Mobile Screen Under Sunlight

The amount of light that reaches our eyes influences how well we can see. We see most objects by the light they reflect (this is what is responsible for color, and why it is harder to distinguish colors in the dark, when there is less light available). Rather than utilizing reflected light, most phone screens produce their own light. This works well when you are observing your phone at night or inside (places where there is significantly less reflective light from the sun).

However, when you are outside in the bright sun, the sun's light is reflected off the objects around you, including your phone screen. The intensity of light outside is substantially greater— by several orders of magnitude (a piece of white paper can be 1,000,000,000 brighter on a sunny day than on a moonless night). As a result, your phone has significantly more light reflecting off it than generated by it. This reflected light obscures the light from your screen. The screen’s brightness compared to everything else in your visual field is very dim.

[On top of this, in bright light your pupils contract. When in the dark, wider pupils allow more light from your phone to reach your eye. Your pupil contraction in daylight decreases the actual total amount of light you receive from your phone screen].

There are screens that can be seen more easily in direct sunlight. Transflective liquid-crystal display (LCD) screens reflect AND transmit light. In daylight the display is mostly reflective— allowing them to be more easily viewed in the sun.

To start off, I didn't have physics in high school and I'm currently studying accountancy, so bare with me if this is wrong, I'm self taught and bound to make mistakes.

The average smartphone screen can be between 200 and 1200 nits. If you look at this fact relative to the time of day, your phone's screen will look brighter the darker it is, or more specifically, the less light hitting your eyes excluding that of your phone's are present.

So this can basically be compared to staring into the night sky in a city with loads of lights and doing so in an isolated area with no light pollution. The light traveling thousands of light years to coincidentally hit your eyes can be flushed out by any background light scattering into your eyes, lowering the visibility of the night sky.

Let's name an observer Person A and the Person holding a match Person B. If A stands 1 kilometer away from B, and B strikes a match, it would be easily visible in a very dark environment. There is close to no other light than that of the match reaching your eye, so it's easy to observe. If the same scenario repeats at day, it would be near impossible to see the match, because of all the background light reaching your eye.

Using the above demonstration, comparing the match with your smartphone and the sun's rays flushing out the amount of light perceived by the observer from the smartphone, it is clear that light pollution is the main cause of your smartphone's screen dimming significantly.

It all depends on contrast,you see.