Image credit: Edmund Optics, Inc. | Thorlabs, Inc.
Long pass and short pass filters are two distinct types of specialized optical filters. Long pass filters transmit electromagnetic radiation with long wavelengths while blocking shorter wavelengths. Short pass filters do the opposite: they pass short wavelengths and block longer ones. Both types are sometimes grouped as edge filters (referring to the steep cut-on and cut-off between transmission and rejection) or barrier filters (due to the rejection of large sections of long or short wavelengths).
The two images below graphically represent the above discussion. Note the steep cut-on / cut-off wavelength of 750 and 600 nm, respectively, and the fact that each filter provides close to zero transmission for their blocked ranges and 100% for their passband.
Long pass (left) and short pass filter transmission.
Image credit: ZC & R Coatings | MIIC Technologies
Long pass and short pass filters are both used in a variety of distinct applications.
Both filters are used in precision spectroscopy applications as band separators. In photometry applications long and short pass filters are used as order sorting or blocking filters, which are applied to a detector's window to eliminate any second- and third-order distortion. Long and short pass filters are also frequently used in Raman spectroscopy.
Long pass filters are frequently employed in fluorescent spectroscopy. Fluorescence, by definition, is the emission of light by a substance that has absorbed electromagnetic radiation; the specific light wavelengths absorbed are termed excitation light. Because fluorescence emitting from an object typically has higher wavelengths than the excitation light that caused it, long pass filters can be used to pass only precise fluorescence wavelengths while blocking leakage from the excitation lamp.
While long pass and short pass filters can be used independently for different applications, they are often used in conjunction to filter a narrow section of light. This is clearly visible in graphs (b) and (c) in the figure below. Note that in graph (c), when a long and short pass filter are aligned to filter the same light source, the sharp cut-offs allow the combination of filters to function as a more precise bandpass filter (graph [b]).
Image credit: Olympus Microscopy
When selecting long pass and short pass filters, it is important to consider three different specifications related to the product's wavelength ranges: cut-on / cut-off wavelength, transmission range, and rejection range.
A filter's cut-on / cut-off wavelength is typically represented as a single wavelength, such as "1000 nm." Barrier filters are also commonly referred to by their cut-on wavelength; for example, a "1000 nm long pass filter" cuts off at 1000 nm and passes all wavelengths higher than this.
Transmission and rejection ranges are closely related to cut-on wavelength and can be determined by considering the filter type along with the cut-on wavelength. For example, a short pass filter designed to operate in the visible spectrum with a cut-on wavelength of 600 nm, the filter would transmit wavelengths from ~390 to 600 nm and block those from 600 to ~700 nm. Therefore, if we know the type of filter and the cut-on wavelength, we can easily determine both the transmission and rejection ranges.
Optical Surface Quality
Optical filter surface quality is specified using MIL-0-13830A standards published in 1963. These scratch/dig ratings consist of two numbers separated by a hyphen, as in x - y; x refers to scratches, while y refers to digs.
A scratch is a surface defect whose length is many times its width. The initial number of the scratch-dig rating describes the maximum allowable width, expressed in tenths of microns, of a scratch.
A dig refers to a defect that is nearly equal in length and width. The second value of the rating refers to the maximum diameter of a dig expressed in hundredths of a millimeter.
For example, a #20-10 indicates that any scratches have a maximum width of .002 mm (or 2 microns), while digs may have a maximum diameter of .10 mm. When considering the two rating values, it becomes clear that smaller values are desirable.