The measurement of heat release rate is of great importance in the study of thermo-acoustic instability occurring in lean premixed combustion and the chemiluminescence emission has been used as an indicator of heat release in combustion instability studies primarily for its relative simplicity. This paper presents results of experimental study of flame chemiluminescence from an atmospheric, swirl-stabilized, turbulent lean premixed flame with a main emphasis on the effect of $CO_2^*$ background level in the $CH^*$ and $OH^*$ band. The test results show that the effect of $CO_2^*$ level in the $CH^*$ band is greater than that in the $OH^*$ band. Also, the background to peak ratio for both $CH^*$ and $OH^*$ bands can be expressed as a function of equivalence ratio, almost regardless of a change in the inlet velocity.