Print layers drifting towards rear

[cliffyk]

Yes, a flat rectangle just as "projected" by the LCD.

The "printout" should be flat (parallel top and bottom surfaces)--if not, and assuming the resin is homogeneous, any variations in the cured thickness have to be the result of unequal exposure to the curing light--thicker in longer exposed areas and thinner in less exposed areas. As the cross-field duration of exposure will be as near to identical as can be (the LCD is only ON or OFF), variations in cured thickness will have to have been caused by the intensity of the light.