Stops in Optical Instruments and Field of View

1
Stops in Optical Instruments and Field of View
Page M37
2
Stops in Optical Instruments and Field of View
Page M37

• Apertures and stops determine total amount of
  light passing through system
• Aperture stop - most restrictive aperture
• Chief ray - most important ray
• Chief ray for any given object point
• passes through center of aperture stop
• defines center of ray bundle through system

3
Apertures Distortion
Page M37

• Distortion demonstrates relationship between
  optical centration (nodal ray) and aperture
  centration (chief ray) in an optical system

4
Apertures Distortion
Page M37

• Distortion ? no image blur
• Due to presence and position of aperture stop in
  system
• Distortion ? variation in lateral magnification
  for off-axis points with distance from the axis

5
Distortion
Page M37

• Isolated thin lens, or thin lens at the aperture
  stop is distortion-free
• Moving aperture stop away from lens alters chief
  ray path ? distortion

6
Other Effects of Stops in Optical Systems
Page M39

• Aperture-dependent aberrations (SA, coma)
• Depth of focus and depth of field
• Field of View
• Image illuminance uniformity of illumination
• Resolving power of optical instruments
  (diffraction)

7
Apertures Stops - Definitions
Page M40

• Aperture stop

8
Aperture Stop
Page M40

• Most restrictive aperture in system
• Determines how much light reaches image
• May be an iris diaphragm or a lens (diameter)
• Aperture stop determined for particular object
  location (may vary for different objects)

9
Aperture Stops
Page M40
10
Aperture Stops
Page M41
11
Aperture Stops - Multiple Lenses
Page M42
12
Aperture Stops - Multiple Lenses
Page M42
13
Aperture Stops Multiple Lenses Aperture
Page M42
14
Apertures Stops - Definitions
Page M43

• Aperture stop

• Field stop

15
Field Stop
Page M43

• Second most restrictive aperture in system
• Restricts passage of oblique rays
• decreases aberrations that arise from oblique
  incidence (oblique astigmatism, curvature of
  field)
• decreases unwanted reflections
• increases uniformity of illumination in image
  plane
• Determines field of view

16
No Field Stop
Page M43
17
Add Field Stop
Page M43
More oblique rays have been vignetted
18
Field Stop
Page M43

• Field stop defines screen diameter
• For camera, edge of film is field stop
• Microscopes and telescopes use iris (often built
  in to eyepiece)

19
Apertures Stops - Definitions
Page M44

• Aperture stop
• Field stop

• Entrance pupil

20
Entrance Pupil
Page M44

• Image of aperture stop seen from object space
• e.g. looking at a persons eye ? see their
  entrance pupil (image of real pupil)

21
Entrance Pupil Multilens System
Page M44
22
Entrance Pupil
Page M45

• Aperture stop and EnP are conjugates for optical
  elements on the object side of the stop
• Total cone of rays (angle ?) just filling EnP
  refracts to just fill the aperture stop
• Ray directed at top of EnP refracts to top of
  aperture stop

23
Locating the Entrance Pupil
NOT in notes
Find the image of the aperture stop seen from
object space Must reverse the system to make the
aperture stop the object
Lens 1
24
Locating the Entrance Pupil
NOT in notes
Lens 1
25
Apertures Stops - Definitions
Page M45

• Aperture stop
• Field stop
• Entrance pupil

• Exit pupil

26
Exit Pupil
Page M45

• Image of aperture stop seen from image space

27
Exit Pupil
Page M45
28
Exit Pupil
Page M45

• Cone of rays just filling EnP ? emerges just
  filling ExP
• Aperture stop and ExP are conjugates for optical
  elements on the image side of the stop
• Total cone of rays (angle ??) just filling ExP
  emerges from system
• EnP and ExP ? conjugates for entire optical system

29
Apertures Stops - Definitions
Page M46

• Aperture stop
• Field stop
• Entrance pupil
• Exit pupil

• Chief Ray

30
Chief Ray
Page M46
31
Chief Ray
Page M46

• Any ray directed at the center of the entrance
  pupil from object space
• Chief ray appears to come from the center of the
  exit pupil as it emerges from the system

32
Telecentric Stops
Page M47

• Optical measuring systems often suffer from
  focusing imprecision due to depth of focus
• Telecentric systems utilize chief ray properties
  to reduce measurement error or to enhance image
  quality

33
Telecentric Stop - Distant Object
Page M47
34
Telecentric Stop Distant Object
Page M47

• Aperture stop at F ? chief ray emerges parallel
  to axis
• System telecentric in image space
• If measuring device not exactly at F? ? stop
  reduces error, because blur circle centered at
  constant height (on chief ray path)

35
Measuring Device at F?
Page M47
h?
36
Measuring Device defocused (not at F?)
Page M47
h?
h? measured from center of upper blur circle to
center of lower blur circle ? still correct
37
Telecentric System Gear Manufacture QC

• Gear manufacture ? tight tolerances for gear size
• Telecentric optics ? assures image always the
  same size (h? not affected by slight defocus in
  system)
• Therefore get accurate measure of actual gear
  size in calibrated system

38
Telecentric System to Enhance Image Quality
39
Telecentric Stop - Near Object
Page M48

• System made telecentric on object side (stop at
  F?)
• Compensates for error in object placement
• Incident chief ray parallel to axis ? refracts
  through second focus centered on stop
• Error in object placement ? incident chief ray
  still parallel (same height)

40
With Telecentric Stop
Page M48
41
Telecentric Stops
Chief Ray
Chief Ray
Page M48
Chief Ray
Chief Ray
42
Image Side Telecentric Stop - Vignetting

• Only parallel incident rays admitted into system
  ? oblique incident rays vignetted by image side
  telecentric stop.

43
Similar principle used in confocal microscopy
44
Aperture Stops in Complex Systems
Page M49
45
Aperture Stops, Entrance Exit Pupils
Page M49
Example 11
Which aperture is the aperture stop?
46
Aperture Stop in Complex Systems

• The aperture or aperture image (viewed from
  object space) that subtends the smallest angle at
  the axial object point is the aperture stop
• Candidates
• Lens 1 (F1)
• Aperture 1 (A1)
• Aperture 2 (A2)
• Lens 2 (F2)

Page M49
47
Aperture Stop in Complex Systems

• Candidates
• Lens 1 (F1)
• Aperture 1 (A1)
• Aperture 2 (A2)
• Lens 2 (F2)

Page M49

• Lens 1 has no preceding optical elements in
  object space.
• If lens 1 is the aperture stop, it is also the
  entrance pupil

48
Lens 1
Page M49

• Lens 1 has no preceding optical elements in
  object space.
• If lens 1 is the aperture stop, it is also the
  entrance pupil

49
Aperture Stop in Complex Systems

• Candidates
• Lens 1 (F1)
• Aperture 1 (A1)
• Aperture 2 (A2)
• Lens 2 (F2)

Page M49

• If aperture 1 is the aperture stop, it must be
  imaged through lens 1 to find the entrance pupil.
• Reverse the diagram to refract A1 through F1

50
Aperture 1
Page M49
51
A1
12 cm
Page M49
3 cm
Object point 40 cm from F1
A1
A2
5 cm
5 cm
3 cm
2 cm
F2 ?5 D
F1 10 D
52
Aperture Stop in Complex Systems

• Candidates
• Lens 1 (F1)
• Aperture 1 (A1)
• Aperture 2 (A2)
• Lens 2 (F2)

Page M50

• Now refract aperture 2 through lens 1 to find its
  image size and position in object space.

53
12 cm
A2
3 cm
2 cm
Object point 40 cm from F1
A2
A1
5 cm
5 cm
3 cm
2 cm
F1 10 D
F2 ?5 D
Page M50
54
Aperture Stop in Complex Systems

• Candidates
• Lens 1 (F1)
• Aperture 1 (A1)
• Aperture 2 (A2)
• Lens 2 (F2)

Page M50

• Finally refract lens 2 (treating it as an
  aperture) through lens 1 to find its image size
  and position in object space.

55
Lens 2
12 cm
Page M50
3 cm
2 cm
Object point 40 cm from F1
A1
A2
5 cm
5 cm
3 cm
2 cm
F1 10 D
F2 ?5 D
56
Which Image Subtends the Smallest Angle?
Object point 40 cm from F1
Apertures
Images
NOT TO SCALE
Page M50
57
Which Image Subtends the Smallest Angle?
Images
F?2
A?2
A?1
F1
Object point 40 cm from F1
Page M50
58
Aperture Stops, Entrance Exit Pupils
Example 11
Page M50
Which aperture is the aperture stop?
A2 is the aperture stop
59
Aperture Stops, Entrance Exit Pupils
Page M50

• Aperture A2 subtends the smallest angle when
  viewed from the axial object point (through lens
  1). This signifies that
• A2 is the aperture stop
• the image of A2 through lens 1 is the entrance
  pupil
• finding the image of A2 through lens 2 locates
  the exit pupil