As a near ideal biometric, iris authentication is widely used and mobile acquisition techniques are known. But iris acquisition on handheld imaging devices, such as smartphones, poses multiple, unique challenges. In this paper, a range of factors that affect the quality of iris images are reviewed. Iris size, image quality and acquisition wavelength are found to be key factors. Experimental results are presented confirming the lower limits of iris size for useful authentication performance. The authentication workflow for handheld devices is described. A case study on a current smartphone model is presented, including calculation of the pixel resolution that can be achieved with a visible-only optical system. Based on these analyses, system requirements for unconstrained acquisition in smartphones are discussed. Several design strategies are presented and key research challenges are outlined together with potential solutions(1).