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We present a methodology for optimizing the output radiance and characterizing a planar waveguide laser utilizing a technique offering “real-time” beam quality measurement. This approach is applicable to lasers in general employing the simultaneous measurement and comparison of a re-imaged cavity-mode beam-waist (near-field) and the power distribution in the far-field (specifically in the Fourier plane of a lens). This approach provides the laser's beam parameter product, and hence M2 beam propagation factor. By exploiting this technique we optimized the performance of a quasi-four-level Nd:YAG planar waveguide laser with an extended stable-cavity configuration, whilst characterizing the in-plane thermal lens. This laser produced a record radiance level of >; 9 TWm-2/sr-1 for this 946-nm transition, with an effective thermal lens focal length >; 400 mm at all incident pump powers up to the maximum level of 87 W.