Technology

By transferring the image-producing step to a proprietary computer programme, the Phase Focus Virtual Lens® not only eliminates the restrictions and performance limitations of today’s conventional focussing devices, but also enables a range of new and improved applications.

The underlying technology of the Phase Focus Virtual Lens® is the Ptychographical Iterative Engine ( PIE; ePIE) that has been pioneered by the company’s Chief Scientifc Officer, Professor John Rodenburg, and his academic colleagues.

While the technology is in principle applicable to the entire electromagnetic spectrum, Phase Focus’s initial developments are directed towards applications in optical, electron and X–ray microscopy.

Rather than using a conventional focussing device such as a glass lens, the Phase Focus Virtual Lens® creates images using the “diffraction pattern” that is generated when an illuminating beam passes through or is reflected from an object. (“Diffraction” is the apparent bending and spreading of waves when they meet an obstruction. It can occur with any type of wave including sound waves, water waves, and electromagnetic waves such as visible light, X-ray and radio waves. Even particles such as electrons can act like waves, and so are also subject to diffraction.)

A series of diffraction patterns is recorded as an illuminating beam moves over a user-defined region of interest on the specimen. The Virtual Lens computer algorithm then processes the diffraction patterns to display images reconstructed from the complex wavefunction (amplitude and phase) that emanates from the chosen area of the specimen.

The algorithm can compute simultaneously the complex wavefunction of the illuminating beam, enabling the illumination to be matched to particular experimental requirements, and allowing the Virtual Lens to operate in situations where the illumination function is highly complex or difficult to model.

Two output images are created:

• The TruWave^TM Magnitude Image is a quantitative measure of how much of the illumination is transmitted through (or reflected from) the specimen and is similar to a conventional brightfield microscope image.
• The TruWave^TM Depth Image is a quantitative image showing how much the specimen changes the phase of the illumination as the wavefront passes through, or is reflected by, the specimen. This image forms the basis of a wide range of analytic options such as refractive index, dielectric constant or thickness measurement, surface topography, cellular analysis, or – in the case of electron microscopy – analysis of magnetic or electronic properties. The image may also be used to create very high contrast between constituents of a specimen that would otherwise be essentially invisible, without the need for exogenous chemical stains.

TruWave^TM Magnitude Image and TruWave^TM Depth Image pairs can together both be focussed in the computer at any user-selected depth in the specimen. This multi-plane z-stack feature can be used to analyse three-dimensional characteristics of the specimen, or to enable post-acquisition focussing (“ perfect focus every time“.)

A summary of the method, and some visible light applications examples, can be found in a recent Imaging & Microscopy magazine article, and also in a recent SPIE newsroom article.