Brightfield microscopy normally only provides a low-contrast image of many transparent biological specimens where few details are distinguished. One way to enhance contrast with brightfield microscopy is to use selective stains, but such stains are often toxic to living cells. A phase contrast light microscope offers a way to view the structures of many types of biological specimens in greater contrast without the need of stains. The contrast method exploits differences in optical density between structures of a specimen that lead to a phase shift of the light that interacts with the specimen and its structures.

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Portions of the ring-shaped light are diffracted by optically dense structures of the specimen and experience a negative phase shift of about λ/4. This phase-shifted, diffracted light bypasses the λ/4 plate. In contrast, the portion of the ring-shaped light that passes directly through the specimen non-deviated will hit the phase plate which causes a positive λ/4 phase shift. As the total difference in phase shift between the light diffracted by the specimen’s structures and that which passes through phase plate will be about λ/2, destructive interference will occur. Consequently, more optically dense structures will appear darker than those that are less optically dense.

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For forensic applications concerning the evidentiary investigation of paints, pigments, textiles, fibers, and human tissues, Leica microscopes offering phase contrast are very useful solutions.

Leica microscopes capable of phase contrast make a difference for the study of transparent and colorless minerals, crystals, and polymers.

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The phase contrast method for microscopy was developed in the 1930s by the Dutch physicist Frits Zernike. After 1942, it became a widely used microscopy technique. In 1953, Zernike was awarded the Nobel Prize for Physics. For more details, refer to the articles: A Brief History of Light Microscopy – From the Medieval Reading Stone to Super-Resolution & Phase Contrast

A phase contrast microscope is similar to a conventional brightfield microscope, except it uses an annular aperture in front of the light source and a quarter-wave phase plate after the objective lens.  For more information, refer to the article: Phase Contrast

Leica microscopes offer phase contrast for the study of cells or tissues concerning various life-science and forensic applications. Phase contrast can also be useful for certain material and earth-science applications.

A phase contrast microscope is similar to a conventional widefield microscope, except it uses an aperture in the shape of an annulus and a quarter-wave (λ/4) phase plate. The annular aperture is placed between the light source and condenser lens and the phase plate after the objective inside the microscope optics. Ring-shaped light that passes through the aperture is focused by the condenser onto the biological specimen to be observed.

PCBboard inspection

After PCB manufacturing, it is necessary to perform inspections to determine whether the quality is compatible with the design requirements.

(1) Check if the surface of the PCB circuit board is smooth, flat, and free from bumps or scratches. Look for drilled holes with missing or misplaced drills or instances where the surrounding copper foil is pierced.

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(2) Assess the integrity of wire patterns by overlaying a photographic film on the PCB circuit board to measure the width and shape of the wires. Check for solder mask voids or breaks, jagged gaps along the edges of the traces, and unintended wire short circuits.

Leica microscopes providing phase contrast are commonly used in life science research for the visualization, analysis, and documentation of biological structures and cellular processes.

Connectivity inspection is required for multi-layer PCB circuit boards. Typically, a multimeter is used to test the connectivity of the PCB circuit board's circuits.

Most often biological specimens and tissues are observed with a phase contrast microscope. A large variety of biological specimens can be observed with phase contrast from fixed specimens to living cells and tissues. For examples, refer to the articles: Phase Contrast & Optical Contrast Methods

The quality inspection of PCB circuit boards generally includes visual inspection, connectivity inspection, and solderability inspection.

This inspection evaluates the solder's ability to wet the PCB circuit board's patterns when soldering components onto it.

Phase contrast is an optical contrast technique for microscopy which makes unstained structures in the cells of biological specimens visible. Cell structures that appear transparent with brightfield illumination can be viewed in high contrast and rich detail using phase contrast. Differences in optical density between structures in the cell can cause light that interacts with them to attain a phase shift. This phenomenon is the basis of phase contrast. As a result, more optically dense structures will look darker than less optically dense ones.

The knowledge portal of Leica Microsystems offers scientific research and teaching material on the subjects of microscopy. The content is designed to support beginners, experienced practitioners and scientists alike in their everyday work and experiments.