compound microscope - objectives lecture

for test 2

optical components - importance

3. substage condenser

2. eyepiece

1. objective

objectives

• largely responsible for image quality, resolution, magnification

• many different types (brightfield, darkfield, DIC, PC) or (dry, HI, LWD)

• different corrections

objective specifications

important to be certain the specs right for the application

Aberration correction

• achromats - achro, achromat or achromatic

• semiapochromat - Fl, fluar, fluor, neofluar, fluotar, or fluorite 

• apochromats - apo or apochromatic 

• field curvature corrections - plan, pl, or Plano

magnification

• objective x tube factor x eyepiece magnification = total magnification

• range from 0.25X to 250X

• empty magnification: 1000 x NA

Numerical aperture

• ability to gather light and resolve detail

• numerical value (no units)

• higher = better

NA = n sin (mu)

NA - numerical aperture

n - refractive index of medium between objective and specimen

mu - ½ A (angular aperture)

Immersion oil

• oil immersion lenses - immersion oil between the objective and the cover slip

specialized optical properties

often designed for specific applications

• DIC - differential interference contrast

• P/Po/POL - polarized light

• SF - strain free

• PH - phase contrast

immersion objectives:

• HI - homogenous immersion

• G - glycerin

• I - immersion

• oil - oil immersion

mechanical tube length

• distance from the nosepiece opening, to the top edge of the observation tubes

• fixed - 160mm, 170mm (cannot interchange)

• infinity - infinity corrected

cover slip thickness

• 0.17 mm is standard

• 0 or - means no cover slip

Parfocal distance

• parfocal distance - measured from the nosepiece objective mounting hole to the point of focus on the specimen

• if different mag. Objectives have same P.D. they will be infocus when “swung in”

aperture and image planes

• conjugated focal planes - planes in focus simultaneously

• two sets of conjugate foci: field planes (normal view through eyepieces) and aperture planes (viewed through bertrand lens or eyepiece telescope)

• normal viewing under microscope also called orthoscopic mode

• viewing through bertrand lens or telescope called conoscopic mode

Conjugate planes

field planes

• field diaphragm 

• object plane

• real intermediate image plane (eyepiece field stop)

• retina

aperture planes

• lamp filament

• aperture diaphragm (condensor)

• back aperture of objective 

• pupil of eye