The Depths of Cast Shadow

Abstract

To improve the accuracy of analysis outputs from remotely sensed images, shadow and illumination effects need to be minimised or removed. Shadow behaviour at different spectral wavelengths needs to be understood to quantify shadow accurately. This study examined whether a normalised spectral signature of shadow is invariant to sun–object–sensor geometry and can be used to quantify shadow depth. A “FieldSpec® Pro FR” Spectroradiometer and a Canon 450D digital SLR camera were used to measure signatures of cast shadow. Our field-based experiment used an occulter to cast shadow onto a ‘Spectralon’ white plate at six incremental zenith angles and evaluated shadow behaviour within and between varying footprints. A white-balanced image of each shadow zenith was taken by the Canon 450D. The FR Spectroradiometer signatures were normalised to unit vector form and compared to longitudinal transect profiles of shadow from normalised camera images using a scattering index (SI). The normalised signatures show that shadow depth is darker and more ‘blue’ at the proximal areas and conversely that image brightness values increases towards distal areas. Since image brightness is a result of sun–object–sensor geometry, we conclude that a normalised spectral signature is invariant to geometry and can be used to quantify shadow depth.