I just replaced the broken screen on a Lenovo t440 running 18.10
Now the screen is so dark I have to use a flashlight pointed right at it to see it.
I tried a couple things, thinking maybe the slider control was not working
And the xbacklight -inc fix suggested in several posts

No luck

Did I just end up with a dud screen? Or is there likely something going on with the graphics card or driver? An Intel Haswell card, btw