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Image contrast measures how distinguishable image intensities are under a stated definition. For a periodic bright-dark pattern, Michelson contrast is $(I_{\max }-I_{\min })/(I_{\max }+I_{\min })$. For a small feature on a uniform background, a relative difference from that background is often more informative.

A high-contrast image can still have poor spatial resolution, and a finely resolved feature can be difficult to detect if noise masks its intensity difference. Illumination, background, optical blur, detector response and display scaling all affect measured contrast. Choosing a formula should follow the imaging task.

Köhler illumination arranges illumination so that the light source is imaged into an aperture plane while the field diaphragm is imaged onto the specimen. This illuminates the observed field without projecting the lamp's structure into the specimen image.

After focusing the specimen, close the field diaphragm, adjust the condenser height until its edge is sharp, center that image and open it just beyond the viewed field. Separately adjust the condenser aperture to set the illumination cone and the balance between image contrast and optical resolution. Confusing the two diaphragms makes brightness changes difficult to interpret.^[ref]

Mass–thickness contrast is image contrast caused by different amounts or strengths of electron scattering along the beam path. In bright-field transmission electron microscopy, a region that scatters more electrons outside the accepted transmitted beam generally appears darker.

A thicker part of one material and an equally thick region containing more strongly scattering atoms can therefore produce similar darkening. The image alone does not separate thickness from composition. Changing the collection aperture changes which scattered electrons are rejected, while electron diffraction in a crystal can add orientation-dependent image contrast that is not explained by mass and thickness alone.

A microscope produces an enlarged representation of a small object. An optical microscope forms its image using light and an objective; an electron microscope uses an electron beam and different focusing and detection methods. The useful result depends on image contrast and optical resolution, as well as magnification.

For a compound optical microscope, the objective forms an enlarged intermediate image and an eyepiece can magnify that image further. Increasing this second magnification alone does not reveal finer detail than the objective has transmitted. The objective's numerical aperture and the illumination wavelength constrain diffraction-limited detail, while specimen preparation determines which structures create image contrast. A practical check compares the visibility of two nearby features, rather than judging only their apparent size.^[ref]

Microscopy makes small structures observable by using an optical microscope, an electron microscope, or a scanned probe such as an atomic force microscope. Each instrument measures a different interaction with the specimen: transmitted light, scattered electrons, or the force on a tip. Consequently two images of the same object need not show the same features.

A useful measurement distinguishes magnification, optical resolution and image contrast. Enlarging a blurred image changes its displayed size without recovering the missing detail. For an optical microscope, imaging small separated objects and recording how their images merge tests optical resolution; changing the illumination while holding the specimen fixed tests whether a feature comes from specimen structure or the imaging method.

The modulation transfer function is the magnitude of the normalized optical transfer function in a linear, shift-invariant incoherent imaging model. It describes how strongly the system transfers sinusoidal image contrast at each spatial frequency, while discarding the transfer function's phase.

For an input $I(x)=I_0[1+m\cos (2\pi \nu x)]$ and a unit-integral point spread function, the output modulation magnitude is $mM(\nu )$. Fine alternating bars contain higher spatial frequencies, so their diminishing visibility illustrates loss of transferred modulation. Measuring image contrast across a calibrated pattern tests this response, but detector sampling, noise and nonlinear image processing can alter the result. A transfer magnitude alone does not locate image features or specify their phase shifts.

Optical resolution is the ability of an imaging system to distinguish spatially separated features under specified observation conditions. A finite numerical aperture spreads a point object into a point spread function; neighboring point images overlap. A criterion such as the Rayleigh criterion assigns a separation to this overlap rather than marking a sharp universal boundary between visible and invisible detail.^[ref]

A practical test images two small sources at known separations, keeping exposure and display image contrast fixed. Decreasing their separation until the chosen criterion fails measures lateral resolution; moving them along the imaging axis tests axial resolution. Magnification alone cannot narrow the specimen-space point spread function.

A phase-contrast microscope makes small specimen-induced wave phase changes visible through wave interference. An illumination annulus is imaged onto a phase ring in the microscope objective's rear focal plane. The ring changes the relative phase and amplitude of largely undeviated illumination compared with specimen-diffracted light, producing intensity differences in the image.^[ref]

Correctly matching and centering the annulus and ring is an apparatus requirement. A transparent unstained specimen can then acquire strong image contrast, but halos and image contrast reversals can occur. Comparing ordinary illumination with the aligned phase system demonstrates that image brightness is not simply a map of optical absorption.

Physical section thickness is the distance through a recovered physical section, measured normal to its faces. For a flat section with parallel faces, write this distance as $t$; a nonuniform section instead requires a local thickness or a stated averaging convention. It is distinct from the nominal section feed commanded to the microtome, the spacing of sampled planes in an image stack, and the spatial resolution of the resulting image.

In ultramicrotomy, a controlled advance presents material to a diamond knife on each cutting stroke. JEOL describes preset feed increments for thin sections and notes that actual thickness depends on the material and temperature.^[ref] Section compression and other deformation can further complicate the relation between the machine setting and the recovered section. A feed setting alone is not an independent thickness measurement.

An original geometric example shows why finite thickness matters. Consider a straight thin feature whose direction makes angle $\theta$ with the section's normal vector. Choose coordinates so the section occupies $0\le z\le t$ and the feature lies alongAn image combining contributions through the section can therefore contain feature positions spanning the lateral distanceFor stipulated $t=100\mathrm{nm}$ and $\theta =30^{\circ }$, this range is $57.7\mathrm{nm}$. It is a geometric range, not a universal formula for the measured image width: image contrast, the imaging response and the feature's own width also matter. Replacing the slab by one mathematical plane discards this depth variation.

For an ideal undeformed series of contiguous $100\mathrm{nm}$ sections, imaging every fifth section gives a centre-to-centre sampling interval of $500\mathrm{nm}$, while each imaged section remains $100\mathrm{nm}$ thick. Record omitted sections and the image stack axis calibration separately. To test a feed setting experimentally, measure recovered thickness with an independent method and stated measurement uncertainty, rather than using the same feed display as both command and verification.

Radiography forms an image from spatially resolved transmission of X-rays or other penetrating radiation. A source illuminates the object, and a detector on the other side records how much radiation reaches each position. Variations in attenuation make internal differences observable without cutting the object.

A thicker region and a more strongly absorbing material can give similar transmission, so one projection does not uniquely determine composition or three-dimensional shape. Unwanted scattering reduces image contrast by adding radiation that did not follow the intended straight path. A reference exposure measures source and detector nonuniformity; multiple viewing directions provide additional geometric information.

Scanning transmission electron microscopy forms an image by scanning a focused electron probe across a thin specimen and recording transmitted or scattered electrons at each probe position. The detector's angular range determines which interaction contributes to the recorded brightness.

A detector including the direct beam gives a different image contrast mechanism from an annular detector collecting scattered electrons. High-angle annular dark-field imaging can emphasize composition and mass-thickness differences, but specimen thickness and channeling still affect interpretation. The probe size limits spatial detail, whereas the displayed-to-scanned length ratio sets magnification; increasing the latter cannot sharpen the probe.

Image contrast from changes in secondary-electron escape and collection as surface slope, edges and detector-facing geometry vary. Secondary electrons originate near the surface, but brightness also depends on material, charging and the beam conditions.

Transillumination examines internal structure using light transmitted through a body region or specimen. The source illuminates one side and the observer or detector records light emerging after propagation through the material. Different optical absorption and scattering properties can create image contrast.^[ref]

A simple physical demonstration illuminates a fingertip from behind and observes the diffuse transmitted glow. Its appearance depends on wavelength, path length and the tissue's optical properties. Strong scattering redistributes the paths, so a brightness difference cannot generally be interpreted as a sharply localized anatomical boundary without an additional model.

A symmetric lossless dielectric slab with arbitrarily small positive index image contrast can support its fundamental even guided mode, but confinement becomes arbitrarily weak as the image contrast vanishes. The field outside decays as $e^{-\kappa |x|}$, so its penetration distance $1/\kappa$ diverges when $\kappa \to 0$.

The existence statement for positive image contrast therefore does not imply a confined mode at exactly zero image contrast: the limiting profile ceases to be normalizable across the infinite transverse direction. This distinguishes a mode's existence from useful confinement. A finite surrounding region or nearby absorbing surface can strongly affect such a weakly confined mode even when the ideal infinite-cladding model predicts a bound solution.