Tips & Tricks for Backlit Symbol Testing Using Radiant Software

Anne Corning

Backlight Units

From computer keyboards to movie theaters, from car dashboards to fasten-seatbelt signs, backlit symbols are all around us. A backlit sign is, quite literally, lit from the back: an illuminated light source called a backlight unit (BLU) is placed behind a surface. Text and shapes cut into the overlaid surface allow light to shine through, creating bright, visible symbols. For example, the EXIT sign at a movie theater is often a simple backlit sign comprised of a BLU and die-cut plastic housing.


Backlit exit sign (left) and backlit computer keyboard (right).

Backlit symbols are commonly used on airplanes and in automobiles to ensure visibility even in dark ambient light conditions. In heavily regulated industries like aerospace and automotive, there can be stringent regulations specifying the brightness, color, and other characteristics of backlit signage, since information presented can be vital to operator and passenger safety.


Backlit symbols on a vehicle instrument panel (left) and backlit airplane signage (right).

To ensure that backlit products adhere to performance requirements and provide the intended visual clarity, legibility, and function, they need to be carefully tested. Brightness (luminance), color, character shape, uniformity (both within a single symbol, and between sets of like-symbols), and other factors must be measured. 

Radiant’s ProMetric® Imaging Photometers or Colorimeters have been applied in this type of rigorous backlit symbol testing for illuminated component manufacturers in various industries, used in conjunction with either ProMetric® Software and TrueTest™ Automated Visual Inspection Software. Today, we’ll take a peek at a few of the tests that can be performed on backlit symbols for quality assurance.

Let’s Get to the Tips & Tricks Already!

In the series of video clips below, Radiant Product Manager Shannon Roberts demonstrates just a few of the different tests and features available for backlit symbol testing in our software packages. 

Automatic Points of Interest (Auto-POI)

The Auto-POI functionality available in both ProMetric and TrueTest platforms enables the automatic application of points of interest on backlit symbols using luminance values and chromaticity coordinates to define measurement thresholds. Once points of interest are defined, users can run pass/fail criteria (using TrueTest) against new values received, such as new parts entering a production line. Since Auto-POI are defined based on light and color values, rather than static regions drawn on the image, symbol sets can accurately be measured regardless of location, position, size, shape, or color without reprogramming POI parameters. 

Watch the video below to see how this works: “Auto-POI: Identifying Points of Interest Using Light and Color to Automatically Measure Illuminated Symbols.”

Pattern Matching on BLU Keyboards

PM-KB™ is a Radiant software module designed specifically for measuring light and color and performing Optical Character Verification (OCV) for illuminated keyboards. In the video below, Product Manager Shannon Roberts walks through inspections of keyboards with various defects, and demonstrates how PM-KB software can be used to catch defects using pattern matching, which is a comparison between a stored reference measurement (representing the ideal pattern) and a received pattern (such as a keyboard on a production line). PM-KB can identify defects in shape, pattern, luminance, and color for each symbol on the keyboard, regardless of language. To learn more, watch the video: “Pattern Matching in the PM-KB® System.”

Segmented Cross-Section Tool

The Segmented Cross-Section Tool in ProMetric software allows the user to draw a custom multi-segment measurement path (cross-section) directly on the image of a backlit symbol to measure inner-symbol luminance (Lv) uniformity and to evaluate variations. To learn more, watch the video: “ProMetric® Software Segmented Cross-Section Tool: Evaluating Inner-Symbol Luminance Uniformity.”

These are just a few of the many features, tools, and tests available in the ProMetric and TrueTest software. If you need fast, accurate BLU measurement, contact Radiant today!

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