What a coating does to light
A coating changes what happens to light at a surface. Uncoated glass reflects part of the light that hits it and transmits the rest, in proportions decided by the glass and the angle. A coated surface changes those proportions deliberately: a thin layer, or a stack of thin layers, can increase transmission in a wanted band, increase reflection in a working region, or block light outside a wanted band. Interference filters are built from such stacks, which is why coating behaviour decides filter behaviour. Understanding coatings at that conceptual level, as deliberate control of transmission, reflection, and blocking, is enough to buy filters wisely, even without mastering coating design. The optical coatings guide covers the same concepts as a buyer's guide.
Transmission, reflection, and blocking, again
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. Slope is how steeply transmission changes between a pass region and a blocked region. Cut-off depth is how completely a blocked region is suppressed. Those five concepts describe almost every coating requirement a buyer will meet, and naming the concept that matters for your application, with the wavelength region it applies to, makes an inquiry far more checkable than asking for a coating in general terms. A fluorescence application cares deeply about blocking and slope, because weak emission must be read against strong excitation. An attenuation application cares about even reduction across a range. A reflector application cares about reflection in its working region. Start from the concept your application cares about, and the coating requirement follows.
The one documented coating on this site
Documented coating evidence on this site exists for one product only. For fluorescence filters, the documented coating process is IAD hard coating, presented on the fluorescence filters page together with blocking figures, a slope figure, excitation and emission examples, dye and probe names, and sizes. That statement belongs to fluorescence filters only. It is not a general coating specification, and it must not be read as one. For every other line, coating is described at category level, and buyers should confirm the coating requirement for their product in the inquiry. That restraint is not a gap in the guide; it is the guide's honesty about what is documented and what is not. A coating claim that is not documented for a product would mislead a buyer into ordering against an unconfirmed behaviour.
Equipment names and photographs
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 names describe the coating and measurement equipment the company lists. They are names only. No photograph on this site is presented as a photograph of that equipment, no factory, workshop, or coating machine photograph is published, and no coating photograph is presented as a record of a specific production run. Buyers who need equipment or process evidence beyond a name should ask for it in the inquiry and judge the reply on its own terms, rather than reading a name as evidence of a process.
Describing a coating requirement
Describe the coating job your element must do: transmit, reflect, or block, in which wavelength region, and with what behaviour at the boundaries. State the transmission, reflection, or blocking behaviour you need in that region, the size and substrate material, and the system the element will sit in. Then state the quantity basis and the delivery basis you need confirmed. That description lets the supplier check the coating requirement against what can be made, without either side assuming a coating behaviour that was never stated. The interference filter guide explains how coating concepts connect to filter selection, and the interference filters overview maps the filter family that coatings create. Please confirm the specification, quantity basis, and delivery basis in your inquiry before ordering.
Coating stacks and why layers matter
A single thin layer changes light behaviour a little; a stack of many layers changes it a lot, and with far more control. Each layer in a stack contributes its own reflection and transmission, and the layers interact, so that the stack as a whole can create a sharp pass band, a deep blocked region, or a steep edge that no single layer could produce alone. That is why interference filters are stacks rather than single coatings, and why their design is a matter of choosing layer materials, thicknesses, and counts together. A buyer does not need to design the stack, but knowing that behaviour comes from a stack helps explain two practical facts. First, a small change in the requirement, such as a shifted band or a deeper block, may need a different stack, not just a thicker single layer. Second, a coating requirement stated only as a material name is incomplete, because the stack, not the material alone, decides the behaviour. State the behaviour you need, in the region you need it, and let the stack follow from the behaviour.
Stacks also explain why angle and uniformity matter for coatings. Light arriving at an angle travels a different path through the layers than light arriving straight on, so the stack's behaviour shifts with angle. A coating specified for straight-on use will behave differently at an angle, and a coating used over a large area must behave evenly across that area, or different parts of the beam will see different coatings. Those are not defects in a particular coating; they are properties of stacks in general. An inquiry that states the angle of use, the beam size, and the area over which behaviour must hold gives the supplier the information needed to judge whether a stack can meet the requirement. An inquiry that omits those items asks the supplier to assume them, and assumptions about angle and area are a common source of later disagreement.
Coating language for inquiries
Use coating language precisely in an inquiry, and avoid language that sounds precise but is not. Transmission, reflection, blocking, slope, and cut-off depth are precise concepts when attached to a wavelength region; without a region, they are only words. A statement such as high transmission is not a requirement until it states transmission of what, in which band, and to what level. A statement such as deep blocking is not a requirement until it states blocking of what, in which region, and to what optical density. The discipline is simple: concept plus region plus level, wherever a level can be stated, and concept plus region plus a request for confirmation where a level cannot yet be stated. That discipline turns coating talk into a checkable requirement, and it is the same discipline that runs through filter selection, custom ordering, and every other page on this site.