Radiometry of the Sun and sky
This is a spectral irradiance measurement of the Sun and sky made on a very clear morning with little haze. The solar altitude was +27.3°.
The plot shows a broad spectrum view of the solar energy distribution. With other spectrometers, I can in principle, measure out to 2,500nm and also into the ultraviolet down to the atmospheric cut-off close to 300nm, but I still have some work to do on the radiometric calibration.
The pale grey line is the solar flux outside the atmosphere (from the Hubble calibration database). Although the legend says 'scaled', the scaling factor is unity.
The orange and red lines show the visible spectrum of the sun/sky measured in the plane perpendicular to the Sun with the Sekonic C-7000 colour meter. The orange is the spectral irradiance which is the power incident on the plane and coming from the whole of the hemisphere (2Pi sr): this adds some of the blue sky back into the measured solar power. If I restrict the solid angle with a tube attached to the device (a toilet roll), I measure the solar spectral irradiance after extinction by 2.17 airmass of atmosphere with only a very small contamination by sky - the red line.
In this case, the orange line does not represent the Sun + the entire blue sky visible to the cosine corrector on the radiometer since parts of the sky were obscured by neighbouring buildings.
The inset photo shows the Sekonic with the adapted toilet roll. This restricts the field of view to a radius of 5.4°, tapering to zero by 10.6° from the Sun.
The underlying spectrum in blue is the solar irradiance after passing through my extinction model for this airmass. The model includes Rayleigh and aerosol scattering and ozone absorption in the ultraviolet and the visible (Chappuis) bands. The only parameters I can vary are the concentration of aerosols (very low in this case) and the ozone column density which is slightly more complicated to choose due to the variation of ozone density with height (my simple model is for a uniform atmosphere).
In this case the Chappuis band only has a small effect around 600nm since the Sun was quite high. The goodness of fit gives confidence in the absolute calibration of the the Sekonic device!
The dashed black line is the Planck function at the correlated colour temperature measured by the C-7000 for the red-line spectrum.
What can we learn from this kind of measurement? Apart from giving a general understanding the effects of atmospheric extinction on the incident sunlight, the model gives a good indication of the aerosol concentration in the atmosphere. This has a big effect on the twilight phenomenon. In this measurement, the best fit model indicates that the aerosols were only about 20% of what is considered a 'normal' atmosphere. Presumably they were washed away by the wettest winter on record!
Radiometry of the Sun and sky
This is a spectral irradiance measurement of the Sun and sky made on a very clear morning with little haze. The solar altitude was +27.3°.
The plot shows a broad spectrum view of the solar energy distribution. With other spectrometers, I can in principle, measure out to 2,500nm and also into the ultraviolet down to the atmospheric cut-off close to 300nm, but I still have some work to do on the radiometric calibration.
The pale grey line is the solar flux outside the atmosphere (from the Hubble calibration database). Although the legend says 'scaled', the scaling factor is unity.
The orange and red lines show the visible spectrum of the sun/sky measured in the plane perpendicular to the Sun with the Sekonic C-7000 colour meter. The orange is the spectral irradiance which is the power incident on the plane and coming from the whole of the hemisphere (2Pi sr): this adds some of the blue sky back into the measured solar power. If I restrict the solid angle with a tube attached to the device (a toilet roll), I measure the solar spectral irradiance after extinction by 2.17 airmass of atmosphere with only a very small contamination by sky - the red line.
In this case, the orange line does not represent the Sun + the entire blue sky visible to the cosine corrector on the radiometer since parts of the sky were obscured by neighbouring buildings.
The inset photo shows the Sekonic with the adapted toilet roll. This restricts the field of view to a radius of 5.4°, tapering to zero by 10.6° from the Sun.
The underlying spectrum in blue is the solar irradiance after passing through my extinction model for this airmass. The model includes Rayleigh and aerosol scattering and ozone absorption in the ultraviolet and the visible (Chappuis) bands. The only parameters I can vary are the concentration of aerosols (very low in this case) and the ozone column density which is slightly more complicated to choose due to the variation of ozone density with height (my simple model is for a uniform atmosphere).
In this case the Chappuis band only has a small effect around 600nm since the Sun was quite high. The goodness of fit gives confidence in the absolute calibration of the the Sekonic device!
The dashed black line is the Planck function at the correlated colour temperature measured by the C-7000 for the red-line spectrum.
What can we learn from this kind of measurement? Apart from giving a general understanding the effects of atmospheric extinction on the incident sunlight, the model gives a good indication of the aerosol concentration in the atmosphere. This has a big effect on the twilight phenomenon. In this measurement, the best fit model indicates that the aerosols were only about 20% of what is considered a 'normal' atmosphere. Presumably they were washed away by the wettest winter on record!