Based on practical requirements for improving the image quality of ao optical instrument

it is demonstrated that the reflection of the cemen- ted surface of the combination lens in an optical system is an important factor of producuing the stray light and influencing the image quality. A new idea that anti-reflection film should be applied to the cemented surface of a combination lens is put out in the article The calculating formula for the total reflection of the cemented surface is derived.The cemented surface reflection of three types of cemented lenses are analysized.The calculations show that the cemented surface reflection is related to the index difference of two leuses having been cemented and the index difference of the glue layer and two lenses having been cemented.The greater the diffrence is

the more seriously the cemented surface reflects.The reflective ratio is 0.1%-1.1%;the cemented surface with anti-reflective film could effectively decrease the reflection

at least by 2 orders of magnitude. Design examples of cemented surface coating and some experimental data are also given.The results are identical with the theoretical calcul alions. Finally

it is put out that the traditional design and working technique of lens cemented surfaces without being coated should be changed. At least the cemented surface with great index difference should be coated with anti-reflective film.This is more important for the optical systems which are complicated

and whose image quality requirments are strict.