27 June 2012

Citius, Altius, Fortius

The Olympic Games will be held in London, England this summer and have the motto of Citius, Altius, Fortius or “Swifter, Higher, Stronger.” At Apogee, we strive to design sensors that perform well in rugged environments and would like to sponsor a friendly competition by surveying where individuals have used our sensors.

Prizes will be awarded for the following categories:
  • highest (altitude)
  • deepest (underwater)
  • hottest
  • coldest
  • farthest north (latitude)
  • farthest south (latitude)

The prize for each category is $100 towards equipment or services offered by Apogee Instruments and a write-up on our website highlighting the research being done.

To qualify, the Apogee Instruments sensor must be deployed in a natural environment (no artificial chambers) and record a measurement. To submit your information (or if you have any questions), send an email to devin.overly@apogeeinstruments.com with the value measured, latitude and longitude of the location, day and time the measurement was taken, any environmental data relevant to the contest, and, if possible, pictures of the sensor in action. We will accept submissions through the last day of the Olympics, 12 August 2012. If a winning entry is submitted after that date, we will highlight it on our website but no other prize will be given.



Devin Overly

General Manager

20 June 2012

Light Intensity Measurements for Light Emitting Diodes (LEDs)

I was working late one evening last week when the phone rang. I debated whether to answer it or not, because it was well after standard business hours and I was trying to finish a project, but I decided I needed a break from my computer screen. I picked up the phone and was greeted by a very polite gentleman, likely from somewhere in the southeastern U.S., as evidenced by the drawl and twang in his voice.

This potential customer said he had recently purchased an LED lighting system for his aquarium, but he was having trouble with coral burning. I’m definitely out of my realm of expertise when it comes to coral, but after talking to him about his proposed application of an Apogee quantum meter to measure the intensity of his new LED system, I figured out that the problem was that the light intensity was too high for the coral.

After explaining his desire to measure the intensity of his LED lighting system with a quantum meter, in hopes of achieving the proper light level for his coral, I started to explain how the quantum meter worked. It became clear, however, that he already knew the meter was designed to measure the total number of photons between 400 nm and 700 nm, the photosynthetically active radiation (PAR) range. What he really wanted to know was the accuracy of the quantum meter when used to measure LEDs. Not surprisingly, with the advancement of LED technology in recent years, this is a question frequently asked by customers.

A previous blog post  provided some qualitative information regarding the use of a broadband device (i.e. quantum/PAR sensors or meters) to measure a narrowband radiation source (i.e. many LEDs currently on the market), where it was stated that a spectroradiometer is the best instrument to accurately measure light intensity of LEDs. While this is true, quantum meters can be used to measure LED intensity, and many customers use them for this application. As a result, an estimate of Apogee quantum meter accuracy for measuring LEDs is very practical.

The error associated with a quantum meter (or sensor) measurement of light from a source that has a different spectrum than the source used to calibrate the meter is called spectral error. Spectral error arises because no quantum meters perfectly match the defined quantum response, meaning they do not respond to all wavelengths of light equally between 400 nm and 700 nm. Apogee quantum meters are sensitive to wavelengths between approximately 370 nm and 665 nm, with a relatively flat response between 450 nm and 650 nm due to the blue pigment used in the diffuser (Figure 1). However, they are not equally sensitive to the wavelengths within the photosynthetically active range (Figure 1). In order to determine spectral error, the spectral responses of the quantum meter, calibration light source, and light source to be measured are required, along with some spectra-dependent calculations (for details, see Federer and Tanner, 1966; Ross and Sulev, 2000).

Apogee quantum sensors and meters for electric lighting are calibrated in a custom chamber filled with T5 cool white fluorescent lamps. LEDs have a very different spectral output than T5 lamps (Figures 2, 3, and 4), thus some degree of spectral error is expected. For the narrowband, broadband, and mixed LEDs shown below, spectral errors are 8 % or less. Apogee quantum sensors and meters are less sensitive to blue wavelengths (near 400 nm) compared to longer wavelengths, and thus read low under blue LEDs. Conversely, Apogee quantum sensors and meters are more sensitive to green and red wavelengths (above 500 nm) compared to blue wavelengths, and thus read high under green and red LEDs. The broadband white LEDs output a small proportion of radiation beyond the upper end of the Apogee quantum sensor/meter sensitivity range (665 nm), and thus yield low measurements for the white LEDs.

IMPORTANT NOTE: LEDs that output a large proportion of radiation above approximately 660 nm will read very low and should not be measured with an Apogee quantum sensor/meter.

I did my best to explain, over the phone, the three preceding paragraphs worth of information to the customer. He thanked me for my time, and then ordered a quantum meter the next day. After hanging up the phone following an after-hours tech support call, I was glad to have helped a customer, and even more glad to be inspired by one at the same time.

LED Error [%]
Blue (448 nm peak, 10 nm FWHM) -8.5
Green (524 nm peak, 15 nm FWHM) 8.0
Red (635 nm peak, 10 nm FWHM) 6.9
Cool White -2.0
Neutral White -3.8
Red, Blue Mixture 4.9
Red, Green, Blue Mixture 5.6

Table 1: Theoretical Spectral Errors for Apogee Quantum Meter Measurements of Multiple LED Sources

Figure 1: Apogee quantum sensor/meter response (blue line) compared to defined quantum response (black line) of equal sensitivity at all wavelengths between 400 nm and 700 nm.

Figure 2: T5 cool white fluorescent spectrum (lamp used by Apogee for electric light calibration of quantum meters; black line) compared to narrowband color LEDs (blue, green, red lines) and defined quantum response (gray line).

Figure 3: T5 cool white fluorescent spectrum (lamp used by Apogee for electric light calibration of quantum meters; black line) compared to broadband white LEDs (cool white fluorescent – blue line, neutral white fluorescent – green line, warm white fluorescent – red line) and defined quantum response (gray line).

Figure 4: T5 cool white fluorescent spectrum (lamp used by Apogee for electric light calibration of quantum meters; black line) compared to mixtures of narrowband color LEDs (red/blue – blue line, red/green/blue – red line) and defined quantum response (gray line).

Federer, C.A. and C.B. Tanner, 1966. Sensors for measuring light available for photosynthesis. Ecology 47:654-657.

Ross, J. and M. Sulev, 2000. Sources of errors in measurements of PAR. Agricultural and Forest Meteorology 100:103-125.


Mark Blonquist

Chief Science Officer

13 June 2012

Signal to Noise Ratio

My last blog explained the difference between single-ended and differential measurements. To follow-up, I want to discuss another key concept in the quest to make accurate and consistent measurements: signal to noise ratio (SNR). I will also reiterate the importance of a differential measurement.

Signal to noise ratio is the ratio of the signal voltage to the noise voltage:

SNR = Vout/Vnoise

SNR is usually averaged, and a good SNR is dependent on the measurement being made and desired accuracy of the measurement.

Noise is usually specified as the root mean square amplitude of a voltage (VRMS). Noise can come from multiple sources, including the datalogger, machines, radio frequencies, and power supply lines. The noise inherent in the datalogger can be less than 1 mVRMS, or even 1 µVRMS. Machines, like motors, could introduce noise with amplitudes greater than 1 VRMS. Some dataloggers, such as those made by Campbell Scientific, filter out the power supply noise by filtering out any noise with a frequency of 50/60 Hz, but the machine and radio frequency noise can be more variable, so it can’t be filtered as easily. Sensors with large signals have more “wiggle” room, whereas sensors with µV-scale signals, such as thermocouples and Apogee SI-100 series sensors, will be much more affected by noise.

Noise can be partially filtered out when differential measurements are made because it will cancel itself out. A single-ended measurement doesn’t have the same luxury. Always grounding the shield wire and always using twisted pair wire can also help mitigate noise. The drain wire is connected to a metal shielding around the twisted pair wire and acts as a Faraday cage blocking electric fields. Twisted pair wire is designed to reduce noise by balancing the signal with an equal and opposite signal, which is then cancelled out by taking a differential measurement. A datalogger designed for low voltage, differential measurements can also help make all the difference when it comes to SNR.

Below is a table describing the best way to check for noise on different sensors from Apogee Instruments.

Sensor Method
SI-100 Series Cover the aperture with a small piece of foil and wait for the foil temperature to equilibrate with the sensor temperature. The target temperature should match the sensor body temperature and be stable to within 0.02 C.
SP, SQ, SU Series

MP, MQ, MU Series
Cover the sensor with a cap or dark cloth, so that no light reaches the detector. The reading should be zero and stable to within 1 mV.



Skif Smith

Electrical Engineer

06 June 2012

Reference Pyranometer Traceability and Recalibrations

For most applications, a pyranometer is only as good as its calibration. This makes the initial calibration process, and subsequent recalibrations, critical steps for accurate measurement of solar radiation. The accepted worldwide standard for global solar irradiance is the World Radiometric Reference (WRR), maintained by the World Radiation Center in Davos, Switzerland. The WRR was introduced in 1980 with the intent of bringing congruity to all solar radiation measurements; it has an estimated accuracy of 0.3% [1].

Apogee maintains a group of four ISO (International Organization for Standardization) classified thermopile (black-body) pyranometers as reference standards for global solar radiation. In order to maintain traceability to the WRR, these reference standards are recalibrated every year (alternating two of the four pyranometers each year) by the National Renewable Energy Laboratory (NREL), specifically the Solar Radiation Research Laboratory (SRRL) in Golden, Colorado [2]. This laboratory has become the North American reference standard for solar radiation. A two year recalibration cycle is typically recommended by manufacturers of thermopile pyranometers due to the potential for sensitivity loss over time (the black-body sensor gradually becomes a gray-body). We maintain this highly uniform group of replicate reference instruments (Figures 1 and 2) not only to allow for an accurate calibration of Apogee’s photodiode pyranometers, but also to provide recalibration services for all types of pyranometers, both thermopile and photodiode models.
Figure 1: Measured incoming shortwave radiation by Apogee’s four reference pyranometers near the summer solstice (22-June-2011) in Logan, Utah. Models include Kipp & Zonen CM21, CM11, CMP11 and Hukseflux SR11.

Figure 2: Comparison of Apogee’s four reference pyranometers near the summer solstice (22-June-2011) in Logan, Utah. The reference for comparison is the mean of all four pyranometers and the time axis has been narrowed to show data with zenith angles less than 70° (8:00 am to 7:00 pm).

Our calibration facility is located in Logan, Utah at 41.7° N latitude. All thermopile pyranometers sent in for recalibration are completed outdoors under cloudless sky conditions. We limit these outdoor calibrations from the vernal (spring) equinox to the autumnal (fall) equinox so that calibrations include a greater range of solar intensity levels and sun angles. A photodiode pyranometer is best calibrated to the sun as well, but with a stable set of replicate transfer standards, these pyranometers can be calibrated indoors under a calibration lamp with minimal uncertainty. This allows for rapid turn-around on recalibrations. The transfer standards used for indoor calibrations are calibrated several times each year, between the spring and fall equinox against the four black-body reference standards.

For further information on recalibrations, including pricing, please click the following link: http://www.apogeeinstruments.com/recalibration.html.

Example calibration certificates that accompany each sensor can be viewed below:
Thermopile Pyranometer (Outdoor)
Photodiode Pyranometer (Indoor)

[1] URL: http://www.pmodwrc.ch/pmod.php?topic=wrc

[2] URL: http://www.nrel.gov/solar_radiation/


Jacob Bingham
Customer Support & Technical Manager