(2012) koehler illumination - portland state university… · koehler illumination i. introduction...
TRANSCRIPT
A. La Rosa Lecture Notes
Portland State University
APPLIED OPTICS ________________________________________________________________________
Koehler Illumination
I. Introduction
We describe an illumination method, introduced by August Koehler in 1893, that allows attaining a uniform illumination on a sample within an optical microscope, despite the eventual use of non‐homogeneous sources (like electrical lamps).
As described in more detail below, the Koehler illumination setup presents two main characteristics:
Creates an evenly illuminated field of view
The light intensity at specimen plane and the size of the illuminated field can be regulated independently
These features are attained by placing two distinct sets of optical elements across the microscope:
a set of “illumination apertures” A1, A2, A3, A4, and
a set of “illuminated field diaphragms” F1, F2, F3, F4.
These two sets are schematically shown in Fig1a and Fig.1b below, but a more detailed explanation follows in the next sections.
f1 f1
Lamp collector
Fila‐ ment
Filament’s image
Condenserlens
fcond fcond
Objectivelens
fobj fobj
Eyepiece lens
fe fe
A1 A2 A3 A4
WD
Object’s image on the user’sretina
Object
Fig. 1a Illumination apertures‐diaphragms A1, A2, A3, A4. (The complementary field‐diaphragms that limit the width of the cone of rays in the system are not shown). The figure shows how a particular cone of light emitted from a point
on the filament is spread out uniformly across the specimen’s plane (a more detailed justification of the ray tracing is given in the main text). In addition, it will be shown that the diaphragm A2 controls the light intensity reaching the sample without modifying the size of the illuminated area on the specimen’s plane.
fcond f1 f1
Lamp collector
Fila‐ment
Filament’s image
Condenserlens
fcon
Objectivelens
fobj fobj
Eyepiece lens
fe fe
F1 F2 F3 F4
Oject’s image on the user’s retina
Object
WD
Fig. 1b Field diaphragms F1, F2, F3, F4. (The complementary illumination apertures that control the intensity reaching the specimen are not shown). Notice in this set up that i) each point on the field‐diaphragm behaves as a point source of light, and ii) the diameter of the field‐diaphragm F1 is imaged at the specimen’s plane. Accordingly, the size of the region illuminated at the specimen’s plane is controlled by the field‐diaphragm F1.
II. The conventional microscope The Koehler illumination is to improve the quality of a conventional optical microscope where:
an objective‐lens forms a first image at the front focal plane of the eyepiece, and the latter forms a virtual image at infinity. This implies that a bundle of parallel rays reaches the user’s eye, and a real final image is formed at the user’s eye retina.
If the illumination on the object is not uniform, the corresponding image will display such low quality light distribution.
It is in this optical microscope setup that we are going to introduce a proper illumination, the Koehler illumination, to a) fully exploit the maximum resolution of the microscope objective (by being able to select the proper numerical aperture of the condenser lens as to match that of the objective lens), b) obtain an uniform illumination over the sample despite using lamps of non‐uniform
brightness, while c) minimizing vigneting (the filament is imaged at the user’s eye lens.)
Objective lens
fobj fobj
Eyepiecelens
fe fe
Exit pupil
First mage’s plane(front focal plane of the eyepiece)
Object’s image on the user’s retina
Aperture stop
Object Optical tube length
Illumi‐nation
WD
fhuman eye
Fig. 2a Basic components of an optical microscope. An object is placed at a proper working distance (WD) from the objective‐lens. The diagram correspond to an old fashion microscope where a tube length specifies where the first image is formed. (Nowadays, infinitely corrected objective lenses are used instead).
Objective lens
fobj fobj
Eyepiecelens
fe fe
Exit pupil
First mage’s plane(front focal plane of the eyepiece)
Object’s image on the user’s retina
Aperture stop
Object Optical tube length
Illumi‐nation
WD
fhuman eye
Fig. 2a Basic components of an optical microscope.
III. Description We show the ray tracing starting from the illumination section towards the
sample stage, and then to the ocular eyepiece. In the final part we outline some hands on “tricks” on how to actually implement the Koehler illumination (we’ll see that the order of descriptions reverses, starting rather from the sample stage and continues with the illuminating lamp set up.)
III.1 Filament and lamp collector
The purpose is to place the image of the filament at the back focal plane of the condenser lens
Notice the magnification of the filament’s image changes with the filament’s position.
In practice, since the illumination set‐up (i.e. filament’s position) is rarely changed, the lamp position is chosen at the very beginning of the setup such that the filament’s image fills the largest diameter of the aperture diaphragm A2. (As a consequence, when working with the aperture diaphragm A2 at lower openings, of the lamp’s light power will be used less efficiently.)
f1 f1
Lamp collector
Filament’s image
Condenser lens
fcond fcond
Oba
Objectlens
fobj fo
F2
A1 A2 F1
Field diaphragm (back focal plane of lamp
collector lens)
GR
P
Aperture diaphragm (or sub stage diaphragm) (Located at the front focal plane of the condenser lens)
Filament
Fig. 3 Filament imaged at the front focal plane of the condenser lens.
Notice that the field diaphragm F1 (located at the back focal plane of the lamp collector) is uniformly illuminated
Indeed, notice in Fig.3 above that it doesn’t matter whether or not the point sources G, R, and P at the filament are of equal intensity. If, for example, one of those sources were off, the overall illumination would certainly be dimmer, but the intensity across the field diaphragm would still be uniform.
(Note: we are not justifying, however, whether a point source like “G”, for example, produces or not a uniform illumination across the plane “F1”.)
III.2 The condenser lens
One of the roles of the condenser lens is to image the field diaphragm (F1) into the object’s plane F2.
Objecback-f
plan
Objl
fobj fobj
A3
G R P
Filament
Filament’s image
Object
Lamp collector
Condenserlens
Field diaphragm (back focal plane of lamp
collector lens)
f1 f1 fcond fcond
A1 A2F1
Aperture diaphragm(or sub stage diaphragm)
(Located at the front focal plane of the condenser lens)
F2
Object’s plane
Fig. 4 Field diaphragm F1 imaged at the object’s plane F2.
The aperture diaphragm A2 controls the numerical aperture of the condenser lens
Notice in the figure below that the smaller aperture A2 the smaller the angle . This is an important feature, since, ideally, the numerical aperture of the condenser lens should match the numerical aperture of the objective‐lens. For a given objective lens, the Khoeler illumination allows using the aperture diaphragm A2 to match that value.
Objecback-f
plan
Objl
fobj fobj
A3
Filament’s image
Object
Lamp collector
Condenser lens
Field diaphragm (back focal plane of lamp
collector lens)
f1 f1
F1
G R P
Filament
fcond
A1 A2 F2
fcond
Fig. 5 The size of the aperture diaphragm A2 determines the condenser’s numerical aperture.
III.3 Independent control of the dimension and light intensity of the
illuminated field area (equivalently, independent control of the illuminated field area and the condenser’s numerical aperture).
The field diaphragm (F1) controls the size of the illuminated area on the object’s plane (F2).
Indeed, notice in Fig. 6 that F1 controls the size of the bundle of rays emitted from a particular point P on the filament. If we decreased the size of the field diagram (F1) then that particular bundles of rays will illuminate a smaller area of the specimen.
A decrease in the field diaphragm (F1) does not affect the light intensity (or power density Watt/m2) received at the specimen.
The reason is that a uniform distribution of light (across the diameter of the aperture) passes through F1. That uniformity will be observed also at the objects’ plane (where the rays in the bundles arrive in parallel). Hence, decreasing the size of diaphragm F1 is translated in a decrease of the illuminated area on the objects’ plane, without changing the uniformity of the illumination.
f1 f1
Lamp collector
Condenser lens
fcond fcond
Objectivback-foca
plane
fobj fobj
A3
A1 A2F1
Field diaphragm (back focal plane of lamp
collector lens)
G R
P
A uniform distribution of light passes across
this aperture
Filament
A uniform distribution of light reaches the
objects plane
F2
Object
Fig. 6 The size of the field diaphragm F1 determines the size of the illuminated area on the object’s plane
(Would “P” and ”R” produce illuminated areas of the same size on the plane F2? )
The aperture diagram (A2) controls the light intensity reaching the object, but without affecting the size of the illuminated area.
Notice in Fig 7 that each point on the aperture generates a bundle that illuminates the object uniformly. (The size of the illuminating bundles controlled by F1.) If we decreased the size of the aperture diagram (A2) then we are decreasing the number of bundles arriving at the object’s plane (hence decreasing the light intensity) but not the size of the illuminated area.
f1 f1
Lamp collector
Condenser lens
fcond fcond
Objective back-focal
plane
fobj fobj
(frontthe
A1 A2 F1
Field diaphragm (back focal plane of lamp
collector lens)
G R
P
Each point on the aperture A2 generated
a bundle.
Filament
All the different bundles illuminate the same area. The number of bundles is
controlled by the A2 aperture.
F2
Object
Fig. 7 The A2 controls the number of bundles arriving to the objects plane (hence controlling the illumination intensity) but not the size of the bundles (hence whthot modifying the area illuminated on the objects plane).
That is, the aperture diagram (A2) and the field diaphragm (F1) affect independently the optical setup.
III.4 Complete optical setup
The above description constitutes the expected arrangement in an Koehler illumination setup. The resulting full ray tracing in the optical microscope is shown below.
A1
f1 f1
Field diaphragm
(Back focal plane of lamp collector lens)
Filament
Filament’s image
Condenser lens
Objectplane
fcond fcond
Objective back‐focal plane
Objective lens
fobj fobj
Eyepiece lens
fe fe
Exit pupil
First‐image’s plane (front focal plane of the eyepiece)
A2 A4
A4
Image on the user’s retina
A3
F1 F2 F3
Lamp Collector
lens
F4
WD
(Front focal plane of the condenser
lens) Fig. 8 Notice how each point source (cone of light) from the filament i) arrive uniformly spread over the specimen object’s plane, and ii) if the diameter of the aperture diaphragm were to be reduced, then less number of point sources from the filament will arrive to the specimen’s plane, however each cone that goes through will illuminate the same specimen’s region. Hence, the diaphragm control the intensity of light reaching the specimen but without altering the size of the illuminated area on the specimen’s plane.
IV. IMPLEMENTATION Starting with the objective lens and the eyepiece in their proper position,
image the field diaphragm F1 in the object’s plane For that purpose, have the field diaphragm quite open and try to obtain a sharp image of the borders. You achieve that placing the condenser lens in the proper position.
Image the filament in the front focal plane of the condenser lens. You can obtain this by placing a piece of upper in the back focal plane of the objective lens. A clear image of the filament should be obtained there.
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V. REFERENCES C. Hammond, "Symmetrical Ray Diagrams of the Optical Pathways in Light Microscopes" The Americas Microscopy and Analysis, Sept Issue, p. 5‐8 (2006). The article considers infinity corrected optics C. Hammond, “A symmetrical representation of the geometrical optics of the light microscope,” Journal of Microscopy. 192, 63–68 (1998). Shinya Inoue, K. R. Spring, “Video Microscopy, the Fundamentals” 2nd Ed. Plenum Press (1997).
Backup figure
A1
f1 f1
Field diaphragm
(back focal plane of lamp collector lens)
Lamp collector
Filament
Filament’s image
Condenser lens
Aperture diaphragm (or substage diaphragm) (In the front focal plane of the
condenser lens)
Object
fcond fcond
Objective back-focal
plane
Objective lens
fobj fobj
Eyepiece
fe fe
Exit pupil
First mage plane (front focal plane of
the eyepiece)
A2 A3 A4 F1 F2 F3 F4
Image on the user’s retina
Backup figure
A1
f1 f1
Field diaphragm
(back focal plane of lamp collector lens)
Lamp collector
Filament
Filament’s image
Condenser lens
Aperture diaphragm (or substage diaphragm) (In the front focal plane of the
condenser lens)
Object
fcond fcond
Objective back-focal
plane
Objective lens
fobj fobj
Eyepiece
fe fe
Exit pupil
First mage plane (front focal plane of
the eyepiece)
A2 A3 A4 F1 F2 F3 F4
Image on the user’s retina