Transflective LCDs make sense when `ambient light level x LCD transmittance^2 x backlight layer reflectivity` is brighter than `brightest white LED x diffuser t

Transflective LCDs make sense when `ambient light level x LCD transmittance^2 x backlight layer reflectivity` is brighter than `brightest white LED x diffuser transmittance x LCD transmittance`. Transmittance for run of the mill LCDs is like 40% so the bar isn't that high. The left part of above formula means white level of transflective LCDs can be as low as 15-20% ambient white. I think what happened was super-bright white LEDs replacing early LEDs and CCFL(Cold-Cathode Fluorescent Light) backlights. iPhone up to 3GS used transflectives, but LCD on 4 and later were transmissive. That was probably the point when cheap sunlight readable backlight became viable.