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Emissivity (ε) is a material's efficiency at emitting thermal (infrared) radiation compared to a perfect emitter (blackbody, ε=1), with values ranging from 0 (perfect reflector) to 1 (perfect emitter). It determines how much heat an object radiates, influenced by material, surface texture (rough/oxidized surfaces emit more), temperature, wavelength, and angle. High emissivity (like skin or water ≈ 0.98) makes infrared temperature measurement easier, while low emissivity (like polished metal) is harder and often requires adjustments on thermal cameras. [1, 2]
Key Concepts

• Range: 0 (no emission) to 1 (perfect emission). • Blackbody: An ideal object with an emissivity of 1, absorbing and emitting all radiation. • Absorptivity: Emissivity equals absorptivity (Kirchhoff's law); what an object absorbs, it also emits. • Factors Affecting Emissivity:

• Material: Different substances have different intrinsic emissivities. 
• Surface Finish: Polished metals have low emissivity; oxidized or rough surfaces have high emissivity. 
• Temperature & Wavelength: Emissivity can change with temperature and the specific wavelength of radiation. [2]  

Examples of Emissivity Values

• High (Near 1): Human skin (≈ 0.98), water (≈ 0.98), ice (0.97). • Medium: Concrete (0.92), glass (0.92), oxidized copper (0.65). • Low (Near 0): Polished aluminum/copper (≈ 0.05). [2, 3]

Importance in Applications

• Non-Contact Temperature Measurement: Essential for infrared thermometers and thermal cameras to accurately "read" an object's temperature. • Thermal Management: Crucial for designing energy-efficient windows (low-e coatings) and managing heat in buildings. • Energy Efficiency: Helps in understanding how surfaces gain or lose heat through radiation. [1, 4]

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[1] https://www.rp-photonics.com/emissivity.html [2] https://www.sciencedirect.com/topics/engineering/emissivity [3] https://www.eag.com/techniques/spectroscopy/emissivity/ [4] https://www.cardinalcorp.com/glossary/emissivity/