What is an Optical Bandpass Filter?

What is an Optical Bandpass Filter?

Optical bandpass filters are optical filters that pass one or more specified wavelength band(s) while blocking others. Bandpass filters are referred to by the wavelength range, also known as the passband, which they are designed to transmit. They are common filters suitable for a wide range of optical applications, including; environmental testing, colorimetry, flame photometry, fluorescence applications, UV sterilization, spectral radiometry, medical diagnostics, chemical analysis, machine vision, biotech instrumentation, medical devices, and laser line separation.

Optical bandpass filters are designed to transmit a well-defined band of energy in the electromagnetic spectrum. Andover offers one of the most extensive listings of standard "off-the-shelf" interference filters in this industry. We have a range of wavelengths range from the ultraviolet through the near-infrared and include many of the primary laser, mercury, biomedical and analytical spectral lines. Standard sizes include 12.5mm Ø, 25.0mm Ø, and 50.0mm Ø. All Andover filters are mounted in black anodized metal rings which provide an added measure of protection against chipping, scratching, and high humidity conditions. In addition, all filters have their part number permanently engraved on their edge and each filter is supplied with a calibrated spectral bandpass data curve at no-charge. Custom spectral data is available and will be quoted upon request. Shorter wavelengths, longer wavelengths, and custom shapes & sizes are available.

Bandpass Filter Fundamentals 

Bandpass filters are one of the simplest and most economical ways to transmit a well-defined band of light and to reject all other unwanted radiation. Their design is essentially a thin film Fabry-Perot Interferometer formed by vacuum deposition techniques and consists of two reflecting stacks, separated by an even-order spacer layer. Each one of these structures is referred to as a cavity, and some filters may contain as many as eight cavities. There are many different variations of the Fabry-Perot type bandpass filter, but for this discussion, we will only consider the all-dielectric and metal-dielectric types. 

The all-dielectric type consists of two highly reflecting mirrors separated by a dielectric spacer layer. These reflecting mirrors are constructed of alternating high and low refractive index materials and the reflectance of the stack is sometimes in excess of 99.99%. By varying the thickness of the spacer layer and or the number of reflecting layers, one can alter the central wavelength and bandwidth of the filter. This type of filter displays very high transmission in the passband, but, has a limited range of out-of-band blocking. To compensate for this deficiency, an additional blocking component is added, which is either all-dielectric or metal-dielectric depending upon the required blocking range. This additional blocking component will eliminate any unwanted out-of-band radiation but it will also reduce the overall throughput of the filter. 

The metal-dielectric type is similar to the all-dielectric type except that it utilizes a metal spacer layer instead of a dielectric layer. Although this type of filter has excellent out-of-band blocking and high passband transmission, it lacks the sharp cut-on and cut-off slopes of the typical two and three cavity filters. The metal-dielectric type is mainly used for bandpass filters in the ultraviolet. However, one version, the induced transmission type, is used as an additional blocking component when rejection is required to the far-infrared.

Our exceptionally high level of stock, and unique processing methods, allow us to ship most items within two days after receipt of an order. If required, rush orders can usually be shipped within one day. All out-of-stock items will be shipped within two weeks after receipt of a purchase order.