Diffuse horizontal irradiance (DHI) is the terrestrial irradiance received by a horizontal surface which has been scattered or diffused by the atmosphere. It is the component of global horizontal irradiance which does not come from the beam of the sun (where “beam” is a 5° field of view concentric around the sun). Much like global horizontal irradiance, DHI is typically measured with a pyranometer; however, in this case the direct light of the sun is blocked in order to remove the beam component of the radiation. The sun may be blocked by a ball or disc which only removes the 5° cone around the sun and must utilize a tracker to continually shade only the sensor of the pyranometer. The pyranometer may also be shaded by a horizontal or vertical shade band (the former requiring no tracker, the latter requiring a horizontal tracker); however, shade bands are generally less accurate as they remove some of the diffuse light. This diffuse light should be measured and corrected by a location- and time-dependent correction factor.
If diffuse horizontal irradiance is not measured directly, it may be calculated in a fashion similar to global horizontal irradiance.