Brightfield Illumination with Ring Lights for Machine Vision

Authors: Thomas Stard & Rebecca Charboneau

Introduction to Brightfield Illumination using Ring Lights

Brightfield ring lights are best when uniform illumination with limited shadowing is needed; especially when the working distance is relatively short, and space is a concern. They mount close to or on the imaging lens and provide coaxial light in a relatively compact form factor. Since brightfield ring lights cast their light at a sharp angle towards the object (Figure 1.), reflective surfaces can cause a circular glare pattern. Furthermore, surface textures or fine surface defects may have poor contrast due to the even lighting from the ring light above the object. The images below show different objects (diffuse and specular) with brightfield illumination using a white LED ring light. 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 Darkfield Ring Light App Note.png — **Figure 1:** Schematic of Brightfield Illumination using a Ring Light

Image Analysis and Evaluation of Brightfield Ring Light Illumination

**Table 1:** Images of different objects illuminated by a brightfield ring light 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

The left column of Table 1 shows images that were captured using the 6mm C Series Fixed Focal Length Lens at a working distance of 200mm. When imaging reflective objects like the printed circuit board (PCB, Panel E) and the medicine vial (Panel A), a ring of LEDs can be observed on the surface of these objects. This glare could interfere with the image processing of the system. For the cap (Panel B), shipping label (Panel C), and the metal gear (Panel D), the uniform light is great for illuminating the surfaces of these less reflective objects. The images in the right column of Table 1 were captured with the 25mm C Series Fixed Focal Length Lens at a working distance of 750mm to maintain the same magnification and field of view of the 6mm C Series Fixed Focal Length Lens. Glare from the ring light LEDs on the PCB surface and the medicine vial can still be seen, but since the working distance is longer for the 25mm C Series Fixed Focal Length Lens the glare is not as evident. This demonstrates the importance of not only defining the illumination geometry, but also considering how working distance and field of view affect the illumination uniformity of the imaging system.

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

0.69X CobaltTL Telecentric Lens
A
B
C
D
E

Table 2 shows images of the same parts captured with the 0.69X CobaltTL Telecentric Lens. Compared to the previous images taken with fixed focal length lenses, the telecentric lens has a much smaller field of view (FOV), so only specific regions of interest of the sample objects are shown. At this small FOV, the effects of any glare are reduced and it appears that the brightfield ring light effectively illuminates all of the objects above. Minor hotspots can still be observed on the reflective parts depending on the topography of the object as seen on the curved solder spots of the PCB.

Conclusion: Key Takeaways for Integrating Brightfield Ring Lights into Vision Systems

Overall, brightfield illumination with a ring light is well suited for diffuse objects and should be cautioned when imaging very reflective objects. This brightfield configuration provides even illumination and minimizes shadows, making it ideal for inspecting flat, matte, or moderately reflective surfaces. When seeking reliable edge detection and feature clarity without complex lighting setups, a ring light provides a cost-effective and easily integrated option that supports consistent imaging performance.

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 2.95" White Advanced Illumination High Intensity Ring Light (#88-484)

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