Hemispherical photography
Encyclopedia
Hemispherical photography, also known as fisheye
or canopy photography, is a technique to estimate solar radiation and characterize plant canopy geometry using photographs taken looking upward through an extreme wide-angle lens (Rich 1990). Typically, the viewing angle approaches or equals 180-degrees, such that all sky directions are simultaneously visible. The resulting photographs record the geometry of visible sky, or conversely the geometry of sky obstruction by plant canopies or other near-ground features. This geometry can be measured precisely and used to calculate solar radiation transmitted through (or intercepted by) plant canopies, as well as to estimate aspects of canopy structure such as leaf area index
. Detailed treatments of field and analytical methodology have been provided by Paul Rich (1989, 1990) and Robert Pearcy (1989).
or whole-sky lens) was originally designed by Robin Hill (1924) to view the entire sky for meteorological studies of cloud formation. Foresters and ecologists conceived of using photographic techniques to study the light environment in forests by examining the canopy geometry. In particular, Evans and Coombe (1959) estimated sunlight penetration through forest canopy openings by overlaying diagrams of the sun track on hemispherical photographs. Later, Margaret Anderson (1964, 1971) provided a thorough theoretical treatment for calculating the transmission of direct and diffuse components of solar radiation through canopy openings using hemispherical photographs. At that time hemispherical photograph analysis required tedious manual scoring of overlays of sky quadrants and the track of the sun. With the advent of personal computers, researchers developed digital techniques for rapid analysis of hemispherical photographs (Chazdon and Field 1987, Rich 1988, 1989, 1990, Becker et al. 1989). In recent years, researchers have started using digital cameras in favor of film cameras, and algorithms are being developed for automated image classification and analysis. Various commercial software programs have become available for hemispherical photograph analysis, and the technique has been applied for diverse uses in ecology
, meteorology
, forestry
, and agriculture
.
understory (Clark et al. 1996), impacts of hurricanes on forest ecology (Bellingham et al. 1996), leaf area index
for validation of remote sensing
(Chen et al. 1997), canopy architecture of boreal forests (Fournier et al. 1997), light environment in old growth temperate rain forests (Weiss 2000), and management of vineyard trellises to make better wine
(Weiss et al. 2003).
). Direct radiation is calculated as the sum of all direct (solar beam) radiation originating from visible (non-obscured) sky directions along the path of the sun. Similarly, diffuse solar radiation is calculated as the sum of all diffuse radiation (scattered from the atmosphere) originating from any visible (non-obscured) sky directions (see diffuse sky radiation
). The sum of direct and diffuse components gives global radiation.
These calculations require theoretical or empirical distributions of direct and diffuse radiation in the open, without canopy or other sky obstruction. Usually calculations are performed for either photosynthetically active radiation
(400-700 nanometers) or insolation
integrated over all wavelengths, measured in kilowatt-hours per square meter (kW h/m2).
The fundamental assumption is that most solar radiation originates from visible (unobscured) sky directions, a strong first order effect, and that reflected radiation from the canopy or other near-ground features (non-visible or obscured sky directions) is negligible, a small second order effect. Another assumption is that the geometry of visible (non-obscured) sky does not change over the period for which calculations are performed.
, are based on calculation of gap fraction, the proportion of visible (non-obscured) sky as a function of sky direction. Leaf area index is typically calculated as the leaf area per unit ground area that would produce the observed gap fraction distribution, given an assumption of random leaf angle distribution, or a known leaf angle distribution and degree of clumping. For further explanation see leaf area index
.
Indirect Site Factor (ISF) is the proportion of diffuse solar radiation at a given location relative to that in the open, either integrated over time for all sky directions or resolved by sky sector direction.
Global Site Factor (GSF) is the proportion of global solar radiation at a given location relative to that in the open, calculated as the sum of DSF and ISF weighted by the relative contribution of direct versus diffuse components.
Indices may be uncorrected or corrected for angle of incidence
relative to a flat intercepting surface. Uncorrected values weight solar radiation originating from all directions equally. Corrected values weight solar radiation by the cosine of the angle of incidence, accounting for actual interception from directions normal to the intercepting surface.
Leaf Area Index is the total leaf surface area per unit ground area.
The lens used in hemispherical photography is generally a circular fisheye lens, such as the Nikkor 8mm fisheye lens. Full-frame fisheye lenses are not suitable for hemispherical photography, as they only capture a full 180° across the diagonal, and do not provide a complete hemispherical view.
In the early years of the technique, most hemispherical photographs were acquired with 35 mm cameras (e.g., Nikon FM2 with a Nikkor 8mm fisheye lens) using high contrast, high ASA black-and-white film. Later, use of color film or slides became common. Recently most photographs are acquired using digital cameras (e.g., Kodak DCS Pro 14nx with a Nikkor 8mm fisheye lens).
When images are acquired from locations with large differences in openness (for example, closed canopy locations and canopy gaps) it is essential to control camera exposure. If the camera is allowed to automatically adjust exposure (which is controlled by aperture and shutter speed), the result is that small openings in closed conditions will be bright, whereas openings of the same size in open conditions will be darker (for example, canopy areas around a gap). This means that during image analysis the same sized holes will be interpreted as "sky" in a closed-canopy image and "canopy" in the open-canopy image. Without controlling exposure, the real differences between closed- and open-canopy conditions will be under-estimated.
Fisheye lens
In photography, a fisheye lens is a wide-angle lens that takes in a broad, panoramic and hemispherical image. Originally developed for use in meteorology to study cloud formation and called "whole-sky lenses", fisheye lenses quickly became popular in general photography for their unique, distorted...
or canopy photography, is a technique to estimate solar radiation and characterize plant canopy geometry using photographs taken looking upward through an extreme wide-angle lens (Rich 1990). Typically, the viewing angle approaches or equals 180-degrees, such that all sky directions are simultaneously visible. The resulting photographs record the geometry of visible sky, or conversely the geometry of sky obstruction by plant canopies or other near-ground features. This geometry can be measured precisely and used to calculate solar radiation transmitted through (or intercepted by) plant canopies, as well as to estimate aspects of canopy structure such as leaf area index
Leaf Area Index
Leaf Area Index The Global Climate Observing System defines LAI as "one half the total green leaf area per unit groundsurface area. On sloping surfaces, the LAI should be projected to the normal to the slope. LAI is expressed in terms of square meters of leaf per square meter of ground...
. Detailed treatments of field and analytical methodology have been provided by Paul Rich (1989, 1990) and Robert Pearcy (1989).
History of Hemispherical Photography
The hemispherical lens (also known as a fisheyeFisheye lens
In photography, a fisheye lens is a wide-angle lens that takes in a broad, panoramic and hemispherical image. Originally developed for use in meteorology to study cloud formation and called "whole-sky lenses", fisheye lenses quickly became popular in general photography for their unique, distorted...
or whole-sky lens) was originally designed by Robin Hill (1924) to view the entire sky for meteorological studies of cloud formation. Foresters and ecologists conceived of using photographic techniques to study the light environment in forests by examining the canopy geometry. In particular, Evans and Coombe (1959) estimated sunlight penetration through forest canopy openings by overlaying diagrams of the sun track on hemispherical photographs. Later, Margaret Anderson (1964, 1971) provided a thorough theoretical treatment for calculating the transmission of direct and diffuse components of solar radiation through canopy openings using hemispherical photographs. At that time hemispherical photograph analysis required tedious manual scoring of overlays of sky quadrants and the track of the sun. With the advent of personal computers, researchers developed digital techniques for rapid analysis of hemispherical photographs (Chazdon and Field 1987, Rich 1988, 1989, 1990, Becker et al. 1989). In recent years, researchers have started using digital cameras in favor of film cameras, and algorithms are being developed for automated image classification and analysis. Various commercial software programs have become available for hemispherical photograph analysis, and the technique has been applied for diverse uses in ecology
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
, meteorology
Meteorology
Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...
, forestry
Forestry
Forestry is the interdisciplinary profession embracing the science, art, and craft of creating, managing, using, and conserving forests and associated resources in a sustainable manner to meet desired goals, needs, and values for human benefit. Forestry is practiced in plantations and natural stands...
, and agriculture
Agriculture
Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...
.
Applications of Hemispherical Photography
Hemispherical photography has been used successfully in a broad range of applications involving microsite characterization and estimation of the solar radiation near the ground and beneath plant canopies. For example, hemispherical photography has been used to characterize winter roosting sites for monarch butterflies (Weiss et al. 1991), effects of forest edges (Galo et al. 1991), influence of forest treefall gaps on tree regeneration (Rich et al. 1993), spatial and temporal variability of light in tropical rainforestTropical rainforest
A tropical rainforest is an ecosystem type that occurs roughly within the latitudes 28 degrees north or south of the equator . This ecosystem experiences high average temperatures and a significant amount of rainfall...
understory (Clark et al. 1996), impacts of hurricanes on forest ecology (Bellingham et al. 1996), leaf area index
Leaf Area Index
Leaf Area Index The Global Climate Observing System defines LAI as "one half the total green leaf area per unit groundsurface area. On sloping surfaces, the LAI should be projected to the normal to the slope. LAI is expressed in terms of square meters of leaf per square meter of ground...
for validation of remote sensing
Remote sensing
Remote sensing is the acquisition of information about an object or phenomenon, without making physical contact with the object. In modern usage, the term generally refers to the use of aerial sensor technologies to detect and classify objects on Earth by means of propagated signals Remote sensing...
(Chen et al. 1997), canopy architecture of boreal forests (Fournier et al. 1997), light environment in old growth temperate rain forests (Weiss 2000), and management of vineyard trellises to make better wine
Wine
Wine is an alcoholic beverage, made of fermented fruit juice, usually from grapes. The natural chemical balance of grapes lets them ferment without the addition of sugars, acids, enzymes, or other nutrients. Grape wine is produced by fermenting crushed grapes using various types of yeast. Yeast...
(Weiss et al. 2003).
Solar Radiation Calculations
Direct and diffuse components of solar radiation are calculated separately (see Earth's radiation balanceEarth's radiation balance
Earth's radiation balance is the equation of the incoming and outgoing thermal radiation.The incoming solar radiation is short wave, therefore the equation below is called the short wave radiation balance Qs:* G = global radiation...
). Direct radiation is calculated as the sum of all direct (solar beam) radiation originating from visible (non-obscured) sky directions along the path of the sun. Similarly, diffuse solar radiation is calculated as the sum of all diffuse radiation (scattered from the atmosphere) originating from any visible (non-obscured) sky directions (see diffuse sky radiation
Diffuse sky radiation
Diffuse sky radiation is solar radiation reaching the Earth's surface after having been scattered from the direct solar beam by molecules or suspensoids in the atmosphere. It is also called skylight, diffuse skylight, or sky radiation and is the reason for changes in the colour of the sky...
). The sum of direct and diffuse components gives global radiation.
These calculations require theoretical or empirical distributions of direct and diffuse radiation in the open, without canopy or other sky obstruction. Usually calculations are performed for either photosynthetically active radiation
Photosynthetically active radiation
Photosynthetically active radiation, often abbreviated PAR, designates the spectral range of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis. This spectral region corresponds more or less with the range of light visible to...
(400-700 nanometers) or insolation
Insolation
Insolation is a measure of solar radiation energy received on a given surface area in a given time. It is commonly expressed as average irradiance in watts per square meter or kilowatt-hours per square meter per day...
integrated over all wavelengths, measured in kilowatt-hours per square meter (kW h/m2).
The fundamental assumption is that most solar radiation originates from visible (unobscured) sky directions, a strong first order effect, and that reflected radiation from the canopy or other near-ground features (non-visible or obscured sky directions) is negligible, a small second order effect. Another assumption is that the geometry of visible (non-obscured) sky does not change over the period for which calculations are performed.
Canopy Calculations
Canopy indices, such as leaf area indexLeaf Area Index
Leaf Area Index The Global Climate Observing System defines LAI as "one half the total green leaf area per unit groundsurface area. On sloping surfaces, the LAI should be projected to the normal to the slope. LAI is expressed in terms of square meters of leaf per square meter of ground...
, are based on calculation of gap fraction, the proportion of visible (non-obscured) sky as a function of sky direction. Leaf area index is typically calculated as the leaf area per unit ground area that would produce the observed gap fraction distribution, given an assumption of random leaf angle distribution, or a known leaf angle distribution and degree of clumping. For further explanation see leaf area index
Leaf Area Index
Leaf Area Index The Global Climate Observing System defines LAI as "one half the total green leaf area per unit groundsurface area. On sloping surfaces, the LAI should be projected to the normal to the slope. LAI is expressed in terms of square meters of leaf per square meter of ground...
.
Indices
Direct Site Factor (DSF) is the proportion of direct solar radiation at a given location relative to that in the open, either integrated over time or resolved according to intervals of time of day and/or season.Indirect Site Factor (ISF) is the proportion of diffuse solar radiation at a given location relative to that in the open, either integrated over time for all sky directions or resolved by sky sector direction.
Global Site Factor (GSF) is the proportion of global solar radiation at a given location relative to that in the open, calculated as the sum of DSF and ISF weighted by the relative contribution of direct versus diffuse components.
Indices may be uncorrected or corrected for angle of incidence
Angle of incidence
Angle of incidence is a measure of deviation of something from "straight on", for example:* in the approach of a ray to a surface, or* the angle at which the wing or horizontal tail of an airplane is installed on the fuselage, measured relative to the axis of the fuselage.-Optics:In geometric...
relative to a flat intercepting surface. Uncorrected values weight solar radiation originating from all directions equally. Corrected values weight solar radiation by the cosine of the angle of incidence, accounting for actual interception from directions normal to the intercepting surface.
Leaf Area Index is the total leaf surface area per unit ground area.
Methodology
Hemispherical photography entails five steps: photograph acquisition, digitization, registration, classification, and calculation. Registration, classification, and calculation are accomplished using dedicated hemispherical photography analysis software.Photograph acquisition
Upward-looking hemispherical photographs are typically acquired under uniform sky lighting, early or late in the day or under overcast conditions. Known orientation (zenith and azimuth) is essential for proper registration with the analysis hemispherical coordinate system. Even lighting is essential for accurate image classification. A self-leveling mount (gimbals) can facilitate acquisition by ensuring that the camera is oriented to point straight up toward the zenith. The camera is typically oriented such that north (absolute or magnetic) is oriented toward the top of the photograph.The lens used in hemispherical photography is generally a circular fisheye lens, such as the Nikkor 8mm fisheye lens. Full-frame fisheye lenses are not suitable for hemispherical photography, as they only capture a full 180° across the diagonal, and do not provide a complete hemispherical view.
In the early years of the technique, most hemispherical photographs were acquired with 35 mm cameras (e.g., Nikon FM2 with a Nikkor 8mm fisheye lens) using high contrast, high ASA black-and-white film. Later, use of color film or slides became common. Recently most photographs are acquired using digital cameras (e.g., Kodak DCS Pro 14nx with a Nikkor 8mm fisheye lens).
When images are acquired from locations with large differences in openness (for example, closed canopy locations and canopy gaps) it is essential to control camera exposure. If the camera is allowed to automatically adjust exposure (which is controlled by aperture and shutter speed), the result is that small openings in closed conditions will be bright, whereas openings of the same size in open conditions will be darker (for example, canopy areas around a gap). This means that during image analysis the same sized holes will be interpreted as "sky" in a closed-canopy image and "canopy" in the open-canopy image. Without controlling exposure, the real differences between closed- and open-canopy conditions will be under-estimated.