Home Spectra News

Optical absorption spectrum of carbonized tissue. by Steven Jacques, Oregon Medical Laser Center, May 1, 1998. When a high power density continuous laser irradiates a tissue, the heating and dessication cause formation of a carbon layer on the tissue surface. The absorption of laser energy by this carbon layer is very efficient and drives tissue vaporization (or "ablation"). This article briefly describes the optical absorption spectrum of such a carbon layer.

A 90-W Nd:YAG laser was used to coagulate some chicken breast from the grocery store (see figure). One site was irradiated until a carbon layer began to form. At a second site irradiation was stopped just prior to the formation of the carbon layer. In both sites, the degree of coagulation was the same.

The optical depth of the carbon layer was measured by a mixed optical fiber bundle carrying light to and from a spectrometer. The optical depth (b, dimensionless) was calculated:

The absorption coefficient, µ_a [cm^-1], was calculated:

where d is the thickness of the carbon layer [cm]. The factor 2 refers to the pathlength for collected photons, one pass through the carbon layer upon entry and a second pass upon escape.

The optical depth of a carbon layer approximately 25 µm thick, a thickness estimate made by Verdaasdonk et al. (University Hospital Utrecht, The Netherlands).

The absorption coefficient of a laser-induced carbon layer on chicken, assuming a 25-µm thick carbon layer. The approximate value of µ_a for a wavelength, nm, expressed in nanometers is

Some values of µ_a for a few selected wavelengths are:

The above values for µ_a are tentative based on the correctness of the assumption of a 25-µm thickness for the carbon layer formed on the chicken. Regardless, the spectral shape as a function of wavelength is correct.