Geoscience Reference
In-Depth Information
P HOTOMICROGRAPHY AND IMAGE ANALYSIS
Photographic images can be taken through the
microscope (photomicrography) by attaching a camera
on to the vertical tube of the microscope's trinocular
head. Until recently 35 mm film cameras were used as
standard for photomicrography. While these provided
high-quality images in the form of photographic prints
and slides, they had the disadvantage of having to wait
for the film to be processed before one could view and
utilize the images. Recent developments in digital image
recording have brought rapid changes to photography
and, in consequence, also to photomicrography. Film
cameras have now been replaced by digital cameras that
provide instantly available images, which can be readily
manipulated and enhanced using computer software, to
suit the required purpose. A basic guide to the practical
aspects of digital light micrography is provided by
Entwistle (2003b & 2004).
As a result of the move towards digital information,
petrographic examination reports are now sent to clients
in digital format using electronic mail, in addition to the
traditional paper copy sent by conventional mail
services. Digital technology has reduced the report
production and delivery times for petrographic
investigations. Having photomicrographs in digital form
also makes it easy to put them up on internet websites,
either for public view, or for clients' private use in a
password-protected area (Entwistle, 2002).
Image analysis is being increasingly used to measure
various properties of construction materials through the
microscope. Measurements are obtained by analysis of
digital photomicrographs using computer software,
which can be highly successful. However, it should be
noted that image analysis techniques are only accurate if
sufficient image contrast is present to allow accurate
identification of the subject being measured. Special
sample preparation may be required to ensure this. Also,
it must be remembered that the results are only valid if
the plane of section analysed is truly representative of
the sample. Applications for image analysis of
construction materials currently under development
include:
• Modal analysis of rock and building stone for
mineralogical composition.
• Determination of textures within rock and building
stone.
• Determination of the shape, size, and particle size
distribution (grading) of aggregates.
• Modal analysis of fine aggregate for mineralogical
composition.
• Modal analysis of concrete for aggregate content,
aggregate particle shape, aggregate grading, cement
content, and air void content.
• Measurement of water/cement ratio of concrete.
• Modal analysis of mortars, renders, and screeds for
aggregate content, aggregate particle shape,
aggregate grading, binder content, and air void
content.
• Assessment of fire damage to concrete by
quantifying heat-induced colour changes.
Complementary techniques
A high-quality petrological microscope can usefully
magnify up to 600 times with a maximum resolution of
around 1 μm (microns). Where closer examination is
required scanning electron microscopy (SEM) is
invaluable as it achieves significantly higher magnifi-
cations of up to 50,000 times and resolutions of around
7 nm (nanometres), by using a beam of high-energy
electrons to replace the light of conventional
microscopes. In addition to their high-resolution
capabilities, electron microscopes have great depth of
field, producing electron images that have a three-
dimensional effect. The on-board electron probe
microanalysis system (EPM) allows inorganic bulk
elemental chemical analysis of characteristic X-ray
spectra emitted by samples. The EMP has two modes of
operation of importance to petrographers. 'Spot
analysis' focuses on selected points of the sample such
as individual crystals to give a semiquantitative
elemental analysis, while 'elemental mapping' scans
along a square line raster over a larger area of the
sample surface (up to 30 mm × 15 mm) to allow
observation of variations in element distributions.
Mineralogical analysis by X-ray diffraction (XRD) can
be helpful for identification of crystalline minerals and
decay reaction products, when the optical properties do
not allow definitive microscopical identification. XRD
has the advantage of actually identifying the minerals
present, although estimation of the relative amounts is
at best only semiquantitative. Other spectroscopic
analysis methods such as X-ray fluorescence (XRF),
atomic absorption (AAS), and atomic emission (ICP-AES)
give very precise determinations of elemental
composition. The elemental composition data from these
techniques require skilled interpretation by the analyst
to identify the proportions of mineral phases present. The
above techniques will only detect inorganic compounds
and, to identify the presence of organic materials,
infrared spectroscopy is the method of choice.
 
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