We propose a new algorithm for optical computed tomography to determin
e the concentrations of absorber in turbid media such as human tissue.
Our method does not require the measurement of a reference phantom, s
o that it is free from significant errors caused by geometric and stru
ctural differences between the target and reference media. We use an i
maginary reference medium for which the optical properties are specifi
ed by the average or quasi-average values measured for the real target
medium. Since we can calculate the weight function and re-emission fo
r such an imaginary medium, tomographic images can be reconstructed ba
sed on the difference in values measured for the target medium and det
ermined for the imaginary reference. The validity of our method was co
nfirmed by measuring an 80-mm-diameter cylindrical, tissue-like phanto
m containing three different absorbers. The image quality such as imag
e distortion and the capability for quantifying the concentrations of
absorber are also discussed.