Lecture 2 Jan 29 2018 Flashcards
The term microscopy used to describe:
Analytical approach in which a microscope plays a central role in maximizing extraction of useful information from a variety of samples.
Identification (ID) of general unknown is greatly facilitated with a______________________.
Preliminary Microscopical Examination
Applications of the microscope in forensics include:
- Document examination
- Tool mark comparison
- Firearms ID
- Serology
- Drug chemistry
- Trace evidence
Microscope used as a symbol of science but seems less exotic than recent instrumentation, in some cases____________________.
old fashioned
________________and______________are the two main purposes of a microscope too create an enlarged image of an object that is clear and distinct.
Magnification
Resolution
Magnification and resolution conflict with each other:
As magnification increases, resolution can decrease
Greater resolution means greater detail, while greater magnification means ________________.
Larger image
A converging (also known as positive )lens can form images in two ways:
Real image
Virtual image
____________ is formed on the side of the lens opposite the object and is inverted with respect to object, light rays converge at a given plane in space and thus image falls on the screen.
- slide projector
Real image
_____________is when a lens is used as a magnifying glass; rays from this type appear to diverge from a plane on the same side of the lens as the object:
- Use of another lens is necessary to convert virtual into real
- When virtual image is examined visually, the lens of the eyes serves this function so that real image is projected onto the retina
Virtual image
___________________ distance from the lens to the principle focus (for thin lens).
Focal length
If an object placed at infinity, the image will form at:
Principle focus
If object placed at principle focus, image will form at _____________.
Infinity
_________ of a simple lens is defined by a straight line running through the centers of curvatures of the two surfaces.
Axis
The on axis point object at infinity on left side of lens, the lens will form real image in the right-hand space known as:
Principle focus of the lens
_____________(for thick lenses) measured from the image for an object at infinity (focal point for incident rays parallel to lens axis) to nearest principle plane.
Equivalent focal length
Magnification (M) for a real image can be defined by the equation:
M = Q/P
If, P=2F=Q, magnification is:
1
If P>Q, then M<1 :
Means there is a reduction
If P1:
We have conditions for magnification by microscope objective
__________________the departures in performance between actual and hypothetical ideal simple lens.
- Aberrations do not rise from defects introduced from manufacturing process
- Inherent even in perfectly made simple lens
Aberrations
The variation of focal length with distance from the lens axis is called__________________.
Spherical Aberration (fuzzy image)
- Focal length will vary with part of lens being used
- Areas or zones of the lens that are closer to its periphery are stronger that have shorter focal lengths than zones near axis of the lens.
__________________refractive substances disperse light to some extent, hence lenses depart from ideal behavior.
Chromatic Aberrations
- Transparent materials do not refract all wavelengths equally.
________________variation of focal length with wavelength.
Longitudinal chromatic aberration
- Image will have out of focus color fringes at edge of objects.
_______________variation of magnification with wavelength.
Lateral Chromatic Aberration
- Size of an object imaged in blue light may be significantly different from the image formed in red light.
Not to be confused with spherical aberration, which is unlike ___________________, an image cannot be sharply focused for any part of the field no matter how much refocusing is done.
Curvature of Field
- Image defect produced by lens exhibits a moderate of curvature which may not be apparent visually.
- Only possible to get acceptable focus for only one area of the field of view (center, edge etc.)
- When attempt is made to photograph such image, difficulty of recording curved image surface on a flat film plane becomes apparent.
If the magnification is not constant across the field of view, the image will not be an exact representation of the object.
Distortion
______________is when magnification fall off, when moving from the center of the field toward periphery.
Barrel distortion
_______________when magnification increases toward the edge of the field.
Pin-cushion distortion
Magnification takes place in one stage, no more than a magnifying glass.
Simple microscope
Ex: hand lenses, fingerprint magnifiers
Magnification of a simple microscope:
M= least distance of vision + 1
focal length of lens
Magnification happens because the lens allows the image of the object to remain focus when the object is placed closer to the eye increasing the angle and resulting in larger image on the retina.
- Distance of closest vision for normal human eye is about 25 cm.
Simple microscope
Modern ______________have magnification in two stages.
Compound microscope (s)
_______________resembles a projector + magnifying glass.
Compound microscope
Last stage of the magnification in the ______________is essentially the same as in a _____________ ( a virtual image is formed) and it takes place in the eyepiece (ocular).
Compound microscope
Simple microscope
Ocular is used to examine a real image produced by the first image-forming lens system, the objective
- objective produces an enlarged real image of the object.
Compound microscope
Other factors for evaluating a microscope are:
Image quality
Visibility
Resolution
Compound microscopes have shorter focal length objectives characteristically have:
High powers
An increase in magnification that continues to reveal finer details in the object is called:
Useful magnification
Additional magnification that merely results in a larger image with no further detail is known as:
Empty magnification
Since in the compound microscope the image produced by the objective is magnified by the ocular, the approximate resultant overall magnification of the system can be found by multiplying the power of the __________by power of the ___________.
Objective
Ocular
For court purposes, common to use a calibrated scale known as ____________________, which is mounted on the microscope slide under the same conditions as specimen.
Stage micrometer
Two most common microscopes found in laboratories:
Biological microscope
Stereoscopic binocular microscope
Biological microscope:
Designed for use with transparent or translucent specimens and is earliest compound microscope.
Stereo microscope:
Most widely used instrument in the crime laboratory.
________________most basic form consists of 2 low to moderate power compound microscopes mounted side by side in a common housing so that the two are directed at the same area of the specimen from slightly different angles.
Stereo microscope
Most common type of illumination employed with the stereo microscope is variously termed:
Incident, reflected and vertical or epi
The______________of the stereo microscope is that microscopic objects tend to have very natural 3D appearances when viewed with the stereo microscope.
Main advantage
Most stereo microscopes have accessory for transmitted light illumination allows light to be directed through the _______________.
Sample from beneath
Consists of light directed onto the specimen from above and is suitable for use with opaque objects as well as certain transparent ones:
Stereo microscope
- incident, reflected and vertical or epi illumination
It is less the microscope than the lighting that provides the analysis with a stereo microscope.
True
Various wavelengths of light can provide luminescence or fluorescence in a sample.
True (Stereo microscope)
Levels of magnification 2.5X - 100X
Stereo microscope
The stereo microscope can be used for ____________or_____________testing.
Detection
Presumptive
Using a stereomicroscope on a _________________allows the microscope to be used on large objects.
Boom arm
Polarized Light Microscope:
Most versatile and powerful tool available for analyzing trace evidence.
________________ microscope is basically a biological microscope with several refinements and accessories added.
Polarized Light
The Polarized Light microscope is composed of a ______________and ____________crucial for optimal performance.
Lamp (illuminator)
Substage condenser
On a standard brightfield microscope, two___________filters are introduced into the light path; one between the sample and light source (polarizer) and one between sample and oculars (analyzer).
Polarizing
Stage on ___________rotates allowing the sample to rotate around the axis of the polarizer, providing responses to the polarized light.
Polarized Light Microscope (PLM)
Compound microscopes can be fitted with thermally controlled stages that can heat samples up to 350 degrees C.
Hot stage
Hot stage is:
Destructive analysis
Hot stage can help identify:
Materials, particularly polymers or to compare materials response to various temperatures.
Comparison microscope is:
Two carefully matched compound microscopes
In a comparison microscope, images produced by the two microscopes are____________in a common intermediate field plane within the ocular.
juxtaposed
- In this manner, 2 objects in two different fields of view can compared directly
The two types of comparison microscopes:
- One designed for opaque objects such as bullets, cartridge cases tool marks; uses incident illumination
- One for hair and fiber comparisons; transmitted light illumination
Transmitted light illumination is when:
The specimen is illuminated from the side opposite of the objective.
Often desirable to employ _______________and _____________methods simultaneously as well as sequentially when examining certain samples.
Transmitted light
Epi
For work with low or moderate power objectives, a crude ________illumination can be employed by simply directing a concentrated beam of light at the top of the specimen.
Epi
Ideal transmitted light illumination system should be capable of illuminating specimen _____________with adequate intensity.
Uniformly
Ideal transmitted light illumination system should allow __________of light paths so that only the area of the specimen being examined is illuminated.
Precise control
Ideal transmitted light illumination system should provide a __________and_________degree of whiteness or color temperature of the light.
Controlled
Predictable
Ideal transmitted light illumination system should be _____________to use without compromising precision.
Convenient
Ideal transmitted light illumination system has two primary components that supply illumination:
Illuminator
Substage condenser
_________________is the first component along the path of the microscope.
Illuminator
August Kohler devised special type of critical illumination called:
Kohler illumination
- For wide variety of applications
___________is the average operating voltage for high quality microscope illuminators.
6 volts
______________is brought to its operating temperature gradually, the life of the lamp will be greatly prolonged.
Lamp filament
__________microscope lamps should not be used at the full voltage unless doing so is necessary for brightness or color temperature considerations.
High quality
Rationale for use of low voltage lamps in high quality illuminators stems from the fact that for 2 filaments of equal wattage output, the one designed to operate at lower voltage can be made more___________.
Compact
The ideal _____________would produce a perfect cone of light with its apex at the specimen plane.
Condenser cone
The term _______________refers to a cone of light that does not exhibit effects of spherical aberration.
Aplanatic cone
Rays following non-prescribed paths manifest as_____________.
Stray light
______________is light that carries no information about the specimen; produces glare and reduction in image contrast.
Stray light
______________ produced is important because its form depends on how well the paths of light rays are controlled by the illuminator and condenser.
Character of the cone of light
Ideal condenser has two variables:
Angularity (angular aperture)
Size of the area of intersection
_____________of a condenser is controlled by substage or aperture diagram.
Angularity
Size of the area of an intersection from a condenser is:
- controlled by the lamp or field diagram
- area of intersection is the area illuminated
- area illuminated should be reduced with high power objective
Typical high power cone of a condenser has:
High angularity with small area of intersection
Typical low power cone of a condenser has:
Low angularity coupled with a large area of intersection
________________third optical component of the microscope.
The objective
- First and most critical image forming part
______________sets the upper limit for the image quality attainable by the microscope.
The objective
A poor __________can degrade image quality produced by the objective, but an excellent ________cannot compensate for a poor or improperly selected objective.
Ocular
Ocular
