Apogee S2-122 Посібник користувача - Сторінка 12

Переглянути онлайн або завантажити pdf Посібник користувача для Аксесуари Apogee S2-122. Apogee S2-122 16 сторінок. Pri sensors

OPERATION AND MEASUREMENT

Connect the sensor to a measurement device (meter, datalogger, controller) capable of measuring and displaying

or recording a millivolt signal (an input measurement range of approximately 0-25 mV is required to cover the

sensor output ranges). In order to maximize measurement resolution and signal-to-noise ratio, the input range of

the measurement device should closely match the output range of the sensor. DO NOT connect the sensor to a

power source. The sensor is self-powered and applying voltage will damage the sensor.

Wiring for S2-100 series

Reflectance and Reflectance Indices (PRI)

Reflectance (ρ) is the ratio of radiance, the calibrated signal returned by the downward-looking sensor, to

irradiance, the calibrated signal returned by the upward-looking sensor, for a specific wavelength range:



= radiance reflected from the surface / irradiance incident on the surface

Typically, a measurement of radiance is used in the numerator of the equation above to determine reflectance.

Theoretically, when ρ is determined from radiance reflected from the surface it is called directional reflectance,

which approximates the bidirectional reflectance factor. When ρ is determined from irradiance reflected from the

surface it is called hemispherical reflectance. Both terms are referred to as reflectance herein.

PRI is calculated as the difference between reflectance at 532 nm (green) and reflectance at 570 nm (yellow)

divided by the sum of these reflectance:

It is important to ensure that paired upward-looking and downward-looking sensors make measurements at the

same time, otherwise temporal changes in sky conditions can result in errors in reflectance and calculated PRI

values.

A single upward-looking sensor can be deployed with multiple downward-looking sensors, and irradiance from the

single upward-looking sensor can be used to calculate PRI at the location of all the downward-looking sensors, if

the downward-looking sensors are in close proximity to each other (e.g., within a research plot).

An approximation of PRI can also be made using only measured radiances and a value of , but this is not

recommended because the difference between green and yellow radiances is small, resulting in high sensitivity of

the PRI approximation to the value of .

−

White: Positive (signal from Yellow (PRI))

Black: Negative (signal from Yellow (PRI))

Yellow: Positive (signal from Green (PRI))

Blue: Negative (signal from Green (PRI))

Clear: Shield