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Ocean Optics Web Book

Photometry and Visibility


Page updated: Aug 31, 2017
Principal author: Curtis Mobley
In general the well defined and objectively measured quantities of geometrical radiometry are used in quantitative studies of radiative transfer. However, there are times when, either unavoidably or by choice, the human eye becomes one of our instruments. Such is the case when we enjoy the beautiful colors of nature or when someone observes the appearance of a distant object as a semi-quantitative measure of the clarity or "visible range" of the atmosphere or ocean. In other instances, the eye-brain system may be the preferred instrument, as in visual searches for underwater objects. We therefore must understand how the human visual system responds to radiant energy. This takes us into the domain of photometry and, more generally, colorimetry. For our present purpose, photometry is defined as the study of the human visual response to the quantities of geometrical radiometry.

Level 1 of this chapter presents the basic ideas of human visual response and visibility. The first topics discussed are the photopic luminosity function, which describes the spectral response of the average human eye, and luminance, which corresponds to the visual sensation of brightness. Luminance is an inherently broad-band (all visible wavelengths) correspondent of monochromatic radiance. An equation for luminance transfer is then developed from the monochromatic radiative transfer equation. This requires the development of photopic (broad-band) replacements for wavelength-dependent inherent optical properties.

Whether or not an object can be seen depends on the visual contrast between the object and its background. This is quantified by the inherent and apparent contrast, and a law of contrast reduction is developed. These tools give us what is needed to understand the classic theory of the Secchi disk and the prediction of the Secchi depth.

Beam and point spread functions are then defined. These are the foundation for rigorous analysis of image propagation through absorbing and scattering media, which will be discussed in Level 2 (under development).

The Level 2 material will also discuss colorimetry, the quantitative description of color. Meanwhile, the basics of colorimetry can be found in Chapter 2 of Light and Water (1994).