How does the presence of a stigma in the Euglena help it to capture light energy?

Stigma or eyespot:

the presence of a pigmented organelle, or eyespot, that allows the organism to orient toward or away from light. This is a sensible adaptation since these organisms carry out photosynthesis. The image to the left show the eyespot . The eyespot itself is not sufficient to help the organism turn toward light since the cell is transparent. So the outside of the eyespot is covered by a black pigmented area. The Euglena determines which way turn turn by turning to the direction in which the eyespot is receiving the least light. In this direction the pigmented eyespot is most directly shaded by the black pigmented area.