In the examples below, the IES Profile is assigned to a Point Light and the diagram in the top left of each panel shows the shape of the given IES Profile. The curve works like a multiplier for the overall brightness of the light, as if you were projecting a texture from a light, but without the overhead of using a texture, or the errors that can occur at some angles. The curve defines light intensity in an arc that 'sweeps' around an axis to make Point, Spot, and Rect Lights appear to be casting light realistically based on the provided real-world data. However, they are not actual texture files. IES Light Profiles are a 1D texture (gradient). This type of photometric lighting is primarily used in Enterprise fields (such as Media and Entertainment or Architecture and Manufacturing), but is often used in games production to achieve realistic lighting effects, too.
It enables them to account for reflective surfaces in the light fixture, the shape of the light bulb, and any lensing effects that happens. These IES Photometric files, or IES Profiles, are a lighting industry standard method of diagramming the brightness and falloff of light as it exists a particular real world light fixture. The Illuminating Engineering Society (IES) has defined a file format which describes a light's distribution from a light source using real world measured data.
