Medical Physics and Biomedical Engineering


Tissue Spectra

Important note

We make an important distinction between specific extinction coefficient and specific absorption coefficient. While both represent the level of absorption per micromole (or millimole) of compound per mm (or cm), extinction is described using base 10 logarithm units and absorption is described using natural logarithm units. The difference between them is a scaling factor of ln10:specific absorption coefficient = specific extinction coefficient x 2.3025851 .

Water Spectrum

A file containing the extinction coefficient of water at 37 degrees over the wavelength range 600 nm - 1050 nm may be downloaded by clicking on the link below:

The spectrum is sampled at 1 nm intervals, and contains a list of ascii pairs, with the format: wavelength (nm), extinction coefficient (OD per cm). The measurement of this spectrum is described by Matcher et al [2].

The following extinction spectra were measured at 37 degrees and the temperature coefficients at each wavelength were measured in the range 28 - 42 degrees. These measurements are described in the Ph.D. thesis of Dr. Veronica Hollis.

Haemoglobin spectra

UCL Spectra

Files containing the specific absorption spectra of haemoglobin (Hb) and oxyhaemoglobin (HbO2) over the near-infrared wavelength range 650 nm - 1042 nm may be downloaded by clicking on the links below:

The spectra are sampled at 1 nm intervals. Each file contains a list of ascii pairs, with the format: wavelength (nm), specific absorption coefficient (per mm per micromolar). More details about the measurement of these spectra can be found in the Ph.D. thesis of Dr. Mark Cope and the publications of Matcher et al [1,2].

Spectra published by Zijlstra et al.

The specific extinction coefficients of various derivatives of both adult and fetal haemoglobin have been published by Zijlstra et al [4]. We have converted the published data from values per haem group to values per functional group (tetrahaem) by multiplying by a factor of four. Each of the following files contains spectra sampled at 2 nm intervals, consisting of ascii pairs with the format: wavelength (nm), specific extinction coefficient (per cm per millimole):

a Generated by polynomial fit to data published in table 8.4 of Zijlstra et al. [4].

Cytochrome Spectra

The difference extinction spectrum (units of OD per cm per millimole) of cytochrome oxidase (Cyt) over the wavelength range 650 nm - 986 nm may be downloaded by clicking on following:

Note that since the total cytochrome oxidase concentration does not change, in vivo near-infrared measurements need only be made of the change in redox state, and therefore it is only necessary to know the difference spectrum between the oxidised and reduced forms of the enzyme. The difference spectrum provided here contains a small contribution from cytochromes other than cytochrome oxidase found in tissue. The contribution of cytochrome to overall absorption in tissue is considerably less than that of haemoglobin because of its lower concentration. Typically, the neonatal brain will have 40-60 micromoles of haemoglobin and 2-3 micromoles of cytochrome oxidase. For useful physiological measurements, one should expect to resolve changes of 1 micromole in haemoglobin and 0.1 micromoles of cytochrome oxidase.

Cytochrome spectra measured at visible wavelengths are also available for download by clicking on the links below. These were all measured from bovine heart mitochondria and are described by Heinrich [3]. Note that in the visible region absorption by the cytochrome oxidase complex is due to the haem a and a3 components, and not the Copper A as for the near-infrared spectra.

The difference spectra for the above set of data are shown in the graph below.


Various other visible cytochrome spectra are available by clicking on the links below:

c This was measured on bovine mitochondria by Springett and Cooper at UCL in 1995.

d This was measured by Prof. Peter Rich at UCL on bacterial cytochrome b and has been red-shifted by 1 nm to be equivalent to mammalian cytochrome.

e This difference spectrum was measured by Liao and Palmer [5].

f Measured by Prof. Chris Cooper at Essex University on porcine heart.

g Described in the Ph.D. thesis of Dr. Mark Cope. Note there is a discontinuity at 640 nm due to a filter change, although this does not appear in the difference spectrum (i.e. oxidised - reduced).

h These data are from bovine heart, measured by Dr. John Moody at the University of Plymouth.


  1. Matcher, SJ, Elwell, CE, Cooper, CE, Cope, M, and Delpy, DT (1995): Performance comparison of several published tissue near-infrared spectroscopy algorithms. Analytical Biochemistry 227(1), 54-68.
  2. Matcher, SJ, Cope, M, and Delpy, DT (1994): Use of the water absorption spectrum to quantify tissue chromophore concentration changes in near infrared spectroscopy. Physics in Medicine & Biology 39, 177-196.
  3. Heinrich, U (1981): Untersuchungen zur qualitativen photometrischen analyse der redox-zustande der atmungskette in vitro und in vivo am beispiel des gehirns. PhD thesis, Abteilung fur Biologie and der Ruhr-Universitat Bochum. These data were kindly supplied by Professor D. W. Lubbers.
  4. Zijlstra, WG, Buursma, A, and van Assendelft, OW (2000): Visible and near infrared absorption spectra of human and animal haemoglobin (VSP: Utrecht). ISBN 90-6764-317-3
  5. Laio, G-L, and Palmer, G (1996): The reduced minus oxidized difference spectra of cytochromes a and a3. Biochimica et Biophysica Acta 1274, 109-111.

Last update: October 18, 2005