The use of India ink as an optical absorber in tissue-simulating phantoms

In studies involving the propagation of light in tissues, phantoms consisting of separate scattering and absorbing components are often used. The optical properties of phospholipid emulsions have been studied extensively but the optical properties of particulate absorbers, such as Indian ink, have not been thoroughly investigated. A common and important assumption has been that ink acts as a perfect black-body absorber. The validity of this assumption has been examined. The results presented show that the optical attenuation coefficient of India ink has a significant scattering component, and it cannot be regarded as a pure absorber. The scattering component is due mainly to a small fraction of micron-sized particles. The non-negligible scattering can cause two types of error in optical phantom work if the ink is assumed to be a pure absorber: (i) it may lead to underestimation of the total phantom scattering if the concentration of ink is high in the scattering medium and, more importantly, (ii) it causes an overestimation of the absorption if this is determined, for example, using spectrophotometry or mu t measurement of the ink solution.