2 Rays and Lenses

Microimaging refers to a group of imaging modalities that go beyond the limits of spatial resolution of the human eye into the microscopic world. The different image modalities within can be divided according to the nature of the signal measured, the dimensions imaged and the nature of the magnification. This review will deepen mostly in the two major modalities of magnification and try to make them clear to the reader through the explanation of the most important microimaging techniques. 1 A view into the invisible world Microimaging is a widely used term for imaging the world of small things. The word itself is formed by the term ”micro”, forged in the ancient Greece meaning small [1] and the term ”imaging”, i.e. the process of producing images [2]. Microimaging is thus the process of making images of small entities. The adjective ”small”, however, is rather subjective, but implicitly includes the dimensions of space: length, area or volume. In order to obtain an objective definition for microimaging a line at the limit of the human eye is traditionally traced. The human eye is an amazing sensor. It allows to image forms by projecting light emitted or reflected by them using an adjustable lens, the cornea, into a light sensitive concave area, the retina 1. Given such a functioning principle, the limit for seeing small structures is given by the number of sensors pro unit of angle on the retina. In the case of humans, this value is such that two point-like objects can be distinguished from each other if the angle projected on the human retina is bigger than 0.17 [rad] (about a minute of arc). The particular minimal angle of a person is indeed used to diagnose eyes under the name of visual acuity [3]. Put in terms of length a normal human eye can differentiate two dots lying at 0.73 [mm] located one meter in front of it [4]. A more interesting comparison is perhaps this separation at 10 [cm], the common limit for the adaptation of the eye lens. At this distance the eye is capable of distinguishing objects at 25 [μm]. Everything below that limit melts and becomes invisible. Microimaging is the science of making invisible things visible. It goes beyond imaging with visible light, but can be applied to almost any signal. In general imaging can be classified according to the signal imaged. This leads to the so called modalities: light imaging, x-ray imaging, γ-imaging, thermal imaging, strain imaging, impedance imaging, flow imaging, etc. A further classification for imaging in general takes place in space. One dimensional signals do not normally classify under images, so images in spatial sense are divided in two and three dimensional imaging. In between there are several modalities that are indeed not two not three dimensional. An example is stereoscopic images. The concept behind is that humans can get a three dimensional impression of the world given the fact that they have two eyes. In this binocular view the world is reconstructed from two two dimensional slightly different images into a three dimensional world. The difference in the images arises from the different position and orientation in space of the two eyes or sensors, and thus the different relative position of image components to each other. Systems that try to emulate this are usually not fully three dimensional, since discretize depth when generating the two two dimensional images in case of visualization or when reconstructing the three dimensional information in acquisition [5]. A second example is three dimensional surface imaging. In this case the object imaged has a two dimensional nature (a surface in three dimensions can be completely described by two parameters), but can be acquired in three dimensions if the physical magnitude of interest is acquire on the surface, giving birth to a pseudo three dimensional imaging. The last classification to be considered in this review is according to the procedure used to achieve the magnification. From 1 it can be seen that in order to magnify distances, a method for increasing angles is needed. This can be achieved by deflecting rays or by miniaturizing the sensor to be used. Since this classification is innate to microimaging and determines directly the resolution of the system, this review will deepen in that classification in the coming sections.