Lambertian reflection (Lambert, 1760) is a widely used model for diffuse reflection. It assumes an ideal diffusely reflecting surface, where the apparent brightness remains constant regardless of the observer’s viewing angle. Consequently, the reflected radiant intensity follows Lambert's cosine law.

In GroIMP a lambert shader is defined as a simplification of a Phong shader

• Lambert JH, Photometria, sive de mensura et gradibus luminis, colorum et umbrae, Augsburg: Eberhard Klett, 1760.