Why Does the Secchi Disk Disappear? An Imaging Perspective

The widely-used Secchi disk method is re-examined from the modulation transfer aspect. Namely, by assuming a volume scattering function and applying small angle scattering approximation, we show that the Secchi depth and horizontal visibility can be determined using the water modulation transfer fun...

Full description

Saved in:
Bibliographic Details
Main Authors: Hou, Weilin, Lee, Zhongping, Weidemann, Alan D
Format: Report
Language:English
Subjects:
DISKS
IMAGING SYSTEMS
MODULATION
MODULATION TRANSFER FUNCTION
OCEANIC OPTICS
OCEANS
Optics
PE0602435N
Physical and Dynamic Oceanography
RADIATIVE TRANSFER
REPRINTS
SEA WATER
SECCHI DEPTH
SECCHI DISK
TRANSFER FUNCTIONS
VISIBILITY
VISION
VISUAL OPTICS
WU73-6867-07-5
Online Access:Request full text
Tags: Add Tag
No Tags, Be the first to tag this record!
Description
Summary:The widely-used Secchi disk method is re-examined from the modulation transfer aspect. Namely, by assuming a volume scattering function and applying small angle scattering approximation, we show that the Secchi depth and horizontal visibility can be determined using the water modulation transfer function and the corresponding spatial frequencies associated with the disk. A basic equation of Secchi disk is reached that is comparable to the radiative transfer approach, in that the Secchi disk is reached that is inversely proportional to the attenuation coefficient (c). With typical values for parameters applied, we demonstrate that the modulation transfer technique produces a horizontal visibility range of about 4.8/c, which is inline with previous studies. The improvement lies in the fact that the current approach correctly addresses the response of all spatial frequencies according to the modeled optical transfer function of the water. In terms of Secchi disk theory, the current approach helps to understand the effect of disk size as well as the role of scattering of the Secchi disk depth. The approach presented provides an understanding of Secchi disk disappearance by showing that the disk is moved away from the obvserver, the spatial frequencies corresponding to the disk size increase, while the modulation transfer dampens contrast at an increased rate. Published in Optics Express, v15 n6 p2791-2802, 19 March 2007. The original document contains color images. All DTIC reproductions will be in black and white.