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Quick Guide to Precision Microscopes
Measuring Instruments
Numerical Aperture (NA) Finite Optical System
The NA figure is important because it indicates the resolving power An optical system that uses an objective to form the intermediate
of an objective lens. The larger the NA value the finer the detail that image at a finite position. Light from the workpiece passing through
can be seen. A lens with a larger NA also collects more light and will the objective is directed toward the intermediate image plane (located
normally provide a brighter image with a narrower depth of focus at the front focal plane of the eyepiece) and converges in that plane.
than one with a smaller NA value.
A point-source on Objective lens Light from point source is focussed
NA = n·Sin the workpiece at the intermediate image plane
The formula above shows that NA depends on n, the refractive index L1 L2 Magnification of the objective = L2/L1
of the medium that exists between the front of an objective and the
specimen (for air, n=1.0), and angle , which is the half-angle of the
maximum cone of light that can enter the lens.
Focal Length (f) unit: mm
Resolving Power (R) The distance from the principal point to the focal point of a lens: if
f1 represents the focal length of an objective and f2 represents the
The minimum detectable distance between two image points, focal length of an image forming (tube) lens then magnification is
representing the limit of resolution. Resolving power (R) is determined determined by the ratio between the two. (In the case of the infinity-
by numerical aperture (NA) and wavelength () of the illumination. correction optical system.)
R = l (µm) O bjecti ve magnification = Focal length of the image-forming (tube) lens
2·NA Focal length of the objective
l = 0.55 m is often used as the reference wavelength E xamp le: 1X = 220000 Example: 10X = 22000
Working Distance (W.D.)
The distance between the front end of a microscope objective and the Focal Point
surface of the workpiece at which the sharpest focussing is obtained.
Light rays travelling parallel to the optical axis of a converging lens
Parfocal Distance system and passing through that system will converge (or focus) to a
point on the axis known as the rear focal point, or image focal point.
The distance between the mounting position of a microscope
objective and the surface of the workpiece at which the sharpest
focussing is obtained. Objective lenses mounted together in the same Depth of Focus (DOF) unit: mm
turret should have the same parfocal distance so that when another
J objective is brought into use the amount of refocussing needed is Also known as ‘depth of field’, this is the distance (measured in the
minimal. direction of the optical axis) between the two planes which define the
limits of acceptable image sharpness when the microscope is focussed
on an object. As the numerical aperture (NA) increases, the depth of
focus becomes shallower, as shown by the expression below:
Working distance
l
D OF = 2· (NA)2 l = 0.55 m is often used as the reference wavelength
Example: For an M Plan Apo 100X lens (NA = 0.7)
The depth of focus of this objective is
Parfocal distance
0.55 m = 0.6 m
Infinity Optical System 2 x 0.72
An optical system where the objective forms its image at infinity and Bright-Field Illumination and Dark-Field
a tube lens is placed within the body tube between the objective and Illumination
the eyepiece to produce the intermediate image. After passing through
the objective the light effectively travels parallel to the optical axis to In bright field illumination a full cone of light is focussed by the
the tube lens through what is termed the ’infinity space’ within which objective on the specimen surface. This is the normal mode of viewing
auxiliary components can be placed, such as differential interference with an optical microscope. With dark field illumination, the inner
contrast (DIC) prisms, polarizers, etc., with minimal effect on focus and area of the light cone is blocked so that the surface is only illuminated
aberration corrections. by light from an oblique angle. Dark field illumination is good for
detecting surface scratches and contamination.
A point-source on Objective lens Apochromat Objective and Achromat Objective
the specimen Image forming (tube) lens
Light from point source is focussed An apochromat objective is a lens corrected for chromatic aberration
f1 at the intermediate image plane (colour blur) in three colours (red, blue, yellow).
An achromat objective is a lens corrected for chromatic aberration in
f2 Magnification of the objective = f2/f1 two clours (red, blue).
Infinity space
J-29
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