The influence of pump radiation parameters such as the polarization and the spectral width of infrared radiation on the conversion of broadband radiation in lithium niobate crystals was investigated. The spectra of converted radiation were calculated for two types of phase matching in the negative uniaxial crystal by taking into account the convergence of the light beam in the crystal. Experimental spectra were obtained and compared with the calculated spectra. 1. Introduction Systems based on thermographic phosphors, digital holograms, and other conversion mechanisms have been widely used to achieve thermal-to-visible conversion [1, 2]. Optical methods for the conversion of broadband infrared radiation are based on frequency-mixing processes such as second harmonic generation and sum frequencies generation in nonlinear optical crystals and are used for the visualization of infrared images of thermal objects [2, 3] and in other applications [4]. These methods appear to offer the most advantage for the analysis of two-dimensional (and three-dimensional) infrared images. The important advantage of optical methods for the visualization of infrared radiation in nonlinear optical crystals, unlike electrooptical transformation methods, is that they preserve the phase information and consequently enable further coherent image processing (e.g., holography in the infrared range with no inertial visualization). Since the late 1960s, studies have focused on problems involved in the conversion of broadband infrared radiation into the visible spectral range by nonlinear optics methods [5]. First, the conversion of broadband infrared radiation in nonlinear crystals was carried out by using narrowband laser pump radiation. However, this method for the visualization of infrared images has some disadvantages such as the spotty image structure caused by the coherence of laser pump radiation, distortion of the image size along one of the directions (in the plane of phase matching), narrow spectral frequency interval of the converted radiation of 20–30？cm？1 (occasionally, of 200–300？cm？1), and sharp deterioration of the converted image when the spectral frequency interval of the infrared radiation is increased or when the pump radiation is focused [6]. At the same time, there are a number of interesting experimental and theoretical results of studies of the conversion of broadband infrared radiation by the use of incoherent pump radiation. It has been shown that when certain conditions are fulfilled, the process of harmonic generation for incoherent pump radiation can be
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