Bandpass optical filters: what are they?

There is a specific range (or ‘band’) of frequencies that optical bandpass filters transmit (or ‘pass’). Infrared and ultraviolet wavelengths are also included here, as well as visible light.

As a result, only the desired alluxa.com frequencies and wavelengths are output, while the unwanted parts of the spectrum are blocked.

Envin Scientific’s high-precision optical filters are capable of blocking and transmitting wavelengths between 175-200 nm and 8 microns.

Bandpass optical filters: how do they work?

By absorbing or reflecting unwanted wavelengths, bandpass filters only transmit those parts of the visible spectrum that are desirable.

There are several characteristics that can be used to define them:

• The effectiveness of blocking unwanted wavelengths is determined by the blocking level.
• The peak transmission is the amount of light transmitted from an incident source.
• Transmission profiles are characterised by a central wavelength.
• Transmission bandwidth between which 50% or more of the light is transmitted at full width at half maximum.

The high transmission across the entire FWHM range, with steep drop-off and very low transmission outside of that range, is an indication of an extremely effective filter with minimal ‘noise’.

What are the steps involved in making optical bandpass filters?

Several methods can be used to manufacture optical bandpass filters:

• A multilayer dielectric coating is vacuum deposited on a substrate to make thin film filters and coated filters.
• By laminating, cementing, and coating thin film layers, spectral and absorption filters are produced.

Combining filters and methods allows us to produce single laminated bandpass filters that have very specific and complex properties, suitable for a wide range of applications in electro-optics.

Envin Scientific’s optical thin-film filters range from 0.1 mm to several centimetres thick, and are available in sizes from 1 mm to over 100 mm.

You can order optical filters in circular, square/rectangular, or any other shape to suit your needs.

How do optical bandpass filters work?

There are countless applications for optical bandpass filters, depending on the wavelengths transmitted, such as:

• Coatings that reduce reflections and glare
• Analyses of chemicals
• Reflectors and dielectric mirrors
• Filters for fluorescent lights
• Applications of infrared technology
• Filters for lasers
• Transmission of only long wavelengths (longwave pass)
• Transmission is limited to short wavelengths (shortwave pass)
• Fibre optics and telecommunications
• Applications of ultraviolet (UV) light

For all kinds of optical applications, we welcome bespoke bandpass filter enquiries and can advise on the most suitable substrates, thin films, and other methods to produce the optical bandpass filters.

Bank of filters

Bandpass filters can be used in multi-scale analysis to generate profiles or surfaces filtered at different cut-off values. In this example, the spectrum is decomposed into individual bands using a “filter bank,” similar to a graphic equaliser for audio. Scales are defined by the number of bands per octave, where an octave is the interval between a wavelength and its double or half.

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With Mountains® 9.2, the new Bandpass-filter bank Operator decomposes a profile or surface into a series of bands that can then be analysed using surface texture parameters. By identifying bands with high correlations, active bands can be identified. When controlling a particular function on a workpiece, this can be helpful in identifying the scale range.