Scanning Electron Microscopy and X-Ray Microanalysis

425 24

for the continuum background significantly changes the
gray-scale image. As shown for the minor Fe constituent in

. Figs. 24.12 and 24.13, the false Fe contrast between the Ni-
rich and Al-rich phases that arises due to the atomic number
dependence of the X-ray continuum is eliminated in the Fe
compositional map.
While rendering the underlying pixel data of quantita-
tive compositional maps in gray scale is a useful starting
point, the problem of achieving a quantitatively meaningful
display remains. Because of autoscaling, the gray scales of
the Al, Ni, and Fe maps in. Fig. 24.12 do not have the same
numerical meaning. The Al and Ni concentrations locally
reach high enough levels to correspond to brighter gray
levels and have sufficient range to create strong contrast
with direct gray-scale encoding, as seen in. Figs. 24.12 and
24.13. The Fe constituent which is present at a concentra-
tion with a maximum of approximately 0.04 mass fraction
(4 wt %) never exceeds the minor constituent range, so the
Fe map appears dark with little contrast in the quantita-
tive  map of. Fig. 24.13. Autoscaling of the Fe-map in
. Fig. 24.14 improves the contrast, but the same limitations

of autoscaling noted above still apply. Various pseudo-color

scales, in which bands of contrasting colors are applied to

the underlying data, are typically available in image pro-

cessing software. Such pseudo-color scale can partially

overcome the display limitations of gray-scale presentation,

but the resulting images are often difficult to interpret. An

example of a five band pseudo-color scale applied to the

compositional maps using NIST Lispix is shown in

. Fig. 24.15. An effective display scheme for quantitative
elemental maps that enables a viewer to readily compare
concentrations of different elements spanning major, minor
and trace ranges can be achieved with the Logarithmic
Three-Band Encoding (Newbury and Bright 1999 ). A band
of colors is assigned to each decade of the concentration
range with the following characteristics:
Major: C > 0.1 to 1 (mass fraction) deep red to red pastel
Minor: 0.01 ≤ C ≤ 0.1 deep green to green pastel
Trace: 0.001 ≤ C < 0.01 deep blue to blue pastel

The quantitative elemental maps are displayed with

Logarithmic Three-Band Encoding in. Fig. 24.16, and the

Raw Fe K-L2,3 + Mn K-M2,3

Fe K-L2,3 after DTSA-II

quantification

20 μm

. Fig. 24.11 Quantitative
compositional mapping with
DTSA-II on the XSI of a deep-
sea manganese nodule from
. Fig. 24.2: direct comparison of
the total intensity map for Fe and
the quantitative compositional
map of Fe; note the decrease in
the apparent Fe in the high Mn
portion in the center of the image
after quantification and local
changes indicated by arrows (yel-
low shows extension of high Fe
region; magenta shows elimina-
tion of dark features)

24.3 · Quantitative Compositional Mapping