Accurate field spectral reflectance measurements are important in the calibration and validation of airborne and space-borne hyperspectral and multispectral imagery. The field portable ASD FieldSpec® 4 spectroradiometer was designed with these requirements in mind. However, even the best instrument can deliver inaccurate data if the measurements are not carried out properly.
One of the most important actions a researcher takes in the field is frequent reference measurements. This paper serves to explain what a reference measurement is in relation to reflectance measurements, why reference measurements are important, and how to properly perform them in the field.
What is a reference measurement?
Before discussing reference measurements, it is important to provide a definition of a reflectance measurement. Reflectance is not a direct measurement. A reflectance measurement is actually a ratio derived from two separate radiance measurements. It is calculated by dividing the radiance of a sample by the radiance from a reference panel of known reflectance, usually having nearly 100% reflectance. This reflectance panel measurement is known as a reference measurement. This relative reflectance spectrum is then sometimes corrected to absolute reflectance by multiplying it be the known reflectance of the reference panel.
Why is a reference measurement so important?
The solar spectrum contains many absorption features, the strength of which are a function of solar elevation angle, altitude, and atmospheric conditions (Fig. 1). By ratioing the radiance of the observed sample to the radiance observed when viewing the reference panel, these features are removed from the resulting reflectance spectrum. An Inherent assumption in this method of ratioing radiances is that the solar irradiance and light scattering effects from the local surroundings is the same for both the sample and reference measurements.
To obtain an accurate reflectance spectrum, the irradiance illuminating the reference panel must be the same as the irradiance illuminating the sample. Any change in the irradiance between the measurement of the reference panel and the sample will result in atmospheric absorption features being imprinted on the measured reflectance spectrum. This is what drives the need to frequently re-measure the reference panel.
Figure 1. Solar radiance reflected from a 99% reflective reference panel. The overall shape of the spectrum is a 5780°K black body with absorption features associated with gases in the Earth’s atmosphere.