Accurate Measurements of Droplet Volume With Coherence Scanning Interferometry

Accurate volume measurement of droplets in inkjet printed organic light-emitting diode (OLED) is important to achieve defect-free manufacturing. Although coherence scanning interferometry (CSI) has been widely used in the 3-D reconstruction of microscale/nanoscale, most existing CSI-based methods pay little attention to the processing of fringe signals with overlap caused by the transparency of the sample. In this article, we propose an effective and accurate volume measurement method for transparent droplets, i.e., wavelet transform-based CSI (WTCSI). First, the envelopes of fringe signals with or without overlap are obtained by wavelet transform (WT). Then, an adaptive multi-Gaussian model (AMGM) is proposed to describe the envelopes. In addition, AMGM uses the zero-crossing count method (ZC-CM) to adaptively distinguish the fringe signals with or without overlap, and the mask of the overlap region corresponding to the droplet region is available as well. Therefore, the upper and lower surfaces of the droplet are separated via weighted least-squares (WLS) on the envelope. In addition, the zero-order fringe peak is identified for a higher accuracy measurement of the droplet profile by combining the envelope peak and phase information. Finally, the volume of the droplet is calculated by integrating the previously measured mask and height. The experimental results demonstrate that the proposed method is effective in the separation of overlapped fringe signals. In addition, the volume measurement error of the proposed method is ±2%, while the relative deviation is better than 0.5%.