When it comes to screen calibration Google have never had much success when it comes to their Nexus device. Whether this is due to poor process or just not having the resources/expertise to do it is unknown. As with many things Nexus related, the developer community are attempting to provide a solution. Developer Myfluxi has come up with kernel code that can be used to reverse engineer the kernel drivers for gamma for the Nexus 5. In keeping with the open source spirit he has made this available to all and from this Faux123 has created an app/tool to adjust these settings so that the Nexus 5’s screen gamma can be reverse engineered and thus used to properly calibrate our Nexus 5 screens.
So what is gamma and why is it important?
The relationship between the input signal and the luminance of a monitor is not linear but exponential with gamma being the exponent. A gamma value of 2.2 has become the de facto standard in Windows, the Internet and the digital photography. This means that only monitors with a gamma value of 2.2 will produce a correct output. quickgamma.de
In a nutshell, to get true colour representation of the input to the screen the output gamma should be 2.2 for all colours. Using the app provided by Faux123, N5 Gamma Calibration the outputs of different colours can be adjusted. At this stage not much is known about what each number does and it is a collaboration attempting to provide proper calibration. Throughout the XDA thread there are examples of differing settings people have used to obtain 2.2 gamma (or close to) for all colours. As not all screens are created equal I recommend you head over and test out each one for yourself and see what works best for you (or you could attempt to adjust it specifically for your own phone if you have enough time). The best results have been seen adjusting the values for row 1/column 1 and row 2/column 5 for each colour. Some users have been adjusting row 3/column 1 as well although when I did this although the gamma was nearly perfect for 2.2 the banding was messed up and my images looked terrible. My best results were achieved using the settings in the thumbnails below.
So what do you need to do to perform this calibration? Firstly you need to have root access to your device (do I need to remind you to perform a nandroid backup again?). Next you need to be running a custom kernel that has the correct committs. At this stage that means a Faux123 beta kernel, a Franco kernel and an Elemental X kernel. Then install the app in the first post of the XDA thread. To test each setting you need to enter the values you want then hit apply. The screen will not change until you turn it off and then on again. To check the gamma output download Display Tester from the Play Store and enter the Gamma detection section. The level your gamma is at is indicated by the bar that is closest in shade to the background colour on each page.
My screen does seem a lot brighter now although that could be the placebo effect. Give it a try and let us know how it goes. As usual please make sure you read through the first post on the XDA thread at least once, perform a nandroid backup and then proceed cautiously. If you manage to achieve 2.2 gamma and feel it made a difference to how your screen looks put your settings you changed in the comments below for others to try. Good luck.