What Does a UV Filter Do? How UV Cut-Off Glass Blocks Light Below 400 nm

A machine-vision station starts producing noisy images right after a lamp is replaced. The lens did not move, the focus did not shift, and the part is the same. What changed is that a narrow band of ultraviolet light around 365 nm now reaches the sensor. A 2 mm UV cut-off glass plate in front of the lens removes that band, and the contrast comes back. That is the entire job of a UV filter: it takes ultraviolet light out of a beam without taking much visible light with it.

The short answer is this: a UV filter blocks wavelengths below roughly 380 to 420 nm and passes the visible band from about 400 to 700 nm. Where exactly blocking turns into transmission depends on the glass composition and the thickness of the part. Two filters sold under the same name can behave very differently if one is 1 mm thick and the other is 3 mm.

People use the phrase "UV filter" for at least four different parts, and mixing them up causes most of the confusion in quotes and specifications:

• A camera lens UV filter, screwed onto the front thread of a lens.
• A UV cut-off glass plate, a bulk-absorbing glass part used inside instruments, sensors and lamp housings.
• A UV-blocking coating, an interference stack deposited on a substrate.
• UV-protective cover glass, glazing used on displays, artwork or equipment enclosures.

What a UV filter does: the short answer

A UV filter reduces the amount of ultraviolet radiation that travels through an optical path. Every practical benefit follows from that one behaviour: haze control in photography, sensor protection in machine vision, colour stability in museum lighting, and UV screening for anyone looking through the glass.

Four consequences matter when you are choosing a part:

• Blocking region. Below the cut-on wavelength, transmittance drops to a low value. The depth of that stop is given as a percentage of transmittance or as optical density, and it is always tied to a stated thickness.
• Passband. Above the cut-on, visible light passes. Typical cut-off glass grades reach high internal transmittance across the visible band, so brightness loss in a well-matched filter is small.
• Edge sharpness. A steep transition of a few tens of nanometres keeps colour rendering intact. A gentle slope pulls violet and deep blue out of the image and shifts white balance.
• Angle stability. Absorption inside the glass works the same at any angle of incidence, while a coating shifts its cut-on toward shorter wavelengths as the beam angle increases.

Absorption in the glass or reflection at the surface?

There are two ways to remove UV from a beam, and they fail in different ways. Knowing which one you are buying prevents most field problems.

Bulk-absorbing glass filters

Coloured optical glass contains dopants that absorb short wavelengths inside the material. The specification therefore follows the part: cut-on wavelength, transmittance in the passband and thickness. Because the filtering happens in the bulk rather than at a surface, performance does not change with the angle of the beam and does not disappear if the surface is cleaned or handled.

Interference coating filters

A dielectric stack reflects UV back toward the source and transmits the visible band. Coatings can produce very steep edges in a thin package, but the cut-on shifts with incidence angle, and the stack can be sensitive to humidity, abrasion and aggressive cleaning. In high-power lamp paths the reflected energy also has to go somewhere.

UV Cut‑off glass filterA UV cut-off glass filter is an optical material specifically designed to block ultraviolet (UV) light. It is widely used in various fields, including optical equipmen...View Product →

For most instrument, sensor and lighting work, absorbing glass is the more repeatable choice, because the curve is a property of the material rather than a surface condition that can be damaged in service.

The UV spectrum in numbers

Ultraviolet is not one thing. The band below 400 nm is divided into three ranges, and a filter that handles one of them well may barely touch another. Ordinary float glass already blocks most UVB and a share of UVA, which is why a dedicated cut-off grade is needed when UVA is the problem.

How to read a UV filter specification

A UV filter is described by a curve, not by a single number. When a supplier says only that a part "blocks UV", ask for the values below. They are the difference between a sample you can evaluate and a quote you cannot compare with anything else.

If the terminology is new to you, this primer on what a cut-off glass filter is walks through the same curve from a buyer's point of view.

Where UV filters earn their place in real equipment

The parts are inexpensive relative to the systems they protect, which is why they show up in so many different product lines.

• Machine vision and inspection. UV LEDs at 365 nm and 405 nm excite fluorescent adhesives and coatings. A cut-off plate keeps the excitation band out of the imaging channel so the fluorescent signal is not swamped.
• Photography. On film, a UV filter reduced blue atmospheric haze. On digital sensors, most UV filtering already happens at the sensor stack, so the practical benefit today is front-element protection and some flare control in bright, hazy conditions.
• Fluorescence microscopy. Excitation light must not leak into the emission channel; a UV cut-off element combined with a bandpass filter does that job.
• Lighting and displays. UV output from lamps degrades polymers, yellows diffusers and fades pigments. Filtering it at the source extends the life of everything downstream.
• Safety windows and curing shields. The glass removes the UV band while keeping the process visible to an operator.
• Architectural and museum glazing. Standard float glass blocks most UVB; a dedicated cut-off grade raises UVA protection for artworks, textiles and displays.

UV visible absorption glassUV-visible absorption glass is a type of optical glass specifically designed to absorb light at specific wavelengths within the ultraviolet (UV) and visible (VIS) ligh...View Product →

Selective grades also let you shape the transition: a gentler edge is sometimes preferred when the imaging system needs to keep a little violet for colour fidelity, while a hard edge is preferred when the UV source is strong.

Mistakes that cost money when buying UV filter glass

Most rejected shipments we see are not caused by bad glass. They are caused by a specification that was never pinned down.

1. Comparing transmittance values measured at different thicknesses. One millimetre and three millimetres of the same grade are effectively two different filters.
2. Assuming that "UV filter" means the same cut-on everywhere. Consumer photography filters, industrial cut-off glass and coated stacks sit in different places on the curve.
3. Over-specifying blocking depth. Going from a modest stop to a very deep one can multiply cost without changing what the system actually needs.
4. Forgetting where the absorbed energy goes. In a high-power lamp path, absorbed UV becomes heat in the glass; that heat has to be managed.
5. Ignoring the visible edge. A cut-on placed at 420 nm removes deep blue and violet, which changes colour rendering in photography, display and inspection work.
6. Putting a coating on an exposed outer surface where it will be cleaned and abraded.
7. Buying UV-transmitting optics when you actually need UV blocking, or the reverse. If the application genuinely needs deep UV to pass through, a UV-transmissive substrate such as Colorless quartz optical glassColorless Quartz Optical Glass is an optical material with excellent transparency, high temperature resistance, and superior optical properties. Composed primarily of ...View Product → is the right answer rather than a cut-off grade.

State the wavelength range, the required blocking level, the passband average and the thickness you can accommodate. With those four numbers, a supplier can match a grade instead of guessing.

Frequently asked questions about UV filters

What a UV filter does is simple, but the part you receive is defined entirely by the numbers you send. Tell us the wavelength range, the blocking level you need, the passband average and the thickness you can fit, and we will match a glass grade to that curve. You can send your requirements to our optical element team and receive a specification for review before ordering.