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INTRODUCTIONA great deal of effort is presently being focused on developing high resolution, high sensitivity medium wavelegnth IR (MWIR) imaging systems for a variety of applications. These range from thermal imaging for industrial applications to military applications for detecting vehicles, missles, etc. The present state-of-the-art method for MWIR imaging consists of fabricating linear and 2D arrays of semiconductor detectors, such as HgCdTe (MCT), InSb, etc., and incorporating these into an appropriate optical imaging system. However, such devices are difficult to make and are very expensive due to low material and device fabrication yields. In the present paper, a new detector media is described which can be fabricated at low cost for use in MWIR imaging. Specifically, the new media is an electron trapping (ETTM) material capable of up-converting MWIR to visible wavelengths, which can be easily detected with a commercial camera system. This paper will describe the specific performance characteristics of the new phosphor material and its application in MWIR imaging. BACKGROUNDBefore describing the specific phosphor optical response characteristics, the physical mechanism believed responsible for its behavior will be discussed. The particular phosphor investigated is an alkaline-earth sulfide host crystal activated with rare-earth çRE) dopants. A luminescent center is created in the phosphor by the introduction of Eu2 and shallow energy electron trapping centers are introduced by the addition of other RE dopants and thermal treatment. These lead to new energy levels within the alkaline-earth crystal band gap between which electron transitions can occur. The transitions responsible for MWIR photon absorption and visible luminescence can be described qualitatively using the simplified energy level diagram shown in Figure 1. First, an electron can be pumped from the Eu2 ground state by short wavelength excitation to its excited 5d state. Once in the 5d state, the electron can either relax radiatively to its ground state, producing prompt fluorescence, or it can tunnel to a neighboring trapping center, leaving behind a Eu3+.Since the electron trap energies are small, electrons trapped in these states can be thermally stimulated to their excited states at room temperature. From the excited states, SPIE Vol. 1243 Electron Image Tubes and Image Intensifiers (1990) / 123 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 05/16/2015 Terms of Use: http://spiedl.org/terms a previously trapped electron can tunnel back to a Eu3+ site to create an excited Eu2+; subsequent relaxation results in the production of luminescence, or more correctly, thermally stimulated luminescence (TSL) with a peak wavelength of 650 nm. In this case, the integrated TSL output is proportional to the initial trapped electron population and the luminescence intensity is proportional to the temperature.By cooling the phosphor to LN2 temperatures, luminescence due to thermal stimulation is substantially reduced and electrons can remain ...

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