Life depends on a beam of light

'I Am Legend' is a 2007 post-apocalyptic science fiction film directed by Francis Lawrence

In September 2012, after a lethal worldwide epidemic, Robert Neville (Will Smith) is left as the last healthy human in New York City.

Only 12 million people were naturally immune to the virus. The rest degenerated into bald, pale, aggressive beings referred to as 'Darkseekers', who hunted down the immune humans as prey. The 'Darkseekers' are so-called for hiding in buildings and dark places during the day due to a painful intolerance to UV radiation.

In this scene, the protagonist and his dog can escape from the 'Darkseekers' thanks to a beam of light that is like a barrier between them and the others.
But it's the sunset and the beam of light is getting smaller...