From LuxRender Wiki
LuxRender supports a variety of light types: point lights, spot lights, area lights (also known as emitters) and environment lights. A scene needs to contain at least one light source. Multiple light types can be combined in a scene.
Area Lights and Meshlights
Area lights are objects that emit light. They can be used to create all kinds of lights and other glowing objects of various colours. For example, modelling a light bulb and assigning an emissive material to the tunsten wire will result in a realistic light bulb.
Objects only emit light in the direction that the normals of the object's faces are pointing. For rendering speed it is best to use as few faces as possible on emissive objects.
Many 3D packages feature an "area lamp" object. This is generally exported as a mesh light of the area lamp's dimensions.
In most cases, it is recommended to use only meshlights/area lights and environmental lighting for your scene, rather than point or spot lights. This is becauase the point and spot lights are unphysical and have no real world equivalent. As a result, they tend to produce unnaturally hard lighting and sharp shadows. Avoiding the point and spot lights will help improve the realism of your scene.
Point lights are infinitely small light sources that emit light in all directions. Apart from their location, the only available setting is colour, which can also be used to regulate intensity.
Spot lights are infinitely small lights that emit light in a cone shape. Apart from colour and location, the width of the cone can be set (as the number of degrees of the centerline it will extend), and there is a setting to feather the intensity towards the edge.
Both point lights and spot lights can distribute their light and colour according to an image. With spot lights, this results in an effect like something being projected from a dia projector or beamer. With point lights, the image map is spherically mapped.
The sunsky light creates a lighting setup that simulates the light of the sun and atmosphere, based on the direction of a sun light in the scene and a parameter named turbidity which defines the clearness of the sky. Both the sun angle and the sky clearness influence the colour of the light.
The intensity of the light can be set with the gain parameter. This can be useful in scenes where a physical sky is combined with emitter lights. If you want softer shadows try adjusting the relative sun size parameter.
It is possible to use this light with only the sky, or only the sun. The former can be useful to light a scene at night. (Note that the night sky itself will not be very realistic looking, as it will lack both clouds and stars, but it should give convincing illumination to the scene) Sun only is useful for some sci-fi scenes which may not take place on planet Earth, although often the distant light is preferable in this case. (see below.)
Infinite (Environment Maps)
The infinite light encloses the scene entirely, lighting it all angles. It is normally used for environment mapping, although it can also be used to add a solid color environment.
Environment maps are high dynamic range images that function as a light source. The maps are projected around the scene and emit light; the colour and intensity of the light depend on the local colour of the map. LuxRender accepts both latlong and angular (light probe) environment maps.
When using environment maps, using a gamma value of 1.0 (instead of the usual 2.2) is recommended. The exception to this is if you want the environment to look exactly like the map itself did in your image editor. This is probably not the case if you are using an actual HDR map.
It is possible to use low dynamic range images (like JPG or PNG) as environment maps, but in this case you may want to add some additional lighting to your scene to avoid getting a poorly contrast render. This is not necessary with HDR environment maps, which can be used as the only source of light to create a realistic lighting.
You can find a lot of free quality HDR maps on the web by looking for "hdr maps"/"hdri maps" or "light probes".
For more information on environment mapping, see the Environment map page
It is also possible to use the infinite light without any sort of environment map to achieve an even-colored world. If you do not have a fully enclosed scene, and don't plan to use sunsky, it is generally a good idea to add a blank infinite light so that there is something to see in reflections.
If you are using the distant light or sun without sky, the infinite light can be useful for adding in a sky, with or without an environment map.
The distant light acts as a point light source beyond all the scene geometry, similar to the sun. You can think of this as a generic version of the sun light (and the infinite light as a generic version of the sky light).
This light is often useful in abstract scenes or in scenes that do not take place on planet Earth. LuxRender's sun light tries to mimic our own Sun's emission spectrum and the color interference of our atmosphere, which would not be correct or desirable in these types of scenes. For scenes that do take place on Earth, it is normally preferable to avoid the distant light and instead use the sun light for the environment, and area lights and mesh lights as support lighting. This is because the Sun is the only light source "on Earth" that would behave the way the distant light does.
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 meshlights and arealights, they can be used to control the light spread angle, much like a spotlight cone. Arealights 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 pointlights (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 meshlight 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)
Some links to IES data files can be find on our external resources page.
Colour and Spectrum
LuxRender calculates light colors using actual spectral data. When using an RGB colour as input, LuxRender will generate a physically plausible spectrum based on the desired colour. The implementation is based on a paper by Brian Smits.
However, you can also define a spectrum for the light source in a variety of ways, using spectrum textures. For more information, see LuxRender Textures Spectrum