FAQs & Alerts

Apogee Sensors: Pyranometers, Quantum Sensors, Infrared Radiometers, and Spectroradiometers
Other Companies: Silicon Pyranometers, Quantum Sensors, and Blackbody Pyranometers

For additional information on recalibration and pricing click here. >

For maximum accuracy, we generally recommend all sensors be recalibrated every two years. You may wish to wait longer between recalibration cycles depending on your requirements. The Clear Sky Calculator may be used as a quick reference to determine if recalibration is necessary for pyranometers and quantum sensors, click here for more information on the Clear Sky Calculator.

The frequency to recalibrate a sensor varies greatly with the application. Generally, we recommend a recalibration cycle of every two to three years, particularly when using the sensor in continuous outdoor applications. But rather than guessing, there is another method that allows the end user to make the determination. The Clear Sky Calculator is designed to calculate the intensity of radiation falling on a horizontal surface, at any time of day, in any location in the world. Essentially, the calculator outputs an estimated value that can be directly compared to the output of the sensor(s) in question. For best accuracy, comparison should be made on clear, non- polluted, summer days within one hour of solar noon. The test sensor(s) should also be leveled and cleaned to help produce consistent measurements.

We recommend recalibration every two years. For information on recalibrating your infrared radiometer click here.

µmol m^-2 s^-1 to footcandles

µmol m^-2 s^-1 to Lux

µmol m^-2 s^-1 to mol m^-2 d^-1

µmol m^-2 s^-1 to Einsteins

µmol m^-2 s^-1 to Watts m^-2 when measuring UV light in sunlight

PPFD to Illuminance Calculator

External Conversion Sites:
Environmental Growth Chambers (EGC) has a comprehensive table and calculator to approximate conversion values for radiation from 400-700 nm from different lamp types.

At Apogee, our solar radiation sensors have a specification for accuracy (5%), uniformity (3%), and repeatability (1%). The accuracy compares each sensor’s output to an absolute reference standard. Uniformity is how consistent our sensors are compared to each other. Repeatability refers to how a sensor performs against itself. Does the same sensor perform consistently under the same conditions? The numbers in our specifications are based on statistical analysis. Large populations of our sensors are compared to a reference, ISO or NIST traceable where available, and the error is measured. We then calculate the mean and standard deviation of the sample from the reference standard. The specifications listed for accuracy, uniformity and repeatability represent plus or minus two standard deviations from the mean (95% of the population). For more information, click here for a knowledge base article on sensor specifications.

Photosynthesis and plant growth depend on the energy in radiation, but only specific wavelengths of radiation cause photosynthesis. We have known since Einstein that one photon excites one electron (the Stark-Einstein Law), which starts photosynthesis. In 1972 a scientist named Keith McCree showed that a meter that counted the number of photons in radiation would more accurately predict photosynthesis than the previously used foot-candle meters. LI-COR (Lincoln, NE) started making meters to measure this radiation and scientists quickly switched to the new measurement system, which is called Photosynthetic Photon Flux (PPF). The energy in a photon is called a quantum so these meters are called quantum meters. A quarter century later, LI-COR has sold thousands of high quality meters, but their least expensive quantum meter with sensor costs $780. The high cost means that they are used only by scientists and large commercial growers. Smaller growers have continued to use foot-candle meters, which measure light for humans. Footcandle meters have errors of up to 45% when used to measure light for photosynthesis.

Solar radiation is a term used to describe visible and near-visible (ultraviolet and near-infrared) radiation emitted from the sun. The different regions are described by their wavelength range within the broad band range of 200 to 100 000 nm (nanometers).

Terrestrial radiation is a term used to describe infrared radiation emitted from the earth. The components of solar and terrestrial radiation and their wavelength ranges are:

Ultraviolet	250 to 400 nm	UV sensor or spectroradiometer
Visible	400 to 700 nm	Quantum sensor or spectroradiometer
Near Infrared	700 to 3000 nm
Infrared	3000 to 100 000 nm	Infrared radiometer

Approximately 99 percent of solar, or short-wave, radiation at the earth's surface is contained in the region from 300 to 3000 nm while most of terrestrial, or long-wave, radiation is contained in the region from 3500 to 50 000 nm.

Outside the earth's atmosphere, solar radiation has an intensity of approximately 1370 watts per square meter. This is the value at mean earth-sun distance at the top of the atmosphere and is referred to as the Solar Constant. On the surface of the earth on a clear day, at noon, the direct beam radiation will be approximately 1000 watts per square meter for many locations.

The availability of energy is affected by location (including latitude and elevation), season, and time of day. All of which can be readily determined. However, the biggest factors affecting the available energy are cloud cover and other meteorological conditions, which vary with location and time.

Historically, solar measurements have been taken with horizontal instruments over the complete day. In the Northern US, this results in early summer values 4 to 6 times greater than early winter values. In the South, differences would be 2 to 3 times greater. This is due, in part, to the weather and, to a larger degree, the sun angle and the length of daylight.