[Nexus 5 Themes and Apps] rajendra82:
It's all trial and error anyway. There is no documentation about what registers apply where. We have the colorimeter experiment results of @mag01 to thank to have any data at all. As for using exponent or not, I stick with exponent where sRGB standard says to use exponent and go linear where it does too. You are looking at content created on many other devices. If you code in behavior similar to the sRGB curve, and apply corrections for an individual panel, based on the assumptions built within the standard, you are least likely to introduce unnecessary distortions than you would by creating your own standard. We can debate all we want about which registers apply where, but the assumptions need to be backed by experimental data. Our eyes are not sensitive enough to notice the difference that small predictable changes in the display that small changes in register values produce. We can notice the changes with the naked eye if we make huge changes to the register values, but at that point the effects of the changes themselves stop being predictable, so what we can see proves little.

P.S. - Some more thoughts on this exponent issue. Take a look at this link to get a better understanding of gamma encoding and decoding, and gamma correction. If you read carefully, you will get a grasp of the issue at hand. Most content being sent to you from around the web as well as that you view locally on the phone is all gamma encoded, because it is the most efficient way to store the images without wasting bits. Only RAW images are stored non encoded, and when put on the screen, the encoding is added as well, or they will look dark. The standard encoding ratio is per sRGB and adobe RGB is around 2.2 above a minimum value. If you encode the images with a higher gamma, they will devote too many bits to the darker colors. The photoshop example you cited a couple of pages back was showing you how to apply a gamma correction. Behind the correction curves was a histogram showing what part of the image was taken up by what part of the luminosity curve. When you apply a gamma correction to the image you are redistributing its bits for more efficient encoding. When you apply a gamma correction grater than 1, you are lightening up the image while a less than 1 gamma correction means you are darkening it. When you download an image from the web, the website doesn't ask you if you are on a Nexus 5 and what color profile you are using. It sends you the image they have, not a special version jus for you. The panel of Nexus 5 has a display gamma, just like every other display in the world. The display gamma is what you are testing with the Display Tester app. The inverse of what you learned about image encoding gamma is true for the display gamma. Set your display gamma too high, and the display content will look darker, set it too low, and the same content will look lighter. Since the standard for image encoding has a gamma of around 2.2, ideally you also want a display gamma of around 2.2, so that the image looks the same to you, as it does to the person that created and edited it. This is true ideally, but since Nexus 5 is a mobile display and viewing conditions vary, the target display gamma of 2.2 may not be appropriate for everyone. If you are a bartended and use your phone most of the time in a dimply lit environment, a display gamma of 2.2 may be too high for you to view images created by office workers sitting florescent lighting. On the other hand if you are a construction worker, using your phone outdoors, a display gamma of 2.2 may be too low for you, as the content will look too dark. What display gamma to target is what we are doing here. If you want to tarhget a different display gamma, that is fine, but the relationship should be exponent relationship, just like that in the sRGB standard, that was used by the people creating the image. Use a different relationship (say linear), and your devices response will not be predictable, and some images could look lighter than intended, and some darker than intended. That is worse than lightening everything, or darkening everything to match the viewing environment. The old CRT makers were lucky, the CRT monitor has a natural display gamma of 2.5, as the Boynton resource listed, so only a small constant correction 1/1.1 brought it 2.2, and everything looked as intended. With an LCD, like we have there is look up table that is used to set the display gamma. What were are editing with these profiles is that look up table.

http://www.cambridgeincolour.com/tutorials/gamma-correction.htm

http://www.cambridgeincolour.com/tutorials/photoshop-curves.htm

http://www.cambridgeincolour.com/tutorials/monitor-calibration.htm#look-up-tables