Light types
LuxRender supports a variety of lamp 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.
All lights have a gain parameter that allows to balance them quickly.
Point light
Point lights are infinitely small light sources that emit light in all directions. Although point lights are widely used in modelling programs, their use is discouraged because infinitely small light sources don't exist in nature and will yield unrealistic results in the form of too sharp shadows.
However point lights are able to make use of IES diagrams available from lighting fixture manufacturers. Those are able to precisely describe light intensity in every direction allowing you to better simulate real lighting conditions.
Spot light
Spot lights are infinitely small lights that emit light in a cone shape. They suffer from the same unphysical limitation as point lights.
Projector light
Both point light and spot light 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.
Area light (or mesh emitter)
Area lights are objects that emit light. They can be used to create all kinds of lamps and other glowing objects of various colours. For example, modelling a light bulb and assigning an emissive material to the tungsten 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.
Area lights intensity can be adjusted to match real world conditions by specifying the power in W absorbed by the light and its luminous efficacy in lm/W (those as standard values available from light fixture manufacturers or lighting schedules).
Area lights also support IES diagrams to overcome point lights limitations. In this case, use two back to back flat polygons to model the emitter.
Environment lights
There are 3 environment lights: sun, sky and image based. LuxRender can accept any number and combination of environment light. However it might only make sense to use a sky or an image based light and eventually add a sun light.
Physical sun light
This will model sun lighting from NASA measurements, including atmospheric absorption adjustable through the turbidity parameter. The size of the sun can be tweak from its reference size with the relsize (relative size) parameter. To get a correct effect, the Z axis must point above, the Y axis is north and X axis is east.
Physical sky light
The physical sky creates a lighting setup that simulates the light of the sun and atmosphere, based on the direction of a sun lamp 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.
Additional parameters called aconst, bconst, cconst, dconst, econst allow you to fine tune the color of the sky to simulate various atmospheric conditions.
Image based light (or environment map)
Environment maps are 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.
Various map format are commonly in use, LuxRender supports both latlong and spherical formats.
It is possible to use low dynamic range images (ie Jpeg or PNG) as environment maps, but in this case you may want to add some additional lighting (like a sun) to your scene to avoid getting a poorly contrasted 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".
Note: LuxRender only supports OpenEXR files for high dynamic range images. The hdr file format is not supported, but can be converted to exr by Blender using the UV/Image editor.
Apart from using an environement map, LuxRender can also use a single color instead. In this case, this color will be used to uniformly illuminate the scene from all directions.
spectrum / light colour
The colour of a light source or object depends on the wavelength of the light, or the combination of wavelengths at which a light source emits light. Although most light sources emit light at a continuous range of frequencies and our eyes are sensitive to a number of different frequencies, we still perceive the light as if it had a single frequency. This means multiple different spectral characteristics can result in the same colour sensation and is called metamerism.
When light sources illuminate surfaces, the resulting perceived colour depends on the spectral distribution of both the light source and the surface. Because spectral characteristics can not be represented accurately by an RGB color, LuxRender provides a number of additional ways to specify the spectrum of light sources.
RGB colour
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.
Blackbody temperature colour
By indicating a colour temperature, LuxRender will use the spectral distribution of a blackbody. This is useful for for example light bulbs (2800-3300K), candles (1850K) and the sun (5000-6500K). For a detailed explanation, see for example techmind.org.
a range of colour temperatures. The middle lamp has a colour temperature of 6500K
Equal energy spectrum
This spectrum emits light with equal power at all frequencies, resulting in a particular white light called E point in white balance adjustments.
Gaussian spectrum
The gaussian spectrum uses a gaussian distribution. It is defined by a frequency (in nanometer) and the standard deviation of the bell curve. The latter controls the width of the curve; low values result in saturated colours while higher values result in less saturated tones.
lamps emitting light around 589nm, with increasingly wide spectrum
Regular data spectrum
This generates a spectrum using numerical data as input. A frequency range should be provided plus a number of values for emissions in Watt at regular frequency intervals withing that range.
An overview of all available lamp spectra can be seen here.
a lamp using spectral data sets for sun light, incandescent light and a low pressure sodium lamp
Irregular data spectrum
This generates a spectrum using numerical data as input. A set of wavelengths and their respective emission power in Watt should be provided.
IES data
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 "iesgen". 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 creates an 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)
Useful IES links: (freeware, for windows or wine)