3D microscope for planetary exploration

Tabletop microscope systems are generally better adapted to use in laboratories than in the field. As planetary missions continue to focus on the mineralogical analysis of other solar system bodies, the need has therefore arisen for an instrument that is sufficiently robust to survive the long travel involved and that is compact and lightweight enough to be fixed on a rover for dedicated analyses of soil and rock samples. Such a microscope must also have remote control abilities, high precision, and the ability to measure the shape of a sample in three dimensions. This type of instrument could be used on Earth for non-destructive inspections in manufacturing microdevices, for surface finish characterizations, and in forensics.

The Canadian Space Agency (CSA) has funded the development of an innovative microscope system to meet the needs of planetary exploration missions. This work has been carried out by MacDonald, Dettwiler and Associates Ltd. (MDA) and the National Optics Institute (INO). The device is designed to capture low- and high-resolution images, to provide multispectral and color images, and for 3D mapping of soil or rock samples.¹ As shown in Figure 1, the Three Dimensional Exploration Multispectral Microscope Imager (TEMMI) is equipped with a 5 megapixel monochrome CMOS sensor, a pattern projector, and an active multispectral illumination system. This device is intended to be mounted on the robotic arm of planetary exploration rovers developed by CSA in the future.

Figure 1. The Three Dimensional Exploration Multispectral Microscope Imager (TEMMI) system (top) and a computer-aided design view of optical subsystems assembly (bottom). DLP: Texas Instruments DLP chip.

In image capture mode, our microscope operates over a wide spectral band (400–900nm). To provide calibrated reflectance images, we illuminate the samples by turning different color LEDs on and off in sequence. We achieve both red, green, blue (RGB), and multispectral imaging capabilities by operating the LEDs in eight different wavebands distributed over the 400–900nm spectral band. Illumination from a 365nm UV LED is also used to induce fluorescence from the sample. Light from the LEDs is transmitted via optical fibers to the three illumination heads located beside the microscope module.

The microscope consists only of fixed optical components. Consequently, changing the numerical aperture and resolution of the microscope is achieved by altering the diameter of the system aperture stop and through pixel binning. We exploit the moiré phase shift method, which requires periodical light patterns to be projected onto the surface of the object, for 3D mapping. These patterns are deformed by the surface shape and are observed with a camera from another viewpoint. In the TEMMI system, a specifically designed video projecter equipped with a DLP chip from Texas Instruments is used to project the moiré patterns, and the microscope acts as the camera. Figure 2 shows an example 3D map of a trilobite fossil that we obtained with TEMMI.

Figure 2. A TEMMI 3D map of a trilobite fossil (the red square on the head of the trilobite measures about 4×5.5mm).

The focusing capabilities of TEMMI allow the generation of high-resolution 3D color or multispectral images with a large depth of field. Such images are patchworks constructed from a set of images captured from different heights. The 3D map is used for the segmentation of the in-focus pixels in each image. This focus-stacking approach also generates color textured 3D maps and virtual models of rock samples that can be subsequently studied by humans.

Both our microscope and projector are based on the reflective Offner configuration that is illustrated in Figure 3. We use a modified Offner layout with a folding mirror to make the microscope more compact. A standard Offner 1× magnification optical relay consists of two concentric spherical mirrors with the object, image, and common center of curvature all located in the same plane. However, TEMMI is designed so that the mirrors are not perfectly concentric. The moiré patterns are projected at an angle of 30^° with respect to the microscope optical axis, which is perpendicular to the nominal plane of the sample surface. The DLP chip is also tilted at 30^° with respect to the projector optical axis to ensure that the projected patterns are in focus in the nominal sample plane. We had to add many compensation components to the projector optical train to compensate for the aberrations introduced by the thick window of the tilted DLP chip. The light source of the illumination system for the projector is a high-power white LED.

Figure 3. Offner reflective optical configurations for the microscope (left) and the projector (right).

We have developed a ruggedized compact microscope system with 3D mapping and multispectral capabilities that can be used to investigate the microscopic structure and morphology of soil and rock surfaces. The system is currently being tested at CSA and at the University of Western Ontario to establish its real potential for space and geological applications.² Further analog site test campaigns conducted by CSA are planned for spring 2013.

The work presented here is supported under a contract from CSA.