Whenever a light has to be emitted in a specific direction, the Spot light should be used. A typical use would be for the headlight of a car. Several additional options are available for the Spot light in the Type menu. You can select from the normal Spot option or one of the following: Square Spot, Parallel Spot or Square Parallel Spot.
"Square" refers to the shape of the light’s cone. This can be compared to the spotlights in a theater whose emitted light can be restricted by flaps on all sides to create a rectangular beam of light. In essence, the cone then takes the shape of an elongated pyramid. "Parallel" refers to the position of the light-emitting light source. The emission of parallel light can be compared to light being emitted perpendicularly from a plane. This makes it possible to, for example, simulate light sources that lie far away from an object without having to actually place the light source far away.
The light’s shape will change to reflect the type of light defined. The image below shows a front and side view of how each of the four types of Spot light appear in the Viewport.
As you can see, each spot type emits its light along the Z axis. Hence, this axis should always point in the direction of the object the light’s beam should illuminate.
Spot Light Details Tab
All Spot types can be modified and fine-tuned using the settings in the Details tab. Depending on whether a normal or a parallel Spot is defined, different parameters will be made available. If a normal Spot is defined, its cone can be modified using the Angle settings. Defining a Parallel Spot will change these parameters to Radius values.
TheInner Angle and Inner Radius values define the area in which the emitted light remains constant. From the outer edge of this value to the Outer Angle or Outer Radius value the light will falloff continuously to zero. This effect is demonstrated using an additional red color in the top image below.
This color effect can be created by enabling the Colored Edge Falloff and Use Gradient options in the Details tab. The gradient’s left edge represents the center and the right edge defines the color at the outer edge of the cone.
If you want to avoid any transition in brightness at all within the cone, disable the Use Inner option. The light’s color and brightness will then be constant until the Outer Radius value is reached, as shown at the bottom of the image above. Also, the Aspect Ratio value can be used to reshape the light’s cone. Values of less than 1 will compress the cone along the Y axis; values greater than 1 will stretch the cone along the Y axis. The image below shows both a distorted round and square spotlight.