Personal Light Source
The only main difference in the Personal Light Source in Underworld compared to Legend is that there's no limit on how long the light is on. In Legend the light lasts for about one minute and 40 seconds, then the light shuts off and recharges.
In Legend an icon appears to the bottom left corner of the screen. This icon shows how much light is left. in Underworld an icon appears only briefly to indicate when the Utility Light is on or off.
From the Official Site
Affixed to the forward strap of her backpack, this light can provide temporary illumination in the darkest corners of Lara's explorations.
Description from the PDA in Tomb Raider Legend
Ultra-bright LED portable light source. Kinetically charged, the PLS will shine for a limited time before shutting off to recharge.
Description from the PDA in Tomb Raider Underworld
The utility light is a rugged, waterproof light source that offers hands-free, high-intensity illumination, and a sensor that detects levels of ambient light and automatically shuts dowsn as necessary.