Thermal characterization of a mercury arc lamp for a projection display system

Abstract

Avionics displays that operate in high temperature, low-pressure environments are challenging to design. Since the trend has been to reduce the physical dimensions of the electronics while increasing the effective display area with higher luminance and optical performance, the thermal design of this type of display product is a critical step in the design process. At the heart of one such display product is a high-pressure mercury arc lamp module, comprising of an arc tube and reflector housing. It dissipates roughly 1/3 to 1/2 of the total power in a display enclosure. The methods used to cool a high power light source could have a dramatic effect on the performance and the reliability of the other electrical components within the display enclosure. This paper will discuss the thermal design of the light source, a custom-designed high-pressure mercury arc lamp module for a projection display used in an avionics application. Computational Fluid Dynamics (CFD) was used to characterize the heat transfer path from the plasma arc to the lamp's outer housing and rest of the electronics within the enclosure. A few of the higher end CFD companies have developed plasma capabilities within their codes. These codes typically do not have the functionality to solve electronics box type problems efficiently. On the other hand, the "electronics specific" CFD codes do not have the higher end computational capabilities or the ability to mesh complex geometry. Because of this, both a general purpose and an electronics specific CFD code were used to accurately predict the temperatures in a projection enclosure used for an avionics display. To establish the complete model, a series of optical measurements was conducted on a typical arc tube and real lamp to obtain the critical parameters that are too complicated or impossible to generate by modeling alone. These parameters include the total radiant power of the lamp, radiant power distribution in different wavelength range, and the optical properties of the optical surfaces. Also measured was the temperature distribution of the lamp at predetermined points in well-controlled conditions. This optical and thermal data are used in the modeling process so the model can produce consistent and convergent results.