An apparatus for measuring a time-varying change in an amount of blood in a tissue volume, the apparatus comprising: means for exciting (12, 12A) a fluorescence agent in the blood; means for acquiring a time-varying light intensity signal during a pulsatile flow of the blood through the tissue volume, the pulsatile flow having a diastolic and a systolic phase resembling a conventional photoplethysmogram; and means for processing the acquired time-varying light intensity signal to obtain a measurement of the time-varying change in the amount of blood in the tissue volume, wherein a modified Beer-Lambert law is applied at the diastolic and systolic phases to obtain: Δ L = ln I e Φ − I m I e Φ − I p εC − 1 where: ΔL is a change in aggregate blood layer thickness within a given tissue volume, I e is an intensity of an excitation light exciting the fluorescence agent in the blood, Φ is a quantum efficiency of the fluorescence agent, I m is an intensity of the time-varying light intensity signal during the diastolic phase minimum of the pulsatile flow of the blood through the tissue volume, I p is an intensity of the time-varying light intensity signal during the systolic phase maximum of the pulsatile flow of the blood through the tissue volume, ε is a molar absorption coefficient for the fluorescence agent, C is an instantaneous molar concentration of the fluorescence agent in the blood, and wherein said processing means is configured to determine C by utilizing a concentration-mediated change in a fluorescence emission spectrum of the fluorescence agent, wherein the utilizing comprises: selecting first and second spectral bands of fluorescence emission spectrum of the fluorescence agent; acquiring by the acquiring means first and second intensities of fluorescence emission integrated over wavelengths in the first and second spectral bands respectively; calculating by the processing means a ratio of the first and second intensities; and deriving by the processing means a value for C from the ratio. means for exciting (12, 12A) a fluorescence agent in the blood; means for acquiring a time-varying light intensity signal during a pulsatile flow of the blood through the tissue volume, the pulsatile flow having a diastolic and a systolic phase resembling a conventional photoplethysmogram; and means for processing the acquired time-varying light intensity signal to obtain a measurement of the time-varying change in the amount of blood in the tissue volume, wherein a modified Beer-Lambert law is applied at the diastolic and systolic phases to obtain: Δ L = ln I e Φ − I m I e Φ − I p εC − 1 where: ΔL is a change in aggregate blood layer thickness within a given tissue volume, I e is an intensity of an excitation light exciting the fluorescence agent in the blood, Φ is a quantum efficiency of the fluorescence agent, I m is an intensity of the time-varying light intensity signal during the diastolic phase minimum of the pulsatile flow of the blood through the tissue volume, I p is an intensity of the time-varying light intensity signal during the systolic phase maximum of the pulsatile flow of the blood through the tissue volume, ε is a molar absorption coefficient for the fluorescence agent, C is an instantaneous molar concentration of the fluorescence agent in the blood, ΔL is a change in aggregate blood layer thickness within a given tissue volume, I e is an intensity of an excitation light exciting the fluorescence agent in the blood, Φ is a quantum efficiency of the fluorescence agent, I m is an intensity of the time-varying light intensity signal during the diastolic phase minimum of the pulsatile flow of the blood through the tissue volume, I p is an intensity of the time-varying light intensity signal during the systolic phase maximum of the pulsatile flow of the blood through the tissue volume, ε is a molar absorption coefficient for the fluorescence agent, C is an instantaneous molar concentration of the fluorescence agent in the blood, and wherein said processing means is configured to determine C by utilizing a concentration-mediated change in a fluorescence emission spectrum of the fluorescence agent, wherein the utilizing comprises: selecting first and second spectral bands of fluorescence emission spectrum of the fluorescence agent; acquiring by the acquiring means first and second intensities of fluorescence emission integrated over wavelengths in the first and second spectral bands respectively; calculating by the processing means a ratio of the first and second intensities; and deriving by the processing means a value for C from the ratio. selecting first and second spectral bands of fluorescence emission spectrum of the fluorescence agent; acquiring by the acquiring means first and second intensities of fluorescence emission integrated over wavelengths in the first and second spectral bands respectively; calculating by the processing means a ratio of the first and second intensities; and deriving by the processing means a value for C from the ratio.