Frequently Asked Questions

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:

Approximately 99 percent of solar, or short-wave, radiation at the earth's surface is contained in the region from 300 to 3,000 nm while most of terrestrial, or long-wave, radiation is contained in the region from 3,500 to 50,000 nm.

Outside the earth's atmosphere, solar radiation has an intensity of approximately 1,370 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 1,000 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 4-6 times greater than early winter values. In the South, differences would be 2-3 times greater. This is due, in part, to the weather and, to a larger degree, the sun angle and the length of daylight.

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 is $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.

From μmol m^-2 s^-1 to footcandles

From μmol m^-2 s^-1 to Lux

From μmol m^-2 s^-1 to mol m^-2 d^-1

From μmol m^-2 s^-1 to Einsteins

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

External Conversion Sites:

The Eppley Laboratory has a conversion area that has "Instantaneous Conversion Calculator" and "Energy Over Time Conversion Calculator"

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

Isopropyl (rubbing) alcohol and a Q-tip work well for cleaning the sensor area. Be careful NOT to use an abrasive cloth on the top as it will scratch the surface of the sensor.

Absolutely. We have quite a few customers that purchase our sensors for continuous underwater applications in aquaculture and aquarium environments. SQ quantum sensor and SP pyranometer sensor are completely sealed and watertight. The cable can be submerged underwater as well.

On MP-200 pyranometer meter, MU-200 ultraviolet meter or MQ-200 quantum meter, you can still submerge the sensor and cable part of these units, but the meter part itself is only splash resistant.

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.

In order to keep condensation from forming on the sensor’s Teflon membrane (where oxygen diffusion occurs), the built-in heater is designed to warm the sensor to a temperature slightly above the ambient temperature. This is particularly important in soil applications where the relative humidity is normally at 100%. For this reason, it is recommended that the heater be continuously powered. Once condensation forms on the membrane, the sensor must be removed from the humid environment and allowed to dry before the condensation evaporates and the signal returns. If the heaters are turned off and condensation forms, the heaters don't supply enough energy to evaporate the condensation once they are turned back on. The heater requires a 12 VDC input and consumes about 74 mW of power. This works out to about 6 mA of current draw.

The AC-100 communication cable is used to download saved measurements from any of our handheld meters. This USB cable includes a built-in circuit board to convert voltage levels to be compatible with the meters – normal USB cables will not work. The AC-100 also comes with a flash drive that includes the necessary computer software, drivers, and instructions. If you already have the AC-100 and just need the software files please click here.

Low or erratic readings are generally due to the sensor window becoming occluded with dirt or other organic deposits. You may want to perform a window cleaning to see if the readings improve. As a reference, when we get infrared sensors back for recalibration we first clean the sensors using a cotton swab dipped in water or a light acid, such as vinegar. The vinegar helps to dissolve salt deposits. Use the cotton swab to clean the inner threads and sensor window. Be careful not to apply too much pressure to the window, as it may actually scratch the anti-reflective coating. We then do a second cleaning to the sensor window using a solvent, such as acetone or ethanol. Many times, this will bring the sensor’s readings within specification. If not, then sending the sensor back for a recalibration may be necessary.

No. All Apogee meters (MO, MP, MQ and MU series) either have sensors built into the meter or attached via two meters of cable.

If you order a sensor only (SI, SO, SP, SQ, SU series) you will need to have your own data logger (or, depending on the sensor, a voltmeter) to collect information from the sensor.

The SP-110 is considered to be self-powered and has been calibrated to 5.0 W m-2 per mV. Use a voltmeter with a mV setting to attain better resolution.  Connect the positive lead of the voltmeter to the red wire of the SP and the negative lead of the voltmeter to the black wire of the SP. Once you are reading the mV output from the sensor, simply multiply this reading by 5.0. This will give you the W m-2 output from the sensor, otherwise known as irradiance.

The SQ-100 and 300 series are considered to be self-powered and have been calibrated to 5.0 µmol m-2 s-1 per mV. Use a voltmeter with a mV setting to attain better resolution.  Connect the positive lead of the voltmeter to the red wire of the SQ and the negative lead of the voltmeter to the black wire of the SQ. Once you are reading the mV output from the sensor, simply multiply this reading by 5.0. This will give you the µmol m-2 s-1 output from the sensor, otherwise known as Photosynthetic Photon Flux (PPF) or Photosynthetically Active Radiation (PAR).

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).