What is HDR or LDR image?

HDR is abbreviation for High Dynamic Range. For a scene, dynamic range refers to ratio between the brightest and darkest parts of the scene. The Dynamic Range of real-world scenes can be quite high - ratios of 100,000:1 are common in the natural world. An HDR image stores pixel values that span the whole tonal range of real-world scenes. Dynamic range of JPEG format image won't exceed 255:1, so it is considered as LDR (Low Dynamic Range). Similarly, dynamic range of CRT monitor won't exceed 100:1. While HDR image (image of 32bit/color channel) shows the dynamic range of real world (natural dynamic range is generally considered to be 100,000:1; the dynamic range human eyes can identify is around 100,000:1. ), which is much higher than that of standard display equipment and images shot with common camera. As a result, HDR image cannot be displayed with this equipment. So from broad sense, image with dynamic range of higher than 255:1 (8 bit per cooler channel) is regarded as HDR image.