Follow the link to find clear discussion on above topic

https://www.researchgate.net/post/Can_Beer_Lambert_law_be_applied_to_fluorescence_based_detection_of_enzyme_activity

Beer Lambert Law is not for fluorescence intensity measurement, Fluorescence intensity usually defined in Arbitrary Unit (A.U.) a Procedure Defined Unit (PDU). The general equation for the measurement of intensity could be like this:

Intensity (Em) = Constant * ((Em + noise (Em)) /(Ex + noise (Ex))

Constant : a constant defined by the maker of spectrometer or by anyone making the spectrometer

Em : Emission intensity measured by PMT (or other sensor)

Ex : Excitation intensity measured by photodiode (or other sensor)

of course Em and Ex will be affected by noise

to obtain more global unit, one of the way is by using intensity calibration such as conversion to Raman Unit (R.U.) as explained in the following paper:

Article Fluorescence Intensity Calibration Using the Raman Scatter P...

Hope this can help.

Sincerely.

thanks so much for your answers