均匀介质构成的圆柱被激光照射后，散射光干涉形成一阶彩虹图像，根据彩虹的频谱结构，可以实现圆柱直径的测量。结合理论及实验对双层圆柱双一阶彩虹的光强分布及频谱特性进行了研究。基于Debye 理论，通过数值计算几种干涉结构的频谱，获知双一阶彩虹频谱中4 个特征峰的成因。将与均匀圆柱叠加波结构频率成因相似的特征频率F₃ 进行单独分析, 发现F₃ 与双层柱体的外直径存在线性依赖关系；而且随着圆柱内外层折射率、内直径等参数的变化，直线斜率发生变化。F₃与内直径无明显的线性依赖关系。通过构建实验系统拍摄了双一阶彩虹，并对其频谱进行了研究。由于双层圆柱中内直径、外层折射率对彩虹图像影响较大，在这两个参数确定的前提下，由F₃ 确定的外直径的相对误差约为2%。

Diameter of a homogeneous cylinder can be determined with the spectrum of the rainbow pattern, formed by the scattering ray interference when the cylinder is illuminated by a laser. As for a coated cylinder, the intensity distribution and spectral characteristics of the twin primary rainbows under different cylinder sizes are investigated via theoretical simulation and experiments. The numerical simulation based on Debye theory is used to obtain the intensity distribution spectra and four characteristic frequency peaks are obtained, and how they are formed is also illustrated. The frequency peak F₃, with its origin similar to that of the ripple structure formed in a homogeneous cylinder, is then analyzed. The results show that F₃ increases with the outer diameter linearly and the slope is also slightly affected by the internal diameter and refractive index of the coated cylinder. However, there is no obvious relationship between the internal diameter and F₃. The experimental research is implemented by building a system to capture the rainbow pattern and its spectrum is analyzed. The relative error of outer diameter about 2% can be determined experimentally, when the internal diameter and the coated refractive index are fixed.