Backlight Illumination for Machine Vision

Authors: Thomas Stard & Rebecca Charboneau

Introduction to Backlight Illumination

Backlights are critical components in precision machine vision systems, offering high-contrast, uniform illumination that enhances edge detection and dimensional measurement. The object of interest is placed on the backlight and the imaging system observes it in high contrast, resulting in a silhouette (Figure 1). This application note explores the use cases for a backlight, highlights its advantages in industrial inspection and metrology, and outlines key considerations for integration into vision systems. The images below show different objects (diffuse and specular) illuminated with a backlight. The images were taken with the 6mm C Series Fixed Focal Length Lens, 25mm C Series Fixed Focal Length Lens, and 0.69X CobaltTL Telecentric Lens using the sensor IMX540 that was resized to a 2/3” sensor format for the C Series Fixed Focal Length Lenses.

**Figure 1:** Backlight Illumination Schematic

Image Analysis and Evaluation of Backlight Illumination

**Table 1:** Images of different objects illuminated by a backlight and captured with fixed focal length lenses

6mm C Series Fixed Focal Length Lens	25mm C Series Fixed Focal Length Lens
A	A
B	B
C	C
D	D
E	E

Table 1 features images taken with fixed focal length lenses and illuminated with a backlight. It is evident from the images that a backlight is ideal for creating a silhouette of an object. This is great for objects that are not transparent like the cap (Panel B), gear (Panel D) and PCB board (Panel E). The text on the shipping label (Panel C) cannot be detected with this type of illumination geometry, making it a poor choice for optical character recognition (OCR) applications. When imaging transparent glass objects like the medicine vial in Panel A, a backlight can be used to look at liquid levels or placement of the label. Overall, this type of illumination is ideal for edge detection or part verification, since it creates a clear difference in gray level intensity between object and background.

**Table 2:** Images of different objects illuminated by a backlight and captured with a telecentric lens

0.69X CobaltTL Telecentric Lens
A
B
C
D
E

Table 2 contains images of the same parts taken with the 0.69X CobaltTL Telecentric Lens. As with fixed focal length lenses, a similar observation can be made: the backlight is best for creating high contrast on the edges of objects. For example, using a telecentric imaging lens and a backlight would allow for the precise measurement of the holes in the PCB (Panel E) or for inspecting the gear (Panel D) and cap (Panel B). The barcodes of the vial (Panel A) and shipping label (Panel C) are only visible due to the high magnification of the telecentric lens compared to the fixed focal length lenses and its ability to collect light.

Conclusion: Key Takeaways for Integrating Backlight Illumination into Vision Systems

Overall, backlight illumination is best suited for edge detection, inspection of transparent materials, and dimensional gauging applications. It produces a high-contrast silhouette, while surface features on the front of the object can only be detected at high magnification and if the object transmits light to some extent. Therefore, it is essential to understand what the object of interest is and what should be imaged and to select the illumination geometry that maximizes the performance of the imaging system.

More Resources

Bill of Materials

Camera Used: LUCID Vision Labs Triton2™ TRT245S-MC, Sony IMX540, 24.5MP, Monochrome Camera (TRT245S-MC) (#28-874)
Lenses Used: EO 6mm C Series Fixed Focal Length Lens (#67-709), EO 25mm C Series Fixed Focal Length Lens (#59-871), and EO 0.69X CobaltTL Telecentric Lens (#15-872).
Light Used: Advanced Illumination 50.8 x 50.8mm High Intensity White LED Backlight (#28-756)

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