4: Emissivity, Reflectivity, and Transmissivity
- Page ID
- 146106
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)- 4.1: Introduction
- This page explains the importance of understanding emissivity, reflectivity, and transmissivity in thermographic measurements. These properties influence how surfaces interact with infrared radiation, which is critical for accurate temperature readings. The page emphasizes that infrared cameras detect radiation, not temperature, and highlights the necessity for Level I thermographers to master these concepts to correctly set up cameras and collect reliable data, thereby minimizing errors.
- 4.2: Emissivity
- This page discusses emissivity, which measures a material's ability to emit infrared radiation, ranging from low (0.0) for reflective materials to high (1.0) for good emitters. Non-metals usually have high emissivity, while bare metals have low. It explains Kirchhoff's Law relating emissivity and absorptivity at thermal equilibrium. Common emissivity values depend on material type, with painted surfaces having high emissivity.
- 4.3: Reflectivity
- This page discusses reflectivity and its impact on measuring infrared radiation, specifically reflected apparent temperature (T-refl). It highlights sources of reflected radiation, such as sunlight and heated objects, which can cause inaccurate readings on shiny surfaces. To ensure accurate T-refl evaluations, techniques such as adjusting camera settings, modifying viewing angles, or using non-reflective shields are recommended.
- 4.4: Transmissivity
- This page discusses transmissivity, which indicates how much infrared radiation passes through materials, usually between 0.0 and 1.0. Most solid materials are opaque to infrared, but some, like specific plastics and thin films, allow transmission. Infrared windows are specially made for safe thermographic inspections and need proper upkeep. Level I technicians must adhere to manufacturer transmissivity values and follow established inspection protocols.
- 4.5: Relationship Between Emissivity, Reflectivity, and Transmissivity
- This page explains the relationship between emission, reflection, and transmission of radiation, stating that their total equals 1.0. It highlights how surfaces with low emissivity reflect more radiation, illustrating the inverse relationship between emission and reflection. This understanding is vital for applications in thermal management and energy efficiency.
- 4.6: Viewing Angle and Measurement Error
- This page discusses the relationship between viewing angle and thermal measurement accuracy, stating that oblique angles lead to decreased emissivity and increased reflectivity, resulting in higher measurement errors. It emphasizes the importance of inspecting surfaces as perpendicular as possible and documenting any deviations, especially in aerial thermography where angles can vary.
- 4.7: Practical Camera Setup Considerations
- This page emphasizes the importance of proper camera configuration for Level I thermographers, highlighting critical factors such as emissivity value, reflected apparent temperature, distance to the target, and atmospheric conditions. It warns that incorrect settings may result in inaccurate temperature readings and unclear thermal patterns.
- 4.8: Common Errors Related to Surface Properties
- This page discusses common mistakes in temperature measurement, such as assuming constant emissivity, neglecting heat source reflections, attempting measurement through standard glass, and failing to document surface conditions. It emphasizes the importance of recognizing these errors as a key responsibility for individuals at Level I.
- 4.9: Summary
- This page covers the critical elements influencing infrared radiation measurement, such as emissivity, reflectivity, and transmissivity. It stresses how surface conditions, viewing angles, and environments affect measurement accuracy and the necessity of proper camera setup for accurate data. The following chapter will focus on infrared camera systems, including details on detectors, optics, and performance specifications.
- 4.10: Review Questions
- This page covers emissivity, important for thermography in obtaining accurate temperature readings. It emphasizes that shiny metal surfaces can mislead due to low emissivity and high reflectivity, causing reflected temperatures to differ from actual ones. Additionally, standard glass is deemed inadequate for thermographic inspections due to its interferential properties, and it points out how viewing angle affects both emissivity and reflectivity.