Use the Toon Directional light with Toon materials for effective work, since it was developed for this MaterialThe representation of the surface or volume properties of an object. type. This adds a Directional light to the scene, much like the native direct light in Maya®, and is suitable for global lighting. Rotation is important, and position does not have any effect on the illumination. The Size In Viewport and Power attributes have no effect on the light source due to the stylistic nature of toon shading. The Texture attribute controls the light source's overall brightness. You can control this by the slider value, or by using a Texture map.
Figure 1: The Toon Directional light