= Raman mapping
{c}
<Raman mapping> records <Raman spectra> at known sample positions and constructs spatial maps from their spectral features. A map can show composition or structural variation that is absent from an ordinary reflected-light image. A point-scanning apparatus combines a focused <laser>, microscope objective, rejection of unshifted light, a <spectrometer> and controlled sample positioning. A primary university facility description identifies single-point acquisition and two-dimensional mapping as measurement modes.<Renishaw Raman Microscope by the National University of Singapore Department of Chemistry>[Instrument arrangement and measurement modes, including two-dimensional mapping]{ref}
The acquired data can be represented as $I(x,y,\widetilde\nu)$, with position coordinates $x,y$ and <Raman shift> $\widetilde\nu$. It is a form of <hyperspectral imaging>. The displayed image is a derived summary of those spectra. For example, a baseline-corrected <Raman band-area map> uses
$$
A(x,y)=\int_{\widetilde\nu_1}^{\widetilde\nu_2}[I(x,y,\widetilde\nu)-B(x,y,\widetilde\nu)]\,d\widetilde\nu,
$$
while a <Raman peak-position map> records a fitted band's position. Different summaries can reveal different properties of the same sample. Manufacturer examples include a silicon-carbide wafer whose spectral map separates inclusions and structural variation.<What Raman images can reveal by Renishaw>[“How do we collect Raman images?”, “How can I analyse Raman images?” and silicon-carbide wafer example]{ref}
For an illustrative20µm-by20µm point map sampled every1µm with both boundaries included, there are $21\times21=441$ positions. At0.10s exposure per position, exposure time alone is44.1s; motion and readout add overhead. Reducing step size increases sampling density, while <optical resolution> remains constrained by the excitation/detection <point spread function> and instrument configuration. A finely interpolated display cannot recover an unsampled small inclusion.
Intensity also depends on illumination, focus and the spectral background. A single-frequency brightness map can mix a desired Raman band with broad background, whereas fitted or integrated features can answer a more specific question. Quantitative composition or stress estimates require an appropriate <measurement calibration>; a colourful spatial contrast is not by itself a calibrated concentration or stress measurement.<What Raman images can reveal by Renishaw>[“How can I analyse Raman images?”, single-frequency intensity and curve-fit parameters]{ref}
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