Note: Mathematical definition: \[\frac{{\rm{d}}n/{\rm{d}}t}{q_{n,\rm{p},\lambda}^{0}}\] when using the incident spectral photon flux, chemical amount basis, \(q_{n,\rm{p},\lambda}^{0}\), with \(n\) (mol) the amount of substance produced or consumed, and \[\frac{{\rm{d}}N/{\rm{d}}t}{q_{\rm{p},\lambda}^{0}}\] when using the incident spectral photon flux, number basis, \(q_{\rm{p},\lambda}^{0}\), with \(N\) the number of entities produced or consumed. The superscript 0 (zero) emphasises the fact that the incident number of photons (prior to absorption) is used.