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Brightness of light for HIDs is usually measured in lumens, or the total amount of light generated. Lumens are the unit of luminous flux, which is the total power of light. To measure lumens, an HID bulb is placed inside a sealed reflective sphere, known as an integrating sphere, and its light is bounced around in all directions in the sphere (integrated) so its power of light can be measured at one point, and then calculated into lumens based on the size of the sphere.Kelvin Scale has nothing to do with "output" that is measured in lumens. Kelvin scale is about color temperature only.
As you move higher up the Kelvin scale more UV light is produced, which is not visible to the human eye, and therefore not "usable light output".
HID Brightness - HID - Research
Nick C.