Incurable genetic retinal blindness is in readiness for clinical trials to attempt restoration of vision. Early trials will involve those patients with the most severe visual losses, but there are no standard methods to quantify extremely abnormal visual perception. Full-field stimulus testing (FST) has been devised to fill this need. The current work advances FST to assay vision in the most severely blind patients using achromatic and chromatic stimuli in an LED-based visual stimulator. Our studies of this new FST in a cohort of retinal degeneration patients show the feasibility of the technique in a clinical setting. Results and variability of the new FST were comparable to our earlier FST device when vision was measurable with both. The extension of the dynamic range of chromatic and achromatic stimuli in the new FST by 2 log units allowed vision that was previously not measurable to be quantified. With this advance, inclusion and exclusion criteria can be developed and safety as well as efficacy outcomes measured in upcoming treatment trials of severe retinal blindness.