HDRI is an acronym for “High Dynamic Range Imaging,” where dynamic range refers to the amount of illumation contained within the final image dataset. Typically used in photography, but indeed CG imagery as well, HDRI imagery contains enough illumination information to determine the final contrast, gamma, color and so on well after the original image is created. In film and digital photography, an HDRI is typically created with “bracketed exposures”, meaning an exposure from several f-stops. Then, in a program that supports deep color or RAW format images, you can then assemble the HDRI image from the separate exposures. Later, you can choose what the white and black point of the image would be, along with several other factors. As long as the image is maintained within a format that offers HDRI benefits, you can continue adjust the image as your purpose suits, all without a loss of data, or a degradation of the quality of the image.