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Radiance HDR is the original high dynamic range image format, created by Greg Ward for his Radiance lighting simulation software in the late 1980s. It uses a clever 32-bit RGBE encoding — three 8-bit mantissa values plus a shared 8-bit exponent — to represent a wide dynamic range in a compact format. It's less precise than OpenEXR but came decades earlier.
The format's primary use today is environment maps for 3D rendering and game development. Those panoramic "light probe" images that surround a 3D scene and provide realistic reflections and lighting? They're often HDR files. The wide dynamic range captures both the dim interior and the bright sky in a single image, allowing the renderer to calculate realistic lighting.
Blender, Unity, Unreal Engine, and most 3D tools read HDR environment maps natively. Photoshop opens them (with tone mapping). For viewing, any HDR-aware image viewer works. The format is niche but essential in its domain — if you work with 3D lighting, you've used HDR files even if you didn't notice the extension.