The incident solar irradiance either can be estimated from direct normal irradiance and diffuse horizontal irradiance values, or incident irradiance can be measured. Global tilted irradiance is needed for non-concentrating solar collectors. A tilted surface will receive irradiance reflected from the ground and will receive reflected irradiance from nearby objects that are within its field of view. Pyranometers were originally designed for the measurement of global horizontal irradiance and installed on horizontal surfaces. In single black detector thermopile pyranometers the conduction of heat from the black detector through the thermopile to the body of the pyranometer is measured to determine the radiant energy incident on the instrument. Pyranometers used to measure tilted irradiance should be in a ventilator or shielded from direct sunlight. The effect of tilt on black-and-white pyranometers is greater than on single black detector thermopile pyranometers. The difference between the photodiode and thermopile pyranometers is divided by the thermopile reading to obtain the percent difference.