Schott KG3 & KG5 Heat Absorbing Filters

Schott KG filter glass

Schott KG Heat Absorbing Filters, Cut to Size in Rye

Kingsview Optical makes custom heat absorbing filters from Schott KG1, KG2, KG3 and KG5 glass. We CNC shape them to your drawing, polish them on our own lines and add AR or gold mirror coatings in our own chambers, so you get one finished part from one UK supplier, from a single prototype to production batches.

Cut and polished in Rye
KG1, KG2, KG3 and KG5
Coated in-house
ISO 9001:2015

Send Your Filter Drawing See the Specification

Schott KG5 heat absorbing filters with gold mirror coating, made by Kingsview Optical — KG5 filters with a gold mirror coating, made in Rye.

Filters made here, start to finish. CNC shaped, polished, coated and measured at our site in Rye.

See all our optical filters →

The basics

What a Heat Absorbing Filter Does

In short: a heat absorbing filter is a piece of glass that passes visible light and absorbs near-infrared. Put one in front of a lamp or LED and most of the heat stays in the filter, while the light carries on into your optics.

Schott KG glasses are coloured right through, so the filtering doesn't depend on a coating and doesn't change with angle of incidence. KG1 absorbs the least infrared and KG5 the most. KG3 is the usual choice, balancing high visible transmission with strong near-infrared absorption.

Because the filter absorbs heat, it gets hot. Large or thick filters in high-power systems can crack from thermal stress if they aren't prepared for it. We can thermally toughen KG filters on site, subject to part size, and design the mount to let the glass expand.

For systems with a lot of infrared, a reflective coating can send some of the heat back before the glass has to absorb it. We can add a gold mirror coating, or pair the KG glass with a sputtered shortpass coating made in-house.

KG glass passes visible light and absorbs infrared heat.

Internal transmittance of Schott KG1, KG2, KG3 and KG5 heat absorbing filter glass at 2mm thick — Internal transmittance of Schott KG1, KG2, KG3 and KG5 at 2mm thick, plotted from Schott data. Thicker glass absorbs more.

Choosing a grade

Which KG Glass Do You Need?

All four grades pass visible light. They differ in how much near-infrared they absorb, and in how much light they let through in the 1.5 to 2.7µm band. The chart above is for 2mm thickness; thickness changes the result, so tell us your heat load and we'll suggest a grade and thickness.

Glass	What it does	Typical use
KG1	The lightest absorption. Passes the most visible light, with some transmission between 1.5 and 2.7µm.	Moderate heat loads where maximum brightness matters
KG2	Light absorption, with the highest transmission of the four in the 1.5 to 2.7µm band.	Where some longer infrared can be tolerated
KG3	Strong near-infrared absorption with high visible transmission. The most widely used grade.	Projectors, illumination optics, IR-cut filters for cameras and sensors
KG5	The strongest absorption of the four, with slightly lower visible transmission.	High heat loads and the strongest IR blocking

Managing the heatAn absorbing filter warms up. Thermal toughening, a mount with room for expansion and some airflow all help large KG filters survive high-power sources.

Reflect before you absorbA gold mirror or sputtered shortpass coating reflects part of the infrared, so the glass has less heat to absorb and runs cooler.

Applications

Where Our KG Filters Are Used

Illumination systemsKeeping heat from lamps and high-power LEDs out of the optical path.

ProjectorsProtecting LCDs, films and slides from infrared heat.

Cameras and sensorsBlocking near-infrared so silicon sensors see a visible-light response.

Medical and dental lightsCooler light on patients and tissue.

Sealed instrumentsRemoving heat without vents or fans, so the housing stays sealed.

Machine visionIR-cut filtering in front of inspection cameras.

Specification

KG Heat Absorbing Filter Specification

These are our standard tolerances. If your design needs tighter, tell us which parameter. We'll tell you plainly whether we can hold it, and what it adds to cost.

Size range	0.8mm to 100mm
Dimension tolerance	±0.1mm
Thickness tolerance	±0.05mm
Parallelism (twin lap)	Better than 10 arcsec
Surface quality (scratch-dig)	60/40 standard, 40/20 on request
Glass	Schott KG1, KG2, KG3 and KG5; Hoya equivalents on request
Shapes	Round, rectangular and custom outlines, CNC shaped from your drawing or STEP file
Coatings	Broadband AR (standard 425 to 650nm, wider on request), wide-angle AR, gold mirror, sputtered shortpass
Finishing	Thermal toughening on site, subject to part size; beam stop wedges; chamfers

Not sure what to specify? Tell us the light source, its power and the operating temperature. Grade, thickness and toughening follow from those, and we'll suggest a combination that survives the heat load.

Options

Made to Fit Your System

CoatingsBroadband AR to lift visible transmission, or gold mirror to reflect infrared

Shapes and edgesRound, rectangular and custom outlines, chamfers and machined beam stop wedges, CNC shaped in-house

Sputtered filtersPair KG glass with a sputtered shortpass or hot-mirror style coating made in our chambers Mounts and assembliesFilters fitted into mounts and housings from our own machine shop

Questions

Heat Absorbing Filter Questions

What is the difference between KG1, KG3 and KG5?

They are all Schott heat absorbing glasses that pass visible light and absorb near-infrared. KG1 absorbs the least and passes the most light. KG5 absorbs the most. KG3 sits between them and is the most widely used grade.

Will a KG filter crack from the heat?

It can, if a large or thick filter sits close to a powerful source. The filter absorbs heat and expands unevenly. We reduce the risk with thermal toughening on site, subject to part size, chamfered edges and a mount that leaves room for expansion. A reflective coating can also lower the heat load.

Can KG3 be used as an IR-cut filter for a camera?

Yes. KG3 is commonly used in front of silicon sensors to block near-infrared and give a visible-light response. Tell us the sensor size and we'll cut and coat the filter to fit.

What thickness should I choose?

Thicker glass absorbs more infrared but also gets hotter and passes slightly less visible light. Our chart is for 2mm. Tell us your source and how much infrared you need to remove, and we'll suggest a thickness.

Can you add a gold or mirror coating?

Yes. We can add a gold mirror coating to reflect infrared, as in the photo at the top of this page, or a sputtered shortpass coating. Both are applied in our own chambers.

What do you need to quote a KG filter?

The outline and size, thickness, glass grade, any coating, whether it needs toughening, and quantities for prototype and production. A drawing helps but isn't essential.

Related Filters and Optics

Custom optical filtersSputtered bandpass, notch, longpass, shortpass and dichroic filters Schott UG UV bandpass filtersUV-pass, visible-blocking glass: UG1, UG11, UG2A and UG5 Optical windowsFlat, parallel windows in optical glass, fused silica and sapphire Optical coatingAR, mirror and sputtered coatings applied in our own chambers

Getting a Quote for KG Heat Absorbing Filters

Send us your drawing, or just the size, thickness and glass you need. An engineer from the team that would make your filters, not a sales team, will review it and tell you plainly what's achievable and what will drive cost.

What to send

Drawing (PDF or STEP), or the size and outline
Glass grade and thickness, or your light source and heat load
Coating: AR, gold mirror or none
Whether the filter needs toughening
Quantities: prototypes and expected production volumes

What happens next

An engineer reviews itWithin one working day, flagging anything that will drive cost or lead time.
We quote the filterFor the prototype, and for production at your volumes.
We cut, polish, coat and deliverAll in Rye, with inspection data if you need it.

Start Your RFQ