Spectral internal transmittance τi (λ) is the ratio of the light intensities at the outlet Φout (λ) and inlet Φin (λ) of the flow cell.
$\tau_{i} \left( \lambda \right) = \frac{\phi_{\text{out}} \left( \lambda \right) }{\phi_{\text{in}} \left( \lambda \right) }$
Fig. 51: Internal spectral transmittance
Unlike the transmittance, in this case the reflection losses occurring at the flow cell windows have to be eliminated. In practice, this is done by running a comparative measurement of the carrier medium without the absorbent product.