We present a method to measure the exciton diffusion length (LD) of optically dark triplet states in organic semiconductor thin films. In order to probe these states, we optically inject triplets via energy transfer from an adjacent phosphorescent thin film. Injected triplet excitons migrate through the full thickness of the material of interest before undergoing energy transfer to a luminescent phosphorescent sensitizer. By measuring photoluminescence from the sensitizer as a function of active layer thickness and sensitizer layer concentration, we are able to extract both LD and the transfer efficiency to the sensitizer. This is in contrast to much previous work that assumes a unity quenching efficiency in measurements of LD. Here, we demonstrate this method on the archetypical organic light-emitting device (OLED) hole transport material N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine) (NPD). An injection layer of tris[2-phenylpyridinato-C2,N] iridium(III) (Ir(ppy)3) is used with a sensitizing layer platinum octaethylporphyrin (PtOEP) diluted into tris(4-carbazoyl-9-ylphenyl)amine (TCTA). In extracting a value of LD = (30±5) nm for NPD, we find a direct correlation between sensitizer concentration and the transfer efficiency. Consequently, it is essential to determine the transfer efficiency to the sensitizer in order to extract the correct value of LD. We expect this technique to be widely applicable with appropriate choice of injection and sensitizer materials.