When a CCFL LCD display shows an image that is visible but dim, and brightness controls do not work, the most likely cause is a failing inverter. The inverter is responsible for converting the laptop's DC power to AC power to light up the CCFL backlight. Without a functioning inverter, the screen would be dim because the backlight isn't receiving the proper voltage. Other answers, such as the battery, power supply, and HDMI port, do not directly affect the brightness of the screen's display.