I believe the reason why some of us are having issues with either yellow blotches, tinge, tint or any thing that's overly yellow where it shouldn't be is due to the yellow phosphor coating.
How LEDs Produce White Light
Conventional Method 2: The Phosphor Method
The Phosphor white method produces white light in a single LED by combining a short wavelength LED such as blue or UV, and a yellow phosphor coating. The blue or UV photons generated in the LED either travels through the phosphor layer without alteration, or they are converted into yellow photons in the phosphor layer. The combinations of the blue and yellow photons combine to generate white light. In some modules, the yellow phosphor is remote.
Phosphor white offers much better color rendering than RGB white, often on a par with florescent sources. Phosphor white light is also much more efficient than RGB white. Because of its superior efficient and color rendering (typically Ra70 to 85), phosphor white is the most commonly used method of producing white light with LED's. Whilst color rendering is good in the pastel shades, the spectral density is not close to daylight and there are problems rendering the more saturated colors such as red (R9). Read more about this in the section on color rendering
In a typical phosphor white manufacturing process, a phosphor coating is deposited on the LED die. The exact shade or color temperature of white light produced by the LED is determined by the dominant wavelength of the blue LED and the composition of the phosphor.
The thickness of the phosphor coating produces variations in the color temperature of the LED. Manufacturers attempt to minimize the color variations by controlling the thickness and composition of the phosphor layer during manufacturing.
Over time, the blue die and the yellow phosphor will degrade. This results in the delivered light shifting in color. It will also produce unexpected colors if the device is operated at a different current or operating temperature.
The 15' retina display is lit by a single strip of leds located at the bottom of the retina display. Image below.
Enlarged
MacBook Pro Retina Display Teardown
Just with the phosphor coating in mind in my opinion could be the cause of the two issues below.
Uniform screen but overly yellow - phosphor coating is too thick
Bottom half of screen yellow while the top portion is uniform - Could be caused by many things.
- Phosphor coating is too thick on a few of the leds, leading to yellow blotches.
- Phosphor coating not covering the entire led.
- Yellow stains on one of the diffuser or prism films.
more info about LEDs and the phosphor layer
Some snippets of info from the above link
Color quality is determined by the amount of phosphor and its distribution over the LED
Phosphor coating and remote phosphor technologies are aiming for less binder and more uniformity by spraying phosphor with minimal binder on die surface or optics surface. The thin phosphor layer thus is formulated on die or optics.
Achieving uniform spray thickness is a key challenge.
In spraying processes the silicone/binder/solvent/phosphor combination is typically a low viscosity (<100 cps) mixture. The chemistry challenge is to keep the phosphor dispersion in suspension and in a uniform mixed. Special conformal coating spray applicators with a combination of process techniques and masks provide the uniform coatings.
Even more info about Light emitting diodes aka LEDs
