  The homogeneity of filter media is important for material selection and
quality control, along with the specific weight (nominal grammage) and the
distribution of the local weight. Cloudiness or formation is a concept used to
describe deviations from homogeneity in filter media. We suggest to derive the
cloudiness index from the power spectrum of the relative local areal weight,
integrated over a selected frequency range. The power spectrum captures the
energy density in a broad spectral range. Moreover, under certain conditions,
the structure of a nonwoven is fully characterized by the areal weight, the
variance of the local areal weight, and the power spectrum. Consequently, the
power spectrum is the parameter that exclusively reflects the cloudiness. Here,
we address questions arising from practical application. The most prominent is
the choice of the spectral band. It certainly depends on the characteristic
"size of the clouds", but is limited by the size and lateral resolution of the
images. We show that the cloudiness index based on the power spectrum of the
relative local areal weight is theoretically well founded and can be robustly
measured from image data. Choosing the spectral band allows to capture the
cloudiness either visually perceived or found to be decisive for product
properties. It is thus well suited to build a technical standard on it.
