Ultraviolet (UV) light interacts with materials in a unique way, enabling features and characteristics to be observed that are difficult to detect by other methods. It tends to be strongly absorbed by many materials, making it possible to visualize the surface topology of many optically transparent or translucent objects without the light penetrating into the interior. Also, because of its short wavelength, it tends to be scattered by surface features that are not apparent at longer wavelengths. Thus, ever smaller features can be resolved or detected via UV light scattered off of them.
Relative to x-ray or IR machine vision, UV machine vision is still in its infancy, but the field is growing as commercial UV hardware drops in price and increases in diversity. New applications are emerging as more users integrate off-the-shelf UV cameras into production environments and experiment with them. For example, Oculus Photonics (Goleta, CA) has begun marketing a digital, compact camcorder, the UVCorder, which has a UV imaging module mounted on a commercial camcorder. The module has a UV response in the 300 to 400 nm range, with a peak response at 370 nm, and rejects visible and IR light. The company is a joint venture between Peter Taylor and Austin Richards of Santa Barbara, CA. Richards is a consultant in the photonics industry specializing in invisible-light imaging.
Richards says that it is important to distinguish between reflected-UV imaging and UV fluorescence imaging. They are different techniques with different characteristics, but because they both involve UV light, they are often confused with one another. Reflected-UV imaging starts with the illumination of a surface with ultraviolet light. The UV light is reflected or scattered and is then imaged by a camera that is sensitive in the UV band. The wavelength of the UV light is not shifted during the process.
Ultraviolet-fluorescence imaging also starts with active illumination of a surface with UV light, but the detected signal is in the visible or infrared band. The fluorescent material absorbs the UV excitation, then re-radiates at a longer wavelength. The emitted fluorescence is not reflected light-it tends to be a diffuse emission.
Scratch the surface
According to Richards, one of the most common applications for reflected-UV imaging is the detection of scratches in a surface. The shorter UV wavelengths tend to scatter more strongly off surface features compared to the visible or near-IR bands. So, for example, scratches not apparent in a visible image may be only visible to a person with excellent eyesight with great difficulty when visible light strikes at a very oblique angle. In contrast, in a UV image taken with 365 nm illumination, the scratches can be seen quite easily.
As a result, UV imaging enables automated systems to detect scratches and digs on optical surfaces such as lenses or windows. In the semiconductor industry, confocal microscopes operating in the deep UV band at 248 or 266 nm can be used to image submicron features with much greater clarity than in the visible band and can be used to find tiny defects in the silicon wafer starting material.
Other reflected-UV applications involve the detection of small amounts of surface contamination. Because UV light tends to be absorbed by organic materials, traces of oil or grease are sometimes detectable on many surfaces, particularly in the deep-UV band. It is also possible to distinguish new paint from old in some situations, even when the two types of painted surfaces look identical in the visible band. The oxidized paint tends to reflect UV, while the organic molecules in the fresh paint absorb UV.
Reflected-UV imaging is a relatively new area of machine-vision technology. Advances in light-source technology, particularly LEDs operating in the near-UV band and laser sources operating in the deep-UV band are driving down costs of systems. Commercial hardware is becoming more available, particularly CCD cameras and lenses specifically designed for UV work. There are still many undiscovered applications for reflected-UV imaging, and it will be interesting to see these applications move from an R&D environment into process inspection over time.
