Film in the sense of dye adsorbed on TiO2 coated FTO glass plate. For solution sample we are using milk powder as a standard and what is the standard used for film?
Peter is also right. You could also follow his suggestion. But ensure that you collect both the IRF and sample decay without changing the orientation.
If you are using the mirror method, then also ensure that you have identical geometry for both the sample and the mirror. Have two identical holders made, place your sample and a small piece mirror on the holders in identical orientations and place them in the instrument for recording the decays.