This is the 16549th most frequent Russian word.
световой
"Световой" means "light-related" or "pertaining to light" in English.
"Световую" describes "схему," indicating that the scheme pertains to lighting arrangements.
Он построил световую схему для театрального шоу.
He created a lighting scheme for the theatrical show.
Here, "световой" modifies "индикатор," meaning that the indicator utilizes light or is related to light.
Световой индикатор на приборе показывает уровень заряда.
The light indicator on the device shows the level of charge.
In this context, "световой" modifies "день" to mean the portion of the day with natural light.