Phase Microscopy

Introduction

Phase-contrast microscopy is a specialised contrast-enhancing technique in transmitted light microscopy that makes it possible to visualise transparent specimens that would otherwise remain invisible under normal bright-field microscopy. It is one of the most common and simplest methods for imaging biological specimens and is ideal for visualising living, unstained cells.

The method was created in 1932 by Frits Zernike (1888–1966) and in 1941 made part of routine microscopic practice by August Köhler and Loos. It quickly transformed research in biology and medicine. For this important discovery, Zernike was awarded the Nobel Prize in Physics in 1953.

Physical Principle

Phase contrast microscopy converts phase differences of light passing through a specimen into visible intensity differences.

When light passes through a transparent specimen, it is phase-shifted due to differences in refractive index and thickness compared to the surrounding medium. These shifts aren’t something the human eye can see directly, so they need to be converted into visible contrast.

This is done using an annular aperture and a phase ring, which change the phase and intensity of the background light. When the light coming from the specimen and the background light recombine, their phase difference is close to 180°, creating interference that makes the structures visible.

working principle of phase contrast microscopy

A typical feature of phase-contrast microscopy is the formation of halo artefacts around structures, which are caused by partial interference of the diffracted light.

Optical Components and Mechanism

Annular Aperture (Condenser Annulus)

• Located in the condenser (front focal plane)
• Translucent plate with a circular transparent ring
• Produces a cone of light that illuminates the specimen

Phase Plate

• Located in the objective lens (rear focal plane)
• Is often integrated into the objective as a ring
• Selectively adjusts the phase and amplitude of undeviated light
• Shifts the phase of undeviated light by ±λ/4
• Reduces the brightness of background light

Mechanism of Contrast Formation

The light passes through the circular opening and forms a hollow cone that illuminates the specimen. During interaction with the specimen, the light is split into diffracted light, which is phase-shifted by the specimen and undiffracted background light.

The undiffracted light passes through the phase plate, where its phase is shifted and its intensity is reduced. Most of the diffracted light passes around the phase plate and remains unaffected.

Both light components then combine in the image plane, where interference occurs between the diffracted and undiffracted light. This turns small phase differences into visible differences in intensity, creating a contrast.

Types of Phase Contrast

Positive Phase Contrast

Positive phase contrast is the most frequently used technique in light microscopy. Thicker cellular structures and organelles, such as the nucleus and mitochondria, appear dark against a bright background.

Negative Phase Contrast

In negative phase contrast, thicker regions of the specimen appear bright against a dark background. This effect occurs when the direct background light is delayed by a quarter wavelength, resulting in constructive interference.

Applications of phase contrast microscopy

Cell Biology

• Most common observation without staining of cells in cultures
• Study of cell morphology (structure and shape)
• Observation of cell movement in real time and division processes
• Analysis of cell mortality

Hematology

• Examination of living blood cells without staining
• Identification of abnormal cell shapes (e.g., in anemia)
• Observation of platelet aggregation and cell dynamics

Microbiology and Infectious Diseases

• Detection of live microorganisms without staining
• Observation of bacterial motility
• Useful for fast early diagnosis of infections

Advantages

• No staining required (cells remain alive)
• Minimal sample preparation
• Real-time observation
• Enhanced contrast in transparent specimens

Disadvantages

• Halo artifacts around structures
• Not effective for specimens with low phase difference
• More expensive than a standard light microscope