Most laptops manufactured before 2014 use a Cold Cathode Luminescent Lamp (CCFL) for lighting the screen (backlight). CCFL lamps have a relatively high energy consumption and short life. On the other hand they have some advantages, like the lack of PWM-caused flickering that cause eye- and headache. Pulse Width Modulation (PWM) is used on any laptop to control the brightness of the screen. PWM does not mean "flickering", but may cause flickering on some displays.
The invertor is a small board situated under the LCD panel that generates high frequency and high voltage AC - over 600V, and from 300Hz to 10kHz. It is the usual suspect when the screen is very dim due to backlight not functioning. Having in mind that nowadays all laptops with CCFL backlight are 5+ years old, it is much more likely the lamp to be exhausted. That's why do not hurry to order an invertor - it may not solve your problem. Better visit a local repair shop for diagnosing the problem.