Introduction to Edge Filters

What is an Edge optical filter?

Edge filters are made up of two specific types:

Short-pass edge filters transmit wavelengths BELOW a certain wavelength, while blocking higher wavelengths.

Long-pass edge filters transmit wavelengths ABOVE a certain wavelength, while blocking lower wavelengths.

There is another special case of edge filter, where the filter is designed for use an angle (usually 45°) – both the transmitted and reflected (blocked) beams are used in the optical design. This type of filter is called a dichroic beamsplitter.

Optical Edge Filter Design and Specifications

Edge location and type (Short-pass or Long-pass): IDEX Health & Science | Semrock offers three types of edge filters. EdgeBasic filters are identified by their 50% transmission wavelength. Their edge steepness is targeted to be 1.5% of this wavelength. RazorEdge and Verona edge filters are typically used with laser sources and are specified with respect to the laser wavelength. The edge steepness of the RazorEdge and Verona filters are 0.5% and 0.2% of the laser wavelength, respectively.

Average transmission: The percentage of light, %T, that passes through the filter averaged over a given wavelength range.

Transition width and edge steepness: These values describe the spectral width from 50% transmission to deep blocking (OD 6). Transition width is the maximum allowed spectral width between the laser line (where OD > 6) and the 50% transmission wavelength, while edge steepness is the actual steepness of a filter, measured from the highest wavelength with OD 6 to the 50% transmission wavelength. These can be specified in wavelength (e.g. 6 nm), percent of the laser wavelength (e.g. 1% of 633 nm), or in wavenumbers (cm^-1). Measuring very steep edges is difficult or impossible with off-the-shelf spectrometers. Our team has designed advanced spectral measurement tools to address this issue. Our advanced KolaDeep and Peregrine SMS measurement systems are described here.

Wavelength to wavenumber conversion note: Wavenumbers are used in Raman spectroscopy to describe the shift of the non-elastically scattered signal from the laser wavelength. Learn more. The lower the wavenumber, the closer to the laser line. Wavenumber (△w) can be calculated by

Here’s an online calculator

Blocking: Measured in units of Optical Density (OD), defined as:

OD = – log ₁₀ (T)

OD_abs designates a minimum blocking requirement over a given wavelength range, while OD_avg designates minimum average blocking over a given wavelength range. Both average and absolute blocking can be specified.

How are Edge optical filters used?

In fluorescence microscopes and other fluorescence imaging systems, like next-generation gene sequencing, dichroic beamsplitter edge filters allow the illumination and detection paths to overlap and separate in different regions of the instrument (Figure 2). This sort of geometry maximizes signal to noise ratios increasing the sensitivity and detection limit of these systems.

Figure 2. The dichroic beamsplitter is used to combine and separate the illumination and detection paths in a fluorescence system.

Microplate readers are commonly used as a clinical diagnostic tool in hospital labs. They are used to detect levels of proteins and other analytes in bodily fluids (blood, urine, etc.) PCR uses fluorescence to detect the presence of DNA that matches a specific probe - it was used widely to detect COVID-19 during the pandemic. Many microplate readers and PCR machines are based on the radiometric measurement of total fluorescence present in a sample well or tube. Long-pass edge filters can be used to reject the excitation light and detect all the emission light over the detector’s range.

VersaChrome Tunable Edge Filters

IDEX Health & Science | Semrock utilizes the angle-dependent blue-shift of our edge filters to offer tunable edge filters. This type of filter is invaluable in instrument prototyping when trying to determine spectral filter tolerances prior to large-quantity orders. The edge wavelength can be lowered up to 12% by increasing the AOI in the beam, while maintaining high edge steepness and transmission. These types of edge filters are also useful in laser applications where the laser wavelength is slightly off of the specification or if the wavelength drifts slightly over time due to thermal or other effects. VersaChrome tunable edge filters allow the user to match the edge wavelength to the laser without changing optics.

Figure 3. A VersaChrome Edge filter with an edge at 628 nm at normal incidence (orange) and at 561 nm (60° – blue) View in SearchLight.

Short-pass and Long-pass VersaChrome tunable edge filters can also be combined to create custom and tunable bandpass filters- another useful prototyping tool. Learn more about using VersaChrome Tunable Edge filters.

Learn More About Our Edge Optical Filter Product Families