Wm2nm

Wm2nm is a shorthand notation for the unit W m^-2 nm^-1, used to express spectral irradiance. Spectral irradiance describes radiant power incident on a surface per unit area per unit wavelength. In this context, Eλ(λ) denotes the spectral irradiance at wavelength λ and has units of watts per square meter per nanometer (W m^-2 nm^-1). The quantity is a differential measure; integrating Eλ over a wavelength interval Δλ yields the total irradiance over that interval, E ≈ ∫ Eλ dλ, with units of W m^-2.

Applications and context

Wm2nm is widely used in spectroscopy, solar energy research, remote sensing, meteorology, and photobiology. It allows scientists to compare how much radiant energy is available or emitted at specific wavelengths, such as in the visible, ultraviolet, or infrared regions. It is particularly important for characterizing light sources, solar spectra, and the spectral response of sensors or biological systems.

Measurement and calibration

Spectral irradiance is typically measured with spectroradiometers or calibrated detectors equipped with dispersive optics. Accurate measurement requires calibration against radiometric references and knowledge of the instrument’s wavelength-dependent response. In practice, measurements are often reported as Eλ at discrete wavelengths or as spectra over a chosen wavelength range.

Conversions and relationships

- Total irradiance: E_total = ∫ Eλ dλ, over all wavelengths.

- Unit conversion: to convert per nanometer to per micrometer, multiply by 1000, since 1 μm = 1000 nm. Conversely, per μm values divide by 1000 to obtain per nm values.

Related concepts include radiance and spectral power distributions, which extend the idea to directional measurements and different units.