Glass - Back to the Future!

Presenting Author:
Fiona Turner

article posted 24 March

Fiona Turner is a Chartered Physicist specialising in instrumentation and metrology. She started her career with British Rail Research, developing rail vehicle based instrumentation for tunnel and track profiling, then went on to work on gas sensing applications, an instrumented aerosol chamber for climate change measurement and laser scanners for anti-counterfeiting. Fiona joined Land Instruments in 2010 as Physics Section Manager, where her main project has been the development of Land’s new SPOT thermometer range with extensive site trials for aluminium extrusion and rolling applications.

Infra-red Temperature Measurement on Thermally Tempered Low Emissivity Glass

S. Fiona Turner*, Richard W. Gagg, Eleanor C. Chalkley, Peter Drögmöller
Land Instruments International, Dronfield, Derbyshire S18 1DJ, England

Heat-strengthened and thermally tempered low emissivity glasses are commonly used architecturally where visual perfection is extremely important. To create visually perfect glass panes or lites, physical distortion must be avoided and temperature precisely controlled. Uniform surface temperature profile across the whole lite must be achieved before entry to the cooling process. Land Instruments specialise in infra-red measurement for challenging industrial process control applications, and in this paper we demonstrate an infra-red measurement solution to generate a thermal map of the product for industrial process control. Badly tempered glass is a nauseatingly obvious indicator of poor manufacturing quality, as can be seen in the right hand image and thermal surface map below.

Infra-red measurements of glass tempering applications are challenging due to the transmissive and reflective characteristics of glass and the largely enclosed industrial tempering installations. Glass measurement solutions normally rely on measurements at 5µm, where the glass becomes opaque with emissivity typically 0.97. However modern low emissivity coatings reduce that emissivity to as little as 0.05. Incorrect estimation of emissivity leads to huge errors in temperature reading. Our proposed solution uses a 5µm line scanner measuring the low emissivity surface through the narrow gap between the furnace and air blowers, aligned with a 5µm spot thermometer taking measurements of the underside of the glass.

The spot thermometer signal is used to compensate for the low emissivity of the upper surface of the glass. A true temperature thermal map of the entire product surface with sub-spot resolution is created in software from successive scan data, which can be used to adjust furnace control parameters and produce visually perfect glass.