Comment by baq

2^20 is just above 1M, so that's how many bits you need to capture the darkest and the brightest pixel without losing information (brightest/darkest = 1M). the assumption is that you tone map those 20 bits onto however many bits you have available (8 for standard display, 10 for HDR, 14 for DSLR sensor, etc.) - at least that's my understanding after consulting wikipedia.