For people who are very sensitive to brightness or bright light, it can be difficult to read on a smartphone display, even though the brightness level is down to the minimum. 

Therefor, Apples iOS operating system has a built in feature, that allows to reduce the brightness level below the minimum for normal use. 

Here is a small how to: 

1. Open the settings app and tap on "General"

2. Tap on "Accessibility" 

3. Tap on "Zoom"

4. Tap on "Zoom Filter" and select "Low Light"

Hope we could help.