Download “Beam Paths” to the “Microscope”

II
From “Beam Paths” to the “Microscope”
October 2008
Rudi Rottenfusser – Carl Zeiss MicroImaging
Understanding Beam Paths

From Pin Hole Camera to the Lens

The three “Thin Lens Laws”

Image Ratio – not the same as Magnification

“Magnification”

The Concept of “Infinity”

The “simple” microscope (Leeuwenhoek, Magnifier, Eyepiece)

The “compound” microscope – Upright / Inverted

Illumination – Transmitted / Reflected

Stereo Microscopes – Greenough / Telescope Types
Geometric Optics – Creating an Image
The Pinhole Camera
Geometric Optics – Creating an Image
Making the aperture larger…
Geometric Optics – Creating an Image
Adding a lens…
How does a lens “bend” light?
Infinite number of prisms
with different angles
Drawing Beam Paths
• Thin Lens Laws (1,2,3)
1) Draw ray through center of lens
(small error is ignored if
glass is very thin)
n1
n2
Exact
path
2) Rays that enter the lens
parallel to the optical axis
cross over at Back Focal Point
(Back) Focal
Point
2b) Rays that enter the lens
from infinity, cross over at
Back Focal Plane
(Back) Focal
Plane
3) Light rays that enter the lens from the
focal point exit parallel to the optical axis.
f
focal distance
(Front)
Focal Point
3b) Light rays that enter the lens from a
point along the focal plane exit parallel ().
Size translates
to angle !

Predicting the behavior of imaging systems
(principle ray technique)
Back Focal Pt
Front Focal Pt
f
Object
f
1) Draw in central ray
Object
1) Draw in central ray
2) In parallel; out via back focal point
1) Draw in central ray
2) In parallel; out via back focal point
3) In via front focal point; out parallel
1) Draw in central ray
2) In parallel; out via back focal point
3) In via front focal point; out parallel
A
B
Image
Intersection
defines image
Image Ratio 
Size of Image
B (Distance of Image to Lens)

Size of Object A (Distance of Object to Lens)
Our eye is a great imaging system.
Its lens provides variable focal lengths
to bring objects in focus at the retina
Magnification – unaided Eye
MB ~ 2x MA
A
B
Objects appear to the eye at different magnifications, depending on
their distance from the eye. Accommodation (lens) makes it possible.
Conventional Viewing Distance
?
250 mm
1x
“Magnification” 1x
250 mm
1x
f = 250 mm
1x
“Infinity Optics” ?
Higher Magnifications via Single Lens
1x
f = 250 mm
Magnifying Glass (Loupe)
M 
5x
Example:
f=50mm
250mm
f Lens
The Leeuwenhoek microscope
The Eyepiece (Ocular) is a “Simple” Microscope
f Eyepiece 
Intermediate Image
250mm
M
Eyepoint (Exit Pupil)
If you need a magnifier, remove eyepiece,
turn upside down and move close to eye;
subject will be about 25mm away from lens
Eye
(Retina)
Eyepiece
Intermediate
Image
The
Compound
Microscope
Objective
Specimen
Eye
(Retina)
Eyepiece
Intermediate
Image
The -corrected
Compound
Microscope
Tube lens
Infinity Space
Objective
Specimen
Advantage of Infinity Correction
Dlat.
Intermediate image is
Dax
1) “in registration”
2) fully corrected
Objective
Finite System
Infinity System
Specimen off-center
Specimen off-center
The -corrected Compound Microscope
Eyepiece
M
f Tube
250mm
250mm


f Objective
250mm
f Eyepiece
M 
Tube Lens
f Tube
f Objective

250mm
f Eyepiece
M Compound Microscope  M Objective  M Eyepiece
Objective
Cross-section through an ∞ corrected Microscope
Eyepiece
Tube Lens
Infinity Space
Objective
Intermediate image
(fully corrected)
The basic light microscope types
Upright microscope
.
Inverted microscope
Illuminating the Specimen
Transmitted Light
The sample must be transparent !
Upright microscope
.
Inverted microscope
Illuminating the Specimen
Reflected (Incident) Light
Eg. Fluorescence, Opaque Samples
Upright microscope
.
Inverted microscope
Mixed Illumination
Upright microscope
.
Inverted microscope
Which Microscope types typically
use these types of illumination?
External Sources, e.g.
?
 Fiber Optics
 Reflectors
 Ring Lights
 Various Combinations
Source
“Couldn’t one build a
microscope for both
eyes, and thereby
generate spatial
images?”
Question addressed to
Ernst Abbe in 1896
by Horatio S. Greenough
1896: Drawing by Horatio S. Greenough
1897 – the first Stereo Microscope in the
world, built by Zeiss according to the
“Greenough” principle
Greenough Type
What happens if we take the objective away from the microscope?
Eyepiece
∞
Tube lens
∞
Objective
M

(Zeiss: f=164.5mm)
f Tube
250mm
250mm
fEyepiece

M Telescope 
f Tube
f Eyepiece
We have created a “Telescope”
Greenough Type
Telescope Type
Introduced first
by Zeiss - 1946
Comparison
Greenough Type:
• 2 separate beam paths
going through centers of
lenses
• Excellent correction
• Economical
Telescope Type:
• One common objective for
both beam paths
• Flexibility to interchange
tubes and objectives
• Possibility to add
intermediate tubes e.g.
for Fluorescence, Coobservation, Drawing
Questions? Short break?
Next:
Dissecting an infinity-corrected microscope