SpotLight

Definition :
SpotLight { 
  exposedField SFFloat ambientIntensity  0 
  exposedField SFVec3f attenuation       1 0 0
  exposedField SFFloat beamWidth         1.570796
  exposedField SFColor color             1 1 1 
  exposedField SFFloat cutOffAngle       0.785398
  exposedField SFVec3f direction         0 0 -1
  exposedField SFFloat intensity         1  
  exposedField SFVec3f location          0 0 0  
  exposedField SFBool  on                TRUE
  exposedField SFFloat radius            100 
}
Explanation :

The SpotLight node defines a light source that emits light from a specific point along a specific direction vector and constrained within a solid angle. Spotlights may illuminate geometry nodes that respond to light sources and intersect the solid angle defined by the SpotLight. Spotlight nodes are specified in the local coordinate system and are affected by ancestors' transformations.

The location field specifies a translation offset of the centre point of the light source from the light's local coordinate system origin. This point is the apex of the solid angle which bounds light emission from the given light source. The direction field specifies the direction vector of the light's central axis defined in the local coordinate system. The on field specifies whether the light source emits light. If on is TRUE, the light source is emitting light and may illuminate geometry in the scene. If on is FALSE, the light source does not emit light and does not illuminate any geometry. The radius field specifies the radial extent of the solid angle and the maximum distance from location that may be illuminated by the light source. The light source does not emit light outside this radius. The radius shall be >= 0.0.

The cutOffAngle field specifies the outer bound of the solid angle. The light source does not emit light outside of this solid angle. The beamWidth field specifies an inner solid angle in which the light source emits light at uniform full intensity. The light source's emission intensity drops off from the inner solid angle (beamWidth) to the outer solid angle (cutOffAngle) as described in the following equations:

    angle = The angle between the Spotlight's direction vector
            and the vector from the Spotlight location to the point
            to be illuminated.

    if (angle >= cutOffAngle): multiplier = 0
    else if (angle <= beamWidth): multiplier = 1
    else : multiplier = (angle - cutOffAngle) / (beamWidth - cutOffAngle)

    intensity(angle) = SpotLight.intensity * multiplier
If the beamWidth is greater than the cutOffAngle, beamWidth is defined to be equal to the cutOffAngle and the light source emits full intensity within the entire solid angle defined by cutOffAngle. Both beamWidth and cutOffAngle shall be greater than 0.0 and less than or equal to PI/2. Figure 6.15 depicts the beamWidth, cutOffAngle, direction, location, and radius fields of the SpotLight node.

SpotLight illumination falls off with distance as specified by three attenuation coefficients. The attenuation factor is 1/max(attenuation[0] + attenuation[1]*r + attenuation[2]*r 2, 1), where r is the distance of the light to the surface being illuminated. The default is no attenuation. An attenuation value of (0, 0, 0) is identical to (1, 0, 0). Attenuation values must be >= 0.0.