Objective Lenses in Dental Microscopy

OBJECTIVE
LENSES
INDIAN DENTAL
ACADEMY
Leader in continuing
Dental Education
www.indiandentalacademy.com

REFERENCES
•Theory and Practice of
Histological Techniques-Bancroft
•Cellular Pathology Technique-
Culling
•Microscope-Basics & Beyond
• J.K. Soneja, Microscopy

Learning objectives:
• Parts of microscope
• Introduction of objective
lens
• Working of objective lens
• Numerical aperture
• Types of objectives
• Colour coding of
objectives
• Properties of objectives

PARTS OF A MICROSCOPE

OBJECTIVE LENSES

MICROSCOPIC OBJECTIVE
LENSES
• The most important component of
an optical microscope.
• They derive their name from the fact
that they are, by proximity, the closest
component to the object being
imaged.

• The main task of the objective is to collect
the maximum amount of light possible
from the object , unite it , and form a
high quality magnified real image, some
distance above.

• On modern microscopes, up to six
objectives are mounted onto a
revolving nosepiece to enable rapid
change from one to another and ideally,
the focus and field location should
require the minimum of adjustment.

• Most objectives are designed for use
with a coverglass protecting the
object.
• A value giving the correct coverglass
thickness should be found engraved
on the objective, usually this is 0.17
mm.

• The microscope objectives tend to be
parfocal.
• This means that while you may change
lenses to a higher or lower focal length
while observing a specimen, it will
always remain in focus.
• Microscope objectives are characterized
by two parameters: magnification and
numerical aperture .

• Magnifying powers or object – to –
image ratios of objectives are from 1:1
to 100:1.
• The ability of an objective to resolve
detail is indicated by its numerical
aperture .

NUMERICALAPERTURE
•The numerical aperture or NA is
expressed as a value.
•The value expresses the product of
two factors and can be calculated as :
NA = n × sin u

NA = n sin u

•Where ‘n’ is the refractive index of the
medium between the coverglass over the
object and the front lens of the objective.
•and ‘u’ is the angle included between
the optical axis of the lens and the
outermost ray that can enter the front
lens.

• In practice the maximum NA attainable
with a dry objective is 0.95.
• Similar limitations apply to water and
oil immersion objectives, theoretical
maximum values for NA are 1.30 and
1.50 respectively.
• In practice, values of 1.20 and 1.40 are
the highest obtainable.

TYPES OF OBJECTIVES
1. Achromat
2. Apochromat
3. Fluorite or semi apochromat
4. Plan achromat
5. Plan apochromat
6. Polarizing objective
7. Phase objective
8. Spring loaded objective

OPTICAL ABERRATIONS
• Chromatic aberration : Failure of
a simple lens to bring light of different
wavelengths to a common focus.

SPHERICAL ABERRATION
•This aberration is due to the curved shape of the
lens
•Light rays entering a curved lens at its periphery
are refracted more than those rays entering the
centre of the lens
•Thus not brought to a common focus

• It is caused by rays from the outer limits of the subject plane
coming to focus nearer to the lens than the axial rays
FIELD CURVATURE
• In this image may be sharp at the center but sharpness
falls at the periphery of the lens.

TYPES OF OBJECTIVES
1)ACHROMATIC-
• Most common
• Least expensive
• Corrected for spherical aberration for
one colour
• Corrected for chromatic aberration for
two colours

Achromatic doublet

2)FLUORITE
/SEMIAPOCHROMATIC OBJECTIVE
• These objectives have fluorite
incorporated into the lens system to
give better colour correction.
• They are corrected chromatically for red
and blue light and spherically for either
two or three colours instead of single
colour.
• Better resolving power and higher
degree of contrast than achromat.

3)APOCHROMAT-
• Finest microscope objective available
• Corrected for chromatic aberration for red
blue and green
• Corrected for spherical aberration for
either two or three wavelengths.
• Used in photomicrograph.

4) PLANACHROMAT OBJECTIVES
• Flat –field objectives
• Particularly useful for cytology screening
• used for correction of field curvature.

5) PLAN APOCHROMAT
OBJECTIVES
•Field of view is almost perfectly flat
•Recommended for photomicrography.
•used for correction of field curvature.

PLAN APOCHROMAT OBJECTIVE

6)POLARIZING OBJECTIVE-
• These are strain free objectives for use on
polarizing microscope.

7)PHASE OBJECTIVE
These objectives contain internal
phase plates which are offered with
a variety of optical correction
factors, ranging from simple
achromats to plan apochromats.

8) SPRING LOADED OBJECTIVES-
Many objectives designed with close
working distances have a spring-loaded
retraction stopper that allows the front
lens assembly to be retracted by pushing
it into the objective body and twisting to
lock it into place.

• Such an accessory is convenient when
the objective is rotated in the nosepiece
so it will not drag immersion oil across
the surface of a clean slide.
• All 20X to 100X objectives have
spring loaded front ends to prevent
slide damage.

COLOUR CODING
• Objective lenses are colour coded and
have numbers written on them.
• Red is 4X, Yellow is 10X, Blue is 40X
and White is 100X.

COLOUR CODING

Immersion Media Color Code
Oil Black
Glycerol Orange
Water White
Special Red

THE PROPERTIES OF
MICROSCOPE OBJECTIVES

Thank You….

Objective Lenses in Dental Microscopy

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