A method of treating a lamp tube having a first end and a second end comprising introducing a first quantity of a luminescent substance into the first end of the lamp tube and introducing a second quantity of a luminescent substance into the second end of the lamp tube is provided. An illumination source comprising a linear tube having a first end and a second end and an inner surface having a luminescent substance distributed thereon, a longitudinal distribution of the luminescent substance having a minimum at a first point of the inner surface and a luminescent substance density greater than the minimum at each of a second and third point of the inner surface, the first point longitudinally located between the second and third points, is provided.