In Doom 3 the shadows are pitch black, making it impossible to see anything without using the flashlight in low light areas. In reality, light bounces around a lot more so that doesn't happen. There's a higher ambient light level.

To increase that ambient light level in Doom 3:

1. download and install RBDoom3 BFG on top of your Doom 3 BFG installation.
2. enable your console if it doesn't already work (edit the RBDoom3bfg.bat file in a text editor and make sure this line is in the batch file):

@echo off
start rbdoom3bfg +seta com_allowconsole 1

3. Start your game using RBDoom3 BFG, start a game, open your console with the ~ key and enter this command:
r_forceAmbient 0.15

press Enter and the ambient light will be raised to 0.15..

3 attached images:
1. Default value of 0.01
2. value of 0.15
3. max value of 0.4.
Any higher than 0.4 doesn't seem to do anything in darker areas, but that's too light anyway.

0.15 works well for my monitor. Start there and go higher or lower to see which setting you prefer.

FYI: the new value you set will be the darkest an area can get. If you go too high you'll never have dark spots. That's why I found the darkest area I could for screenshots and set the values based on that. That way Doom 3 is still a dark game, but realistically and appropriately dark, not impossible to see anything dark.

Or you can use also DOOM BFA which is using the same rendering code plus various improvements & fixes plus a launcher that allows you to set up these parameters quite easier (to some extent)