The three curves in the figure above shows the normalized response of an average human eye to various amounts of ambient light. The shift in sensitivity occurs because two types of photoreceptors called cones and rods are responsible for the eye's response to light. The curve on the right shows the eye's response under normal lighting conditions and this is called the photopic response. The cones respond to light under these conditions.
As mentioned previously, cones are composed of three different photo pigments that enable color perception. This curve peaks at 555 nanometers, which means that under normal lighting conditions, the eye is most sensitive to a yellowish-green color. When the light levels drop to near total darkness, the response of the eye changes significantly as shown by the scotopic response curve on the left. At this level of light, the rods are most active and the human eye is more sensitive to the light present, and less sensitive to the range of color. Rods are highly sensitive to light but are comprised of a single photo pigment, which accounts for the loss in ability to discriminate color. At this very low light level, sensitivity to blue, violet, and ultraviolet is increased, but sensitivity to yellow and red is reduced. The heavier curve in the middle represents the eye's response at the ambient light level found in a typical inspection booth. This curve peaks at 550 nanometers, which means the eye is most sensitive to yellowish-green color at this light level. Fluorescent penetrant inspection materials are designed to fluoresce at around 550 nanometers to produce optimal sensitivity under dim lighting conditions.
References: Robinson, S. J. and Schmidt, J. T., Fluorescent Penetrant Sensitivity and Removability - What the Eye Can See, a Fluorometer Can Measure, Materials Evaluation, Vol. 42, No. 8, July 1984, pp. 1029-1034
Dye penetrant inspection (DP), also called liquid penetrate inspection (LPI) or penetrant testing (PT), is a widely applied and low-cost inspection method used to check surface-breaking defects in all non-porous materials (metals, plastics, or ceramics).
https://en.wikipedia.org › wiki › Dye_penetrant_inspection
Test will first coat the material being tested with a solution that contains a visible or fluorescent dye. Inspectors then remove any extra solution from the material's surface while leaving the solution in defects that “break” the material's surface.
Liquid or dye penetrant testing (PT) is a non-destructive material testing method which uses capillary forces to find surface cracks or pores and make them visible. It can detect surface-breaking flaws such as cracks, laps, porosity.
Dye Penetrant Inspection (DPI) is widely used to detect surface breaking flaws. This non-destructive testing technique, also known as liquid penetrant inspection (LPI), is a cost-effective method used to locate surface breaking flaws such as cracks, porosity, laps, seams and other surface discontinuities.
Penetrant materials come in two basic types: Type 1 - Fluorescent Penetrants: they contain a dye or several dyes that fluoresce when exposed to ultraviolet radiation. Type 2 - Visible Penetrants: they contain a red dye that provides high contrast against the white developer background.
Penetrant method of non-destructive testing is dedicated to detecting surface defects invisible to the naked eye. It is based on use of capillary properties of liquids (namely, on capillary penetration of special indicator liquids inside defects).
There are limitations to DPI. One disadvantage is that it does not go below the surface and so can only identify surface defects. Another is that it requires a fairly clean surface as testing on rough surfaces can lead to false indications. Porous materials can't be inspected with dye penetrants.
Visible penetrants contain a red dye that provides high contrast against the white developer background. Fluorescent penetrant systems are more sensitive than visible penetrant systems because the eye is drawn to the glow of the fluorescing indication.
Disadvantages of the penetrant method of Non-Destructive Examination are: Can only detect defects open to the surface. Preparation, before testing, can be time consuming and costly. The method takes time and can rarely be completed in less than 30 minutes.
How do I pursue a career in non-destructive testing? To become an NDT Technician, you must have hours in the classroom and on-the-job training (OJT) hours in the field. UTI can provide the classroom hours, and upon graduation and securing employment your employer can provide the OJT.
What are the commonly used methods of non-destructive examination (NDE)? The most commonly used NDE method in structural steel fabrication is visual (VT). VT includes observation before, during, and after welding.
Basic liquid penetrant process: (a) step 1, apply penetrant; (b) step 2, remove excess; (c) step 3, apply developer. In the first step, a penetrating liquid containing dyes is applied to the surface of the object to be inspected. The part surface must be clean and dry before applying the penetrant.
The five most common testing methods in NDT are Visual Testing (VT), Ultrasonic Testing (UT), Radiographic Testing (RT), Magnetic Particle Testing (MT), and Liquid Penetrant Testing (PT).
Emerging technologies in NDT include artificial intelligence, robotics, and advanced sensors that are revolutionising the way inspections are conducted. These technologies offer increased accuracy, efficiency, and safety in NDT testing procedures.
ASTM E1417 “Standard Practice for Liquid Penetrant Examination” governs the liquid penetrant inspection process used in manufacturing and maintenance operations. ASTM E1417 addresses personnel requirements, equipment requirements, penetrant application, and penetrant removal methods.
What is the Difference between a Dye Penetrant Test and a Liquid Penetrant Test? Dye penetrant testing (DPT) and liquid penetrant testing (LPT) are the same thing. The process is also known by other names, including liquid penetrant inspection (LPI) liquid penetrant examination (LPE), or simply penetrant testing (PT).
The principle of liquid penetrant testing is that the liquid penetrant is drawn into the surface-breaking crack by capillary action and excess surface penetrant is then removed; a developer (typically a dry powder) is then applied to the surface, to draw out the penetrant in the crack and produce a surface indication.
LPT can be used to locate and evaluate defects throughout an items life-cycle, such as: Manufacturing defects open to the surface (such as cracks) Lack of fusion. Porosity (gas pockets)
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