Having worked in the vision measurement industry for over two decades, I have seen many customers repeatedly encounter a particular problem: measuring transparent or translucent materials often yields unsatisfactory results. Glass, acrylic, transparent plastic parts, and some optical elements with special coatings may appear ordinary, but under a vision measuring system, they often cause significant challenges.
Many customers, when testing glass or acrylic parts for the first time, find that edges are blurred and it’s nearly impossible to distinguish the true boundaries in the image. Some report seeing double contours, where the same part produces different detected edge positions, leading to large variations in dimensional measurements. This problem is especially common in optical glass, transparent screens, lenses, and precision housings.
The root cause lies in the optical properties of these materials. Vision measurement relies on reflected light from the surface to form clear edges. Transparent parts allow most light to pass through, with only a small portion reflecting back to the camera, naturally resulting in blurred edges. Furthermore, transmitted light can refract and reflect multiple times between surfaces, producing overlapping “false edges” in images, sometimes even more pronounced than the real edge, which misleads software. Semi-transparent or frosted surfaces scatter light strongly, creating uneven grayscale distributions and causing algorithms to be disturbed by noise.
One client experienced this while measuring a glass cover plate. No matter how they adjusted the system, the image edges remained blurred, and the measured dimensions fluctuated by tens of microns. Initially, they suspected equipment instability. After analysis, we determined the issue was not the machine, but overlapping signals from the top and bottom surfaces due to light transmission, which prevented the software from correctly identifying the true edge. Without proper handling, the measurement results cannot serve as reliable process references.
To address these challenges, I typically advise customers to take several measures:
1. Adjust the lighting. Standard ring lights are often ineffective for transparent parts. I usually recommend using backlighting. Coaxial backlighting can present the outline as a silhouette, producing very clear edges and is a common solution for measuring transparent parts. For semi-transparent or highly scattering materials, low-angle diffuse light or combined lighting can enhance edge contrast.
2. Temporary surface treatment. With customer approval, applying a developer spray or attaching a temporary frosted film can convert transmitted light into diffuse reflection. One client measuring transparent plastic parts found that after using a washable spray, image edges became clear and measurement repeatability significantly improved. These methods add extra steps, so customers must weigh the benefits against production requirements.
3. Optimize software algorithms. Transparent part measurement relies heavily on algorithms. Multi-threshold fitting can better distinguish true edges from false contours, and filtering can remove unstable points caused by optical noise. For double-edge issues, geometric criteria can help select only the edge that meets process requirements. Sub-pixel fitting also improves repeatability, even if edges are slightly blurred.
4. Consider the environment and fixturing. The background can significantly affect transparent part imaging. I usually recommend using a black stage to increase contrast. Fixtures should avoid entering the optical path, as they can create additional reflections and interfere with edge detection.
In summary, the main challenges in measuring transparent and translucent materials stem from light transmission, refraction, and scattering. Understanding these optical phenomena, combined with proper lighting, surface treatment, and algorithm optimization, allows most problems to be resolved. Over two decades of experience have shown me that vision measurement is not just about equipment precision—it requires a deep understanding of material properties and optical principles. Only by addressing both aspects can we reliably measure transparent parts with accuracy and consistency.