I've got an official display, and having found other questions about setting the brightness I know I can change the brightness, and make a script that will run on boot, but that means the screen will start at default brightness and eventually be turned down. Is there a way to set the boot-time brightness value (such as a config.txt setting)?
The value that seems to be 'just right' for me is 12, so having it start at 255 (not sure if that's the default) is far too bright.
As a workaround for not being able to lower the brightness at boot you could make the whole boot sequence black.
Disable the rainbow by adding this on a new line in /boot/config.txt
disable_splash=1
Disable the splash image
sudo systemctl mask plymouth-start.service
Or just make a darker version by replacing /usr/share/plymouth/themes/pix/splash.png.
Remove the RPi logo(s) by adding this on the same line in /boot/cmdline.txt
logo.nologo vt.global_cursor_default=0
It will at least stay black until the sysfs brightness setting is applied.
Set brightness with sysfsutils which you'll have to install with:
sudo apt-get install sysfsutils
then add to /etc/sysfs.conf the following line:
class/backlight/rpi_backlight/brightness = 12