IN THIS CHAPTER
Achromat, semi-apochromat and apochromat
objectives
Decipher the hieroglyphs on an objective
Chapter 5
Types of Objectives
Objective lenses are the optical elements closest to the sample. They gather
light from the sample, which is focused to produce the real image that is
seen through the eyepiece. Objective lenses are the most complex part of
the microscope due to their multi-element design.
Objective Lenses and Color Correction
Light transmitted through a lens generates a certain color aberrations (color fringes),
due to the different refractive behavior of different wavelenghts in glass.
Different
types of objective lenses show a different degree of color correction.
Depending on their ability of chromatic or color correction, objectives are assigned
into three different classes:
✔✔Achromat: »Achro« is Greek and means »colorless«. Chromatic objectives
are axially corrected for two colors, typically red and blue. Physically spoken,
these objectives bring the two wavelengths for red and blue into the
focus on the same plane. Achromatic objectives are furthermore corrected
for spherical aberration for green. They don’t produce strong color fringes
around the details of the image (chromatic aberration). The letter »A« on
objectives indicates that this objective is achromatic. These objectives are
most common and widely used on laboratory microscopes, providing lower
apertures, excellent working distances and a strong contrast.
✔✔Semi-apochromat: Like achromat objectives, semi-apochromats are corrected
chromatically for red and blue light. Additionally, they are corrected
spherically for two or three colors. Due to this fact these objectives come
up with a higher numerical aperture, resulting in brighter images. Plus,