In-Situ Close-Range Imaging with Plenoptic Cameras

This paper discusses the concept of plenoptic hand lens imagers for in-situ close-range imaging during planetary exploration missions. Hand lens imagers, such as the Mars Hand Lens Imager on-board the Mars rover Curiosity, are important cameras for in-situ investigations, e.g. of rock layer, minerals or dust. They are also important for the preparation and documentation of other instrument operations and for rover health assessment. Due to the small working distance between object and the camera's main lens, significant physical limitations affect the imaging performance. Most evident is the limited depth of field of a few millimeters for working distances of a few centimeters. This requires a highly accurate positioning of the camera and also limits the in-focus content of an image significantly. Hence, in order to have an extended object completely in focus, a sequence of images, each being focused to a different distance, is required. A single, passive camera is insufficient to compute depth from a single shot; only the combination of multiple images, either taken from different vantage points or at different focal settings, allows this. To overcome those limitations, we propose the use of plenoptic cameras as hand lens imagers. From a single exposure, they allow to create an extended depth of field image and at the same time a metric depth map while maintaining a more open aperture. These and other advantages might make it possible to omit space grade focus mechanisms in the future. A plenoptic camera is achieved by adding an additional matrix of lenslets shortly in front of the image sensor of a conventional camera. Hence, available space camera hardware can be used to form a new type of sensor. Due to its recording concept, a plenoptic camera maintains the depth of the scene as it is projected into the camera by the main lens. Thanks to the parallax between the lenslets, it is possible to compute depth via triangulation for each image point as well as a high resolution 2-D extended depth of field image. This paper provides an overview of the state of the art of hand lens imaging from which we derive a set of common requirements for future devices. We briefly introduce the plenoptic camera technology and provide first experimental results on the imaging performance based on samples of test targets and rocks. The results show that our preliminary plenoptic camera setup can comply with the requirements for in-situ hand lens imaging in terms of image quality, depth estimation and the usability for planetology.