Question 8 of 9: Optical and Dielectric Property Measurement of Agricultural Materials

Question 8: Optical and Dielectric Property Measurement of Agricultural Materials (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Part (a) — colour (three-parameter) measurement. Colour is measured with a tristimulus colorimeter (or a spectrophotometer with colorimetric software), which illuminates the sample with a standard CIE illuminant (e.g. D65) and measures the light it reflects (or transmits) through three broadband filters that mimic the human eye's response, or by full-spectrum reflectance combined with the CIE standard observer functions. The result is reported as three coordinates in a standard colour space — most commonly CIE L*a*b* (or the closely related Hunter L, a, b): L* is lightness (0 = black, 100 = white), a* is the red–green axis (positive = red, negative = green), and b* is the yellow–blue axis (positive = yellow, negative = blue). The instrument is first calibrated against a white and a black reference tile of known L*a*b* values, then the sample is measured directly (for an opaque or granular agricultural material) or through a fixed-path-length cell (for a liquid, e.g. juice colour).

Part (a) — %transmittance at a wavelength. Percent transmittance is measured with a spectrophotometer set to a single wavelength of interest (chosen for the constituent being probed — e.g. a pigment absorption band): the instrument first records the incident intensity I₀ through a reference blank, then the transmitted intensity I through the sample at the same wavelength and path length, and reports %T = (I/I₀)×100. This is routinely used, for example, to assess the clarity of a juice or the maturity/ripeness of produce via absorbance at a wavelength associated with a specific pigment (chlorophyll, anthocyanin, etc.), since %T = 10^2−A is directly related to the Beer–Lambert absorbance A = εCl of that pigment.

Part (b) — dielectric property measurement. The dielectric constant ε′ (energy storage) and loss factor ε″ (energy dissipation, which governs microwave/RF heating) of an agricultural material at a specified wave-length (frequency) are measured with an open-ended coaxial probe (or parallel-plate capacitor cell for lower frequencies) connected to an impedance or network analyzer. The probe is placed in contact with (or the sample loaded into) the measurement cell, and the analyzer measures the complex impedance/reflection coefficient of the probe-sample system at the target frequency. The sample's capacitance (relative to the same cell measured empty, i.e. relative to a vacuum/air reference of known geometry) gives ε′ = C_sample/C_vacuum, and the measured conductance (dissipation) gives ε″ through the loss tangent tanδ = ε″/ε′. The whole measurement is repeated at each frequency of interest to build a dielectric spectrum, which is what a microwave heating (or RF drying) process design ultimately needs, since the volumetric power absorbed by the material is proportional to f·ε″·E² at the operating frequency f and field strength E.