While the earliest doors in history were constructed out of wood or metal, once sheet glass was invented it has found application in doors – where it has fitted into small windows.
In the 16th century European cathedrals were designed incorporating large stained glass windows, held in place by lead beading, and decorated with depictions of religious stories or heraldic symbols of great families.
Over the last 200 years, glass has been used in doors, initially for decorative purposes or to allow in light, and more recently as a means of monitoring visitors at the door and controlling access to the building. The glass is secured within the door by means of “vision panels” (or lites in American English) handcrafted from wood or metal by the door manufacturer or installer and were of varying quality and fitness for purpose. For example the UK-based glassmaker Pilkington invented wire glass to make glass more secure when smashed, which was then found to effective for fitting into fire-rated doors, required by insurance companies to protect buildings.
In medieval buildings, louvres made of stone, glass, metal or wood were used on rooftops to allow the passage of air or smoke – and the sound of bells in the case of a church belfry – from inside a building.
In the late 20th century, fire regulations became more stringent spurred on by several major fires such as the MGM Grand Hotel in Las Vegas, US, in 1980, where 85 lives were lost. Strict standards were then set for the manufacture and testing of fire doors in public buildings and required the compulsory fitting of permanent labels on the door, vision frame and glass indicating UL or WHI (or BS in the UK) showing test compliance to the US-based National Fire Protection Association (NFPA-80) standard for fire doors and fire windows.
The vision frame and louvre thus became a high-performance sub-assembly of the door, with some specialist manufacturers such as US-based Anemostat – a leading manufacturer of metal vision frames and door louvers – launching a number of innovative benchmark products. These products were rigorously tested by UL or WHI in partnership with door manufacturers and were sold directly to the door manufacturer or through distributors of door hardware systems.
New quality certification standards such as the ADA in the US and the DDA in the UK have recently been introduced to adapt doors to suit the needs of disabled or wheelchair-bound people. These new regulations improve visibility with the installation of long narrow door vision panels. There are also new tough standards in the US governing fire-rated glass to be also rated for impact resistance, in compliance with the glass safety standards of CPSC Category I or II for the International Building Code 2006 version.
Recently, fire resistance standards have recently been raised even further with the adoption in North America of UL 10C positive pressure and hose stream testing of the fire door after a duration of up to 90 minutes, and in the UK to BS476.22 positive pressure testing for up to 240 minutes (no hose stream required). In Europe the new CE standard is being adopted, which is applied to factory-fitted vision panels capable not only of resisting fire (integrity) but also limiting the heat transmitted through the door and glass (insulation) to EN-1634. This has resulted in the vision panel manufacturer having to adapt and test its products successfully, whilst ensuring that they are easy to install, cost-effective and aesthetically acceptable.
There is a growing requirement for specialised or custom vision panels to address the need for added door security – that is bullet, vandal or hurricane resistance – energy-conservation (double glazed units), corrosion resistance, hygiene (such as for pharmaceutical or high-tech clean rooms), and sound transmission resistance. In addition, clients also want special custom shapes, such as stars or keyholes.
How big a vision frame can be fitted to a door? This depends on whether it is a wood or steel door. Steel doors can usually be fire -rated for longer periods of time, with larger sizes of glass, with proper test evidence. The type of glass will also be a factor because ceramic or multi-layered intumescent glass can resist fire longer and in larger sizes than traditional wire glass. If in doubt, consult the vision panel manufacturer or its trained local distributor, who will provide expert advice.
* Sherwood, responsible for US-based Anemostat’s sales to Europe and the Middle East, has 25 years of global experience in the door and hardware industry. Los Angeles-headquartered Anemostat is one of the largest manufacturers of metal vision frames and door louvres and is currently expanding into other countries such as Canada, Mexico, UK, Europe, the Middle East and Australia with a complete product line-up of fire-rated vision frames, door louvres, metal edges and astragals, security vision frames and louvres for wood and steel doors.