NDT - UV Inspection
Consider UV Inspection
November 2, 2012
Advances in UV luminescence sensors have resulted in faster response times, higher resolution and increased operating range.
Ultraviolet (UV) inspection techniques can be applied to a wide range of processes, from medical to automotive to glass manufacturing and product packaging. It often involves verification of the completeness of a process, for example a continuous glue seam along the diameter of a clear medical syringe or verification of a process step, such as the application of a UV blocking layer in a clear packaging film necessary to prevent degradation of UV sensitive products. Many processes require manual sampling or inspection on a representative sample pulled from a production line. Often this is a time-consuming and sometimes subjective process.
Regardless of the specific application, it all begins by understanding the benefits that high-speed, on-line, real-time process monitoring can provide to improve throughput, reduce scrap and, perhaps most importantly, to prevent defective products from reaching your customers.
The materials and processes that can benefit from the use of UV inspection techniques are those that possess fluorescent characteristics or absorb (block) UV light, or where UV tracer may be added to an existing process to provide a reliable detection method. Fluorescence is the emission of visible light as a result of excitation by ultraviolet (UV) light, the emission ceasing when the UV light is removed. Signs, paints and other objects that glow under a black light are fluorescent.
UV inspection is based on the use of sensors that provide a light output in the near ultra violet range (“black light”), from about 320 to 380 nanometers (nm). These sensors may provide various light spot sizes ranging from one to 10 millimeters and operate at sensor-to-target distances of 25 to 300 millimeters. The target material has a fluorescent characteristic, such as glue with a UV tracer (pigment), which emits visible light when positioned in the sensor’s light spot. This visible light is measured by the sensor and an output signals external processes equipment, such as a programmable logic controller (PLC), to the presence of the target material. Since these types of sensors sample at rates of up to 40 kilohertz (kHz), or 25 microseconds, high spatial resolution allows small defects to be detected and high-speed processes to be monitored. In addition, some of these types of sensors also provide an analog output that is proportional to the strength of the visible light emitted by the target material. This output may be used as an input to the PLC to indicate relative variation in the amount of UV tracer.
Many commonly used packaging and production materials use luminescent tracers as a means of providing presence verification. Among these materials are adhesives, gums, films, inks and greases. Since many of these materials are clear or nearly clear, other types of sensors are not suitable for reliable verification. A luminescent mark can be printed using an invisible ink anywhere on a label without affecting the aesthetics of the label, while allowing a luminescence sensor to detect the mark to verify presence, orientation and positioning of the label. Often, clear films such as those used in tamper-evident seals contain optical brighteners that cause them to luminesce in the presence of a UV light source, allowing presence verification to be accomplished.
Advances in UV luminescence sensors have resulted in faster response time from object present to output activated, higher resolution and increased operating range. High-speed sampling, resulting in faster response times in the range of 25 microseconds, allows automated processes to run at higher speeds. For example, a 25 microsecond response time for a process that runs at 2,000 feet per minute results in a completed sensor measurement cycle every 0.01 inch of linear travel. Each measurement cycle indicates presence or non-presence of the target via its discrete output line. In conjunction with the sampling speed increases, higher resolution 10-bit measurements allow for finer distinction between presence and non-presence conditions. The higher resolution measurement benefits applications such as the detection of a glue (with a UV tracer) applied to a material that also has some degree of luminescence. New, higher output light sources and photodetectors with increased sensitivity have resulted in UV sensors that can operate at up to 600 millimeters sensor-to-target distances.
A typical medical application of applying a clear adhesive to a clear syringe sleeve can be a difficult process to verify. Sensors and vision systems do not detect clear materials reliably.
However, use of an adhesive with a UV tracer and a UV sensor with a small spot size focused onto the location of the adhesive to verify the process on each syringe. As each syringe moves through the sensor’s light spot, the sensors output signal indicates presence or non-presence of the adhesive to the control system (PLC) resulting in 100% of the products being checked.
Another application involves continuous monitoring of clear packaging film with a UV blocking (absorbing) layer. Since the UV blocking characteristic is vital to providing the protective characteristic of the packaging (prevents degradation of contents from exposure to UV light, sunlight) and the material is clear to allow the contents to be seen, the verification can be difficult.
However, use of a photo sensor with a remote UV light source allows positioning of the light source on one side of the film and the receiver on the other. When the film contains the UV blocking layer, the film absorbs the UV light and the receiver indicates a very low signal level. When the UV blocking layer is missing, the UV light is transmitted through the film, resulting in a high signal level at the receiver. The receiver signals the PLC of the missing layer and the appropriate action can be initiated immediately, such as stopping the process and alerting the operator.
Similarly, UV blocking agents applied to clear plastics such as preforms or bottles can also be verified using the same technique.
Another application involves automotive glass often coated on one side with paint, tint or other special treatment. In the process of manufacturing float glass, the finished glass has an air-side and a tin-side. When secondary processes are required it is necessary to identify or differentiate between the air-side and the tin-side. This is often accomplished by use of a handheld UV lamp and a visual inspection. The handheld devices use 254 nm, UVC (ultraviolet type C) lamps that are considered hazardous and must be used with protective eyewear.
Recent advances in UV LED technology in the UVC ranges now allow on-line sensing of the tin-side of float glass. These types of sensors are application specific, designed exclusively for the special detection requirements necessary at the UVC wavelengths. The UV light beam is projected onto the surface of the glass, when the tin-side is present fluorescence occurs and the resulting visible light is measured by the sensor. The sensor provides a discrete output to the PLC indicating tin-side presence. When the air-side is present, the UV light is absorbed by the glass, no fluorescent occurs and no visible light is present.
These examples are only a small representation of the many applications for UV inspection that can help to improve the quality of products and processes in many industries including medical, automotive and aerospace. Automating production inspection points often allows 100% verification of products during manufacturing processes, improving quality; reducing scrap costs and assuring satisfied customers.