Measurement of Direct and Diffuse Irradiance

Direct and diffuse solar irradiance are important components in a wide variety of remote sensing applications including environmental and climate research. These measurements allow scientists to fully characterize light and energy affecting an environment. These are critical metrics when researching climate change subjects like glacial retreat and alternative energy sources like solar panels. It can also be used by environmental scientists studying biomes and measuring the amount of light reaching ground level through a canopy. There is no shortage of applications for this data in environmental science, climate research, agriculture, and other industries. Spectral Evolution provides the tools to collect this data and supports the pursuit of better understanding of our rapidly changing environment.

Direct solar irradiance is the solar radiation that passes directly through the atmosphere from the sun without being scattered or absorbed by the atmosphere. Diffuse solar irradiance is the light that has been scattered by atmospheric components such as air and water molecules, dust, or clouds but not directly from the solar disc. Global solar irradiance is the total amount of solar irradiance on an upward-facing horizontal surface and is the sum of the direct and diffuse solar irradiance. The downwelling total solar irradiance provides information on the total radiance that affects the earth’s weather and climate system and environment. Measurement of direct and diffuse solar irradiance can provide information on the transformation of energy on the earth and in the atmosphere, analysis, and distribution of the properties of atmospheric components and verification of satellite measurements of radiation.

These measurements are made easy using a spectroradiometer like the Naturaspec™ and a selection of accessories like a Gershun tube and an in-line cosine diffuser. An in-line diffuser allows measurement of 180° FOV. This makes it possible to measure total horizontal solar irradiance and diffuse solar irradiance when shaded. Direct solar irradiance can be measured using a Gershun tube giving a 1° FOV.

Global Horizontal irradiance (GHI), Diffuse Horizontal radiation (DHI), Direct Normal irradiance (DNI), z = solar zenith angle.

Spectral Evolution field spectrometers such as the PSR-1100f, PSR+, RS-3500, Naturaspec, SR-6500, and RS-8800 can be used to measure direct and diffuse solar irradiance. These ground-based instruments provide advantages such as high spectral resolution, portability, and ability to adjust optical alignment of the measurement apparatus and target. This allows for fast, easy, and accurate measurements.

Here we present field measurements using a Naturaspec with an in-line diffuser for measuring diffuse solar irradiance and global horizontal radiation. These irradiance measurements were collected accurately and efficiently with a leveled tripod mount, pistol grip, and in-line diffuser. The field collection only took a few minutes and the Darwin software calculated the energy measurements very easily emphasizing its field portability and efficiency.

Direct Horizontal Irradiance, Diffuse Horizontal Irradiance spectra using in-line diffuser.

DARWin™ SP Data Acquisition software, which comes with every Spectral Evolution instrument provides a tool accessed by a pull-down menu for integrated energy measurement. The pull-down utility allows users to calculate integrated energy levels as a function of wavelength interval (user adjustable).

Direct Horizontal Irradiance, Diffuse Horizontal Irradiance Integrated energy values.

Spectral Evolution spectroradiometers provide:

• Fast, full-spectrum UV-VIS-NIR measurements – 350-2500nm with just one scan
• Superior reliability in the field with 100% diode array optics with no moving parts
• Bluetooth interface – ditch the rat’s nest of cables
• Lithium-ion rechargeable batteries for field use
• Potential remote-control capabilities (RS-8800)
• Lightweight and rugged
• Standalone operation – internal memory holds 1000 scans (PSR+)
• Optional rugged handheld tablet for field use running DARWin LT on Windows 10 with digital camera, and GPS