Automotive interior lighting is lighting inside of a car that provides comfort and safety for the driver and passengers.

Automotive interior lighting includes features such as:

  • Instrument panels
  • Infotainment display
  • Satellite navigation
  • Multimedia displays
  • Glovebox light
  • Mirror lighting
  • Overhead lighting on passenger side
  • Car door illumination / Puddle light
  • Overhead lighting
  • Storage lights
  • Reading lights, back seat lights
  • Trunk light
  • Registration plate light
Interior lighting examples in a car | Synopsys

How Is Automotive Interior Lighting Helpful?

Broadly, automotive interior lighting falls into one of three categories:

  • Displays and indicators 
  • Illumination
  • Accent lighting
Car instrument panel - What is automotive interior lighting? | Synopsys
Dome Light - What is Automotive Interior Lighting? | Synopsys

The driver uses displays and indicators to obtain information, which could be standard dashboard instrumentation information or more complex information from a navigation system, a menu-driven display, or a multimedia device. For display and indicator lighting, the key metrics are color, luminance, and luminance uniformity.

Interior Illumination systems provide light to enable the driver or passengers to see something else. These systems include dome lights, mirror lights to illuminate a face, map and reading lamps, storage lights, and so on. For Illumination systems, the key metrics are the geometry of the illuminated area, illuminance levels, and uniformity. Color can also a factor, both for visibility issues and for custom "mood" lighting. 

The primary purposes of accent lighting is to provide styling cues, create a desired ambience and to locate various controls, such as power window controls and radio knobs,  and other features, such as cup holders and charging ports, in a darkened cabin. Accent lighting generally has a much lower intensity than interior illumination fixtures since the goal is not to illuminate other objects in the cabin, but just to be visible itself. The main design concerns with this type of lighting are color, visual uniformity, and appropriate light levels.

Automotive interior showing various display and accent lighting | Synopsys

How Do You Design a Dome Light?

Unlike specifications for exterior automotive lighting, which come from a government agency  (e.g., the ECE in Europe), specifications for interior lighting often come directly from the manufacturer—General Motors, Toyota, Volkswagen, or Hyundai, for example. For a dome lamp, the specifications might include the color for the light, as well as a set of illuminance test points specified in lumens per square meter, or lux, that you measure with a lux meter.  

The figure below shows a circular light guide designed for an automotive dome light using the Light Guide Designer in LightTools. The LightTools Optimization Module was used to control the size, shape, and placement of prism textures in the light guide that direct the light from the light source to the target area for the best results.

Circular light guide dome lamp designed using the LightTools Light Guide Designer | Synopsys

How Do You Design Lighting for a Speedometer?

While the design of something like an illuminated needle for a speedometer may seem simple enough, to do it well the designer must carefully craft the needed elements, and, of course, using the right design tool is a great help.

Speedometer and dashboard modeled in LightTools | Synopsys

Typically, items in an instrument cluster such as the speedometer, shift panel (PRNDL), temperature controls, and radio controls have manufacturer test point specifications for luminance (the illumination metric most closely associated with brightness) as well as color (u',v' or x,y color coordinates). Often, the numbers or the needles will have several spread out test points that a quality engineer measures in a dark room with a spectral photometer (or radiometer), which is also called a spot meter.

Usually, the needle is wider at the bottom than it is at the top. A well-engineered needle will have uniform spatial appearance (luminance) along the length. This can be particularly challenging to design because you need a non-linear extraction of light along the needle to achieve the uniform light distribution. Fortunately, LightTools helps you at each design challenge faced in a system like this. The design process may go something like this:

  • Get the specifications – This might be a set of test points, the face of the speedometer as an appliqué, and some or all of the needle geometry. For this example, the goal is to have the white letters be approximately 10 nits (lumens per steradian per meter-squared).

  • Design the backlight – In this example, shown in the figure below, the backlight has an LED providing light at its top edge. Tools such as the Backlight Pattern Optimizer in LightTools help you to find the best pattern or optimize the 3D textures in such a way that the light is emitted uniformly from the backlight toward the driver. The appliqué is placed in front of the backlight so that the desired pattern is created.
Model with full backlight in LightTools | Synopsys

  • Design the light sources, base, and fold mirror for the needle
    • First, make sure you can get light from the source (often an LED mounted on a flat circuit board) into the needle. This can be accomplished by using the “stalk” of the speedometer knob as a light guide so light gets to the top of the structure.
    • At the top of the circular section, you can use a carefully designed notch (a TIR fold mirror) to turn the light by 90 degrees, as shown on the left in the image below.
    • To find the best angle for the fold mirror, we used the LightTools Parameter Analyzer feature.
    • We also used the Parameter Analyzer to study how much the light reaching the needle changed as the needle turned around. The results showed that the design worked much better with two LED sources.
Designing with One or Two LED Sources | Synopsys

  • Design light extractors for the needle
    • Light that gets into the needle needs to be extracted out of it and toward the driver's eye
    • There are several ways to accomplish this -- using a pattern of textures or paint dots, for example; LightTools has several tools for designing and optimizing the pattern (e.g., the Backlight Pattern Optimizer and the Light Guide Designer), or you can adjust the pattern by hand or with the Parameter Analyzer.
    • For this example, we applied a white paint stripe pattern on the underside of the needle and optimized it so that the light the driver sees is spatially uniform.
Design light extractors for the speedometer needle | Synopsys

  • The final design – True color plot approximating what the driver will see.
Final speedometer design in LightTools | Synopsys

What Solution Does Synopsys Offer?

For designing vehicle interior lighting, LightTools illumination design software from Synopsys provides a complete set of design, simulation, and analysis tools for designing superior automotive optical designs.

LightTools illumination design software | Synopsys

Left: Side door puddle lamp with logo, designed using the Freeform Designer in LightTools. 

Center: Linear light guide created using the Light Guide Designer in LightTools. 

Right: Prismatic lens map lamp design created by optimizing prism structures in LightTools.

Explore Synopsys Automotive Lighting Design Software