WIP article based on RTW/BI only so far.
This file effects all the lighting settings for battle maps regardless of climate and season, separate settings are used for day and night battles though. Extract from lighting section of file:
min_light_intensity 1.0 max_light_intensity 1.4 ambient_reduce_proportion 0.75 ambient_red_increase_proportion 0.25 ambient_green_increase_proportion 0.25 ambient_blue_increase_proportion 0.5
min_light_intensity 1.0 -This one effects the lighting globally. Lowering this number will provide darker shadows, but also darkens entire environment (according to experiments)
max_light_intensity 1.4 -This value controls the maximum amount of lighting on the battlefield. Changing only this value and not changing the "min_light_intensity" accordingly could result in washed out colors and less details on the battlefield and units.
ambient_ _ increase proportion - lines affect how much the colour of the sunlight as set in descr_daytypes.txt is altered for shadow areas. The vanilla example shown will increase the amount of blue in the shadows more than the other colours.
A high ambient_ _increase_proportion eg. 0.9 can actually produce more light in the shadow areas than in the lit areas. A low value can make most of map very dark. The equivalent lines in the unit lighting section seem to have negligible effect.
Also to be noted, the battle map lighting and fog control contains many more values and options to control the visuals of the battlefield, especially after the expansion pack of Barbarian Invasion, which added even more controls and dynamic lighting effects to the game. For instance a separate section for night lighting effects.