Darkfield Illumination with Ring Lights for Machine Vision

著者: Thomas Stard & Rebecca Charboneau

Introduction to Darkfield Illumination using Ring Lights

In machine vision applications, lighting plays a critical role in capturing high-quality images for precise inspection and measurement. Using a ring light as a darkfield light source offers an effective way to enhance surface feature visibility by directing light at low angles, creating high contrast and revealing fine details without overwhelming the subject with direct illumination (Figure 1). This lighting technique helps reduce reflections and shadows, making it ideal for inspecting textured or reflective surfaces. This application note will provide an overview of when to use a darkfield ring light and the best practices for integrating them into your systems. The images below show different objects (diffuse and specular) with darkfield illumination using a white ring light. The images were taken with the 6mm C Series Fixed Focal Length Lens and 25mm C Series Fixed Focal Length Lens using the sensor IMX540 that was resized to a 2/3” sensor format for the C Series Fixed Focal Length Lenses.

Image Analysis and Evaluation of Darkfield Ring Light Illumination

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 and the right column images 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. The darkfield ring light evenly illuminates the objects and highlights surface defects such as the scratches on the metal gear (Panel D). Compared to a brightfield ring light, the darkfield ring light provides a uniform illumination profile for reflective surfaces like the printed circuit board (PCB, Panel E). It also works exceptionally well with diffuse objects like paper and a matte label on the medicine vial (Panel A). One important thing to note with a darkfield ring light is that it must be close to the object (~50mm) to achieve proper illumination. This may be difficult for wide angle imaging lenses as the ring light itself will be imaged if the inner diameter of the light is not large enough.

0.69X CobaltTL Telecentric Lens
A
B
C
D
E

Table 2 shows images taken with the 0.69X CobaltTL Telecentric Lens using the darkfield ring light. It can be observed that there are minimal hotspots on all objects that were imaged. When combined with a telecentric lens, the darkfield ring light is the ideal choice for optical character recognition of small text etched into a plastic or metal surface. Additionally, due to the shorter working distance of a telecentric lens, a darkfield ring light is a great option as it will typically fit within the working distance range of the lens and be out of the field of view.

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

Overall, darkfield ring lights are a great choice to highlight the edges of objects and to inspect engravings or deep scratches. By directing light at a low angle relative to the surface, it enhances contrast for features that scatter light while leaving smooth areas dark—ideal for inspecting polished or reflective materials.

More Resources

• Application Note: Backlight Illumination for Machine Vision
• Application Note: Brightfield Illumination with Spot and Bar Lights for Machine Vision
• Application Note: Brightfield Illumination with Dome Lights for Machine Vision
• Application Note: Brightfield Illumination with Ring Lights for Machine Vision
• Application Note: In-line Telecentric Illumination for Machine Vision
• Application Note: Common Illumination Types
• Application Note: Illumination Mounts for Machine Vision Applications
• Application Note: Relative Illumination, Roll-Off, and Vignetting

Bill of Materials

• Camera Used: LUCID Vision Labs Triton2™ TRT245S-MC, Sony IMX540, 24.5MP, Monochrome Camera (TRT245S-MC) (#28-874)
• Lenses Used: Edmund Optics 6mm C Series Fixed Focal Length Lens (#67-709), Edmund Optics 25mm C Series Fixed Focal Length Lens (#59-871), and Edmund Optics 0.69X CobaltTL Telecentric Lens (#15-872)
• Light Used: CCS 132mm Low Angle Ring Light White (LDR2-132SW2-LA) (#21-790)