The contrast between the dark and dreary part of the journey does not explicitly mention light, but the fact that he can see, and note the whiteness of the fruit, implies the contrast between the vision before he prayed, and what he sees after that prayer for mercy.
What he sees is a tree. Lehi did not call this the tree of life. Nephi gives it that name in his vision (1 Nephi 11:25). It is appropriate there, but not here. Lehi knew the scriptures and knew what the tree of life was all about. This tree was similar, but certainly not the same. It didn’t heal or give lasting life—it made one happy.
Of course, we can argue that eternal life and divine joy are the same thing, but Lehi doesn’t mention life. The point of his vision is joy. When he partakes of the fruit, he is not healed. He is not made immortal. Knowing what the scriptural story of the tree of life was, Lehi saw this as different. After partaking, his soul is filled “with exceedingly great joy.”
After partaking of that fruit and joy himself, he desired what all good parents desire—that their families have that same joy.