HP L1506 15-inch LCD Monitor - Manual (Page 60)

LCD Monitor Quality and Pixel Policy
To locate defective pixels, the monitor should be viewed under
normal operating conditions, in normal operating mode at a
supported resolution and refresh rate, from a distance of
approximately 50 cm (16 in.).
HP expects that, over time, the industry will continue to improve
its ability to produce LCDs with fewer cosmetic imperfections
and HP will adjust guidelines as improvements are made.
D–2 User Guide