Light Spectra for the iPad mini 4 - iPad Air 2 - iPad Pro 12.9    

The Backlights for almost all LCD displays are white LEDs, which are made by using a Blue LED together with a yellow phosphor to transform some of the light for the Red and Green primaries. The spectrum of an LCD display is just the spectrum of its white LED Backlight filtered through the individual Red, Green and Blue sub-pixel color filters within the LCD panel, resulting in broad rolling spectra for LCDs. The figure below shows the Light Spectra for each of the iPad Tablets.

Some recent research has shown that blue light in the 460-482nm range (received by melanopsin receptors in the eye) may delay sleep onset for late-night display users (such as Tablets, Smartphones, TVs, Laptops, Computer Monitors, etc). The melanopsin cells control pupil dilation and melatonin release from the pineal gland, which is responsible for regulating the circadian rhythm.

The 460-482nm range is marked in light gray in the figure below. The amount light energy in this range is considerably greater than in the solar spectrum, which is why displays can interfere with the onset of sleep after night use.

Because of the fixed broadband peak for the blue primary spectrum, the only way to significantly reduce the light energy in the 460-482nm spectral range is to significantly turn down the brightness for the entire blue primary, which is the tall spectral peak on the left. This will give all images a significant yellow color cast because they are lacking blue (since yellow is the complementary color to blue). Just slightly turning down the blue brightness, which is what a number of Apps do, is not likely to have a significant effect other helping together with placebo and a conscious modification of behavior.

One innovative high-tech way to solve this problem would be by using Quantum Dots which can be tuned during manufacture to avoid the 460-482nm range, while still delivering blue light so that the display doesn't look yellow and provides a full Color Gamut.

Light Spectra for each of the displays
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iPad 2015 Display Technology Shoot-Out Article
Mobile Display Shoot-Out Article Series Overview and Home Page