The AM1.5g standard spectrum on the previous page is suitable for flat PV panels that face a hemisphere, i.e. they see a
large portion of the blue sky.
Concentrating systems have a far smaller acceptance angle than a hemisphere (unless it is a fluorescent concentrating system)
and hence cannot exploit the radiation from the blue sky or from reflections at the clouds. Therefore, such systems usually
need a sun-tracking assembly, so the sun shines from a perpendicular angle on the system.
The standard direct (AM1.5d) solar spectrum  – as opposed to the global 1.5g spectrum – includes
radiation coming only from small surroundings of the sun disc (5.8° aperture angle) and is projected orthogonally onto the cell.
Calculations with the same atmospheric conditions as for the 1.5g spectrum then yield a total irradiance of 90 mW/cm2 .
The difference between the AM1.5d and AM1.5g spectra is mainly the blue sky, which is characterized by Rayleigh scattering,
see the page "Scattering of sunlight in the atmosphere".
The concentrating power of such PV systems varies within a broad range: from low (e.g. lenses integrated in architecture), to
medium (e.g. trough systems) to high (parabolic systems). These systems have – accordingly – a broad range of acceptance
angles and put different demands on a standard spectrum. For these reasons, there is an ongoing discussion in the PV community
about appropriate standard spectra for various types of concentrating systems.