Lighting With IES Files
IES files contain information about the light distribution of a lighting fixture. These files are typically provided by lighting fixture manufacturers.
The main use of IES is measuring real world lamp models, many manufacturers sites offer their IES library for free. Generic IES can also be made with applications like "ies generator". Since IES profiles can be applied to mesh lights and area lights, they can be used to control the light spread angle, much like a spotlight cone. Area lights with the IES profile of a spot will illuminate like a spot but will have a physical size. This will make them easier to balance in intensity with other physical lights. However, to have the physical intensity of a specific IES scaled correctly, the IES must be used with point lights (mesh and planes will still make the correct 'shape', but the intensity will be altered by their power, efficiency and size)
On the left: a single quad mesh light using a "narrow cone" IES : resulting light (from a cylinder) makes "petals" as if the sides were separate spotlights, on the right: a "wide cone" IES makes almost a uniform ring shape.
And these are the corresponding IES diagrams:
These simple shapes make light "cones", complex shape make various effects (wallwashers , multiple beams..) The diagram: what you see in IES viewers or iesgen4 reads like this: in the center there is the light source. Around it, infinite rays going in every direction (generally represented in 2d, the 3d shape is obtained by visualizing the spinning of the 2D graph around a line in the plane)
Each ray represents the light going out of the lamp in a particular direction (towards the floor, walls, ceiling). The length of the ray is the intensity of light in that direction. The shape you see is made by the tips of all rays.
A circle means intensity is the same in all directions, a thin and tall ellipse means a narrow light cone going downwards (or up). Intensity scale (units): the intensity at all directions affects the overall brightness of the lamp; a light sending rays of maximum intensity (1.0) on a small angle (like a ellipse diagram) will be dimmer than a light with intensity 1.0 in all directions (a circle diagram). Most ies files have a 2D diagram meaning that the light distribution is symmetric all around the lamp axis. Some files have 3d diagram, made by two sections (the 2nd also made on the lamp axis but rotated 90°) they're visible in iesviewer in red and pink. For placing these asymmetric lights in your scene, rotation on vertical axis counts.
There is unfortunately no standard for intensity. It can mean that of a specific bulb (i.e. 40W), a generic power (i.e. 100W and you have to use "gain" in the exporter as multiplier to adjust it). Or in generic IES files can have intensity 1.0 and "gain" is used to specify their power (IES suppliers specify what rule they used on their site or documentation).
Diagram types: that above is a "polar" visualization of the IES with the rays drawn around the center (used in iesviewer and iesgen4) it's the easier to read as it resembles the actual shape of the light. The other possible visualization is a Cartesian (XY) diagram, as used by iesgen3, shows ray intensity on Y and the direction on the X axis (on the left it's intensity in the center of the cone (the light axis), on the right the intensity perpendicular to that ) this is less intuitive but makes easier to draw simpler curves for generic IES.
Top: the same "narrow cone" ies as before seen in iesgen 3 (xy diagram) Bottom: an IES with similar cone angle but "sharper" (smaller penumbra angle)
LuxRender can also use IES files with both vertical and horizontal angle data.
IES files can also be generated by hand, as the 3D IES file used in the image above was generated by hand. A good place to start is to open an existing IES file and start modifying it. You can find standard IES file format specifications by doing a quick internet search.
Some links to IES data files can be find on our external resources page.