Based on the space-invariant linear system, the analytical formula in Ref. 1 only describes a partial influence of stray light on image quality. Hence it is necessary to measure the veiling glare index of an optical system in order to evaluate its veiling glare level.However, in order to measure the veiling glare index of a long focal length optical system, we put forward a new divided extended bright field method, in which the black patch is hung up in front of the extended bright field and its dimension is strictly limited. A new design method of black patch is then presented.