Focus Measure in a Liquid-filled Diaphragm (LFD) Lens Using Passive Auto-focus Method

Auto-focusing in imaging systems depends on the determination of the correct image focus criterion. In this research, the image captured by a liquid-filled diaphragm (LFD) fluid lens was analyzed to determine a focus measure criterion that can be used to establish the correct focus and thus quantify the image quality. The LFD lens was actuated using a stepper-motor driven syringe mechanism. The lens diaphragm was made of polydimethylsiloxane (PDMS) polymer that exhibit good optical properties. The lens focal length was controlled by varying the fluid volume within the diaphragm lens. A CCD camera was attached to the fluid lens to capture live images of a binary target. The edge slope width (ESW) of the pixel intensity profile across the white-to-black transition region in a binary target was used as the focus measure. The experiments carried out showed the viability of the proposed focus measure criterion for automatically focusing the image formed by a diaphragm-type fluid lens.