The flux density or irradiance is a physical measurement quantity for radiation. It quantifies the energy flux (Joule per second, or Watt) that is incident on a given horizontal surface. Irradiance therefore bears the unit Watt per square meter and depends on the angular distribution of the radiation field. Each photon contributes to irradiance with a weight that is determined by the cosine of its angle of incidence.

The Sun emits radiation which at the top of our atmosphere amounts to 1368 W m^-2 (on average). Interaction processes (scattering, absorption) with the particles of the atmospheric gases as well as with aerosol, cloud, and ice particles change the intensity and the direction of the solar radiation. These processes depend on the particle properties as well as on the wavelength of the radiation. A slight dependence on the polarization of the radiation is also observed. Spectral measurements of the flux density (which is then expressed in units of W m^-2 nm^-1) allow conclusions about the current state of the atmosphere, but also set high requirements for the instrumentation. Our measurement system is designed to meet these demands to a high degree.

The actinic flux density quantifies the energy flux through a spherical surface. Therefore, all photons contribute equally, regardless of their angle of incidence. Hence, the actinic flux density is the relevant quantity, e.g., for chemical processes in the atmosphere. For instance, dissociation processes of gas molecules such as ozone and nitrogen oxides are driven by photons of a certain energy (wavelength), but this process is independent of the angle of incidence of these photons.

As the relationship between actinic flux density and irradiance is not trivial, special optical sensors are required for measuring the actinic flux density. These sensors detect radiation from all directions equally (isotropically).

The actinic flux density is often measured in the ultraviolet range to describe chemical decay processes of trace gases, such as ozone, nitrogen oxides a.o. In addition, theory shows that with knowledge of both irradiance and actinic flux density at two altitudes one can derive the mean absorption coefficient of the layer between those two altitudes - and from this the heating rate of this layer. For this purpose, we have used combined airborne measurements of irradiances and actinic flux densities during several experiments.

Note the difference between irradiance and actinic flux density. On the left: Irradiance is related to a horizontal detection area. Photons are weighted with the cosine of their angle of incidence. This is why a low sun provides less heat to the Earth surface than a high sun. — On the right: Actinic flux density, for instance, as seen by an ozone molecule in the centre. Actinic flux density is detected isotropically, as it is of no importance to the molecule where the photons are coming from. As soon as a proper photon arrives, no matter whence, the molecule dissociates.

The radiance also describes a radiant energy flux. In contrast to the irradiance, however, it refers only to a certain solid-angle portion of space rather than an entire hemisphere. Consequently the unit of radiance is W m^-2 sr^-1.

This quantity is what is measured by satellites. Remote sensing of atmospheric parameters (e.g., meteorological quantities, trace gases, aerosol particles, clouds) are based on these satellite measurements.
In theory, irradiance and actinic flux density can be derived from radiances when they are measured for a sufficient number of solid angles. But it is not very practical, therefore irradiance and actinic flux density  are measured directly. For the reverse case, irradiances can be translated into radiances only for rare isotropic conditions.

Several parameters can be derived directly from the upwelling and downwelling irradiance measurements, such as the albedo α, the layer reflectance R, transmittance T, absorption A, and the absorbed irradiance Fabs. more

The surface albedo is a measure for the interaction of the surface of the Earth with atmospheric radiation. It is defined as the ratio of irradiance reflected from the surface and irradiance incident on the surface. A fresh snow cover has a surface albedo of approx. 0.8-0.9, i.e., 80 to 90 percent of radiation are reflected. The lowest albedo values are observed for water surfaces; oceans reflect less than 10 percent of radiation. A variety of physical effects, such as the changing colours of vegetation, make surface-albedo measurements scientifically valuable.

For local investigations it is sufficient to place a spectrometer on a tripod at a height of 1-2 meters above ground. In this way, albedo spectra can be determined for surfaces such as 6-days-old oak leaves, soft fresh snow, granite, or a stubble field. However, for regional and global investigations like the atmospheric energy budget, a more macroscopic view is necessary. This is achieved by measurements from aircraft or satellites.

Satellite measurements of the optical properties of the atmosphere also strongly rely on a spectral knowledge of the surface albedo. The signal (radiance) detected by the satellite instrument is a mixture of contributions from the Earth surface and from the atmosphere. For instance, it is difficult to detect aerosols over bright surfaces such as the Sahara desert or the snow-covered Arctic. Various approaches have been developed to this purpose, but uncertainties remain. We can (and do) provide airborne measurements of the surface albedo with our SMART-Albedometer. Our measurements can help the evaluation process of such approaches.

The photolysis frequency of a certain gas molecule is calculated on the basis of the spectral actinic flux density. The process of dissociation is caused by absorption of photon energy. The speed of the reaction complies with the photolysis frequency J which serves as a description of photochemical reactions in the atmosphere. The photolysis frequency depends on the available actinic flux density F_a, the molecule's specific absorption cross section σ, and the quantum yield φ: