Rainbow refractometry is widely used to measure the radius and real part of refractive index of a cylinder. However, studies on the detection of imaginary part of the refractive index with rainbow technique were scarce. This paper presents a new method for simultaneously measuring the radius, real and imaginary part of the refractive index of a cylinder, on the basis of the Airy theory and the Bouguer theory. The rainbows produced by the illuminated cylinder at a capillary exit are captured by a CCD camera in a lab- scale system, and then processed by the proposed method. Experimental results showed that the radius, real and imaginary part of the refractive index can be accurately determined when the SNR （signal to noise ratio） of the ripple structure is sufficiently high. However, the SNR of the ripple structure gradually decreases with decreasing scattering intensity of the cylinder, leading to larger measurement errors of the radius and real part of the refractive index. The relative error of the imaginary part of the refractive index derived from the measurement errors of the radius and real part of the refractive index, is less than 3.4%.
