Kopp Glass, Pittsburgh, PA

Incandescent lamps have been used as the primary light source in airfield lighting designs for many years. With recent environmental awareness of the general population and government regulation forcing greater energy efficiency, the lighting industry is rapidly transitioning to the use of light-emitting diodes (LEDs). LEDs offer greater energy efficiency by using a greater portion of the input electrical energy for light output, thereby reducing the heat output generated due to electrical losses.

Snow-covered taxiway light.
With the reduction in the infrared (IR) signature of the airfield LED lighting fixtures, adverse weather conditions can prevent visibility of critical airfield lights that provide visual cues essential to the safety of aircrews and passengers. Snow, ice, fog, water mist, and frozen condensation can alter the light distribution patterns and significantly decrease lamp visibility. In the past, snow and ice was melted by the heat generated from use of the incandescent bulb. How ever, the use of LEDs lacks adequate heat output to melt the snow and ice.

Solution

LED boundary lens colors.
Kopp Glass, Inc., an ISO 9001:2008 certified company, has engineered a heated glass lens design that is capable of preventing the accumulation or removing accumulated frozen precipitation from airfield lighting lenses. Glass is a low thermal conductivity material, which inhibits removal of frozen precipitation when heated indirectly. How ever, when heating the glass lens directly, the fixture can remain energy efficient by avoiding unneeded heating of the lighting fixture and open volume within the lamp. Lenses can be heated regardless of design or size. The heated application is suitable for use with colored LEDs that meet the aviation chromaticity requirements of SAE AS25050. While currently available “arctic kits” address the accumulation problem, they lack the energy efficiency of the direct heating method.

Figure 1. LED blue glass boundary lens change in absolute temperature as a function of electrical power input. Heating was initiated after four hours at -20°C and illustrated for a 40 minute heating duration. Electrical power is noted for correlation to the lens thermal response.
The aviation colored glass lens is coated with a Kopp Glass proprietary transparent conductive oxide (TCO) coating on the interior surface of the lens. This coating provides an electrically conductive surface on an electrically resistive glass lens. Metallic electrodes and leads are attached to provide the needed electrical field to power the heating application. Upon application of the electric field, the electricity flows through the conductive coating and transfers heat to the glass lens through conduction. This thermal conduction transfers the heat throughout the bulk glass lens and radiates the heat from the surfaces of the glass. This thermal radiation is capable of preventing snow and ice accumulation on the lens and is capable of meeting arctic testing requirements outlined in FAA Engineering Brief No. 67C.

Design Options

Table I. Electrical inputs and corresponding change in lens temperature of AP-3522/K3007 LED blue glass boundary lens after 30 minutes of heating. FAA Engineering Brief No. 67C requires a 15°C temperature rise in 30 minutes.
The heated lens application has proven successful in heating roundel glass lens (AP-3510) and airfield boundary lens (AP-3861, AP-3522, AP-2562) designs suitable for use in L-804 guard light and L-861 elevated edge light fixtures. Other lens applications and designs are likely suitable for this technology, as lens size only alters the power required to heat the lens area exposed to the environment.

The heated lens can be made in an assortment of color options to meet aviation chromaticity requirements. The prerequisite of a colored LED that meets the aviation chromaticity requirement is essential for the heated lighting application. The heated lens is provided as an annealed glass lens for lighting fixture manufacturer use.

Electrode designs are critical to the efficiency and effectiveness of the lens heating. Kopp Glass has specifically designed the electrode configurations for uniform and efficient heating. The electrode is printed using a highly stable, inert metal that will avoid oxidation. Electricity can be provided using standard wire leads or connectors, depending on the needs of the client.

Engineering Data and Technical Details

Using the colored LED sources provides a greater photopic transmission when using an aviation chromaticity lens as compared to similar lens colors using an incandescent light source. This increased transmission provides the additional opportunity to reduce the electrical power input to the light source. Since there are refractive index differences between the glass lens materials and the TCO coating, transmission losses of 8-10% are likely to be observed in addition to the typical chromaticity associated transmission losses of the lens design.

Powering the TCO lens heater requires an elevated voltage and low current as compared to the electrical supply provided for lighting on the airfield. Power levels of 14-20W will prove adequate to meet the minimum temperature changes outlined in arctic testing methods per FAA Engineering Brief No. 67C.

The surface modified lens designs (AP-3861, AP-3522, AP-2562) provide ample heating capacity that is in excess of the minimum temperatures changes required to meet the typical arctic kit testing methods according to the results of internal laboratory testing. Figure 1 illustrates the AP-3522 boundary lens in the Kopp Glass LED blue (K3007) design heating capability and electrical efficiency when exposed to an arctic testing environment. Electrical power inputs used to drive the heated lens boundary application in laboratory testing are provided in Table I.

As demonstrated in Figure 1, laboratory test results indicate the heated lens design is capable of heating beyond requirements if powered appropriately. This temperature change observed upon powering of the lens is dependent upon the electrical resistance of the semiconductive TCO coating and the coated area of a lens.

Conclusion

The Kopp Glass heated glass lens design provides airfield lighting manufacturers alternate ice and frozen precipitation removal options. The heated lens conducts thermal radiation throughout the bulk of the glass, providing uniform and direct heating of the entire lens. This direct heating method avoids the need to heat the entire lighting fixture and open lamp volume, thereby offering the opportunity for additional electrical energy savings.


Lighting Technology Magazine

This article first appeared in the May, 2011 issue of Lighting Technology Magazine.

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