Spectral signature sampling

When operating in the spectral domain, as it is possible with airborne scanners and satellite imagery, it is of interest to be able to characterize properties like down- and up-welling radiance, attenuation and reflectance in the spectral domain as well.

The background for the interest in spectral signatures is that the spectral signatures may characterize the targets being observed, and that they are influenced by optical properties of the media and surfaces being observed. The composition of a medium or surface thus influences how radiance is transmitted, absorbed, scattered and/or reflected. The spectral reflectance signatures may thus help in determining in whether two different targets types can be distinguished from each other, and if so in which part of the spectrum the spectral characteristics differ.

For the observation of the spectral characteristics spectroradiometric observations are used. In the following, a few types of observations are described. The common characteristic being that the observations results in spectral signatures rather than a single value. Some of the values may be measured directly using a Spectroradiometer, e.g. FieldSpec, others are derived from a set of spectral signatures.

In the following, the radiometric definitions are briefly described. For a more in depth description of the various terms, please refer to (Kirk, 1994).

Basic definitions:

Radiant flux is the amount of radiation coming from a source per unit time. Unit: W

Radiant intensity is the radiant flux leaving a point on the source per unit solid angle of space surrounding the point. Unit: W sr^-1

Radiance is the radiant intensity emitted (or scattered) by a unit area of source (surface). Unit: W m^-2 sr^-1

Irradiance is the radiant flux incident on a receiving surface from all direction per unit of area. Unit: W m^-2 nm^-1.

Reflectance is the fraction of incident flux that is reflected by a surface.

All the above mentioned terms may have the prescript spectral added in which case the units are per wavelength unit, e.g. nm.

Measurements

When referring to how the measurements are taken, the outset has been the use of a FieldSpec spectroradiometer, where the radiation is led by an optic fibre into the detector system. The various types of radiation can be measured by the proper combination of fore-optic and calibration routines, which are described in the following.

The spectral radiance is commonly measured by pointing the tip of the optic fibre towards the target. The solid angle of observation may be adjusted by attaching specific calibrated fore-optics.

The spectral irradiance may be measured directly by attaching a cosine receptor to the spectroradiometer and pointing the receptor parallel to the surface for which the irradiance is to be measured. In the case of measuring down-welling solar irradiance the sensor head is pointed upwards and kept horizontal. An indirect way of measuring irradiance is to point the fibre optic (without cosine receptor) vertical down on a perfect lambertian reflector placed horizontal. The method requires the measured radiance to be multiplied by 2 p to convert to irradiance. A Spectralon panel may be used as an almost perfect lambertian reflector.

Ideally the reflectance is measured / calculated based on simultaneous measurement of radiant flux towards and away from the target. This does however require a dual sensor set-up, which is far more expensive than single sensor. In the single sensor set-up spectral reflectance is instead measured by first pointing the tip of the optic fibre vertical down towards a target of known reflectance, e.g. a Spectralon panel. This measurement is called the white reference. The following measurements are then made as relative to the white reference. The post-processing then involves the correction for the non-perfect reflectance of the white reference panel.

In the case of reflectance measurements over or even below the water surface the differences in radiance levels may require that different integration times are applied for the white reference and the target to be observed. In this case the reflectance in not measured directly but rather calculated from observations of radiance or irradiance.

In any case where the single sensor set-up is used it is a natural pre-requisite that the illumination is constant over the time of measurements. Be careful to measure the down-welling radiance both prior to and after the measurements of targets.

Parameters to observe

If possible the following measurements should be taken in parallel with over-flight or close to over-flight. The purpose of the spectroradiometric measurements is to characterise the actual illumination situation and the vertical beam attenuation of the water as the composition is at the time of image acquisition.

The downward illumination may be described briefly by two signature types: the downwards spectral irradiance and the diffuse proportion of the downwards spectral irradiance.

The downwards spectral irradiance measurement is made as a radiance measurement of the radiance reflected by a horizontal Spectralon panel as described above, the measurements should are taken above the surface.

The diffuse proportion of the downward irradiance may be estimated by a reflectance measurement using a bare tip of the fibre optic. The fibre tip is pointed towards a position in the sky which 90 degrees away from the sun and in the sun-observer zenith plane.

Spectral downward irradiance measurements at various depths are used for determining the vertical beam attenuation. It is in general assumed that the downward irradiance available at a given depth can be described as:

E_d = E₀ e^{-k d}

E₀ is the irradiance just below the surface and E_d is the irradiance at depth d, k is the vertical beam attenuation coefficient.

The spectral reflectance signatures should in the ideal world be independent of the illumination situation, it is however always preferential to measure the spectral reflectance signatures at conditions similar to the situation when the image data are acquired. Observations needed to calculate the reflectance signatures are up- and down-welling irradiance just above the target in question.

Observations for reflectance signatures should be made over a variety of surfaces, where e.g. the water depth and density of vegetation vary.