocking properties (optical density >6 ). They are available in common wavelengths for popular fluorophores.

Shanghai Optic’s hard coated fluorescence filters are ideal for many applications including fluorescence microscopy, co-localization fluorescence measurements, and fluorescence in situs hybridization (FISH). They also can be used for gel and spot imaging comparisons and for comparative genomic hybridization (CGH). These feature high passband transmission (greater than 90%) as well as excellent blocking (optical density greater than 6).

Other dichroic filters are available as well. These high quality filters feature a sharp transmission from reflection to transmission, and are available in many longpass and shortpass options.

Our laser line filters are dichroic filters designed for laser-based fluorescence instrumentation, Raman spectroscopy, and analytical or medical laser systems. They feature a narrow bandwidth centered around the laser, and are ideal for suppressing underground plasma and other secondary emissions while allowing the laser line to pass unhindered. These filters have peak transmission of greater than 90%. The optical density of greater than 5.0 means they will do an excellent job of blocking unwanted light.

Raman filters are hard-coated dichroic filters with an ultra steep edge, designed to measure even the smallest Raman shifts. They have deep blocking (OD>6.0) for maximum laser rejection, and are a good choice for precise work.

We also produce laser safety filters, which transmit visible wavelengths while blocking laser light.

Specification in Optical Filters

There are a number of specifications you will want to keep in mind as you select the optimal optical filter for your application.

Cut-on Wavelength is the wavelength at which the transmission has reached half, or 50 percent, in a longpass filter. Similarly, cut-off wavelength is the wavelength at which the transmission has reached 50 percent in a shortpass filter.

The slope of an edge filter is given as the percentage of the cut wavelength with designated starting and end points. For instance, if the start and end points are considered to be 10 percent and 80 percent transmission, a 600 nm filter with 1 percent slope would transition from 10 to 80 percent transmission over a 6 nm bandwidth.

Optical density relates to the amount of light attenuated by a filter, and is calculated by the transmission percentage in the blocked region. Since the optical density is calculated with a negative log, a higher optical density value will indicate lower transmission.  For fluorescence microscopy, you may need a filter with optical density of 6 or greater. Laser separation and chemical detection require optical densities in the 3-4 range, while filters with OD of less than 2 are best for such applications as color sorting or spectral separations.

The Blocking Range tells you the wavelength range blocked by the filter; typically, the range where transmission is less than 50 percent.

Bandwidth describes the portion of the spectrum is transmitted through the filter. It is also called Full Width Half Maximum (FWHM), and describes the area where the filter achieves more than 50 percent transmission.

The Central Wavelength is the wavelength of the central point, or midpoint, of the FWHM. It may be the point of maximum transmission, especially for traditional coated optic filters.

Your choice of optical coatings will depend on the primary transmission range of your filter.  We have a wide range of optical coatings suited to different wavelengths.

Shanghai Optics filters are designed for a wide variety of applications, and we are able to customize as needed to fit your requirements.

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