Spectroradiometers are used to measure the spectral power distributions of light sources, monitors, and other illuminants. They are used to evaluate and categorize commercial and residential lighting, and to calibrate liquid crystal displays (LCDs) and cathode ray tube (CRT) displays on laptop computers and high definition televisions (HDTVs). In addition to measuring visible light, spectroradiometers may be used in radio astronomy and remote Earth sensing applications. A subfield of astronomy, radio astronomy detects celestial sources of radio frequencies such as stars and galaxies. Earth remote sensing uses sensors and satellites to collect geological and climactic data about the planet. Spectroradiometers are also used in agricultural applications to monitor nitrogen in the soil and indicate when fertilizers are required.
Spectroradiometrers are designed to measure absolute radiometric quantities in narrow bands of wavelength. Like photometers, these spectrometers can be used to measure light visible to human eye. Yet spectroradiometers can also measure light in the ultraviolet (UV) and infrared (IR) bands. In radiometry, quantities include radiant energy, radiant flux, and radiant intensity; radiance and irradiance; radiant exitance or radiant emittance; radiosity; and both spectral radiance and spectral irradiance. Radiance is the power per unit solid angle, per unit projected source area. Irradiance is the power incident on a surface. Radiant flux or radiant power is the radiant energy per unit time. Units of measure in spectroradiometry include joules, watts, watts per steradian, and watts per square meter.
There are three main performance specifications for spectroradiometers: spectral range, spectral resolution, and spectral accuracy. Wavelength or spectral range is the dispersion of the grating across the linear array, or expressed as the size of the spectra on the array. Wavelength or spectral resolution is the width of an analytical peak at half its height. Measured in nanometers, it measures the ability of spectroradiometers to separate two overlapping peaks. At the point of resolution, two peaks of equal height are resolved if there is a visible dip between the tops of the peaks. Wavelength or spectral accuracy is the agreement between a mean measured value and a true or accepted value as quantified by error. With spectroradiometers, spectral accuracy is related to determinate errors and quantified by bias.