**General terms:**

- Emissivity ϵ(λ) – The ratio of the spectral exitance, Mλ(T), from an object at temperature T to the exitance from a blackbody at the same temperature MλBB(T) (unitless).

`ϵ(λ)=(M_λ(T))/(M_λBB(T))`

- Absorptivity – The ability of the material to remove electromagnetic flux from the system by converting incident flux to another form of energy (e.g.) thermal energy. The absorptance α can be expressed as the ratio of the flux per unit area incident on the surface that is converted to another form of energy (Mα) to the irradiance on the surface.

`α=M_α/E_i`

- Reflectivity – The ability if the material to turn incident flux back into the hemisphere above the material and the reflectance r can be expressed as the ratio of the exitance from the front of the sample (Mr) to the irradiance on the front of the sample.

`r=M_r/E_i`

- Transmissivity – The ability of the material to allow the flux to propagate through it. The transmittance or transmission (τ) can be expressed as the unitless ratio of the exitance from the back of the sample (Mτ) to the irradiance on the front of the sample (Ei).

**Hemispherical measurements:**

- Radiant flux density – The amount of radiant flux (or power) crossing a unit of area, and is measured in W m-2. Radiant flux density is also known as intensity or irradiance.

- Irradiance (E) – The rate at which radiant flux is delivered to a surface (e.g. the responsive surface of a detector).

`E=E(x,y)=dϕ/dA [Wm^(-2)]`

- Radiant exitance – The flux per unit area away from the surface (e.g. the power per unit area radiated by a source or reflected from a surface.

`M=M(x,y)=dϕ/dA [Wm^(-2)]`

- Radiant spectral flux density – Flux per unit wavelength interval and designated with a wavelength subscript.

- Spectral irradiance – Function of photon wavelength (or energy), denoted by F, is the most common way of characterising a light source. It gives the power density at a particular wavelength. The units of spectral irradiance are in Wm-2µm-1. The Wm-2 term is the power density at the wavelength λ(µm).

- Spectral radiant exitance – Funtion of photon wavelength, flux of a particular wavelength per unit area away from the surface (e.g. the power per unit area radiated by a source or reflected from a surface.

**Directional measurements:**

- Radiant intensity – Both the irradiance and exitance provide spatial information about the flux, but no angular or directional information. The simplest term used to describe directional or dispersive information about the flux is the radiant intensity (I):

`I=I(θ,ϕ)=dϕ/dΩ [W〖sr〗^(-1)]`

- Radiance – Radiant intensity provides directional information, but not spatial information. Radiance (L [wm-2sr-1] overcomes this limitation. It is the most complicated of the radiometric terms, but also the most useful. It is defined in the equation below where x and y define a location in the plane of interest, and θ and ɸ are angles that define the direction of interest relative to the normal to the plane. The radiance is the flux per unit projected area (at the specified location of the plane of interest) per unit solid angle (in the direction specified relative to the reference plane).

`L=L(x,y,θ,ϕ)=(d^2 ϕ)/dAcosθdΩ=dE/dΩcosθ=dI/dAcosθ=dM/dΩcosθ`

- Spectral radiance – The radiance of a surface per unit frequency or wavelength, depending on whether the spectrum is taken as a function of frequency or of wavelength.

- Solid-angle () – A two-dimensional angle in three-dimensional space that an object subtends at a point. It is a measure of how large the object appears to an observer looking from that point. In the International System of Units (SI), a solid angle is expressed in a dimensionless unit called a steradian (symbol: sr).

- Radiant energy – The energy of electromagnetic waves

- Radiant flux – The rate of flow of energy (rays/photons passing), and is the first derivative of the radiant energy with respect to time (t):

`ϕ=dQ/dt [Watts,W]`

- Radiation – The emission or transmission of energy in the form of waves or particles (photons) through space or through a material medium

- Energy – Each photon carries energy dependent on the wavelength of photon:

`q=hc/λ`

- Where:
- h = Planck’s constant (6.6256*10-34 [joules*sec])
- c = Speed of light (2.99792458*108 [m/s]
- λ = wavelength