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Filters & Dichroics

Precision Wavelength Selectivity

What are Filters & Dichroics?

Optical Filters and Dichroics are components which are used to transmit or block specific wavelengths of light, for the purpose of controlling light in an optical system.

Optical filters and dichroics operate based on thin-film interference. Multiple layers of dielectric materials with varying refractive indices are deposited on to a substrate using evaporation or sputtering. The constructive and destructive interference of light waves at these layers results in selective reflection and transmission of different wavelengths.

Dichroic filters, also known as dichroic mirrors, are specialized optical filters that selectively transmit light of certain wavelengths while reflecting others. They are designed to work at specific angles of incidence, typically at 45 degrees.

Types of Filters & Dichroics

Bandpass (BP)

Bandpass filters (also called band pass filters, BP filters or band-pass filters) have been a mainstay of Omega's business since the early days when we produced some of the first bandpass filters for supermarket scanners back in the mid-1970s. They are designed to have high transmission over a certain wavelength range while blocking light from other wavelengths to provide good signal-to-noise in the transmitting region.

Longpass (LP)

Longpass filters (sometimes called edge filters, long pass, long-pass, or LP) transmit wavelengths above (LP) a certain wavelength. This wavelength is called a cuton wavelength for longpass filters. LP filters can be used instead of bandpass filters in fluorescence applications to maximize signal when using a single fluorescent label, or to block the laser in Raman applications.

Shortpass (SP)

Shortpass filters (sometimes called edge filters, short pass, Short-pass, or SP) transmit wavelengths below (SP) a certain wavelength. This wavelength is called a cutoff wavelength for shortpass filters. SP filters are often used in digital cameras to block undesired near-infrared (NIR) signals from reaching the camera. These types of filters are often called hot mirrors.

Multi-band

Multi-band filters and dichroics are used when an optical system design cannot support multiple filters. These multi-band filters can be configured as bandpass or notch (band-reject) for desired ranges, and in dual, triple, quad, penta or even octo-band (8 bandpasses!).

Narrowband (Notch)

Omega routinely produces filters which selectively transmit or reject wavelength bands of less than 1nm and even less than 0.1nm

Neutral Density

Reflective ND filters provide a linear spectral transmission across the visible region. They control light intensity without selectively absorbing energies at specific wavelengths. They are, therefore, generally used to attenuate the intensity of a beam of light over a broad spectral region. This property is often required to prevent saturation or damage to a wide variety of detectors and even the human eye.

Linear Variable

Linear variable filters are bandpass filters that change center wavelength across one dimension of the part. They can be used as simple spectrometers or in order-sorting applications. These elements are very useful for Hyperspectral Imagers

Filters on Fiber Tips

In applications which demand spectral selectivity on the entrance or exit of a fiber, Omega has a solution to deposit one or more filter recipes to a fiber (or fiber bundle).

Patterned filters are coated onto a single substrate using shadow masks (cheapest) or lithography techniques

Patterned / Multizone

Patterned filters are useful in applications where lateral space constraints require discrete wavelength selection. This is often the case with Earth Observation filters.

Absorption

Omega supplies, and designs custom filters using absorption color glass.

Cold Mirror

A specialized version of a long-pass filter, typically designed to reflect the ultraviolet (UV) or visible (VIS) portion of the spectrum while passing longer infrared (IR) wavelengths.

Hot Mirror

A specialized version of a short-pass filter, typically designed to transmit the ultraviolet (UV) or visible (VIS) portion of the spectrum while reflecting longer infrared (IR) wavelengths.

Ultraviolet Filters

Omega has been manufacturing UV filters for more than 85 years. UV light (in the 200 nm - 400 nm range) has many commercial and consumer-level applications. UV light can be used in photolithography or dentistry to photoset polymers, and in biomedical applications to excite molecules during surgery. It can be used to measure DNA and to sterilize our water and the air we breathe.

Infrared Filters

Omega Optical has the capability to produce traditional Bandpass and Notch filters in the near-infrared (NIR), shortwave (SWIR), mid-wave (MWIR), long-wave (LWIR), and far infrared (FIR).

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Questions About optical filters & dichroic

Which Optical Specifications should you consider?

Transmission & Optical Density- the steeper the edge between transmission and reflection, the thicker the film and the higher the cost. Omega defines measures edge steepness from the 50% transmission wavelength to the OD5 wavelength. This transition is often measured in wavenumbers (cm-1). Most of our edge steepnesses are within 1-5% of the 50% transmission wavelength (200-1000 cm-1 at 500 nm).
Out-of-band blocking/ Signal-to-Noise Ratio- Blocking of wavelengths over a very large range (i.e. all visible wavelengths) can increase costs. For an LED or laser-cleanup filter where the wavelengths to be filtered are close to the peak wavelength, a large blocking wavelength range is not usually necessary. Deep blocking (>OD 6) in a narrow wavelength range (< 20 nm), as in a rejection (notch) filter, can also increase costs.
Full-Width at Half-max (FWHM)- FWHM is directly proportional to the center wavelength- narrower filters are more difficult to achieve at longer wavelengths. We often specify FWHM as a percentage of the center wavelength.
Angle of Incidence (AOI) & Polarization – When a filter or dichroic is used at an angle, the two polarizations are affected differently. This difference can be minimized or enhanced during the design process.
F# or numerical aperture (NA)- A filter can be designed to work at a range of incoming angles (around the AOI), but the larger the range, the more expensive the filter will be. It is easiest to design and manufacture filters that work in a narrow range of angles (typically +5 degrees)
Image Quality (total transmitted/ reflected wavefront distortion) and parallelism- thin substrates are often warped by the coating process or by mounting if they are large. We can help you determine what thickness is appropriate for the filter size you are using.
Surface Quality- tight scratch/dig tolerances (like E/E) reduces yield, requires extra inspection time and increases cost. Our standard surface quality specifications are F/F (MIL C-48497A)