Optical system to detect volume of medical samples in labeled test tubes

A novel system for liquid-level detection is reported. The system uses two different wavelengths of light to detect the liquid level of medical samples in tubes that are covered by an unknown number of paper labels. By measuring the intensity of the transmitted light at two distinct wavelengths and computing their ratio, which is compared to a threshold value, a system was developed that is self-compensating for the number of tube labels and the type of medical sample to be analyzed. A laboratory prototype was built, and the test results were analyzed using different types of medical samples. Based on a series of experiments, the system was found to detect the liquid level of 0.4 mL with a maximum allowable tolerance of ±0.1 mL with better than 99.73% reliability and a total test time of 0.5 s. These results were achieved with test tubes that had up to six layers of labels attached to the outside of the tube, thereby making the tube completely opaque.