Assertion (A): A large measurement aperture is preferred for textured samples.
Reason (R): It averages the variation across the surface rather than sampling one point.
Answer: A
Both are true and the reason gives the reason, and several readings at different positions also help. Rotating the sample between readings averages directional effects. Several positions are measured as a matter of course.
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Rotating a fabric sample between successive measurements is done to
Answer: B
Pile fabrics in particular measure differently in different orientations. Averaging several orientations gives a far more representative value. Pile fabrics are strongly directional. Averaging orientations is therefore essential.
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State whether the following is true or false.
A pile fabric can give measurably different readings depending on its orientation.
Answer: C
The direction of the pile alters how light is scattered back. Averaging over several orientations is standard practice for such materials. Pile direction alters the scattering measurably. Several orientations must be averaged.
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A transmission measurement differs from a reflectance measurement in that it measures
Answer: D
The sample is placed in the beam rather than at a reflectance port. Regular and diffuse transmission geometries are both used. The sample sits in the beam rather than at a port. Regular and diffuse modes are both available.
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Match each measurement mode with its usual application.
List I
A. Reflectance
B. Regular transmission
C. Diffuse transmission
D. Total transmission
List II
1. translucent and scattering samples
2. opaque solids such as textiles and paint
3. the sum of regular and diffuse components
4. clear liquids and transparent films
Answer: A
The mode must suit the optical character of the sample being measured. Using regular transmission on a scattering sample loses much of the light entirely. The mode must suit the optical character. A scattering sample needs the diffuse mode.
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Assertion (A): Regular transmission geometry is unsuitable for a scattering sample.
Reason (R): Light scattered out of the direct beam never reaches the detector at all.
Answer: B
Both are true and the reason gives the loss, and the sample appears darker than it is. Diffuse transmission collects the scattered light as well. Regular geometry would report the sample too dark. Scattered light must be collected too.
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An abridged spectrophotometer differs from a full one in that it
Answer: C
Correct answer: samples the spectrum at a limited number of wavelengths
An abridged instrument uses a set of filters or a small detector array to sample the spectrum at a limited number of points rather than scanning it continuously. It still reports spectral data and still needs calibration; what it gives up is resolution, which matters for samples with sharp spectral features and matters little for ordinary colourants.
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A diode array instrument differs from a scanning monochromator instrument in that it
Answer: D
Simultaneous detection makes the measurement much faster and avoids drift between wavelengths. Resolution and stray light performance may be poorer than a good scanning instrument. Speed is gained at some cost in performance.
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Consider the following statements about diode array instruments.
1. They acquire the whole spectrum simultaneously.
2. They avoid drift occurring between successive wavelengths.
3. They always have better resolution than scanning instruments.
Answer: A
The first two are correct and are the advantages of the design. The third is not generally so, and resolution is often the compromise accepted. Simultaneity is the principal advantage. Drift between wavelengths is avoided entirely.
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A portable instrument differs from a bench instrument principally in
Answer: B
Sphere size, source stability and aperture are all constrained by portability. It is chosen where the sample cannot be brought to the laboratory. Sphere size and stability are both constrained. Performance is traded for portability.
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