Phase contrast microscopy is an optical microscopy
illuminationIllumination, an observable property and effect of light, may also refer to:*Illumination , the use of light sources*Illumination , the use of light and shadow in art*Illumination , the artistic decoration of hand-written texts...
technique in which small phase shifts in the light passing through a transparent specimen are converted into
amplitudeAmplitude is the magnitude of change in the oscillating variable, with each oscillation, within an oscillating system. For instance, sound waves are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
or
contrastContrast is the dissimilarity or difference between things:* Contrast , expressing distinctions between words* Contrast , the difference in color and light between parts of an image.* Contrast...
changes in the image.
A phase contrast
microscopeA microscope is an instrument to see objects too tiny for the naked eye. The science of investigating small objects using such an instrument is called microscopy. Microscopic means invisible to the eye unless aided by a microscope.-History:An early microscope was made in 1590 in Middelburg, The...
does not require staining to view the slide. This type of microscope made it possible to study the
cell cycleThe cell cycle, or cell-division cycle, is the series of events that take place in a cell leading to its division and duplication . In cells without a nucleus , the cell cycle occurs via a process termed binary fission...
.
As light travels through a medium other than
vacuumIn everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty," but in reality, no volume of space can ever be perfectly empty...
, interaction with this medium causes its
amplitudeAmplitude is the magnitude of change in the oscillating variable, with each oscillation, within an oscillating system. For instance, sound waves are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
and
phaseThe phase of an oscillation or wave is the fraction of a complete cycle corresponding to an offset in the displacement from a specified reference point at time t = 0. Phase is a frequency domain or Fourier transform domain concept, and as such, can be readily understood in terms of simple harmonic...
to change in a way which depends on properties of the medium.
Phase contrast microscopy is an optical microscopy
illuminationIllumination, an observable property and effect of light, may also refer to:*Illumination , the use of light sources*Illumination , the use of light and shadow in art*Illumination , the artistic decoration of hand-written texts...
technique in which small phase shifts in the light passing through a transparent specimen are converted into
amplitudeAmplitude is the magnitude of change in the oscillating variable, with each oscillation, within an oscillating system. For instance, sound waves are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
or
contrastContrast is the dissimilarity or difference between things:* Contrast , expressing distinctions between words* Contrast , the difference in color and light between parts of an image.* Contrast...
changes in the image.
A phase contrast
microscopeA microscope is an instrument to see objects too tiny for the naked eye. The science of investigating small objects using such an instrument is called microscopy. Microscopic means invisible to the eye unless aided by a microscope.-History:An early microscope was made in 1590 in Middelburg, The...
does not require staining to view the slide. This type of microscope made it possible to study the
cell cycleThe cell cycle, or cell-division cycle, is the series of events that take place in a cell leading to its division and duplication . In cells without a nucleus , the cell cycle occurs via a process termed binary fission...
.
As light travels through a medium other than
vacuumIn everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty," but in reality, no volume of space can ever be perfectly empty...
, interaction with this medium causes its
amplitudeAmplitude is the magnitude of change in the oscillating variable, with each oscillation, within an oscillating system. For instance, sound waves are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
and
phaseThe phase of an oscillation or wave is the fraction of a complete cycle corresponding to an offset in the displacement from a specified reference point at time t = 0. Phase is a frequency domain or Fourier transform domain concept, and as such, can be readily understood in terms of simple harmonic...
to change in a way which depends on properties of the medium. Changes in amplitude give rise to familiar absorption of light which gives rise to colours when it is wavelength dependent. The human eye measures only the energy of light arriving on the retina, so changes in phase are not easily observed, yet often these changes in phase carry a large amount of information.
The same holds in a typical microscope, i.e., although the phase variations introduced by the sample are preserved by the instrument
(at least in the limit of the perfect imaging instrument) this information is lost in the process which measures the light. In order to make phase variations observable, it is necessary to combine the light passing through the sample with a reference so that the resulting interference reveals the phase structure of the sample.
This was first realized by
Frits ZernikeFrits Zernike was a Dutch physicist and winner of the Nobel prize for physics in 1953 for his invention of the phase contrast microscope, an instrument that permits the study of internal cell structure without the need to stain and thus kill the cells.-Biography:Zernike was born in Amsterdam,...
during his study of diffraction gratings. During these studies he appreciated both that it is necessary to interfere with a reference beam, and that to maximise the contrast achieved with the technique, it is necessary to introduce a phase shift to this reference so that the no-phase-change condition gives rise to completely destructive interference.
He later realised that the same technique can be applied to optical microscopy. The necessary phase shift is introduced by rings etched accurately onto glass plates so that they introduce the required phase shift when inserted into the optical path of the microscope. When in use, this technique allows phase of the light passing through the object under study to be inferred from the intensity of the image produced by the microscope. This is the phase-contrast technique.
In optical microscopy many objects such as cell parts in protozoans, bacteria and sperm tails are essentially fully transparent unless stained. (Staining is a difficult and time consuming procedure which sometimes, but not always, destroys or alters the specimen.) The difference in densities and composition within the imaged objects however often give rise to changes in the phase of light passing through them, hence they are sometimes called "phase objects". Using the phase-contrast technique makes these structures visible and allows their study with the specimen still alive.
This phase contrast technique proved to be such an advancement in microscopy that Zernike was awarded the
Nobel prizeThe Nobel Prize is a Sweden-based international monetary prize. The award was established by the 1895 will and estate of Swedish chemist and inventor Alfred Nobel. It was first awarded in Physics, Chemistry, Physiology or Medicine, Literature, and Peace in 1901...
(physics) in 1953.
Background
The technique was invented by
Frits ZernikeFrits Zernike was a Dutch physicist and winner of the Nobel prize for physics in 1953 for his invention of the phase contrast microscope, an instrument that permits the study of internal cell structure without the need to stain and thus kill the cells.-Biography:Zernike was born in Amsterdam,...
in the 1930s for which he received the
Nobel prizeThe Nobel Prize is a Sweden-based international monetary prize. The award was established by the 1895 will and estate of Swedish chemist and inventor Alfred Nobel. It was first awarded in Physics, Chemistry, Physiology or Medicine, Literature, and Peace in 1901...
in
physicsPhysics is a natural science; it is the study of matter and its motion through spacetime and all that derives from these, such as energy and force...
in 1953. Phase-contrast microscopy is a mode available on most advanced light microscopes and is most commonly used to provide contrast of transparent specimens such as living cells or small organisms.
Explanation
A practical implementation of phase-contrast illumination consists of a phase ring (located in a conjugated
apertureIn optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...
plane somewhere behind the front lens element of the objective) and a matching annular ring, which is located in the primary
apertureIn optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...
plane (location of the condenser's aperture).
Two selected light rays, which are emitted from one point inside the lamp's filament, get focused by the field lens exactly inside the opening of the
condenserCondenser may refer to:*Condenser , a device or unit used to condense vapor into liquid. More specific articles on some types include:*Condenser , a range of laboratory glassware used to remove heat from fluids....
annular ring. Since this location is precisely in the front focal plane of the condenser, the two light rays are then refracted in such way that they exit the condenser as
parallelParallelism is a term in geometry and in everyday life that refers to a property in Euclidean space of two or more lines or planes, or a combination of these. The existence and properties of parallel lines are the basis of Euclid's parallel postulate. Two lines in a plane that do not intersect or...
rays. Assuming that the two rays in question are neither refracted nor diffracted in the specimen plane (location of microscope slide), they enter the objective as parallel rays. Since all parallel rays are focused in the back focal plane of the objective, the back focal plane is a conjugated aperture plane to the condenser's front focal plane (also location of the condenser annulus). To complete the phase setup, a phase plate is positioned inside the back focal plane in such a way that it lines up nicely with the condenser annulus.
Only through correctly centering the two elements can phase contrast illumination be established. A phase centering telescope that temporarily replaces one of the oculars is used, first to focus the phase element plane and then center the annular illumination ring with the corresponding ring of the phase plate.
An interesting variant in phase contrast design was once implemented (by the microscope maker C. Baker, London) in which the conventional annular form of the two elements was replaced by a cross-shaped transmission slit in the substage and corresponding cross-shaped phase plates in the conjugate plane in the objectives. The advantage claimed here was that only a single slit aperture was needed for all phase objective magnifications. Recentring and rotational alignment of the cross by means of the telescope was nevertheless needed for each change in magnification.
Technical Details
To understand how phase contrast illumination works, we study two wave fronts (see the figure to the right). This figure simplifies a few things. First, the condenser annulus is just a small aperture located in the center (see the plane labeled '1') and the phase plate is also just covering a small aperture (located in the plane labeled '3'). Second, the optical system is greatly simplified by showing only two single lenses to represent all optical elements.
The plane labeled '1' is the front focal plane of the condenser. The light emanating from the small aperture 'S' is captured by the condenser and emerges as light with only parallel wavefronts from the condenser. When these plane waves (parallel wave fronts) hit the phase object 'O' (located in the object plane labeled '2'), some of this light is diffracted (and/or refracted) while moving through the specimen. Assuming that the specimen does not significantly alter the amplitudes of the incoming wavefronts but mainly changes phase relations with respect to the "unperturbed" wavefronts, newly generated spherical wave fronts that are retarded by 90° (λ/4) emanate from 'O' (see the purple area that contains now "unperturbed" plane waves and spherical wave fronts). It is important to note that there are now two types of waves, the surround wave or S-wave and the diffracted wave or D-wave, which have a relative phase-shift of 90° (λ/4). - The objective focuses the D-wave inside the primary image plane (labeled '4'), while it focuses the S-wave inside the back focal plane (labeled '3'). The location of the phase plate 'P' has now a profound impact on the S-wave while leaving most of the D-wave "unharmed". In what is known as positive phase contrast optics, the phase plate 'P' reduces the amplitude of all light rays traveling through the phase annulus (mainly S-waves) by 70 to 90% and advances the phase by yet another 90° (λ/4). However, the phase plate leaves most of the D-waves "untouched". Hence the recombination of these two waves (D + S) in the primary image plane (labeled '4') results in a significant amplitude change at all locations where there is a now destructive interference due to a 180° (λ/2) phase shifted D-wave. The net phase shift of 180° (λ/2) results directly from the 90° (λ/4) retardation of the D-wave due to the phase object and the 90° (λ/4) phase advancement of the S-wave due to the phase plate. Without the phase plate, there would be no significant destructive interference that greatly enhances contrast. With phase contrast illumination "invisible" phase variations are hence translated into visible amplitude variations. The destructive interference is illustrated in the figure to the right. Blue and orange indicate D-wave and S-wave, respectively. The resulting wave (D + S), indicated by yellow, has a reduced amplitude.
Implementing phase contrast function using a 4F correlator
We can see how the phase contrast principle works by considering the figure below, which shows a 4F correlator (see
Fourier opticsFourier optics is the study of classical optics using techniques involving Fourier transforms and can be seen as an extension of the Huygens-Fresnel principle...
) that implements the phase contrast function (in this figure, magnification is unity, so it cannot really be called a microscope in the usual sense).
With reference to this figure, we assume a plane wave incident from the left and a phase transmittance function of the form:
The term, in the exponent is known as the
phase (see
phase (waves)The phase of an oscillation or wave is the fraction of a complete cycle corresponding to an offset in the displacement from a specified reference point at time t = 0. Phase is a frequency domain or Fourier transform domain concept, and as such, can be readily understood in terms of simple harmonic...
) of the transmittance function. If this "phase object" is thin, so that then,
Film (or detectors) respond to variations in amplitude, not phase. The transmittance function above will have very small variations in amplitude, since the two terms in the transmittance function are in phase quadrature. For maximum contrast, we will prefer to have these two terms in-phase (not in quadrature phase), so that variations in directly impact the amplitude of the transmittance function. We accomplish this by selectively multiplying one term in the equation above by a factor of
j, thus bringing the two terms in-phase.
We can accomplish this using the 4F correlator in the following way. We assume a plane wave field incident on the "input plane" of the correlator (on the far left in the diagram). The
Fourier transformIn mathematics, Fourier analysis is a subject area which grew out of the study of Fourier series. The subject began with trying to understand when it was possible to represent general functions by sums of simpler trigonometric functions...
(FT) of the phase transmittance function
is formed in the back focal plane of the first lens as
where is the
Point spread functionThe point spread function describes the response of an imaging system to a point source or point object. A more general term for the PSF is a system's impulse response, the PSF being the impulse response of a focused optical system. The PSF in many contexts can be thought of as the extended blob...
(PSF) of the lens. The PSF is basically just a small dot in the FT plane (the back focal plane of the first lens), whereas the function will be more spread out. Since the PSF is localized to a small region about the optic axis (the horizontal axis) in the FT plane, we may place a small, quarter-wavelength thick dot there. This dot will impart a quarter wavelength phase shift to the term, while leaving the term relatively unaffected.
So, behind the dot, the field has the form:
and both terms are now in-phase. We now FT this field distribution using the second lens, to produce the following field in the "output plane" (the rightmost plane) of the 4F correlator system:
where we now neglect the factor of
j common to both terms. Now the phase function, directly modulates transmittance amplitude, making for better contrast in the image.
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