Although both the Apple Watch and Apple Watch Sport have the same OLED display, they have significant differences in optical performance due to their glass covers. In the dark they both perform the same but in ambient light the reflectance of Sapphire and Glass results in performance differences.
For the Apple Watch Sport with Ion-X Glass we measured the Screen Reflectance to be 4.7 percent, while for the Apple Watch with Sapphire we measured 8.2 percent Screen Reflectance, which is 74 percent higher than Glass. Both values are about 0.6 percent higher than just pure sapphire and glass alone, indicating that Apple has done an excellent job in optically bonding both the glass and sapphire to the OLED display without an air gap.
DisplayMate notes that while reflectance plays a big part in the display quality of current generation Apple Watches, the reflectance of both Sapphire and Glass will be reduced significantly in the near future, thanks to new technologies. For now the 74% higher reflectance of the Apple Watch with Sapphire means that it will reflect almost twice as much surround light.
Contrast Ratios are infinite at 0 lux absolute darkness but they decrease rapidly with ambient light. At mid-range indoor lighting of 500 lux the contrast ratio of the glass Apple Watch Sport is just 64 and the sapphire Apple Watch is 38. This is with the watch at maximum brightness with the ambient light sensor fooled. At moderate outdoor lighting in the shade (2000 lux) the contrast ratio is just 17 with glass and 10 with sapphire. For full daylight (10000 lux) the contrast ratios are just 3 and 2.
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Hit the link below for even more analysis of the displays.