It’s all Greek to me!
Submitted by J Heath on 19 September, 2014.
When I was writing the Dictionary of Microscopy in 2005 I asked a very distinguished microscopist on my reviewing panel for a definition of diffraction. “The interaction of a wave of any kind with an object of any kind,” came the reply. Although I didn’t use it (sorry David!), saying instead “the scattering of waves by an object,” it makes the point that all objects diffract light (and electrons) and hence a diffraction pattern contains all the spatial and phase information about the object.
If the object has a periodic structure, such as a butterfly wing scale or a semiconductor, a clear diffraction pattern with several orders of spots is formed by interference of the diffracted waves in the back focal plane of the microscope objective lens. In a light microscope you can see this pattern by using a Bertrand lens to view the back focal plane; in a transmission electron microscope you switch on the diffraction lens to see it on the screen.
In developing his classic theory of image formation in the (light) microscope Ernst Abbe realized that the more orders of the diffraction pattern captured by the lens, the higher the resolution of the image. The resolving power or information limit of high-resolution electron microscopes is typically demonstrated with Young’s fringes and power spectra; in modern fully aberration-corrected machines that limit is now better than 50 picometers.
Of all the myriad imaging techniques in microscopy, the one with the strangest name must be ptychography. (Even if you are a Greek scholar its etymology still doesn’t give you a clue as to its meaning: ptycho = fold + graphy = write.) The term was coined by German physicist and electron microscopist Walter Hoppe (1917-1986) to describe a technique of coherent diffractive imaging that uses convolution, the mathematical process of folding two functions together.
Ptychography measures the phase and amplitude of a scattered wavefield. An object is scanned by a coherent beam and diffraction patterns are recorded. Because each diffraction pattern is obtained from a region that overlaps another there is a large degree of redundancy in the data which is used to calculate the phase of the diffraction patterns. Knowing the phase of thin transparent objects allows the reconstruction of very high-resolution images and hence 3D models. Because in principle ptychography does not require a lens it has huge potential to revolutionise electron and X-ray imaging.
Dr Julian Heath
Top: Ptychography of Spirogyra. (Image and ptychography definition courtesy of Tim Godden and John Rodenburg)
Below: Diffraction pattern (left) and power spectrum (right) showing resolution of a 200 kV TEM. (Courtesy of JEOL)
ADDENDUM 13 October 2014
Following the publication of this blog, I received the following comment from George Pantazopoulos of the ELKEME Hellenic Research Centre for Metals in Athens, Greece, an M&A reader and contributor, on the etymology of the words ptychography, microscopy and analysis.
Congratulations for the condensed and well thought editorial of Microscopy and Analysis, September/October 2014 issue, under the title “It’s all Greek to me!” Please allow me the opportunity to slightly extend this discussion for the term “ptychography” to a more broader basis.
The Greek origin of this narrow but very promising field of microscopy science (ptycho-graphy ≡ πτυχο-γραφία : πτυχή = fold + γραφή = writing / scripture) is not an exception but the rule in classical and modern scientific terminology. As a representative paradigm, the title of the very popular edition Microscopy and Analysis is absolutely Greek. The etymological sources of those words are highlighted as follows:
a. Microscopy: consisted of the Greek words Μικρός (small, little) + Σκοπώ (observe carefully, study), meaning in a more liberal translation the process or science of observation of the (small scale) matter, such as cells, crystals, molecules/atoms. From this topic, relevant terms are produced, such as microscope and microscopic. Many of the modern scientific terms are synthesized having as a second end component Greek words such as –scopy and -graphy e.g. Spectroscopy, Endoscopy, Fractography, Topography, Tomography, Crystallography, and many, many others. Another huge family of Greek origin complex words display the end component –logy (≡ -λογία originated from the Greek noun λόγος and the verb λέγω which means to speak, to say,…, producing also the word λέξις meaning word and standing also as a linguistic term/see lexicon). So, the word “λόγος” / logy – logist, comes as the end component of complex words frequently met in the core of sciences and philosophy: Methodology, Geology, Mineralogy, Cosmology, and so many others.
b. Analysis. Consisted of the Greek words Ανά - Λύσις (the latter originated from the verb λύω which has a plethora of meanings: unbound, solve, etc.). Analysis is a genuine term and in a more general sense means a study process that aims to unfolding and deep understanding of the basic constituents of the matter, but also the elemental aspects of a theory or an idea with an abstract or complex background. This term has a tremendous value in conveying the meaning of many processes and actions in modern and classical science and philosophy, reflecting the diachronical essence of the scientific roots of study of matter composition, presenting exceptional references to Aristotelis (Aristotle) and basic four elements (earth, air, fire, water), Demokritos (Democritus) and Ancient Greek Atomic Philosophers, who had an outstanding influence on the development of the modern science tracing back to the Alchemists’ endeavors and Medieval and Renaissance philosophy, continued by the birth of Experimental Science based on logical reasoning and validation based on experiment (Galileo Galilei).
Epimythion: The predominance of Ancient Greek language in modern and classical science and philosophy, not only contributes to precisely transferring the meaning of the terms, using the original/genuine words tracing back to the Antiquity 2,5-3 millennia ago and more, but it has also embraced the entire scientific world epitomizing the roots and foundations of human free thinking/intellectual progress and the perpetual effort in seeking the elements of truth and knowledge – at every level of existence (matter, man, universe).