余弦辐射体，也称为朗伯辐射体(Lambert radiator)，指的是发光强度的空间分布符合余弦定律的发光体（不论是自发光或是反射光），其在不同角度的辐射强度会依余弦公式变化，角度越大强度越弱。

In optics, Lambert's cosine law says that the radiant intensity or luminous intensity observed from an ideal diffusely reflecting surface or ideal diffuse radiator is directly proportional to the cosine of the angle θ between the direction of the incident light and the surface normal. The law is also known as the cosine emission law or Lambert's emission law. It is named after Johann Heinrich Lambert, from his Photometria, published in 1760.