The distribution of light in a tropical mudflat

collection

The distribution of light in a tropical mudflat

Dataset

Division of Ecology & Evolution

10.25911/wcxh-ga12

English

English

Natural scene analysis has been extensively used to understand how the invariant structure of the visual environment may have shaped biological image processing strategies. This paper deals with four crucial, but hitherto largely neglected aspects of natural scenes: (1) the viewpoint of specific animals; (2) the fact that image statistics are not independent of the position within the visual field; (3) the influence of the direction of illumination on luminance, spectral and polarization contrast in a scene; and (4) the biologically relevant information content of natural scenes. To address these issues, I recorded the spatial distribution of light in a tropical mudflat with a spectrographic imager equipped with a polarizing filter in an attempt to describe quantitatively the visual environment of fiddler crabs.

The environment viewed by the crabs has a distinct structure. Depending on the position of the sun, the luminance, the spectral composition, and the polarization characteristics of horizontal light distribution are not uniform. This is true for both skylight and for reflections from the mudflat surface. The high-contrast feature of the line of horizon dominates the vertical distribution of light and is a discontinuity in terms of luminance, spectral distribution and of image statistics. On a clear day, skylight intensity increases towards the horizon due to multiple scattering, and its spectral composition increasingly resembles that of sunlight. Sky-substratum contrast is highest at short wavelengths. I discuss the consequences of this extreme example of the topography of vision for extracting biologically relevant information from natural scenes.

A compact spectrographic imager (CASI - Compact Airborne Spectrographic Imager, Itres Research, Canada) was used to record 360° horizontal panoramas in an open tropical mudflat at Cungulla, Queensland, Australia (approximately 19° 25.6' latitude, 147° 6.9' longitude), in the Bowling Green National Park, south of Townsville. The CASI is a pushbroom imager and was mounted approximately 20 cm above ground on a motorized turntable, which was carefully levelled. The assembly of turntable, power supply, instrument control unit, and connecting cables allowed scans of 220° in azimuth at any one time, which took about 10 to 15 minutes each. The imager was equipped either with a lens that had an 80.87° field of view (FOV), or with a 37.8° FOV lens fitted with a polarizer, which could be rotated in 30° steps. Integration times and turntable rotation speeds were adjusted to generate square pixels across and along the scan direction, resulting in a resolution of 0.158° with the 80.87° FOV lens or of 0.071° with the 37.8° FOV lens. Spectral resolution varied between 8 and 12 nm, over a wavelength range between 418 nm and 963 nm, or between 380 nm and 920 nm. Recordings were done in October 1997 and September 1999. For more detail of data acquisition, calibration and analysis see Zeil J (2023) Views from ‘crabworld’: The spatial distribution of light in a tropical mudflat. Journal of Comparative Physiology A (in press)

jochen.zeil@anu.edu.au

Research School of Biology, The Australian National University, 46 Sullivans Creek Road, Canberra ACT2601

02 62477515

Jochern Zeil

Javaan Chahl, Martin Hofmann, Katharina Siebke

310906 - Animal neurobiology

280102 - Expanding knowledge in the biological sciences

Visual ecology, visual system specialization, environmental light, fiddler crabs

Pure basic research

2023

2023

The Australian National University Data Commons

Open Access Allowed

Open

Creative Commons Licence (CC BY or CC BY-SA or CC BY-ND or CC BY-NC or CC BY-NC-SA or CC BY-NC-ND) is assigned to this data. Details of the licence can be found at http://creativecommons.org.au/licences.

CC-BY - Attribution

Indefinitely

12

14.5 GB

No