principles of light microscopy
TRANSCRIPT
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Principles of Light MicroscopyPrinciples of Light MicroscopyPeter EvennettPeter Evennett
[email protected]@microscopical.co.uk
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True or False ?• Light microscopy is out of date now that we have electron microscopes.
• All graduates are taught how to use the light microscope.
• Because their fittings are standardised, most objectives and eyepieces can beinterchanged and used on any microscope.
• Oil immersion is necessary only for high magnifications.
• It is best to use thick coverglasses because they are stronger.
• It is best to use very thin coverglasses for top-quality work.
• Microscopes are fitted with diaphragms designed to control the intensity ofillumination.
• A good microscope provides a higher magnification than a poorer one.
• An image of the lamp filament should not occur anywhere in the microscope.
• Light microscopy is so much simpler than electron microscopy, that it isunnecessary to attend a course on it.
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Robert Hooke 1635 - 1703Micrographia 1665
Objective lens
LampPowersupply
Condenserlenses
Focusingmechanism
Mechanicalstage
Eyepiece
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Thin slice of Cork
I could exceedingly plainlyperceive it to be all
perforated and porous,much like a Honey-comb…
…these pores, or cells...
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Robert Hooke on
Simple and Compound microscopes
“ … ‘tis possible with a single Microscope to
make discoveries much better than with a
double one, because the colours which do
much disturb the clear Vision in double
Microscopes is clearly avoided and prevented in
the single.”
1665
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Bacteria
Humansperm
Antony van Leeuwenhoek,1632 - 1723
A different approach – using a single lens
50mm
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LamphouseFocus knobs
Condenser lensStage
Objective lens
Eyepiece
Binocular
Camera tube
Revolving nosepiece
Stand
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Like paintings, photographs or sculptures in an artgallery, the microscope image is a representationor a likeness, an artefact, and it will be differentfrom the original object.
In order properly to understand microscope images,we need to understand these artefacts, and howthey are produced.
you look at an image of the specimen.
With a microscope you don’t look at thespecimen:
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The input requirements of the eyedetermine the output requirements of the
microscope:
• The image must be presented atinfinity, so that the image-forming raysare parallel on entering the eye
• Exit pupils must be 3-5mm diameter,to match the pupils of the eyes…
• and separated by about 65mm tomatch the interocular distance.
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MicroscopyMicroscopy
Microscopes must provide:• Resolution
ability to carry information about fine detail inthe specimen to the image
• Contrastdifferences in the image between featuresand their surroundings
• Magnificationto make the image large enough for the eyeto appreciate the resolved detail
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The limit to resolution
All optical systemscameras, telescopes, eyeballs, microscopes
have a limit to the smallness of eachimage point – like ‘optical pixels’.
The smaller they are and the more youhave, the better.
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All optical systems - the eye,cameras, projectors, telescopes,
microscopes - image a point of lightas a disc of light surrounded by
bright and dark rings.
And the diameter of these rings is related to the aperture of the lens.
Because of Diffractionat the image-forming lens:
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Airy disc
The limit to resolution
The aperture of the lens
The size of the Airy disc depends on:
The wavelength of the imaging radiation
Larger aperture and shorter wavelength
smaller disc better resolution
Image of a pointsource:
The Airy Pattern
This is caused byDiffraction
Original object
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Image of a point source:the Airy Pattern
Small aperture Larger aperture
Airy discs
with most of its brightness in the centralAiry disc
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Diffraction in the microscope
• Diffraction occurs whenever light or otherwave motion encounters any kind of obstacle
• The Airy pattern is the result of diffraction atthe objective lens
• Diffraction also occurs at the specimen.• Whether you consider resolution to be limited
by diffraction at the objective lens or thespecimen, the result is the same.
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Points resolved Not resolved
Two Airy patterns: brightness plots
The Rayleigh Criterion states that two objects can beresolved in an image when they are separated by
distance 0.61λ / n sin α
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The Rayleigh Criterion for Resolutioncalculated from aperture of objective lens
The peak of one curve falls (approximately) over thecentre of the first dark ring of the other
a bAmplitudesof a and b
Combined amplitudesof a and b
Just detectableby the human eye
radius = λ / 2 n sin α
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Light passes throughthe specimen
Directly into the eye
And the specimen is coveredby a layer of glass
Transmitted-light, Bright-field
A most unusual way oflooking at things!
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Light passes throughthe specimen
Directly into the eye
And the specimen is coveredby a layer of glass
Transmitted-light, Bright-field
A most unusual way oflooking at things!
And the microscopeuses light for a job that‘it wasn’t designed for’- looking at things thatare around the same
dimensions as thewavelength of light
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The job of an ideal lens• To accept as many rays as possible from each point in an object
• To reassemble all the rays from each point at corresponding pointsin the image…
• In such a way that the distance travelled by all the rays from eachobject point to its corresponding image point is the same- so that they all arrive ‘in phase’.
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A lens has two focal planes
Focal point
Focal point
Rays entering lens parallel...
...are converged on theopposite side of the lens
Rays entering lensthrough a focal point... …emerge parallel on the
opposite side of the lens
as with a projector
as with a camera
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Ray diagrams - three simple rules
Focal point
1. Rays entering lens parallel to axis...… cross the axis at the focal point on the opposite side of the lens
2. Rays entering lens through focal point... leave the lens parallel to axis
3. Rays passing through centre of lens are undeviated
Focal pointObject
Image
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What can lenses do??
Camera- forming a reduced-size, real image, close to the lens
Projector- forming an enlarged, real image, distant from the lens
Magnifying glass- not forming a real image; parallel rays to infinity
Lenses can act in a way similar to those of three familiar optical devices:
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Object in focal plane
f f'
2f 2f'
Camera -portrait
Projector
Camera -landscape
Camera -macro lens
Magnifyingglass
Object at infinity Image in focal plane
Image at infinity
Image on retina
Focal length of lens
Image smaller than object
Image same size as object
Image larger than object
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SpecimenObjective lens
Primary imageEyepiece
EyeImage on retinaThe objective lens works like a
projector lens and forms thePrimary Image
10mm below the top of theviewing tube
and the eyepiece acts as amagnifying glassand examines the
centre of this image
Ground glass on top of
microscope tube
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Magnification
Depends on:Image distance
longer higher mag
Objective focal lengthshorter higher mag
Eyepiece focal lengthshorter higher mag
...and in recent microscopes on thefocal length of the tube lens (more later)
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The objective lens
The most important lens of the microscope
Is the microscopeThe other parts support its function
and adapt the image to the receiving device
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Objectivelens
The importance of Aperturein the Microscope
Information
collectedInformation
wasted
Consider that every rayleaving the object carries
some information aboutfine detail in the objectSome of these rays
– and some information –will be collected by the objective
and some rays– and some information –
will NOT be collected,and will be wasted
Resolution will therefore dependon the angular aperture
of the objective -the larger the imagingaperture the higher the
resolution
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Objective lens
Specimen
Medium ofrefractive index
n
NA = n sin α
Numerical NA Aperture
α
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NumericalAperture
CoverglassMountantSlide
72°
67°39°
NA = 1 x sin 72° = 1 x 0.95
= 0.95
NA = 1.515 x sin 67° = 1.515 x 0.92
= 1.4Refraction makes
angle α in air
representless-oblique raysat the specimen
- which is where itreally matters!
Immersion ObjectiveDry Objective
Oil
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‘Milky glass’block
Objective lens
Light passing down microscope from objective into glass block
0.3
Magnification 12.5NA 0.3
Magnification 25NA 0.65
0.650.3
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Magnification 25NA 0.65
0.65 0.65
Magnification 40NA 0.65
Same aperture, different magnifications
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25 / 0.65
40 / 0.65
Same aperturedifferent magnifications
Same magnificationdifferent apertures
0.65 0.95
40 / 0.95
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25 / 0.65
40 / 0.65
40 / 0.95
90 / 1.32
1.32
Oil
0.650.95
0.3
12.5 / 0.3
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Half-aperture angle
Refractive index of medium
Wavelength ofimaging radiation
Minimumresolved distance
Inscription on Ernst Abbe’s memorial
Numerical Aperture
Minimum resolved distance is now commonly expressed asd = 0.61 λ / NA
d
λ
α
n
Why is Numerical Aperture Important?
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• Resolution depends on NA
• Light transmission of objectivedepends on NA2
• Depth of field of objective is (approximately)inversely proportional to NA2
Why is Numerical Aperture Important?
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Once you have bought the objective lenses,there is little you can be done to improve
resolution…
…but it can easily be made worse by poorillumination of the specimen
Microscope Illumination
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• Light up the specimen uniformly
– over a controllable area
• Illuminate the objective aperture uniformly
– over a controllable angle
What are we trying to do whenilluminating a microscopical specimen??
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Microscope Illumination
Two basic methods of illumination:
Source-focused (or ‘Critical’) Illumination:Light-source imaged on to specimen
Köhler Illumination:Light-source imaged in the aperture of thecondenser
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Bench lampimaged on
ground glasson stage bycondenser
lens Ground glassImage of
bench lamp
Source-focused Illumination
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Light sources suitable for source-focusedillumination:
Uniformly-illuminated skyFlame of oil-lampSurface of opal light bulbUniformly-illuminated white paper or ground glass
*note that these are really ‘secondary sources’
Condenser lens acts like a camera lens- throws an image of source on to underside of slide
**
*
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gave an image of thestink-pipe on theChemistry Building
But looking for aregion of uniformlyilluminated sky inLeeds…
…when themicroscope was set up correctly
Source-focused Illumination
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Using a normal electric lampgives an image of the writing
on the end of the bulb
…and a modern halogen lamp iseven worse
Source-focused Illumination
Köhler Illuminationsolves this problem
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SpecimenObjective lens
Primary imageEyepiece
EyeImage on retina
An image of the object
forms the primary imageand this is transferred
to the retina
These are three
conjugate planes - successive images of one
another
… and there are more.
Conjugate planes
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August Köhler1866 - 1948
publishedA new system of
illumination forphotomicrographic
purposes
(in German) in 1893.
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The back focal plane of theobjective
Objective lens
Image of objects at ‘infinity’in back focal plane
of objective
Specimen
Ground glassFocal length
(approx)
Back focal planeFocal length
(approx)
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Condenser
Object
Objective
vvvvv Back focal planeof objective
Imagine a lightsource in the firstfocal plane of the
condenser
Light will passparallel through
the object
And be brought intofocus in the
back focal plane of the objective
Into the objective
Condenser andobjective lenses
vvvvv
How do we light up thespecimen uniformly?
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Lamp collector
vvvvv
Filament
vvvvv
Condenser
Object
Objective In Köhler Illumination an extra lens, theLamp collector lens
throws an image of the filament into thefirst focal plane of the condenser
Back focal planeof objective
This image of the filament falls also on the
aperture diaphragm of the condenser,the Illuminating aperture diaphragm
The Illuminated field diaphragmfitted just after the lamp collectoris imaged on to the object by the
condenser lens
vvvvv
In this situation the lamp collectorlens appears to be uniformly filled
with light
vvvvv
vvvvv
Object
Illuminatingaperture
diaphragm
Illuminatedfield
diaphragm
To imageHow do we light up the
specimen uniformly?
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Note that
• Each point in the objectreceives light from
many points on the filament
and that
• Each point of the filamentprovides light to
many points on the object
Condenser
Lamp collector
Object
Objective
vvvvv
vvvvv
Aperture diaphragmof condenser
To image
vvvvvBack focal planeof objective
How do we light up thespecimen uniformly?
Filament
Object
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Light up the specimen uniformly over a controllable area?
It is unnecessary, and often detrimental, to illuminateparts of the specimen outside the field of view
– some specimens are light-sensitive, and could be damaged
– light can be scattered into field of view from outside this area
– illuminating a large area of specimen produces a largeprimary image, and light can reflect from internal walls ofmicroscope, reducing contrast in the image
Why is it necessary to…
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vvvvv
Tube
Condenser
Objective
Eyepiecelens
Lens of eye
Lamp collector
Field diaphragmof eyepiece
Aperture diaphragmof objective
Aperture diaphragmof condenser
Object
Primaryimage
Retina
Why control thearea illuminated ?
Large area ofobject illuminated
provides largedisc of light atprimary image
causing reflectionsfrom walls of
microscope andreduction in contrast
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vvvvv
Condenser
Objective
Eyepiecelens
Lens of eye
Lamp collector
Field diaphragmof eyepiece
Aperture diaphragmof objective
Aperture diaphragmof condenser
Tube
Illuminated FieldDiaphragm
Retina
1. An adjustable diaphragm here
3. And the disc of light at theprimary image is kept off the walls
of the microscope
How do we control thearea illuminated ?
2. Is imaged on to the specimen,so that the area illuminated is
restricted
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Objectivelens
Information
collected
Consider that every rayleaving the object carries
some information aboutfine detail in the objectSome of these rays
– and some of the information –will be collected by the objective
Illuminate the objective aperture uniformly over a controllable angle?
Why is it necessary to…
and some rays– and some information –
will NOT be collected,and will be wasted.
Informationwasted
Resolution will thus dependon the angular aperture ofthe objective - the larger theaperture the higher theresolution
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Objectivelens
‘Common sense’ suggeststhat if we expect toreceive light over a
large angle, it isimportant for good
resolution that most ofthe objective apertureshould be illuminated
Why is it necessary to…Illuminate the objective aperture uniformly over a controllable angle?
But why just most ?Why not all ?
Why not a very widecone of light ?
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Objectivelens
If the illuminatingaperture is too large,light will be scatteredfrom the edges of theobjective lens, thusreducing contrast.
Why is it necessary to…Illuminate the objective aperture uniformly over a controllable angle?
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Worse
If the illuminating aperture istoo small,
resolution will be reducedand
image quality will beimpaired
though contrast will beincreased.
Illuminate the objective aperture uniformly over a controllable angle?
Why is it necessary to…
Objectivelens
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ObjectivelensSo for best resolution the
illuminating apertureshould approach theimaging (objective)
aperture
60 to 75% is oftenrecommended
Why is it necessary to…Illuminate the objective aperture uniformly over a controllable angle?
60%
100%
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vvvvv
Objective
Eyepiece
Lens of eye
Lamp collector
Field diaphragmof eyepiece
Aperture diaphragmof objective
Aperture diaphragmof condenser
Illuminated FieldDiaphragm
Retina
Object
1. Closing thecondenser diaphragm
2. Narrows the angleof rays passing though
the object
Objective
Condenser
vvvvv
vvvvv
View down tube(objective back focal plane)
How do we control the angle ofillumination ?
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vvvvv
Objective
Eyepiece
Lens of eye
Lamp collector
Field diaphragmof eyepiece
Aperture diaphragmof objective
Aperture diaphragmof condenser
Illuminated FieldDiaphragm
Retina
Object
vvvvv
vvvvvBack focal plane
of objective
Lamp filament
Illuminating ApertureDiaphragm
vvvvv
The aperturediaphragm of the
condenser thus actsas the Illuminating
Aperture Diaphragm
– so called because itis the diaphragm
which regulates theIlluminating Aperture
How do we control the angle ofillumination ?
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vvvvv
Illuminated FieldDiaphragm
Illuminating ApertureDiaphragm
Condenser
Objective
Eyepiecelens
Lens of eye
Lamp collector
Field diaphragmof eyepiece
Aperture diaphragmof objective
Aperture diaphragmof condenser
Object
Primaryimage
Retina
Back focal planeof objective
Exit pupilof eyepiece
Filament
vvvvv
vvvvv
vvvvv
Field set ofconjugate planes
Aperture set ofconjugate planes
vvvvv
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Illuminated Field Diaphragm
Condenser lens
Image of IF Diaphragm on specimen
IF Diaphragm also imagedin primary image plane- prevents light fromreflecting from inside oftube
What are the diaphragmsfor?
![Page 63: Principles of Light Microscopy](https://reader031.vdocuments.us/reader031/viewer/2022012010/613c6b02f237e1331c516827/html5/thumbnails/63.jpg)
Illuminating aperturediaphragm
Illuminating aperturediaphragm imaged in
back focal planeof objective…
…and also inExit Pupil
of eyepieceWhat are the diaphragmsfor?
![Page 64: Principles of Light Microscopy](https://reader031.vdocuments.us/reader031/viewer/2022012010/613c6b02f237e1331c516827/html5/thumbnails/64.jpg)
Condenser lensIlluminating aperture
diaphragm openIlluminating aperture
diaphragm closed
1. Closing thecondenser diaphragm
2. Narrows the angleof rays passing though
the object
Objective
Condenser
View down tube(objective back focal plane)
What are the diaphragmsfor?
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The Illuminating Aperture Diaphragm
sets the angle of the cone oflight illuminating the specimen,
and is adjusted according toobjective NA
The diaphragms are NOTintended for adjusting thebrightness of the image
The Illuminated Field Diaphragmsets the area of specimenilluminated, and is adjustedaccording to magnification
What are the diaphragmsfor?
![Page 66: Principles of Light Microscopy](https://reader031.vdocuments.us/reader031/viewer/2022012010/613c6b02f237e1331c516827/html5/thumbnails/66.jpg)
Illuminating system completely out ofadjustment
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Image ofIlluminated
FieldDiaphragmnot sharplyfocused onspecimen
Fuzzy edge
Centre ofimage of
diaphragm
Illuminating system completely out ofadjustment
Illuminated Field Diaphragm closed;its image is not centred on field of view
![Page 68: Principles of Light Microscopy](https://reader031.vdocuments.us/reader031/viewer/2022012010/613c6b02f237e1331c516827/html5/thumbnails/68.jpg)
Condenser focus adjusted
Image ofIlluminated
FieldDiaphragmnow sharplyfocused onspecimen
Sharp edge
![Page 69: Principles of Light Microscopy](https://reader031.vdocuments.us/reader031/viewer/2022012010/613c6b02f237e1331c516827/html5/thumbnails/69.jpg)
Image of Illuminated Field Diaphragmnow centred on field of view
![Page 70: Principles of Light Microscopy](https://reader031.vdocuments.us/reader031/viewer/2022012010/613c6b02f237e1331c516827/html5/thumbnails/70.jpg)
Image of Illuminated FieldDiaphragm centred on field of view
How?• moving the condenser lens in x-y• moving the Illuminated Field Diaphragm in x-y• waggling the microscope mirror• moving the entire lamp about on the bench• centring the objective lens
Most commonsystem
![Page 71: Principles of Light Microscopy](https://reader031.vdocuments.us/reader031/viewer/2022012010/613c6b02f237e1331c516827/html5/thumbnails/71.jpg)
Illuminated Field Diaphragm openedto illuminate full field of view
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Back Focal Plane of objective
Lamp notcentred
withcollector
lens
![Page 73: Principles of Light Microscopy](https://reader031.vdocuments.us/reader031/viewer/2022012010/613c6b02f237e1331c516827/html5/thumbnails/73.jpg)
Lampcentred
withcollector
lens
Back Focal Plane of objective
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Diffuser inserted between lamp andcollector lens
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Illuminating Aperture Diaphragmclosed to c75% of objective aperture
75%
100%
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Illuminating Aperture too large
Image hazy and ‘washed out’
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Illuminating Aperture correct
Image contrast optimal
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Illuminating Aperture too small
Image contrast too high; artefacts present
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Correct Too small
Illuminating Aperture
75%
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Illuminating Aperture too small
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Illuminatingaperturetoo small
Illuminatingaperturecorrect
Note this object
This is what it‘should’ look like!
![Page 83: Principles of Light Microscopy](https://reader031.vdocuments.us/reader031/viewer/2022012010/613c6b02f237e1331c516827/html5/thumbnails/83.jpg)
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Illuminating aperture wide open:much larger thanobjective aperture
Gradually close illuminatingaperture diaphragm.
Illuminating aperture now equal toobjective aperture
Removing stray light
Optimum setting:Stray light removed but
still have adequateilluminating aperture
Imagebrightness
Illuminating aperturemuch smaller thanobjective aperture
Reducing illuminating aperture
X
Setting the Illuminatingaperture diaphragm –
a simple way
Poor image!
![Page 85: Principles of Light Microscopy](https://reader031.vdocuments.us/reader031/viewer/2022012010/613c6b02f237e1331c516827/html5/thumbnails/85.jpg)
Illuminating aperture too large
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Illuminating aperture correct
![Page 87: Principles of Light Microscopy](https://reader031.vdocuments.us/reader031/viewer/2022012010/613c6b02f237e1331c516827/html5/thumbnails/87.jpg)
Illuminating aperture too small
![Page 88: Principles of Light Microscopy](https://reader031.vdocuments.us/reader031/viewer/2022012010/613c6b02f237e1331c516827/html5/thumbnails/88.jpg)
Illuminating aperture much too small – an extreme example
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Köhler Illumination provides
Control of Area illuminated by theIlluminated Field Diaphragm,
which is adjusted according to magnification.
Control of Angle of illumination by theIlluminating Aperture Diaphragm
(the condenser diaphragm),which is adjusted according to objective aperture.
![Page 93: Principles of Light Microscopy](https://reader031.vdocuments.us/reader031/viewer/2022012010/613c6b02f237e1331c516827/html5/thumbnails/93.jpg)
Epi-illumination
Imagine illuminating system is ‘folded over’about the object plane, so that ...
Condenser
Objective
Illuminated FieldDiaphragm
Primaryimage
vvvvv
vvvvv
vvvvv
vvvvv
Lamp collector
Eyepiecelens
Filament
Condenser falls on Objective
Illuminating Aperture Diaphragm falls inBack Focal Plane of Objective
Illuminated Field Diaphragm falls inPrimary Image plane
Lamp collector falls on Eyepiece lens
![Page 94: Principles of Light Microscopy](https://reader031.vdocuments.us/reader031/viewer/2022012010/613c6b02f237e1331c516827/html5/thumbnails/94.jpg)
Condenser
Objective
Illuminated FieldDiaphragm
Primaryimage
vvvvv
vvvvv
vvvvv
vvvvv
Lamp collector
Eyepiecelens
Filament
![Page 95: Principles of Light Microscopy](https://reader031.vdocuments.us/reader031/viewer/2022012010/613c6b02f237e1331c516827/html5/thumbnails/95.jpg)
vvvvv
vvvvv
Primaryimage
vvvvv
Illuminated Field Diaphragm inserted in planecorresponding to Primary Image Plane
Objective lens actingas its own Condenser
Reflectorinsertedat 45°
Cannot insert Illuminating Aperture Diaphragm here - would also restrict imaging aperture
Illuminating Aperture Diaphragm inserted here.Filament must now be repositioned
![Page 96: Principles of Light Microscopy](https://reader031.vdocuments.us/reader031/viewer/2022012010/613c6b02f237e1331c516827/html5/thumbnails/96.jpg)
vvvvv
vvvvv
Illuminated FieldDiaphragm
Primaryimage
Reflectorvvvvv
Objective lens actingalso as Condenser
Cannot insert Illuminating Aperture Diaphragm here - would also restrict imaging aperture
Illuminating ApertureDiaphragm
vvvvv
Filament