(\varepsilon_{HbO}^{\lambda_{0}}\frac{\Delta OD^{\lambda_{0}}(k)}{DPF^{\lambda_{0}}}) - (\varepsilon_{HbO}^{\lambda_{0}}\frac{\Delta OD^{\lambda_{0}}(k)}{DPF^{\lambda_{0}}})}{l^{i}(\varepsilon_{HbR}^{\lambda_{0}}\varepsilon_{HbO}^{\lambda_{0}} - \varepsilon_{HbR}^{\lambda_{0}}\varepsilon_{HbO}^{\lambda_{0}})},$$ 