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Point irradiance radiometer

IrradianceRadiometer measures spectral irradiance within a circular field of view at a specified position. It is a non-perturbing point receiver with an ideal cosine response, suitable for investigating platform shading and instrument orientation over water or other optical surfaces.

import less

scene = less.Scene()
scene.size = 1000
scene.terrain = less.Terrain(property=less.WaterSurface())
scene.illumination = less.Illumination(
    source=less.Sun(zenith=30, azimuth=180),
    atmosphere=less.NoAtmosphere())

downwelling = scene.simulate(less.IrradianceRadiometer(
    position=(500, 500, 5), orientation="up", fov=180,
    bands=[450, 550, 650], quality=32768))
upwelling = scene.simulate(less.IrradianceRadiometer(
    position=(500, 500, 5), orientation="down", fov=180,
    bands=[450, 550, 650], quality=32768))
print(downwelling.irradiance, downwelling.units)
print(upwelling.irradiance, upwelling.standard_error)
upwelling.save("upwelling.npz")

Direct sunlight and diffuse skylight

The receiver includes both direct sunlight and diffuse skylight according to the scene illumination. The NoAtmosphere() example above includes direct sunlight only; it does not automatically add diffuse skylight. To include both, replace the illumination settings with:

scene.illumination = less.Illumination(
    source=less.Sun(zenith=30, azimuth=180),
    atmosphere=less.PrescribedAtmosphere(
        direct_beam_transmittance=0.75,
        diffuse_horizontal_transmittance=0.25))

These values are illustrative, not estimates for a particular site or weather condition. direct_beam_transmittance is direct solar beam transmittance. diffuse_horizontal_transmittance is the ratio of surface diffuse horizontal irradiance to top-of-atmosphere horizontal solar irradiance, not the diffuse fraction of total surface illumination. This configuration provides a uniform sky. See Atmosphere and Surface–Atmosphere Transport for other sky distributions.

An upward-facing receiver integrates visible sunlight and skylight within its FOV. A downward-facing receiver measures light reflected by water and other objects, including reflected skylight. A platform can obstruct part of the sky and change the illumination reaching the water. Therefore, completely blocking the sun does not necessarily reduce the reading to zero: skylight or light reflected by other objects may still reach the receiver. The output is total irradiance; separate direct-sun and diffuse-sky components are not currently provided.

Parameters and results

Positions use metres and the LESS Z-up coordinate system. orientation="up" measures downwelling irradiance; "down" measures upwelling irradiance. An optional nonzero three-vector normal overrides this orientation for a tilted receiver. fov is the full cone angle in degrees, greater than zero and at most 180. bands specifies sampling wavelengths in nm. quality is the number of path samples (at least two).

The one-dimensional irradiance and standard_error arrays have units of W/m²/nm; wavelengths identifies their spectral samples. Standard errors describe Monte Carlo sampling uncertainty only, not uncertainty in scene geometry, water properties, illumination or instrument calibration.

The result is a cosine-weighted angular integral, not average radiance. A restricted FOV is not extrapolated to a hemisphere. Its upwelling/downwelling ratio is an apparent restricted-FOV ratio, not hemispherical albedo. Broadband albedo requires spectral-response-weighted integration of both measurements, not an arithmetic mean of spectral ratios. Broadband SRF configurations are currently not accepted by this receiver.

Platform shading

Measure both directions without a platform, then add the platform geometry and repeat at the same position with unchanged illumination and water parameters. Differences include both obstruction and platform reflection. Add instrument housings and supports explicitly if their obstruction matters; the receiver itself does not create geometry. Check sea-surface extent, especially for elevated receivers with near-hemispherical fields of view.

The receiver uses backward path tracing and is distinct from IrradianceMap, which records radiation exchanges inside the scene.

Solar source and limitations

A finite uniform solar disk is importance sampled for direct views from the receiver. This supports partial solar occultation by platform edges or the receiver FOV. Direct illumination of surfaces still uses the directional sun: platform shadows on the water do not include solar-disk penumbrae. This model is therefore not suitable for studies focused on that fine shadow-edge structure. Water reflection results also depend on the chosen water model and its range of applicability.

Finite receiver area, calibrated angular-response curves and polarization response are not modeled.

  • less.IrradianceRadiometer, less.IrradianceProduct
  • less.Scene.simulate(), less.WaterSurface
  • less.Illumination, less.Sun, less.PrescribedAtmosphere
  • API reference