Optical filters are used across a variety of sciences including electronics, acoustics, mechanics, chemistry and optics. Broadly defined, an optical filter is a device that allows only certain portions of a signal to pass through a device while preventing unwanted portions of the signal from passing by either absorbing, destroying or reflecting the unwanted signal.
In optics, optical filters transmit specific light wavelengths or ranges of light wavelengths. They are passive devices and the non-transmitting light wavelengths are either absorbed or rejected by the filter using the reflection and interference.
Optical filters tune light signals which makes them ideal for a variety of applications. Optical filters are used in stage lighting, photography, videography, communications, instrumentation, and life sciences. Precision optical filters are used in spectrometry, microscopy, laser systems and machine vision.
There are four basic types of optical filters:
- Bandpass filter: Transmits light wavelengths between a lower and upper threshold while blocking light wavelengths below the lower threshold and above the upper threshold.
- Notch filter: An inversion of a bandpass filter where light wavelengths between the upper and lower threshold are blocked and all other light wavelengths are transmitted.
- Long pass or long wavepass filter: Transmits light wavelengths above a cut-on threshold.
- Short pass or short wavepass filter: Transmits light wavelengths below a cut-off threshold.
Long pass and short pass filters are often referred to as edge filters. It should also be noted that bandpass or notch optical filtering can be accomplished by combining a short pass and a long pass filter in series. This article focuses on short wavepass filters.
Characteristics of Short Wavepass Filters
Short pass filters are useful for isolating shorter wavelengths. For example, an infrared (IR) filter might be used to filter out all light wavelengths above 700nm, which are in the infrared region. These are sometimes called heat-absorbing or heat-rejecting filters. One use of IR filters is in imaging devices to protect sensors that are sensitive to infrared light.
Figure 1 shows the transmission curve* for an 800nm short wavepass filter. The filter absorbs infrared light above 800nm, above the near-infrared range, and may be used as an excitation filter for applications such as fluorescence microscopy where a fluorescent dye is excited at 800nm or shorter wavelengths.
*A transmission curve is a graph of a mathematical function that characterizes optical filters expressed as the percent or fraction of light transmitted versus wavelength.
Edge filters are named according to the wavelength where the filter transitions from transmitted light to blocked light. For a high performing filter, this transition is sharp and steep. For a short pass filter the cut-off wavelength is the wavelength where transmission drops to 50%, which is ~800nm for the filter depicted in Figure 1.
Peak transmittance is the maximum transmission and highly efficient filters exhibit close to 100% peak transmission. Peak transmittance is affected by the filter’s substrate material, construction, and surface finish. Polymer filters tend to have lower peak transmittance than glass filters and absorptive filters have lower transmittance than interference filters.
Average transmission, the bounds between peak transmission and the lowest transmittance over the transmitted range, is also used to characterize short pass filters. This value is important for applications with a minimum acceptable transmission requirement.
Design of a Short Wavepass Filter
Fabrication of optical filters falls into two broad categories: hard coated and traditional coated. Hard coated optical filters have a high-performance optical coating on a substrate while traditional coated optical filters use a laminated stack of coatings—interference, absorbing or metallic—between two layers of substrate.
Traditional coated optical filters are made by depositing thin films on the surface of the substrate using deposition methods such as physical vapor deposition. Substrate shape and thickness are selected to meet application requirements, but the choice of substrate also affects the performance of the filter. A secondary substrate is epoxied onto the thin film to protect the layers.
The absorptive or reflective properties of each subsequent layer determines the filters transmission and blocking range. Figure 2 shows the construction of a typical short pass traditional optical filter that may be characterized as a 500nm filter.
While the construction of traditional coated optical filters is more complex, they tend to have lower peak transmittance and higher environmental sensitivity than similar hard coated filters. However, traditional coated optical filters are more cost effective.
In addition, the performance of an optical filter relies heavily on the surface quality of the filter as characterized by the standard MIL-0-13830A, which specifies scratch/dig ratings. Surface defects with a length many times its width (in tenths of microns) is a scratch. A dig is a defect with length and width nearly equal specified as the maximum diameter in hundredths of a millimeter. The smaller the scratch/dig the better the filter performs.
Absorptive vs Dichroic Filters
Optical filters are either absorptive or dichroic. Both transmit certain light wavelengths while blocking others, but differ on how they accomplish this.
Absorptive filters contain organic or inorganic additives in the optical material that absorb and attenuate certain wavelengths while allowing others to transmit. An absorptive Infrared (IR) filter absorbs IR wavelengths to reduce heat in optical devices and systems. These filters look like blue tinted glass due to additives such as flouro-phosphate. While the performance of all filters is dependent on the angle of incidence (AOI), absorptive filters are less sensitive to it. Absorptive filters are also easier to manufacture.
Dichroic filters utilize thin films to block specific wavelengths from transmitting through the material by reflecting certain light wavelengths off the internal interfaces between layers. The rejected light wavelengths are reflected and dissipated through destructive interference.
For dichroic filters, layers of dielectric materials (such as refractory oxides) of specified thickness and various refractive indices are applied to the surface of an optical material. The dielectric films are either hard sputtered via a magnetron (sputtering) or chemically deposited (chemical vapor deposition aka CVD) on the surface of the optical material.
Dichroic filters are highly dependent on the angle of incidence of incoming light with the transmission curve shifting left with increasing angle of incidence.
Applications of a Short Wavepass Filter
Short wave pass filters are used in a variety of applications in life sciences, medical diagnostics, environmental testing, photography, imaging. They are suitable for applications where color separation of input light is required such as fluorescence microscopy as excitation filters.
Short pass filters can also be used for optical sorting and identification applications. For example, a short pass filter can be used for a simple pass/fail device to sort items, such as pharmaceuticals or chemicals, according to the color of light emitted through or reflected off the surface of an item. Similarly, short pass filters may be used in an optical switch activated by wavelengths below the cutoff wavelength.
A variety of filters can be used to fine tune the performance of optical systems. Short pass filters are often used in combination with long pass filters to create custom bandwidth filters that operate between the long pass filter’s cut-on wavelength and the short pass filter’s cut-off wavelength. This technique offers more flexibility in tuning the bandpass performance.
Optometrics offers a wide range of short wavepass filters that are traditionally coated (laminated PVD) absorptive optical filters with a scratch/dig surface quality of 80/50. They are ideal for life science applications and in optical instrumentation. Typical substrates include quartz, fused silica, soda lime float glass, borofloat or borosilicate glass, BK-7, and about 20 different kinds of colored glasses. Polished “flat” glasses are used in imaging applications. Infrared applications use specialized materials such as Germanium (Ge), Sapphire, Zinc Sulfide (ZnS), Zinc Selenide (ZnSe) and Chalcogenides among others.
Standard and Custom Short Wavepass Filter
In addition to offering a standard assortment of short wavepass filters in a variety of cut-off wavelengths from 450nm to 1,000nm, Optometrics can also help you design and manufacture a custom short wavepass filter. It manufactures 500,000+ precision replicated optics each year. So, whether you need a few or tens of thousands to support your high-volume requirements please get in touch!