- Reflectance Definition: Reflectance is defined as the ratio of the radiant flux reflected from a surface to the incident radiant flux, and it is unitless.
- Types of Reflectance: There are two types of reflectance: specular (mirror-like) and diffuse (scattering).
- Reflectivity Definition: Reflectivity is the property of a material to reflect light or radiation and remains consistent regardless of the material’s thickness.
- Reflectance Measurement: Reflectance can be measured relatively using a reference plate or absolutely by comparing to the light source.
- Solar Reflectance Index: This index indicates a material’s ability to reflect solar energy, ranging from 0 to 1.
What is Reflectance
Reflectance is reflected radiant flux фr divided by incident radiant flux фi under specified spectral, polarisation and geometric conditions. The standard symbol is ρ; the alternative p shown here is a legacy typesetting error.
![\[ \rho = \frac{\phi_r}{\phi_i} \]](https://www.staging.electrical4u.com/wp-content/ql-cache/quicklatex.com-dcc923bc58067969de200157185e3ab3_l3.png "Rendered by QuickLaTeX.com")
Reflectance is a dimensionless ratio of the same radiant-flux quantity. It can vary with wavelength, incidence angle, collection direction, polarisation, surface condition and temperature. Reflectance and transmittance enter the energy balance with absorptance.
Total hemispherical reflectance can be separated into regular or specular reflectance ρs and diffuse reflectance ρd, provided the measurement geometry collects both components.
Specular reflectance is the fraction reflected in the mirror direction under the specified angular aperture. A smooth mirror is a familiar example.
Diffuse reflectance is the fraction scattered outside the regular beam into other directions. A matte cinema screen produces mainly diffuse reflection.
Total reflectance is the sum of the collected specular and diffuse components when those components partition the same reflected hemisphere without overlap.
Reflectivity
In precise radiometric terminology, reflectivity is a special material property measured as the reflectance of an optically smooth, sufficiently thick opaque specimen. General usage is less consistent, so a report should define the quantity and geometry rather than treat the terms as interchangeable.
For the ideal smooth, homogeneous, semi-infinite specimen used in that definition, measured reflectance represents material reflectivity. Finite thickness, roughness, coatings, substrates and layered interference make specimen reflectance depend on more than the bulk material.
Reflectance vs Reflectivity
Reflectance describes a specimen and its measurement conditions. Reflectivity, under the strict definition used here, describes the limiting material property obtained with a smooth, opaque and sufficiently thick specimen.
A thin layer can reflect from more than one interface. The reflected fields interfere, so wavelength, refractive index, absorption, angle and layer thickness can all change the measured reflectance.
Reflectivity is the limiting reflectance under the defined smooth, opaque and semi-infinite specimen conditions and still depends on wavelength, angle and polarisation.
Reflectance is the fraction of incident electromagnetic power reflected by a specimen under stated conditions. Reflectivity is the special material-property case described above.
Reflection Measurement
Reflection can occur at the surface and within a scattering material. A measurement may collect the specular component, the diffuse component or both.
The figure shows light incident at angle θ. In optics, incidence angle is normally measured from the surface normal, so the reference direction must be clear.
Smooth surfaces tend to produce a narrow specular lobe, while roughness and volume scattering spread energy into diffuse directions. Many real specimens produce both components.
Total hemispherical reflectance requires collection over the reflected hemisphere, often with an integrating sphere. A limited detector aperture measures only the directions it accepts.
Reflectance can be measured by comparison with a calibrated reference or through an absolute instrument method with a traceable measurement model.
A relative measurement compares the sample signal with a mirror or diffuse reference such as a barium-sulfate plate under matching geometry. The reference does not have 100% reflectance; apply its calibrated spectral value to convert the signal ratio into specimen reflectance. The protected equation below omits that calibration factor.
![\[ Relative \; Reflectance (\%)= \frac{Amount \; of \; Light \; Reflected \; from \; the \; sample}{ Amount \; of \; Light \; Reflected \; from \; the \; Reference \; Plate} \times 100 \]](https://www.staging.electrical4u.com/wp-content/ql-cache/quicklatex.com-d3382435d82e595606a29c34cd8949d7_l3.png "Rendered by QuickLaTeX.com")
An absolute measurement determines reflected-to-incident flux through a calibrated geometry and instrument response. It does not assume 100% reflectance of air. Some absolute methods still use auxiliary standards to calibrate detectors and geometry. The protected equation is only the defining ratio.
![\[ Absolute \; Reflectance (\%) = \frac{Amount \; of \; light \; Reflected \; from \; the \; sample}{Amount \; of \; Light \; Used} \times 100 \]](https://www.staging.electrical4u.com/wp-content/ql-cache/quicklatex.com-963754ec873e367c8985cb069f45528f_l3.png "Rendered by QuickLaTeX.com")
Spectral Reflectance Curve
A spectral reflectance curve plots reflectance against wavelength for stated incidence and collection geometry, polarisation and sample conditions.
The curve shows the reflected fraction across wavelength. Different specimens can have similar curves, and one specimen can produce different curves when geometry, surface condition or polarisation changes.
Visible spectral reflectance contributes to colour, but perceived or calculated colour also depends on the illuminant, observer functions and viewing geometry. Reflectance spectra can extend beyond the visible band.
The figure compares example spectral reflectance curves. It should not be treated as a universal signature for each named surface.
Solar Reflectance Index
Solar Reflectance Index (SRI) is a calculated indicator of the relative steady-state surface temperature of an opaque horizontal or low-sloped surface under standard conditions. ASTM E1980 combines measured solar reflectance with thermal emittance; SRI is not another name for either property.
ASTM E1980 defines a standard black surface as SRI 0 and a standard white surface as SRI 100. Results can be below 0 or above 100. Solar reflectance itself normally ranges from 0 to 1, but SRI also includes thermal emittance and standard heat-transfer conditions.
Attenuated Total Reflectance (ATR)
Attenuated Total Reflectance (ATR) is an infrared sampling technique for materials in optical contact with a high-refractive-index crystal. Accessories can use one or several internal reflections, so multiple reflections are not required by the definition.
When an infrared beam travels inside a higher-refractive-index crystal towards a lower-index sample interface, part or all of it can undergo internal reflection. Refractive index, not “reflective index,” sets the critical-angle condition.
Total internal reflection occurs when light travels from higher to lower refractive index and the internal incidence angle exceeds the critical angle. In the ideal lossless interface, all propagating power is reflected.
An electromagnetic field extends a short distance beyond the crystal surface and decays exponentially into the sample. This non-propagating near field is the evanescent wave.
At wavelengths absorbed by the sample, interaction with the evanescent field reduces the internally reflected beam. The resulting spectrum is called Attenuated Total Reflectance. Penetration depth depends on wavelength, angle and both refractive indices.
Reflectance Spectroscopy
Reflectance spectroscopy measures reflected radiation as a function of wavelength to infer composition or optical behaviour. Some instruments operate without contact, while ATR and other accessories require sample contact.
Elastic reflectance spectroscopy measures the spectral and angular distribution of reflected light without a wavelength shift. Fluorescence and other inelastic emission require separate treatment because the outgoing wavelength changes.
Instrument dynamic range is finite and method-specific. A reflectance spectrum does not directly equal an absorption coefficient when scattering, roughness or layered interference matters; a suitable optical model and calibration are required.
Reflection and Transmission Coefficients
The reflection coefficient is the generally complex ratio of reflected wave amplitude to incident wave amplitude, including phase. For optics it is usually an electric-field amplitude ratio. Reflectance is the corresponding power ratio and, for one reflected mode, equals the squared magnitude of that coefficient.
In optics, Fresnel reflection coefficients depend on refractive indices, incidence angle and polarisation. In electrical engineering, the same amplitude-and-phase concept describes waves reflected by impedance discontinuities on transmission lines.
Reflection and transmission amplitude coefficients follow boundary conditions, but their squared magnitudes are not generally interchangeable with power fractions. Transmitted power also needs an impedance or refractive-index and angle factor.
The transmission coefficient is the generally complex ratio of transmitted wave amplitude to incident wave amplitude. In a lossless interface the correctly calculated reflected and transmitted power fractions sum to one; absorption or other channels change that balance.
