Radiometric Measurements and Practical Applications

Radiometry – the measurement of electromagnetic radiation – is critically important for many environmental studies and is applicable to the development of illumination sources for commercial and industrial use.  Photometry concerns the measurement of radiometric sources and their interaction with the human eye.  Spectroradiometry involves the measurement of absolute radiometric quantities over specific wavelength bands.

Avantes offers a suite of spectrometers and accessories to support even the most demanding spectroradiometric and photometric measurements. Our instruments are trusted worldwide for their suitability for measurements of radiometric sources ranging from the sun and solar simulator through UV germicidal sources and commercial and residential lighting.  Read more to explore real-world applications for radiometric measurements.

Radiometry is essentially the measurement of the electromagnetic radiation or wave that is generated by the oscillation of photons. The frequency of these waves, or the wavelength, determines the amount of energy carried by the photons, and hence the ‘colour’ of the radiation. The full electromagnetic range extends from X-rays to the deep UV, through the visible light spectrum, and out to the far IR and beyond into microwaves and radio waves.   Avantes instruments support radiometric measurements in the UV, visible light, and the Near-Infrared from 250-2500 nm.

To collect radiometric data, a detector in a spectrometer collects photons and converts them to an electrical charge according to a known relationship.  To determine the radiometric quantities such as those detailed in Figure 1, the spectrometer must be calibrated against a reference source from NIST/PTB or other standards body such that the detector’s response is correlated to this absolute standard. 

Radiometric units allow us to make absolute statements about the power of sources ranging from the sun to light bulbs or LEDs.  This is critical given that the sensitivity of any radiometric measurement system is subject to the variation of all of the components such as the detector and optical components.  Through the process of calibration, we are able to reference the absolute standard and derive calibrated radiometric units.  It is possible to find examples of radiometric measurement applications in dozens of industries and research projects.

The Baseline Surface Radiation Network (BSRN) is a subprogramme of the World Climate Research Programme (WCRP), the international body that coordinates climate research around the globe. Data from the BSRN, a network of more than 60 manned sites that meet programme standards, is collected at the World Radiation Monitoring Center, currently hosted at the Alfred Wegener Institute in Bremerhaven, Germany. 

The mission of the BSRN programme, established in the late 1980s, is to develop a robust historical record of long-term variability in radiative load around the world according to internationally agreed-upon standards and instrument calibration procedures. This surface radiation data is used to monitor variations in radiation flux over time, provide validation for satellite-based radiation estimates, and contributes to high-quality computational models used by researchers to predict climate response.

To apply for recognition as a BSRN site, a participating observatory or monitoring installation must meet several geographic and institutional prerequisites, including the validation of radiometric instrumentation traceability of calibrations to world standards.

The Tartu Observatory in Tartu, Estonia, originally established in 1810, is an accepted BSRN site. This observatory conducted exploratory studies over a period of three years of a radiometric system that featured an Avantes spectrometer for narrow-band continuous computer-assisted spectrometry. This system collected a spectral measurement every 15 minutes, recording erythemal UV Index (UVI), spectral irradiance at 306 nm, and the ratio of UVA to UVB radiation. This installation was widely studied and researchers reported comparable results to those obtained with filter instruments and with LibRatran (Library of Radiative Transfer) codes at the Tartu site.

Recent advances in CMOS technology mean that the latest Avantes instruments for radiometric measurements have even better dark-noise performance and ultra-low stray light than earlier models.

The Pandora programme started in 2005 with the mission to create a fleet of high-quality, low-cost spectrometers which can be distributed in large quantities to measure air-quality and validate satellite measurements of the atmosphere. The feature of the Pandora systems that stands out is the ability to record total column profiles of the atmosphere, measuring all the layers at once.

A Pandora spectrometer unit is relatively small and portable, which has allowed these systems to be placed in installations all over the world: from Finland and Thessaloniki to the University of Alaska and atop the NOAA station in Boulder, Colorado. A specialized version can also be installed aboard a seagoing vessel even under moderate seas using software that corrects for motion to keep the sun centred in the field of view. The spectrometer at the core of the Pandora system is the Avantes AvaSpec-ULS2048x64-EVO back-thinned CCD spectrometer. Click to read more about the Pandora programme for radiometry in previous Avantes posts.

Several varieties of fish and sea-life are phototaxic, which means they move toward lights. This effect has been carefully studied, and over time several varieties of light sources for fishing have been developed. Incandescent and halogen lamps gave way to metal-halide lamps which provided the high-light intensity required to lure fish, but increased power consumption exponentially. These lamps also produce UV radiation that would be harmful to fishermen.

Researchers at the Korea Maritime University in Busan, Korea are working with Light Emitting Diode (LED) light sources to develop an alternative light source that is more efficient to operate – and therefore more cost-effective – and does not cause harmful radiation. Their work studied the effects of wavelength and intensity on the behaviour of squid using the AvaSpec-ULS4096CL-EVO spectrometer to characterize performance of the light sources reviewed.

These researchers determined that the squid in the study reacted most to light in the blue spectrum between 450 and 490 nm, which penetrates seawater the farthest. Matching the intensity of the single metal-halide lamp at 1.5 kW required multiple LEDs, however. The final system employed six blue LED modules and consumed 180 W: one-eighth of the power requirements of the MHL and one-seventh of the weight.

In many parts of the world, indoor farming is on the rise, but these systems carry high investment costs to design and install. Researchers at the Chinese Agricultural University in Beijing studied the growth response of hydroponically grown lettuce under varying light intensity, photoperiod, and light quality conditions.

The experimental set employed four settings for Photosynthetic Photon Flux Density (PPFD) at 150, 200, 250, and 300 µmol/m²*s as well as three variations in light quality using fluorescent bulbs at a red:blue (R:B) ratio of 1.8 and LED sources at R:B 1.2 and 2.2. Finally, researchers also varied the photoperiod between 12 hours/day and 16 hours/day Researchers determined the daily light integral (DLI) for each set of parameters which were tightly controlled and measured throughout using an Avantes spectrometer system available in China, but comparable to the AvaSpec-ULS2048CL-EVO.

The sun is the source of all energy in our solar system, but it is also a source of danger. Solar flares are huge explosions taking place in the top layers of the sun which expel ionized radiation outward into the universe. If the earth were in the direct path of a large-scale coronal burst, it could prove disastrous for society as we know it, rendering many of our modern conveniences unusable.

Physicists, astrophysicists, and astronomers are still learning about the complicated electrochemical reactions and plasma characteristics of the sun and its many layers. The Great North American Eclipse that crossed the United States in 2017 gave solar researchers a unique opportunity to analyze the sun’s corona. Avantes was on hand at the National Center for Atmospheric Research (NCAR) observation base in Casper, Wyoming. Read more about Avantes support for these ground-breaking experiments in the Solar Eclipse Field Report.

Solar panel manufacturers utilize solar simulators to quality control their products for quantum efficiency.  The characterization of solar simulators is a well-documented procedure which is described in the ASTM method 927-05. 

Avantes instruments and systems support the ASTM method for either pulsed or continuous wave solar simulators.

Avantes is trusted around the world to deliver near real-time radiometric measurements for applications ranging from semiconductors to agriculture with a suite of modular spectrometers, light sources, and accessories. Discover how we can support your unique application by consulting with a knowledgeable Sales Engineer or trusted Avantes distributor today.

Ansko, I., et al. ‘Study of Suitability of AvaSpec Array Spectrometer for Solar UV Field Measurements.‘ Atmospheric Chemistry and Physics 8.12 (2008): 3247-3253.

Choi, J. S., et al. ‘Photoreaction analysis of squids for the development of a LED-fishing lamp.’ Proceedings of the 2nd international conference on maritime and naval science and engineering. 2009.

Kuusk, Joel, and Andres Kuusk. ‘Hyperspectral radiometer for automated measurement of global and diffuse sky irradiance.’ Journal of Quantitative Spectroscopy and Radiative Transfer 204 (2018): 272-280.

Zhang, Xin, et al. ‘Effects of environment lighting on the growth, photosynthesis, and quality of hydroponic lettuce in a plant factory.’ International Journal of Agricultural and Biological Engineering11.2 (2018): 33-40.