Figure 5a White Spectrum
Figure 5b Night Shift Spectra
LCD Displays have Backlights consisting of 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.
OLED Displays are emissive devices so their spectra are just the sum of the individual Red, Green and Blue OLED spectra, modified slightly by the touchscreen layer and anti-reflection absorption layer through which their light must pass. As expected the iPhone XS Max OLED spectra are relatively narrow with deep notches between the primaries, which results in highly saturated colors that are adjusted with display Color Management to provide high color accuracy for each of the iPhone XS Max Gamuts.
The relative heights of the Red, Green, and Blue peaks determine the resulting White Point of the display.
The Night Shift mode is designed to change the color balance of the display in order to reduce the amount of Blue Light produced by the display, which some recent research indicates can affect how well users sleep afterwards. As the user adjustable Night Shift slider is increased, the on-screen images take on an increasingly yellowish tint because amount of Blue Light emitted by the display decreases, which is seen in Figure 5b below. With the Night Shift slider at its Maximum setting, the Blue Light component is reduced by 80 percent.