When light pass through the object, the diffraction happens according to the object’s own RI(refractive index) and some properties of light including wavelength and phase shift, etc. also change. If the light that passes through the object is mixed up with the original light(reference), we can observe brightness changes in images according to the change of phase shift. This is a fundamental theory of phase contrast microscopy. Phase contrast microscopy is usually used for observation of the transparent objects (ex. Unstained biological sample, especially live cell.) which have difficulties to see without any stain or labeling.
ODT(optical diffraction tomography) is the technology that makes a 3D image (tomogram) with the RI and the 3-dimensional location which can be obtained by calculating phase shift in hologram taken around the specimen 360°. The similar technology is CT(computed tomography) using X-ray and MRI(magnetic resonance imaging) using nuclear magnetic moment.
Schematic light path
A sample is located on a stage between an objective and a condenser lens. A laser is split into a specimen and reference arm. The sample and the reference arms then generate a 2-D hologram, which is recorded by a digital image sensor. The laser illuminates the sample with the incident angle of 53°(HT-1S) or 63°(HT-1H), which rotates 360° with respect to the optical axis. A 3-D refractive index (RI) tomogram of the sample is then reconstructed from the measured multiple holograms with various illumination angles.
In a DMD, individual mirrors can be tilted to the angle of ±10-12°, which results to an on or off state of the reflected light. HT series utilizes a DMD in order to systematically control the illumination angle of a laser beam impinging onto a sample. Without any mechanically moving parts, the use of a DMD enables highly stable, fast, and reliable control of beam paths.
A digital micromirror device, or DMD is an MEMS-based optical light shaper from Texas Instruments™. A DMD has on its surface several hundred thousand micromirrors arranged in a rectangular array.