Ambient reflection is widely present in many applications of computer graphics and image processing, which is traditionally modelled as a constant free from environmental factors. This paper reconsiders ambient reflection modelling of Lambertian and Phong surfaces and calculates it as reflection integrations of infinitesimal incident beams from the environment. It reveals that ambient reflection exists in variable forms for Lambertian surfaces of non-convex objects and Phong surfaces of all objects. For convex objects with Lambertian surfaces, ambient reflectance coefficient is actually the diffuse reflectance coefficient. Generalised ambient reflection models are proposed to calculate ambient reflection using the same reflection model as used in calculation of other reflections. Based on this analysis, new ambient reflection formulations of Lambertian and Phong surfaces are derived to enable efficient computations in computer graphics and image processing.