What an interference filter is
An interference filter is an optical element built from thin coatings that select light by wavelength. Light that enters the coating stack is partly transmitted and partly reflected at each layer boundary, and the layers are arranged so that wanted wavelengths add up in transmission while unwanted wavelengths cancel or are reflected away. The result is a filter that passes some wavelengths and rejects others, with a transmission curve that shows a pass region, a transition, and a blocked region. That curve is the basic language of interference filters, and learning to read it, even in general form, makes every later selection decision easier. The interference filter guide explains the same concepts as a buyer's guide, and the interference filters overview maps the whole family on this site.
The family on this site
The interference filter family on this site includes narrow band pass filters, fluorescence filters, high reflection filters, beam splitting filters, color splitting filters, IR sensor filters, short pass filters, neutral density filters, and the special filters category name, together with CCD gel imaging and IPL beauty machine filter categories. Each member does a different optical job, and the job is the right way to tell them apart. Narrow band pass filters pass a narrow band and reject on both sides of it. Fluorescence filters separate excitation light from emission light in fluorescence systems. High reflection filters return a large share of light in a working region. Beam splitting filters combine light of two or more wavelengths into the same optical path, acting as semi-transmitting mirrors. Color splitting filters separate light by colour into different paths. IR sensor filters select the infrared band a sensor must see. Short pass filters reflect the short wave region and transmit the long wave region. Neutral density filters reduce light intensity evenly across a range. Special filters appears only as a category name from the listed production lines, without an application description on this site.
Transmission, reflection, and blocking
Three concepts describe what a filter does with light. Transmission is how much light in a wanted band passes through. Reflection is how much light is sent back. Blocking is how strongly light outside the wanted band is kept out, often expressed as an optical density figure. A good filter description states all three where they matter, with the wavelength region each one applies to. A description that states only transmission leaves the buyer guessing about rejection, and a description that states only a centre wavelength leaves the buyer guessing about the band's width and edges. The slope between a pass region and a blocked region matters too: a steep slope keeps the two apart cleanly, while a shallow slope blurs the boundary and loses signal at the edges. For fluorescence filters, blocking and slope are documented explicitly, because fluorescence readings measure weak emission against strong excitation. For other lines, transmission, reflection, and blocking remain inquiry items until a deep specification page documents them.
Centre wavelength, bandwidth, and cut-off
Centre wavelength is the centre of the band a filter passes, or the reference point for a cut-off edge. Bandwidth is the width of the band that passes. Cut-off depth is how completely light outside the wanted band is rejected. Cut-off edge is the wavelength region where transmission changes from passing to blocking. Those four items, together with size and substrate material, are the standard inquiry list for interference filters, and they appear on every line page on this site. They appear as inquiry items rather than values for most lines, because documented deep specifications exist only for fluorescence filters and neutral density filters. That distinction is important for a beginner: an inquiry list is not a specification table, and a value that appears on one product page does not transfer to another product. The fluorescence blocking figures, for example, belong to fluorescence filters only and must not be read as general interference filter specifications.
Coatings and equipment, in plain terms
Interference filters are made by coating: thin layers are deposited on a substrate in a controlled stack, and the stack's design decides the filter's behaviour. Equipment names listed by the company are Optorun OTFC 1300 and Leybold Syrus 1350 automatically operated coaters, and Cary 5000 and Cary 7000 high performance spectrophotometers. Those are equipment names only. No photograph on this site is presented as a photograph of that equipment, and coating is explained at category level in the optical coatings guide, except for the documented IAD hard coating statement for fluorescence filters. A beginner does not need to master coating design to buy a filter, but knowing that coatings create the behaviour helps explain why a filter cannot be described by a single wavelength number: the whole stack, and the whole curve, decide the result.
How to describe what you need
Describe the optical job first: what must pass, what must be blocked, what must be split or combined, or what must be attenuated, and in which wavelength region. Then list the inquiry items: centre wavelength or wavelength range, bandwidth or pass band behaviour, cut-off depth where it applies, size and shape, and substrate material. Then describe the system the filter will sit in, including the instrument type and, for fluorescence work, the dye or probe name. Then state the quantity basis and the delivery basis you need confirmed. That description is checkable, and a supplier can review it against the listed lines. A description that names only a product category, without a job or a wavelength region, cannot be checked, and it will only generate further questions. Please confirm the specification, quantity basis, and delivery basis in your inquiry before ordering.
Pass bands, edges, and curves
Every interference filter can be described by its curve, and every curve has the same anatomy: a pass region where transmission is high, a blocked region where transmission is low, and a transition between them. Narrow band pass filters have a pass region bounded on both sides by blocked regions. Short pass filters have a pass region on one side of an edge and a blocked region on the other. High reflection filters have a reflection region rather than a transmission region, but the anatomy is the same in mirror form: a working region, a transition, and the rest. Learning that anatomy helps a buyer ask better questions, because most selection confusion comes from describing only one part of the curve. A buyer who states only a centre wavelength has described a point, not a curve. A buyer who states a centre wavelength, a bandwidth, and a cut-off depth has described the curve well enough to be checked.
Curves also explain why the same product family can serve very different instruments. A fluorescence system, a spectrometer, a sensor, and a camera all use curves, but they use different parts of the curve and they tolerate different transitions. A fluorescence system tolerates almost no excitation leakage, so it demands deep blocking and a steep slope. A spectrometer tolerates a gentler transition if the band is well placed, because its measurement reads within the band. A sensor system tolerates a broader band if the sensor's own response is selective. Those tolerances are application facts, not filter facts, which is why an inquiry should state the application alongside the curve requirement. The curve belongs to the filter; the tolerance belongs to the system; and a good inquiry states both, so that the supplier can judge fit rather than guess it.