A visual assessment is not enough to ensure the quality of illuminated LED or OLED control panels & indicators components. We need to use luminance measuring devices in order to verify the luminance level, i.e. the perceived brightness of these elements.
by Jacek Dylak and Miko Przybyla I GL Optic
Conventional devices for measuring luminance are the so-called spot luminance meters, with a relatively simple construction, using a lens with a fixed focal length and a photodiode with a V-Lambda filter. These devices were used after measuring various flat luminous emitting surfaces. However, currently produced electronic devices with backlit components of various shapes and sizes require more advanced analysis than just luminance measurement at a single point. Therefore, Imaging Luminance Measuring Devices ILMDs using a lens and a high-resolution CMOS sensor with a V-Lambda filter are becoming more and more popular. These devices, combined with specialized image analysis software, offer great opportunities for R&D and production quality control.
How to take measurements
To measure the backlit controls, it is worth using a imaging luminance measure device, i.e. a luminance camera. When using such a device we can perform a detailed image analysis based on a photo taken and obtain measurement data of components of various shapes and sizes.
Luminance [cd / m2] is a photometric quantity corresponding to the impression of the brightness of the luminous surface in the observers’ eye. The measurement should take place in a darkroom to avoid the influence of stray light on the obtained results. For small elements, it is advisable to use a lens with the longest possible focal length in order to obtain the highest image resolution. In the case of GL Opticam 1.0 and 2.0 systems, a lens with a focal length of 50 mm will be appropriate.
Steering wheel control luminance image.The analysis software allows you to mark selected areas for detailed analysis of rectangular Regions Of Interest – ROI . Since the shape of the controls is irregular, a special tool allows you to set a lower luminance threshold to remove the background signal from the analysis. The gray color on the image (Pic.2) indicates pixels excluded from the analysis. In this way, we only analyze the actually luminous area of the icon.
Basic luminance statistical analysis such as mean value, minimum and maximum value, luminance range are displayed for each region. In addition, a measure of uniformity is added, calculated as the ratio of the minimum value to the average luminance value. In the attached example, the ROI no. 2 is characterized by the lowest uniformity. It is also visible in the pseudo-color scale image.
The statistical values also allow you to determine, if the average luminance values for all the controls that are lit are similar or if there are any differences. Here in region 2 you can see that the average and maximum luminance values are significantly higher than in the others.
The histogram, i.e. the graphical visualization of the cross-section of the luminance value distribution, enables the visualization of the obtained results. Here, too, you can see the area of indicator 2 is clearly different from the others.
The three-dimensional visualization of the luminance value is a spectacular but also easy to interpret tool for presenting the luminance distribution. It also allows you to quickly detect any inequalities.
Visualization with isocandelas graphically shows the uniformity of the illumination of the control. Isocandelas are lines connecting points with the same luminance level. The shape of the isocandela on the controls clearly indicates the brightest areas closest to the light emitting diode.
Color measurement
With elements illuminated with diodes of different colors, there is a need to measure the colorimetric characteristics.
In order to analyze the color characteristics of the highlighted symbols, the spectral measurement of each of them was performed with the GL Opticam Spectis 1.0 Touch spectroradiometer. The measurements were made with the use of a Pen-type optical probe, which enables very precise measurements. The results were assigned for each region, which allowed to plot their colorimetric coordinates over the color space.
Moreover, thanks to the measurements of the spectral distribution of individual diodes, it was possible to correct the luminance values for individual symbols based on the estimation of errors resulting from the mismatch of the V (λ) filter – so-called Missmatch Correction. The luminance correction range was from 2.5% to over 5%.
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