Sapphire screen cover hampers Apple Watch display quality, expert says

In his detailed testing of the new Apple Watch display, Raymond Soneira of DisplayMate Technologies found that the scratch resistant sapphire found on the mid- and high-end Apple Watch models "significantly affects the optical performance" of the OLED display. Compared to traditional glass, the sapphire is said to perform poorly in high ambient light and at large viewing angles.

"In order to increase the use of sapphire for displays, the sapphire industry will need to modify the optical properties of sapphire without significantly affecting its scratch resistance and other mechanical properties," Soneira wrote. It can't be done using traditional anti-reflection optical coatings which scratch easily."

Instead of sapphire, the entry-level Apple Watch Sport utilizes a less expensive "Ion-X" strengthened glass display, which is the same technology used on the iPhone 6 and the iPhone 6 Plus.

Compared to the Ion-X display, the sapphire cover reflects about twice as much ambient light. The sapphire on the Apple Watch is said to reflect about 8.2 percent of light, compared to just 4.6 percent with the Ion-X glass.

Underneath the protective cover, all Apple Watch models use an OLED panel, which is a first for Apple. The OLED display earned high marks from Soneira, who said the screen has largely the same colors, color calibration, and color accuracy as the iPhone 6 and iPhone 6 Plus.