Combining Computer Vision with Photometry for More Effective Backlit Symbol Inspection
The automotive industry places safety as one of its highest considerations in the design and manufacture of vehicles. As technology, consumer expectations, and market forces have shifted over the decades, safety is still in the forefront. Critical to the safety of every vehicle are its lighted components. Lighting communicates vital information to the driver and facilitates safe vehicle operation in any ambient-light or environmental condition.
The dashboard is a central hub of visual information, which typically includes illuminated gauges, symbols, indicators, and warning lights to help the driver operate the vehicle and monitor its status. Illuminated components are also common in center stacks with their many backlit controls, illuminated buttons and dials, and displays that contain GPS, infotainment, and environmental indicators.
In recent years, advancements in LED technology, new display technologies such as OLED, and new freeform/curved shapes have pushed automobile designers to incorporate a wider variety of displays, backlit instrument panels, controls, and lighting into all areas of the vehicle interior. At the same time, a proliferation of sensor systems on the vehicle are serving an increasing amount of real-time information to the driver—for example, vehicle proximity alerts as an icon on a display or in a sideview mirror.
Industry standards require that illuminated vehicle elements such as warning lights and indicators meet strict specifications for luminance, chromaticity, clarity, and shape. These elements must be tested against standards for safety and performance as well as internal quality specifications; as the technology has evolved, so has the complexity of inspection regimens.
Typical backlit icons and symbols on an automotive dashboard.
Inspection Requirements for Illuminated Components
Testing the visual performance and quality of all lighted elements in the vehicle requires an inspection solution that provides:
- Absolute measurement of luminance (brightness) and chromaticity (color)
- Measurement of the uniformity of luminance or chromaticity across a backlit area
- Evaluation of the shape and completeness of an illuminated area, such as a symbol (identify defects such as inclusions or exclusions)
- Ability to assess a symbol’s position even if rotated or moved (“dynamic registration”)
- Inspection data for reporting to standards oversight and certification entities
- Ability to perform rapid, automated evaluations in a production environment
Existing Systems. Typically, automakers have used a combination of methods to perform the needed inspections. However, each method only provides a partial solution:
- Spot Meters – provide absolute luminance and chromaticity measurement, but only for a single small area (spot) at a time. These meters have no ability to evaluate shape completeness or detect dimensional defects.
- Human inspectors – can quickly discern symbol shape, orientation, and completeness, but not absolute luminance and chromaticity. Additionally, these inspectors are costly to employ and subject to fatigue and error.
- Machine vision cameras – can match characters, shapes, and symbols accurately compared to a golden sample for completeness inspection and defect detection. However, they cannot measure absolute luminance and chromaticity. These systems also may not be able to perform a test if symbol location and orientation changes or if the part under inspection is not perfectly aligned—as frequently happens with the laser-cut components on the production line. This issue requires complex fixturing for the inspection set up.
Both color (chromaticity) and brightness (luminance) of backlit indicators must be uniform and meet precise regulatory specifications.
A New Inspection Solution: Machine Vision Capabilities + Photometry
Radiant’s decades-long history of developing photometry-based solutions for light and color measurement in the automotive industry has put us in a unique position. While we have had photometry as our foundation, additional inspection needs around lighted components have provided an opportunity for us to develop expertise in addressing some of the more challenging machine-vision problems in areas such as surface and glass inspection and assembly verification. Bringing our experience and capabilities together, a new solution was born: VIP™ (Vision Inspection Pack) Software.
VIP is a set of machine vision tools built on top of our TrueTest™ Automated Inspection Software platform. Combined with a ProMetric® Imaging Photometer or Colorimeter, VIP provides complete measurement of component luminance, chromaticity, and defects in a single image. VIP brings together the science of photometry-based imaging and the toolset of machine vision in one solution, giving automotive manufacturers a complete array of options to address inspection needs.
Perceiving a Need for Better Symbol Inspection
There are dozens of types of illuminated components in automobiles, often manufactured by different suppliers. This can introduce slight variations between different parts in the same vehicle, different production runs by the same supplier, and between lots of the same part received from different suppliers.
As just one example, consider blind-zone indicators: the warning symbols that light up to alert a driver if there is another vehicle in the blind spot, helping to prevent a collision. The indicators are typically manufactured by laser cutting a stencil layer that will overlay an illuminated panel, with light shining through to illuminate the symbol.
The blind zone indicator (right illuminated image of two vehicles positioned together) from a Buick Lucerne’s right-side mirror. (Image: NHTSA1)
Blind-zone indicators are a critical vehicle safety component, thus are subject to strict requirements for luminance and chromaticity values, shape, and position. They must be clear, visible, and legible under all ambient conditions. Yet they are particularly challenging to inspect, where fixturing (holding a part in place for inspection) can be difficult. Millimeter differences in laser cutting or part placement for inspection can interfere with effective measurement by machine vision systems.
Typically, blind zone indicator components have been subjected to inspection in two phases:
- The part is inspected by a machine vision system to ascertain shape and completeness of the symbol (no defects such as inclusions or exclusions).
- The part is inspected with a photometric system to measure luminance and chromaticity values.
Backlit symbols must be inspected for defects in their shape and formation, such as gaps (exclusions, left) or extra elements (inclusions, right).
In practice, this multi-phase inspection requires a manufacturer to move each part from one inspection station to another, in essence, inspecting it twice. This is time consuming, with multiple pieces of equipment required, and has the potential to introduce variations from one inspection to the next. With VIP, however, comprehensive symbol inspection for shape, orientation, illumination, and color can all be performed by a single system, in a single measurement image. With VIP, manufacturers have been able to consolidate two inspection lines into one, thus reducing inspection time and labor.
At the same time, VIP captures a complete set of data needed for both internal quality control and for reporting to regulatory bodies that monitor standards for instrument display such as, in the US: MIL-DTL-7788 (formerly Society for Automotive Engineers SAE AS7788) and Federal Motor Vehicle Safety Standard FMVSS 111; and equivalent global standards such as those from JASO (Japan) and DFF (Germany).
The Power of Dynamic Registration
Traditional machine vision systems are limited in their ability to accurately assess illuminated regions that may be rotated or appear outside of a fixed position. Another advantage of VIP is its ability to perform dynamic registration. Regardless of slight changes in position or orientation of components or component features, VIP can locate and register regions of interest (ROI), simplifying component placement and fixturing.
VIP automatically registers icons and shapes based on trained registration regions. Global registration (left) ensures icons are accurate relative to one another. Local registration (right) ensures icons can be accurately measured and inspected regardless of location or orientation.
The aviation industry also has inspection requirements for backlit icons and symbols, such as these passenger fasten safety belt icons. In this case, quality is determined by measuring the symbol at defined POI (point of interest) locations (green circles). The position of these POI must be maintained for accurate inspection, regardless of whether the symbol is rotated relative to the inspection system.
For automakers tasked with ensuring the quality of illuminated components, the equivalent need for light measurement, completeness inspection, and defect detection has been a long-standing challenge in need of an efficient solution. VIP Software provides the only solution on the market with all these capabilities combined—and offers the unique flexibility to perform dynamic registration for additional ease-of-use. VIP was recently recognized with a Vision Systems Design 2021 Innovators Award, providing independent confirmation of the distinctive, breakthrough nature of this solution.
Learn more about VIP (Vision Inspection Pack) Software.
Radiant ProMetric Imaging Photometer and the VIP Software toolkit.
- Forkenbrock, G., et al., Blind Spot Monitoring in Light Vehicles – System Performance. Report No. DOT HS 812 045, July 2014, Washington, DC: National Highway & Traffic Safety Administration.