image using the soft x-rays emitted from a synchrotron. Recently, the use of soft x-rays emitted from laser-produced plasmas rather than synchrotron radiation is becoming more popular. Analysis Additionally, X-rays cause fluorescence in most materials, and these emissions can be analyzed to determine the chemical elements of an imaged object. Another use is to generate diffraction patterns, a process used in X-ray crystallography. By analyzing the internal reflections of a diffraction pattern (usually with a computer program), the three-dimensional structure of a crystal can be determined down to the placement of individual atoms within its molecules. X-ray microscopes are sometimes used for these analyses because the samples are too small to be analyzed in any other way. Biological applications One early applications of X-ray microscopy in biology was contact imaging, pioneered by Goby in 1913. In this technique, soft x-rays irradiate a specimen and expose the x-ray sensitive emulsions beneath it. Then, magnified tomographic images of the emulsions, which correspond to the x-ray opacity maps of the specimen, are recorded using a light microscope or an electron microscope. A unique advantage that X-ray contact imaging offered over electron microscopy was the ability to image wet biological materials. Thus, it was used to study the micro and nanoscale structures of plants, insects, and human cells. However, several factors, including emulsion distortions, poor illumination conditions, and low resolutions of ways to examine the emulsions, limit the resolution of contacting imaging. Electron damage of the emulsions and diffraction effects can also result in artifacts in the final images. X-ray microscopy has its unique advantages in terms of nanoscale resolution and high penetration ability, both of which are needed in biological studies. With the recent significant progress in instruments and focusing, the three classic forms of optics—diffractive, reflective, refractive optics—have all successfully expanded into the X-ray range and have been used to investigate the structures and dynamics at cellular and sub-cellular scales. In 2005, Shapiro et al. reported cellular imaging of yeasts at a 30 nm resolution using coherent soft X-ray diffraction microscopy. In 2008, X-ray imaging of an unstained virus was demonstrated. A year later, X-ray diffraction was further applied to visualize the three-dimensional structure of an unstained human chromosome. X-ray microscopy has thus shown its great ability to circumvent the diffractive limit of classic light microscopes; however, further enhancement of the resolution is limited by detector pixels, optical instruments, and source sizes.