Narration script

Unit seven. On its way down, sunlight is absorbed by particular molecules, such as ozone, oxygen, water vapour and carbon dioxide, and it is scattered by air molecules and dust. How much of each happens depends on the path length through the air, which we call the air mass. The industry's standard ground spectrum, A M one point five G, describes sunlight on a tilted surface after passing through one and a half atmospheres' worth of air, and it is normalised to one thousand watts per square metre.

You will learn the main absorbers and their wavelengths, why the sky is blue, how to compute air mass from the Sun's angle, the difference between direct, diffuse and global light, and exactly what conditions define the standard spectrum.

Every efficiency on a rooftop panel datasheet is measured under this spectrum. Factory flash testers are graded by how well they match it. Understanding it means understanding the reference point for the whole industry.

Molecules absorb photons whose energies match their internal transitions. Ozone removes ultraviolet below about three hundred to three hundred and twenty nanometres. Oxygen has a sharp band at seven hundred and sixty. Water vapour carves out bands near seven twenty, eight twenty, nine forty, eleven thirty, fourteen hundred and nineteen hundred nanometres. Water is the big variable, because humidity changes with the weather. Carbon dioxide absorbs further out in the infrared.

This is the most important chart in the unit, plotted from the real A S T M G one seventy-three data. The top curve is sunlight in space. The middle curve is global sunlight on the ground, and the lower curve is the direct beam only. Look at the labelled dips: the ozone cutoff on the far left, the oxygen line at seven sixty, and the deep water bands in the infrared, some going almost to zero. Over this wavelength range, space sunlight integrates to about thirteen forty-eight watts per square metre, the global ground spectrum to one thousand, and the direct beam to nine hundred.

Now scattering. Rayleigh scattering by air molecules goes as one over wavelength to the fourth power. So blue light at four hundred and fifty nanometres is scattered nearly six times more than red at seven hundred. That scattered blue fills the sky, which is why the sky is blue. The direct beam loses blue, so the Sun looks yellow-white at midday, and red at sunset when the path is long. Dust and haze scatter all colours more evenly, which makes a whitish sky. And crucially, scattering does not destroy light; it redirects it, and much of it still reaches the ground as diffuse light.

Air mass is the path length through the atmosphere compared with the straight-down path. For a flat atmosphere it is one over the cosine of the zenith angle, the angle of the Sun from straight overhead. Sun overhead, air mass one. Sun sixty degrees from vertical, air mass two. Air mass one point five corresponds to forty-eight point one nine degrees.

This sketch shows the geometry. At forty-eight degrees from vertical, sunlight crosses one and a half times as much air as it would straight down.

This graph shows air mass against zenith angle. The simple formula, in blue, shoots up to infinity at the horizon. A better formula, from Kasten and Young, accounts for Earth's curvature, and stays finite at about thirty-eight at the horizon.

Let's try Chicago, at about forty-two degrees north. At noon on the equinox, the air mass is about one point three four. At noon in late December, about two point four. At noon in late June, about one point oh five. So the standard air mass of one point five is a reasonable mid-latitude middle value.

Next, three components. Direct normal irradiance is the beam from the Sun's disc, measured facing the Sun. Diffuse irradiance is scattered light from the rest of the sky. Global irradiance is the total on a surface, the direct beam projected onto that surface plus the diffuse. On a clear day diffuse might be ten to twenty percent of global. Under overcast skies it is nearly all of it. Flat panels use both; concentrators use only the direct beam.

To compare cells fairly, the industry fixed one reference spectrum. Here are its defining conditions: a surface tilted thirty-seven degrees toward the Sun, air mass one point five, a standard atmosphere with rural aerosols, one point four two centimetres of precipitable water, and specified ozone and haze. The global version integrates to about one thousand watts per square metre, and the direct version to about nine hundred. The international testing standard, I E C sixty nine oh four part three, uses the same global spectrum.

We can quantify what the atmosphere removes. Going from space to the ground standard loses about three hundred and forty-eight watts per square metre, roughly a quarter. Ultraviolet drops from seven point six percent of the total to four point six percent. And the share beyond silicon's cutoff drops from about twenty-five percent to nineteen, because the water bands remove so much of the near infrared.

Real spectra are not the standard. They get redder in the morning and evening, deeper water bands on humid days, and bluer under haze and clouds. Wide band gap cells respond better to bluer light; silicon is fairly tolerant. These spectral effects matter for energy-yield modelling.

Misconceptions. A M one point five does not mean one point five kilowatts; it is a path length. Scattered light is not lost. Clouds do not block all light; overcast days still give roughly ten to thirty percent of clear-sky irradiance. And the standard spectrum is a reference, not what your panel sees at any given moment.

In the factory, sun simulators are classified by how closely they match this spectrum in wavelength bins. Reference cells calibrated to it set the flash tester's irradiance every shift. And mismatch factors correct for differences between test and reference cells.

Try the review questions. In the next unit, you will download this very spectrum and integrate it yourself, with real numbers.