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Why use borosilicate?

Borosilicate is a commonly used float glass typically consisting of roughly 80% silica, 10% boric oxide and a small amount of alkaline sodium oxide, potassium oxide and aluminium oxide. The material has excellent chemical and thermal properties and is used extensively in the optics and chemical industries. Closer to home, borosilicate glass is used to make glass ovenware under the trade name Pyrex.


Applications for borosilicate glass

For optical applications, borosilicate has consistently high transmission from 310nm well into the NIR. The material is chemically resistant to acidic and saline solutions and will not degrade under normal conditions. Borosilicate has a low thermal expansion meaning the optic can survive rapid changes in temperature while maintaining its structural integrity where other materials might crack or shatter. A maximum operating temperature of 500°C makes borosilicate ideal for high power light source applications and as a substrate material for hot and cold mirrors. Because it holds its shape well borosilicate can also make a cost effective alternative material for optical flats. Please click on the link below for more information on optical and mechanical properties.

Custom borosilicate windows from Knight optical.

Knight Optical offers an extensive custom manufacturing service supplying cost effective, bespoke optical solutions. We are able to produce custom borosilicate parts in almost any shape, size or thickness. Knight Optical have supplied custom borosilicate optics to a large number of customers including tubes for solar applications, scratch resistant camera protection windows, wafer substrates, underwater windows and much more.

Borosilicate is an ideal substrate for a range of coatings including BBAR, AR, dichroic and mirror coatings for applications from the UV to NIR. The material can be optically polished to a high surface quality. For precision (λ/4 and better) surface quality we would typically recommend using BK7 or a derivative as the cost of precision polishing typically outweighs the low cost of the borosilicate material. For applications requiring transmission beyond 2700nm we would recommend a dedicated IR material.

Our typical custom borosilicate window capabilities:

Dimensions: 2mm to 300mm+
Dim. tolerence: ±0.05mm
Thickness: 0.7, 1.1, 1.75, 2.0, 2.75, 3.3, 3.8, 5.0, 5.5, 6.5, 7.5, 9.0, 11.0, 15.0, 16.0, 19.0, 21.0 and 25.4mm thk.
Flatness: < 1/20th wave @ 633nm
Parallelism: < 3 arcminutes
Surface Quality: < 10:5
Coating Options: AR coated single/both sides
Mounting Options: Mounted or unmounted

Knight Optical stock borosilicate sheets 1150x850mm. 

Our standard stock window specification can be found below:

Material: Borosilicate (LEGB)
Diameter: +0, -0.2mm
Thickness ±0.2mm (float) ±0.1mm (polished)
Flatness: Float type (typical) 1λ over 10mm dia
                  Polished grade λ/2 over 90% of aperture
Surface quality: <40-20 scratch/dig
Parallelism: <1 arc minute (polished type) 

Knight Optical offers two standard ranges of stock borosilicate windows. 

Float grade borosilicate windows

Float grade borosilicate windows are cut from standard float sheet and offer an affordable solution to basic optical window applications. The surface quality of our borosilicate range is typically not as high as our precision BK7 windows and has a nominal flatness of 1λ @ 633nm per 10mm diameter. Float grade borosilicate is ideal for basic optical applications such as display lighting and substrates for coating where surface quality is not critical. Stock float grade borosilicate windows are supplied uncoated.

Polished grade borosilicate windows

Polished grade borosilicate windows are precision cut from float sheet and optically polished to a high standard for improved surface quality. Our stock precision borosilicate windows are polished to λ/2 @ 633nm over the clear aperture and are more tightly tolerance than their unpolished counterparts. Borosilicate is scratch resistant and can offer a cheap alternative to sapphire and BK7 for exposed optics such as camera protection windows and viewports. Chemical resistance makes borosilicate ideal in pharmaceutical applications. Stock polished grade borosilicate windows are supplied uncoated.

ISO Standard

Knight Optical is very proud to be certified for ISO14001 for Environmental Management and ISO9001 for Quality Management Systems. These certifications demonstrate our commitment to maintaining competence and reliability in the supply, metrology testing and inspection of our products.

ISO 14001: Environmental Management we strongly believe that all businesses should be mindful of their environmental impact and seek to manage this effectively in their day to day activities. Green energy, recycling, carbon footprints and other environmental issues are very real concerns and we feel it is important to demonstrate to our customers our commitment to minimising our environmental impact.

Knight Optical has adopted and been certified for ISO 14001: Environmental Management System to comply with our environmental obligations. Our EMS framework allows us to have a systematic approach to the way we handle the challenges brought on by our commitment to environmental standards.


ISO 9001: Quality Management Systems quality should be consistent and customers should expect the same high quality service and products every time they order from us. In being certified for ISO9001, we are able to constantly monitor the way we work and ensure a consistently high level of service. We have well defined and documented procedures to ensure consistency and allow for corrective measures to be quickly applied should faults be found.

Our ISO9001 procedures give our management a clear and concise picture of how the company is performing. This mechanism of reviewing goals and performance aids us in our efforts to push forwards and improve as our company grows.


At Knight Optical we pride ourselves on the quality of our products and service. Every component is individually tested by our highly skilled technicians in our state-of-the-art metrology lab to ensure all parts meet our high-quality standards. 

To learn more about how our Quality Borosilicate Windows can improve your application and your supply chain experience, contact our multilingual technical sales team by email: 

info@knightoptical.com or call direct on
0044 1622 859444 (UK, EUR or ROW)
001 401 583 7846 (USA & Canada) 

For more information on our metrology and testing capabilities

Inside Knight Optical’s catalogue you’ll find details of the industries we serve as well as the many other services we can offer. It also showcases hundreds of our most popular products, supported by useful technical information, to help you choose the right part for your application.

Our 2018/2019 Catalogue

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