A reference standard for tissue-simulating phantoms, i.e., a phantom with well known and stable optical properties,
reproducible, and easy to be found, would be very useful for many applications based on measurements of
diffused light. Although many tissue-equivalent phantoms have been proposed, to our knowledge none of them
has been characterized sufficiently well to be suggested as a reference standard. Based on the results of measurements
of optical properties we carried out at visible and NIR wavelengths, the use of Intralipid 20% diluted
in water as diffusive medium, and of India ink as absorber, is here suggested as a first step towards a diffusive
reference standard for tissue-simulating phantoms. As for Intralipid 20%, measurements carried out on samples
from nine different batches with expiry dates spreading over ten years showed surprisingly small batch-to-batch
variations. For the reduced scattering coefficient the maximum deviation from the value averaged over the nine
batches was of about 2%, and the results for the absorption coefficient were very close to those for pure water.
As for India ink measurements on samples from different batches and from five different brands showed large
inter-brand and inter-batch variations for both the absorption and the extinction coefficient. On the contrary,
small variations have been observed for the ratio between the absorption and the extinction coefficient. Intralipid
20% and Indian ink can be therefore easily mixed to obtain liquid phantoms with well known optical properties.
This phantom can be a first step towards a reference standard for optical tissue phantoms.