In this paper we report a new set of accurate measurements of guided mode resonances coupled in a negative photonic crystal slab. Narrow peaks are visible in the reflection spectrum with a full-width at half maximum (FWHM) of less than 2 nm. In addition to the traditional measurements of the reflected signal, we present the imaging of the coupled radiation propagating into the slab. Finally, by comparison with the already known phenomenological analysis [1] we propose a new physical model of the phenomenon. The experimental data shows an excellent agreement with mentioned theory.