Direct Solar Irradiance and Horizontal Direct Solar Irradiance
Direct solar irradiance – also known as direct normal irradiance (DNI) – is a measure of the rate of solar energy arriving at the Earth's surface from the Sun's direct beam, on a plane perpendicular to the beam, and is usually measured by a pyrheliometer mounted on a solar tracker. The pyrheliometer has a finite field of view typically with a maximum field of view of 5°. Horizontal direct solar irradiance is the component of direct solar irradiance that would be measured on a horizontal surface, obtained by multiplying the direct solar irradiance by the cosine of the Sun's zenith angle.
Diffuse Solar Irradiance
Diffuse solar irradiance is that component of incoming solar irradiance on a horizontal plane at the Earth's surface resulting from scattering of the Sun's beam due to atmospheric constituents; it does not contain the direct solar irradiance component. Diffuse solar irradiance is measured by a pyranometer, typically with dual glass domes shaded from the Sun's beam. The shading is accomplished either by an occulting disc or a shading arm attached to a solar tracker. The angle subtended by the shading disc of the diffuse pyranometer should be the same as the field of view of the pyrheliometer.
Global Solar Irradiance
Global solar irradiance is a measure of the rate of total incoming solar energy (both direct and diffuse) on a horizontal plane at the Earth's surface. A pyranometer sensor can be used to measure this quantity with limited accuracy. The most accurate measurements are obtained by summing the diffuse solar irradiance and the vertical component of the direct solar irradiance.
Downward Infra-Red (Terrestrial/Longwave) Irradiance

Any body with a temperature greater than 0 K (Kelvin) will radiate energy over a range of wavelengths, called the body's radiation spectrum. Most of the Sun's spectrum lies in the wavelength range of 0.25 - 4.0 µm (1 µm (micrometre) = 10-6 m), the so-called short wave range. Downward infra-red irradiance is a measurement of the irradiance arriving on a horizontal plane at the Earth's surface, for wavelengths in the range 4 - 100 µm, which corresponds to the wavelength emitted by atmospheric gasses and aerosols. It is related to a "representative atmospheric temperature" of the Earth's atmosphere by the Stefan-Boltzmann Law:

E = σ T4,    where:
 E = irradiance measured. [Wm-2]
 σ = Stefan-Boltzmann constant. [5.67 x 10-8 Wm-2deg-4 ]
 T = representative atmospheric temperature. [K]

Consequently, this quantity will continue to have a positive value, even at night time, and is measured using a pyrgeometer. As in the case of diffuse solar irradiance measurement, it is required that this instrument is shaded from the direct beam of the Sun during the day, since the Sun's beam can heat the pyrgeometer dome and increase the uncertainty in the measurement. The measured downward infra-red irradiance can be use to derive "representative temperature" of the sky. This derived estimate is dependent on a number of factors, but typically larger values are recorded when middle to low level clouds cover the sky than those recorded during clear skies.

Sunshine Duration
Sunshine duration is defined to be the sum of all time periods during the day when the direct solar irradiance equals or exceeds 120 Wm-2 ±20%. The range of irradiance within this definition reflects the variation in response characteristic of the traditional Campbell-Stokes sunshine duration recorder, which uses direct beam sunlight focussed by a glass sphere to burn a trace on a paper chart. Where the Bureau has measurements of direct irradiance with a pyrheliometer sunshine duration is calculated for every second of the day.
Sun's (or Solar) Zenith Distance
The solar zenith distance is the angular distance from directly overhead (i.e. the zenith) measured in degrees, and is sometimes used instead of solar altitude or solar elevation within calculations based on the horizontal coordinate system. The zenith distance is the complement of solar altitude (i.e. 90°- altitude).