Glossary
1. N.A. (numerical aperture) N.A. determines resolving power, depth of field, and luminosity of the image . The larger the N.A. the higher is the resolving power and smaller is the depth of field. N.A.= n • Sin θ n is the index of refraction of the medium in which the lens is working. n=1.0 for air. θ is the half-angle of the maximum cone of light that can enter or exit the lens.
7. F (focal length) Distance between a principal point and a focal point. f 1 is the focal length of an objective, f 2 is the focal length of a tube lens. Magnification is determined by the ratio of the focal length of the tube lens to that of the objective. (For an infinity-corrected optical system)
Focal length of tube lens Focal length of objective
Magnification of objective =
200 (mm) 20 (mm)
200 (mm) 200 (mm)
(E.g.) 1X =
(E.g.) 10X =
Objective Air (n=1) Workpiece
θ
8. Field number and FOV (real field of view) The field number of an eyepiece is determined by the field stop diameter of the eyepiece and it is expressed in mm. FOV is the area of specimen observable and is determined by the field number of the eyepiece and magnification of the objective.
Stage glass
2. R (resolving power) Minimum distance between points or lines that are just distinguishable as separate entities. Resolving power is determined by N.A. and wavelength λ .
Field number of eyepiece Magnification of objective
FOV (mm )=
λ 2 · N.A.
R (µm) =
( E.g. Using an eyepiece of field number 24)
24 1
FOV for 1X objective = = ø24 (mm)
3. W.D. (working distance) Distance between the surface of the specimen and the front face of the objective when in focus. 4. Parfocalizing distance Distance between the surface of the specimen and the objective's seating surface when in focus.
24 10 FOV for 10X objective = = ø2.4 (mm) Area of specimen observable on TV monitor
Area of camera image element (VxH) Magnification of objective
Area of specimen observable on TV monitor
=
Indication magnification on TV monitor
Diagonal line length of monitor indication Diagonal line length of camera image element
Parfocalizing distance
= Magnification of objective
Indication magnification on TV monitor
x
Working distance
5. Infinity-corrected optical system An optical system in which the image is formed by an objective and a tube lens with an 'Infinity Space' between them, into which optical accessories can be inserted.
Note: Size of camera image element (V x H x Diagonal) 1/3 inch image element: 3.6x4.8x6.0 mm 1/2 inch image element: 4.8x6.4x8.0 mm 2/3 inch image element: 6.6x8.8x11.0 mm
Infinity space
9. DOF (depth of field) Vertical distance in the specimen, measured from above and below the exact plane of focus, which still yields an acceptable image. The larger the N.A., the smaller the depth of field.
Objective
Imaging (tube) lens
Objective focal point (object plane)
Imaging lens focal point (image plane)
f 1
f 2
Eyepiece observation (Berek's)
Magnification = f 2 /f 1
λ 2x (N.A.) 2
ω x 250.000 N.A. x M
λ = Standard wavelength (550 nm)
+
±DOF (µm) =
6. Finite-corrected optical system An optical system in which the image is formed only by an objective lens.
ω : Resolution of human eye (Visual angle: 5 minute) M: Total magnification (Objective mag. x Eyepiece mag.)
Objective
Object point
Image point
TV monitor observation
λ 2x (N.A.) 2
λ = Standard wavelength (550 nm)
L 1
L 2
±DOF (µm) =
Magnification = L 2 /L 1
42
Made with FlippingBook. PDF to flipbook with ease