Forensic Microscopy Exam 1

Resolution

Minimum distance between two features the microscope can differentiate

Limited by Aberrations + Diffraction

Airy Pattern

minimum point of light that can be sustained by a circular aperture due to diffraction

Rayleigh Criterion

𝑹 = 0.61𝝀 / 𝑵𝑨_obj

Abbé Criterion

𝑹 = 1.22𝝀 / 𝑵𝑨_obj + 𝑵𝑨_cond

Field of View

Diameter of the circle detected

FOV (mm) = Field number of eyepiece / magnification of objective

Abberations: Errors of Focus

No image point for any object point in a focal plane

• Defocus, decenter and tilt

• Longitudinal chromatic aberration

• Spherical aberration

• Field curvature

• Astigmatism

Abberations: Errors of Magnification

No equal image scale across the image plane

• Lateral chromatic aberration

• Coma

• Distortion

• Astigmatism

Defocus

Detector not in the focal plane

Error of Focus

Decenter

The optical element is not centered on the optical axis

Error of Focus

Tilt

The optical element is not perpendicular to the optical axis

Error of Focus

Longitudinal Chromatic Aberration

Different colors focus at different points (due to dispersion – RI depends on wavelength)

Error of Focus

Lateral Chromatic Aberration

Different colors image at different height (leading to different magnification)

Error of magnification

Spherical Aberration

Variation of focus with aperture

Error of Focus

Coma

Variation of magnification with aperture

Correct by using diaphragm

Error of magnification

Field Curvature

The plane for imaging is not flat

Error of Focus

Distortion

Failure to magnify the image equally as the distance increases from the axis

Error of magnification

Astigmatism

The image of a point out of axis has two different focal points because of the asymmetry of the lens

Snell’s Law

𝒏𝟏 𝐬𝐢𝐧 𝒊 = 𝒏𝟐 𝐬𝐢𝐧 𝒓

Light bends more in the higher RI

Critical Angle

sin(i_c) =𝑛₂/𝑛₁

𝑛_{2 is always the smaller angle}

The Becke Line

Moves toward the higher refractive index medium

The Lens Equation

𝟏/𝒒 − 𝟏/𝒑 = 𝟏/𝒇

𝒒: distance from the image to the lens

𝒑: distance from the object to the lens

𝒇: focal length (from the focal point to the lens)

The Eye Image formation

Cornea + lens=convex lens

Focal lens is adjustable by the muscles

Image always formed on the retina

Magnification Equation

M = q/p

-M means image is reversed orientation

M< 1 means the image is smaller than the object

Illuminating Light Path

Light source

Condenser diaphragm

Objective exit pupil

Iris

Imaging Light Path

Field of view diaphragm

Sample

Real intermediate image

Retina