Introduction to phase contrast microscopy
A phase contrast microscope is a microscope that converts the phase difference (or optical range difference) produced when light passes through an object into a change in amplitude (light intensity). It is mainly used to observe living cells, unstained tissue sections or stained specimens lacking contrast.
The human eye can only identify changes in the wavelength (colour) and amplitude of visible light, but not phase changes. Most biological specimens are highly transparent, and the amplitude of the light wave is basically unchanged after it passes through, and only the phase change exists.
Phase contrast microscopy basically turns the optical range difference of the visible light through the specimen into the amplitude difference, which improves the contrast between various structures and makes various structures clearly visible. Light through the specimen after refraction, deviated from the original light path, while being delayed by 1/4 λ (wavelength), if then increase or decrease 1/4 λ, the optical range difference becomes 1/2 λ, the two beams of photosynthesis axis after interference to strengthen the amplitude increases or decreases to improve the contrast.
From the structural point of view, phase contrast microscope is different from ordinary optical microscope:
1. ring-shaped diaphragm with a ring-shaped aperture of the diaphragm, installed between the light source and the concentrator, the role is to make the light through the concentrator to form a hollow cone of light, focusing on the specimen.
2. phase plate phase contrast microscope in the objective lens inside the increase of magnesium fluoride coated phase plate, the role of direct or diffracted light phase delay 1/4 λ. Phase plate has two areas, direct light through the part called "conjugate surface", diffracted light through the part called "compensation surface". Compensation surface". The phase plate is divided into two types according to its working effect:
(1) A + phase plate: direct light delayed 1/4 λ, two groups of light waves co-axial light wave superposition, amplitude increase, the specimen structure is brighter than the surrounding medium, the formation of bright contrast (or negative contrast).
(2) B + phase plate: diffracted light delayed 1/4 λ, two groups of light waves after the merging of the axis of the light wave phase reduction, the amplitude becomes smaller, the specimen structure is darker than the surrounding media, the formation of dark contrast (or positive contrast). Objective lens with a phase plate is called phase contrast objective lens, often in the objective lens housing with the word "Ph".
