20-08-2012, 04:27 PM
PRINCIPLES OF RADIATION MEASUREMENT
RADIATION MEASUREMENT[.pdf (Size: 372.27 KB / Downloads: 46)
Much confusion has existed regarding the measurement of radiation.
This report presents a comprehensive summary of the
terminology and units used in radiometry, photometry, and the
measurement of photosynthetically active radiation (PAR).
Measurement errors can arise from a number of sources, and
these are explained in detail. Finally, the conversion of radiometric
and photometric units to photon units is discussed. In this
report, the International System of Units (SI) is used unless
noted otherwise (9).
RADIOMETRY
Radiometry (1) is the measurement of the properties of radiant
energy (SI unit: joule, J), which is one of the many interchangeable
forms of energy. The rate of flow of radiant energy, in the
form of an electromagnetic wave, is called the radiant flux (unit:
watt, W; 1 W = 1 J s-1) Radiant flux can be measured as it flows
from the source (the sun, in natural conditions), through one or
more reflecting, absorbing, scattering and transmitting media
(the Earth's atmosphere, a plant canopy) to the receiving surface
of interest (a photosynthesizing leaf) (8).
PHOTOSYNTHETICALLY
ACTIVE RADIATION
In the past there has been disagreement concerning units and
terminology used in radiation measurements in conjunction with
the plant sciences. It is LI-COR's policy to adopt the recommendations
of the international committees, such as the Commission
Internationale de I'Eclairage (CIE), the International Bureau of
Weights and Measures, and the International Committee on
Radiation Units. The International System of Units (SI) should
be used wherever a suitable unit exists (9).
Units
The SI unit of radiant energy flux is the watt (W). There is no
official SI unit of photon flux. A mole of photons is commonly
used to designate Avogadro's number of photons (6.022 × 1023
photons). The einstein has been used in the past in plant science,
however, most societies now recommend the use of the mole
since the mole is an SI unit. When either of these definitions is
used, the quantity of photons in a mole is equal to the quantity
of photons in an einstein (1 mole = 1 einstein = 6.022 × 1023
photons). Note: The einstein has also been used in books on
photochemistry, photobiology and radiation physics as the quantity
of radiant energy in Avogadro's number of photons (5). This
definition is not used in photosynthesis studies.
Terminology
LI-COR continues to follow the lead of the Crop Science
Society of America, Committee on Terminology (10) and other
societies (11) until international committees put forth further
recommendations.
PHOTOMETRY
Photometry refers to the measurement of visible radiation (light)
with a sensor having a spectral responsivity curve equal to the
average human eye. Photometry is used to describe lighting
conditions where the eye is the primary sensor, such as illumination
of work areas, interior lighting, television screens, etc.
Although photometric measurements have been used in the past
in plant science, PPFD and irradiance are the preferred measurements.
The use of the word "light" is inappropriate in plant
research. The terms "ultraviolet light" and "infrared light"
clearly are contradictory (8).
The spectral responsivity curve of the standard human eye at
typical light levels is called the CIE Standard Observer Curve
(photopic curve), and covers the waveband of 380-770 nm. The
human eye responds differently to light of different colors and
has maximum sensitivity to yellow and green (Figure 3). In
order to make accurate photometric measurements of various
colors of light or from differing types of light sources.
MEASUREMENT ERRORS
At a Controlled Environments Working Conference in Madison,
Wisconsin, USA (1979), an official from the U.S. National
Bureau of Standards (NBS) stated that one could not expect less
than 10 to 25% error in radiation measurements made under
non-ideal conditions. In order to clarify this area, the sources of
errors which the researcher must be aware of when making
radiation measurements have been tabulated. Refer also to the
specifications given with each sensor for further details.
Absolute Calibration Error
Absolute calibration error depends on the source of the lamp
standard and its estimated uncertainty at the time of calibration,
accuracy of filament to sensor distance, alignment accuracy,
stray light, and the lamp current measurement accuracy. Where
it is necessary to use a transfer sensor (such as for solar calibrations)
additional error will be introduced. LI-COR quantum and
photometric sensors are calibrated against a working quartz
halogen lamp. These working quartz halogen lamps have been
calibrated against laboratory standards traceable to the NBS.
Standard lamp current is metered to 0.035% accuracy.
Microscope and laser alignment in the calibration setup reduce
alignment errors to less than 0.1%. Stray light is reduced by
black velvet to less than 0.1%.
Readout Error
This error is due to the readout instrument as distinguished from
the sensor. Zero drift, temperature, battery voltage, electronic
stability, line voltage, humidity and shock are all factors which
can contribute to readout error. The use of electronic circuitry
such as chopper-stabilized amplifiers and voltage regulators in
LI-COR meters largely eliminates many of these problems: zero
drift, temperature, battery voltage, electronic stability, line
voltage.
Total Error
The errors given are largely independent of each other and are
random in polarity and magnitude. Therefore, they can be
summed in quadrature (the square root of the sum of the
squares). The total error is shown below for an LI-190SA
Quantum Sensor and LI-COR meters when used for measuring
lighting in a typical growth chamber or natural daylight over a
temperature range of 15° to 35°C.