Quality Control of Lighting
After a component has been properly processed, it is ready for inspection. While automated vision inspection systems are sometimes used, the focus here will be on inspections performed visually by a human inspector, as this is the dominant method. Proper lighting is of great importance when visually inspecting a surface for a penetrant indication. Obviously, the lighting requirements are different for an inspection conducted using a visible dye penetrant than they are for an inspection conducted using a fluorescent dye penetrant. The lighting requirements for each of these techniques, as well as how light measurements are made, are discussed below.
Lighting for Visible Dye Penetrant Inspections
When using a visible penetrant, the intensity of the white light is of principal importance. Inspections can be conducted using natural lighting or artificial lighting. When using natural lighting, it is important to keep in mind that daylight varies from hour to hour, so inspectors must stay constantly aware of the lighting conditions and make adjustments when needed. To improve uniformity in lighting from one inspection to the next, the use of artificial lighting is recommended. Artificial lighting should be white whenever possible and white flood or halogen lamps are most commonly used. The light intensity is required to be 100 foot-candles at the surface being inspected. It is advisable to choose a white light wattage that will provide sufficient light, but avoid excessive reflected light that could distract from the inspection.
Lighting for Fluorescent Penetrant Inspections
When a fluorescent penetrant is being employed, the ultraviolet (UV) illumination and the visible light inside the inspection booth is important. Penetrant dyes are excited by UV light of 365nm wavelength and emit visible light somewhere in the green-yellow range between 520 and 580nm. The source of ultraviolet light is often a mercury arc lamp with a filter. The lamps emit many wavelengths and a filter is used to remove all but the UV and a small amount of visible light between 310 and 410nm. Visible light of wavelengths above 410nm interferes with contrast, and UV emissions below 310nm include some hazardous wavelengths.
Standards and procedures require verification of lens condition and light intensity. Black lights should never be used with a cracked filter as output of white light and harmful black light will be increased. The cleanliness of the filter should also be checked as a coating of solvent carrier, oils, or other foreign materials can reduce the intensity by up to as much as 50%. The filter should be checked visually and cleaned as necessary before warm-up of the light.
Since fluorescent brightness is linear with respect to ultraviolet excitation, a change in the intensity of the light (from age or damage) and a change in the distance of the light source from the surface being inspected will have a direct impact on the inspection. For UV lights used in component evaluations, the normally accepted intensity is 1000 microwatt per square centimeter when measured at 15 inches from the filter face (requirements can vary from 800 to 1200 µW/cm2). The required check should be performed when a new bulb is installed, at startup of the inspection cycle, if a change in intensity is noticed, or every eight hours of continuous use. Regularly checking the intensity of UV lights is very important because bulbs lose intensity over time. In fact, a bulb that is near the end of its operating life will often have an intensity of only 25% of its original output.
Black light intensity will also be affected by voltage variations. A bulb that produces acceptable intensity at 120 volts will produce significantly less at 110 volts. For this reason it is important to provide constant voltage to the light. Also, most UV light must be warmed up prior to use and should be on for at least 15 minutes before beginning an inspection.
When performing a fluorescent penetrant inspection, it is important to keep white light to a minimum as it will significantly reduce the inspectors ability to detect fluorescent indications. Light levels of less than 2 fc are required by most procedures with some procedures requiring less than 0.5 fc at the inspection surface. Procedures require a check and documentation of ambient white light in the inspection area. When checking black light intensity at 15 inches a reading of the white light produced by the black light may be required to verify white light is being removed by the filter.
Light intensity measurements are made using a radiometer. A radiometer is an instrument that translate light energy into an electrical current. Light striking a silicon photodiode detector causes a charge to build up between internal layers. When an external circuit is
connected to the cell, an electrical current is produced. This current is linear with respect to incident light. Some radiometers have the ability to measure both black and white light, while others require a separate sensor for each measurement. Whichever type is used, the sensing area should be clean and free of any materials that could reduce or obstruct light reaching the sensor. Radiometers are relatively unstable instruments and readings often change considerable over time. Therefore, they should be calibrated at least every six months.
Ultraviolet light measurements should be taken using a fixture to maintain a minimum distance of 15 inches from the filter face to the sensor. The sensor should be centered in the light field to obtain and record the highest reading. UV spot lights are often focused, so intensity readings will vary considerable over a small area. White lights are seldom focused and depending on the wattage, will often produce in excess of the 100 fc at 15 inches. Many specifications do not require the white light intensity check to be conducted at a specific distance.