[AT&T Samsung Galaxy S 4 General] Slade8525:
As it seems there are a few threads on auto brightness and color issues, i figure i should do the best i can to explain how it works on the S4, and mobiles in general. I work in Television/Film and have been shooting for almost a decade.

ANY QUESTIONS, feel free to PM me, i'll probably ask for very specific photo's (ISO/WB and other data) so i can help you in a professional manner, and i'll try to reply within 6-8 hours. No one should live with a bum screen!

To fully understand what im trying to express, load the attached image onto your phone, and on a bright sunny afternoon, print out the attached image on a small piece of decent photo paper, grab your phone, and find a room in your house that has only fluorescent lights and close any windows or shutters.

Look at the photo in the sunlight; note the colors. Now go inside to the fluorescent dark room, note the change in what the colors look like, and that white is still pretty white (thats your brain), then turn your camera flashlight on (LED), and note the changes in color again (some reds may look purplish or greens bluish), but white still looks kinda white (should look very light blue). Now repeat and look at the print out versus the same image on your phone screen; it should match best under 6500k lighting, but still be off (thats a printed image vs monitor thing though). Also depends on your printer ink type (dye/pigment), color space, etc etc etc. Your eyes take raw data in, but your brain does the magic, and says 'nope, thats white', so you perceive it as 'white' or 'white enough' and you 'know' it is meant to 'represent' white.

Hardware - This includes an ambient light sensor. The way these work is similar to metering systems in cameras; they measure the amount of light hitting a photodiode. Even in high end (DSLR) cameras, sometimes the light meters are junk. On cell phones, they are typically pretty crappy. The iPhone 5's have excellent ALS (ambient light sensors) and they ramp up/down smoothly; much better than any other device i've used.

The way the ALS works in most devices is simple; there are several photodiodes, each tuned for a specific spectrum of light. Say two for 3200k, two for 6500k, two for whatever else, etc, and they basically average the reading, apply a curve, and adjust the screen appropriately. They do NOT accurately measure color temperature very well, and 'see' brightness only in limited spectrums, meaning their idea of what 'bright' and 'dim' is may be vastly different from the human eyes' perceived 'bright' or 'dim'. Also, the 'curves' applied don't match up that well with how the human eye perceives brightness; its really amazing we can fake it as close as we can, really, but most of the magic lies in your brain.

Lighting - this is kind of tricky; and i don't feel like getting into it too deeply, but what you perceive as 'white' is mostly dependent on your brain; not your eyes or the lights around you. Think about reading a book (a real book) inside a library under fluorescent lights; the pages are white, even though the color temperature might be 5000k or 6500k. Now that same book outside; thats 5800k. Now under some old incandescent lights; that might be 2800k. Yet you know it SHOULD be white. Thats your brain. And thats reflected light; its based on the ambient color temperature around you; so reflected light 'looks' white or blue or whatever.

Your phone screen (galaxy S IV) is basically white balanced at 6600k , so it will appear most white when around lighting around 6500k, which is on the bluer side of daylight/cloudy and indoors.

Screen Mode - The Galaxy S IV has a feature that allows it to dynamically change based on the ALS, or can be overridden by the user, which mostly affects color gamut, and either amps up contrast and saturation while shifting hues slightly, or flattens them to a more realistic (usually most people prefer high saturation/high contrast images) image.

Sampling frequency - i have no idea what the ALS sensor samples at, or any definitive specs on the exact sensor and how it interacts with auto brightness, its priority, etc (yet, ive emailed Samsung and i wouldn't mind coming up with a simple app to adjust this if i can figure out the sensor specs and their software). But certain lights (fluorescent mostly) flicker at either 50hz or 60hz depending on their AC current, which may, at times, interfere or give bad readings to the ALS depending on the ALS sensor reading timings (again i do not know exactly what those values are, i suspect they are long as my S III and S IV typically react about a half-second behind lighting changes).

DETERMINING IF YOU HAVE A BAD DISPLAY

All together, it works pretty darn good, if a bit slowly, but there seems to be a lot of confusion and people wondering if their screen is bad. The BEST and EASIEST way is to go to any cell store or mall, and compare YOUR screen and the SAME image to another S IV, with the same settings. The best settings for this are to turn power saving OFF, turn off ALL power saving apps, turn auto brightness OFF, turn brightness up ALL THE WAY, go to 'settings', 'display', 'screen mode', and change the setting to 'movie'. Compare several IDENTICAL images or pages. Then change the 'screen mode' to 'standard' and compare the same images.

While doing so, be sure to check out images such as the one i provided and make sure the colors match, grays are grays or at least the same slight caste of pink/green/etc, and while doing so, TILT THE SCREENS at various angles together (level surface, side by side, tilt to 45 degrees at the same time on all 4 axes), and look for discoloration or bleeding; if you see big differences between two or three other phones and yours, you got a bum screen. I haven't seen a 'bad' one yet, and i made the poor AT&T guy open up five of them and let me play with them (AMOLED also has jet black splotches with full blacks in a fully black room; i wanted the screen with the least noticeable splotches).