LED Technology
Where light really starts: PCB-to-LES distance
By Amandine Cocq from Gaggione
What is the PCB-to-LES distance / focal point?
The PCB-to-LES distance refers to the vertical distance between the Printed Circuit Board (PCB) on which the LED is mounted and the Light Emitting Surface (LES) of the LED. The LES is the point or area on the LED package where the light is emitted when an electric current passes through it. This distance is a key mechanical parameter, as it defines the optical origin of the light source relative to the board. The definition of PCB-to-LES is especially important in systems where precise light positioning and direction are required, such as in optical systems with lenses.
Why do we need to know the distance PCB-to-LES? The LES must coincide with the optical system focal point for optimal performances. So, understanding the distance from the PCB-to-LES is essential for accurate optical and mechanical design when working with LEDs. This measurement directly impacts optical alignment, especially in applications involving lenses, reflectors, or light guides. If the PCB-to-LES distance is incorrect or unknown, it can lead to poor light performance, inefficiency, homogeneity issues, or even mechanical
integration issues. Knowing this value allows designers to properly position the LED in their system to achieve optimal light output and beam shape. In short, knowing the PCB-to-LES distance is a small detail that has a big impact on the overall performance and reliability of your LED-based product.
How to get the PCB-to-LES distance?
When designing with LEDs, one of the most frequently asked questions is: how to get the PCB-to-LES distance? This
34 February 2026 Components in Electronics Illustrative examples of a datasheet
crucial measurement is determined entirely by the LED itself.
To find this information, the first step is to download the LED datasheet provided by the manufacturer. Once you have the datasheet, navigate to the section detailing the LED mechanical design. A well-documented datasheet typically includes the PCB-to-LES distance as part of the dimensional drawings. However, in some cases, this key detail might be vague or even missing altogether. So, what should you do if the datasheet doesn’t provide it?
1 General case – rule of thumbs
For chip-on-board (COB) solutions, the LES is on top of the LED.
For flat top LEDs, the LES is on top of the LED.
For domed LEDs, the LES is at the curvature centre of the dome.
Be careful for some LED references refer to the “_info.pdf” provided by the manufacturer, the LES may not be at the curvature centre of the dome.
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