The light_source is not really an object. Light sources have no visible shape of their own. They are just points or areas which emit light. They are categorized as objects so that they can be combined with regular objects using union. Their full syntax is:

LIGHT_SOURCE:
    light_source
    {
        <Location>, COLOR
        [LIGHT_MODIFIERS...]
    }
LIGHT_MODIFIER:
    LIGHT_TYPE | SPOTLIGHT_ITEM | AREA_LIGHT_ITEMS |
    GENERAL_LIGHT_MODIFIERS
LIGHT_TYPE:
    spotlight | shadowless | cylinder | parallel
SPOTLIGHT_ITEM:
    radius Radius | falloff Falloff | tightness Tightness |
    point_at <Spot>
PARALLEL_ITEM:
    point_at <Spot>
AREA_LIGHT_ITEM:
    area_light <Axis_1>, <Axis_2>, Size_1, Size_2 |
    adaptive Adaptive | jitter Jitter | circular | orient
GENERAL_LIGHT_MODIFIERS:
    looks_like { OBJECT } |
    TRANSFORMATION fade_distance Fade_Distance |
    fade_power Fade_Power | media_attenuation [Bool] |
    media_interaction [Bool] | projected_through

LIGHT_TYPE        : pointlight
falloff           : 70
media_interaction : on
media_attenuation : off
point_at          : <0,0,0>
radius            : 70
tightness         : 10

The different types of light sources and the optional modifiers are described in the following sections.

The first two items are common to all light sources. The <Location> vector gives the location of the light. The COLOR gives the color of the light. Only the red, green, and blue components are significant. Any transmit or filter values are ignored.

Note: you vary the intensity of the light as well as the color using this parameter. A color such as rgb <0.5,0.5,0.5> gives a white light that is half the normal intensity.

All of the keywords or items in the syntax specification above may appear in any order. Some keywords only have effect if specified with other keywords. The keywords are grouped into functional categories to make it clear which keywords work together. The GENERAL_LIGHT_MODIFIERS work with all types of lights and all options.

The simplest kind of light is a point light. A point light source sends light of the specified color uniformly in all directions. The default light type is a point source. The <Location> and COLOR is all that is required. For example:

  light_source {
    <1000,1000,-1000>, rgb <1,0.75,0> //an orange light
  }

Normally light radiates outward equally in all directions from the source. However the spotlight keyword can be used to create a cone of light that is bright in the center and falls of to darkness in a soft fringe effect at the edge.

Although the cone of light fades to soft edges, objects illuminated by spotlights still cast hard shadows. The syntax is:

SPOTLIGHT_SOURCE:
    light_source
    {
        <Location>, COLOR spotlight
        [LIGHT_MODIFIERS...]
    }
LIGHT_MODIFIER:
    SPOTLIGHT_ITEM | AREA_LIGHT_ITEMS | GENERAL_LIGHT_MODIFIERS
SPOTLIGHT_ITEM:
    radius Radius | falloff Falloff | tightness Tightness |
    point_at <Spot>

Default values:

  radius:    30 degrees
  falloff:   45 degrees
  tightness:  0

The falloff, radius, and tightness keywords control the way that light tapers off at the edges of the cone. These four keywords apply only when the spotlight or cylinder keywords are used.

The falloff keyword specifies the overall size of the cone of light. This is the point where the light falls off to zero intensity. The float value you specify is the angle, in degrees, between the edge of the cone and center line. The radius keyword specifies the size of the "hot-spot" at the center of the cone of light. The "hot-spot" is a brighter cone of light inside the spotlight cone and has the same center line. The radius value specifies the angle, in degrees, between the edge of this bright, inner cone and the center line. The light inside the inner cone is of uniform intensity. The light between the inner and outer cones tapers off to zero.

For example, assuming a tightness 0, with radius 10 and falloff 20 the light from the center line out to 10 degrees is full intensity. From 10 to 20 degrees from the center line the light falls off to zero intensity. At 20 degrees or greater there is no light.

Note: if the radius and falloff values are close or equal the light intensity drops rapidly and the spotlight has a sharp edge.

The values for the radius, and tightness parameters are half the opening angles of the corresponding cones, both angles have to be smaller than 90 degrees. The light smoothly falls off between the radius and the falloff angle like shown in the figures below (as long as the radius angle is not negative).

The tightness keyword is used to specify an additional exponential softening of the edges. A value other than 0, will affect light within the radius cone as well as light in the falloff cone. The intensity of light at an angle from the center line is given by: intensity * cos(angle)tightness. The default value for tightness is 0. Lower tightness values will make the spotlight brighter, making the spot wider and the edges sharper. Higher values will dim the spotlight, making the spot tighter and the edges softer. Values from 0 to 100 are acceptable.

You should note from the figures that the radius and falloff angles interact with the tightness parameter. To give the tightness value full control over the spotlight's appearance use radius 0 falloff 90. As you can see from the figure below. In that case the falloff angle has no effect and the lit area is only determined by the tightness parameter.

Spotlights may be used anyplace that a normal light source is used. Like any light sources, they are invisible. They may also be used in conjunction with area lights.

The cylinder keyword specifies a cylindrical light source that is great for simulating laser beams. Cylindrical light sources work pretty much like spotlights except that the light rays are constrained by a cylinder and not a cone. The syntax is:

CYLINDER_LIGHT_SOURCE:
    light_source
    {
        <Location>, COLOR cylinder
        [LIGHT_MODIFIERS...]
    }
LIGHT_MODIFIER:
    SPOTLIGHT_ITEM | AREA_LIGHT_ITEMS | GENERAL_LIGHT_MODIFIERS
SPOTLIGHT_ITEM:
    radius Radius | falloff Falloff | tightness Tightness |
    point_at <Spot>

Default values:

  radius:     0.75 degrees
  falloff:    1    degrees
  tightness:  0

When performing adaptive sampling POV-Ray starts by shooting a test ray at each of the four corners of the area light. If the amount of light received from all four corners is approximately the same then the area light is assumed to be either fully in view or fully blocked. The light intensity is then calculated as the average intensity of the light received from the four corners. However, if the light intensity from the four corners differs significantly then the area light is partially blocked. The area light is split into four quarters and each section is sampled as described above. This allows POV-Ray to rapidly approximate how much of the area light is in view without having to shoot a test ray at every light in the array. Visually the sampling goes like shown below.

While the adaptive sampling method is fast (relatively speaking) it can sometimes produce inaccurate shadows. The solution is to reduce the amount of adaptive sampling without completely turning it off. The number after the adaptive keyword adjusts the number of times that the area light will be split before the adaptive phase begins. For example if you use adaptive 0 a minimum of 4 rays will be shot at the light. If you use adaptive 1 a minimum of 9 rays will be shot (adaptive 2 gives 25 rays, adaptive 3 gives 81 rays, etc). Obviously the more shadow rays you shoot the slower the rendering will be so you should use the lowest value that gives acceptable results.

The circular command has been added to area lights in order to better create circular soft shadows. With ordinary area lights the pseudo-lights are arranged in a rectangular grid and thus project partly rectangular shadows around all objects, including circular objects.
By including the circular tag in an area light, the light is stretched and squashed so that it looks like a circle: this way, circular or spherical light sources are better simulated.

A few things to remember:

• Circular area lights can be ellipses: the AXIS_1_VECTOR and AXIS_2_VECTOR define the shape and orientation of the circle; if the vectors are not equal, the light source is elliptical in shape.
• Rectangular artefacts may still show up with very large area grids.
• There is no point in using circular with linear area lights or area lights which have a 2x2 size.
• The area of a circular light is roughly 78.5 per cent of a similar size rectangular area light. Increase your axis vectors accordingly if you wish to keep the light source area constant.

The orient command has been added to area lights in order to better create soft shadows. Without this modifier, you have to take care when choosing the axis vectors of an area_light, since they define both its area and orientation.
Area lights are two dimensional: shadows facing the area light receive light from a larger surface area than shadows at the sides of the area light.

Actually, the area from which light is emitted at the sides of the area light is reduced to a single line, only casting soft shadows in one direction.

Between these two extremes the surface area emitting light progresses gradually.
By including the orient modifier in an area light, the light is rotated so that for every shadow test, it always faces the point being tested. The initial orientation is no longer important, so you only have to consider the desired dimensions (area) of the light source when specifying the axis vectors.
In effect, this makes the area light source appear 3-dimensional (e.g. an area_light with perpendicular axis vectors of the same size and dimensions using circular and orient simulates a spherical light source).

Orient has a few restrictions:

1. It can be used with "circular" lights only.
2. The two axes of the area light must be of equal length.
3. The two axes of the area light should use an equal number of samples, and that number should be greater than one

These three rules exist because without them, you can get unpredictable results from the orient feature.

If one of the first two rules is broken, POV will issue a warning and correct the problem. If the third rule is broken, you will only get the error message, and POV will not automatically correct the problem.

Using the shadowless keyword you can stop a light source from casting shadows. These lights are sometimes called "fill lights". They are another way to simulate ambient light however shadowless lights have a definite source. The syntax is:

SHADOWLESS_LIGHT_SOURCE:
    light_source
    {
        <Location>, COLOR shadowless
        [LIGHT_MODIFIERS...]
    }
LIGHT_MODIFIER:
    AREA_LIGHT_ITEMS | GENERAL_LIGHT_MODIFIERS

shadowless may be used with all types of light sources. The only restriction is that shadowless should be before or after all spotlight or cylinder option keywords. Do not mix or you get the message "Keyword 'the one following shadowless' cannot be used with standard light source". Also note that shadowless lights will not cause highlights on the illuminated objects.

Normally the light source itself has no visible shape. The light simply radiates from an invisible point or area. You may give a light source any shape by adding a looks_like { OBJECT } statement.

There is an implied no_shadow attached to the looks_like object so that light is not blocked by the object. Without the automatic no_shadow the light inside the object would not escape. The object would, in effect, cast a shadow over everything.

If you want the attached object to block light then you should attach it with a union and not a looks_like as follows:

  union {
    light_source { <100, 200, -300> color White }
    object { My_Lamp_Shape }
  }

Presumably parts of the lamp shade are transparent to let some light out.

with d being the distance the light has traveled.

You should note two important facts: First, for Fade_Distance larger than one the light intensity at distances smaller than Fade_Distance actually increases. This is necessary to get the light source color if the distance traveled equals the Fade_Distance. Second, only light coming directly from light sources is attenuated. Reflected or refracted light is not attenuated by distance.

Normally light coming from light sources is not influenced by fog or atmospheric media. This can be changed by turning the media_attenuation on for a given light source on. All light coming from this light source will now be diminished as it travels through the fog or media. This results in an distance-based, exponential intensity falloff ruled by the used fog or media. If there is no fog or media no change will be seen.

Note:in POV-Ray 3.0 this feature was turned off and on with the atmospheric_attenuation keyword.