A shortpass filter is a type of optical filter that transmits light with wavelengths shorter than a certain cut-off wavelength, and blocks or reflects light with longer wavelengths.
Shortpass filters are useful for applications that require selective filtering of light sources, such as fluorescence microscopy, spectroscopy, laser systems, machine vision, and photography.
Shortpass filters can be made of different materials and techniques, such as colored glass, interference coatings, or dichroic mirrors.
Comparison of different type of filters
Key Specifications of Shortpass Filter
The most important specification of a shortpass filter is the cut-off wavelength, which defines the boundary between the transmission and blocking regions of the filter.
The transmission region is the range of wavelengths that are passed by the filter with high efficiency, typically above 80% or 90%.
The blocking region is the range of wavelengths that are rejected by the filter with high optical density (OD), typically above 2 or 3.
The slope or transition region is the range of wavelengths where the filter changes from transmission to blocking, or vice versa. The slope is usually measured by the bandwidth between the 10% and 90% transmission points, or the 50% cut-on and cut-off points.
Other specifications that may affect the performance and quality of a shortpass filter include surface quality, wavefront distortion, angle of incidence, temperature dependence, and durability.
OD and FWHM of a shortpass filter
OD stands for optical density, which is a measure of how much light is attenuated by a filter. It is defined as the negative logarithm of the transmittance ratio
A higher OD means a lower transmittance and a better blocking ability. For example, an OD of 2 means that only 1% of the incident light is transmitted, while an OD of 4 means that only 0.01% is transmitted.
FWHM stands for full width at half maximum, which is a measure of the bandwidth or width of a spectral curve at half of its peak value. It is commonly used to describe the spectral bandwidth of a bandpass filter or a light source.
For a shortpass filter, FWHM can also be used to describe the slope or transition region of the filter, as shown in Figure 1.
A smaller FWHM means a steeper slope and a sharper transition between transmission and blocking. For example, a FWHM of 10 nm means that the filter changes from 50% to 10% transmission within a 10 nm range.
OD and FWHM are important parameters that affect the performance of a shortpass filter. A high OD ensures that unwanted light is effectively blocked, while a small FWHM ensures that desired light is efficiently transmitted with minimal loss or distortion.
Performance and Standard Filters Difference
A high performance shortpass filter is designed to have a high transmission in the passband, a high OD in the out-of-band, and a small FWHM in the slope region. It also has low reflection, low wavefront distortion, and high temperature stability.
An example of Performance shortpass filter
Cut-off wavelength: 1500 nm
Transmission: ≥91% from 400 to 1450 nm
OD: ≥4 from 1550 to 2200 nm
FWHM: <1%
Surface quality: 40-20 scratch-dig
Wavefront distortion: λ/4 at 633 nm
Angle of incidence: 0°
A standard shortpass filter is designed to have a moderate transmission in the passband, a moderate OD in the out-of-band, and a moderate FWHM in the slope region. It has lower requirements for reflection, wavefront distortion, and temperature stability than a high performance filter.
