The image quality indicator, unassuming yet crucial in radiographic testing

Release Date:

2022/09/17

In radiographic testing, the image quality indicator (IQI) is one of the most commonly used and critical inspection tools. Often unassuming—sometimes even tucked away in a corner and easy to overlook—it may appear at first glance as nothing more than a few metal wires or thin metal sheets. Yet it plays the role of the “unsung hero,” serving as an essential accessory for controlling and ensuring the quality of radiographic inspections. This leads us to pose a profound question: Do you remember whether you have ever made a mistake simply because you were not familiar with it? 1. What is an IQI? An IQI, also known as a radiographic image quality comparator, is primarily a tool used to inspect and quantitatively evaluate the image quality of radiographic films. It is also referred to by various other names, such as IQI or penetrameter. The material requirements for an IQI generally stipulate that it must be made from a material that is either identical to the test piece’s material or has similar radiographic absorption characteristics. IQIs incorporate artificial structures with controlled thickness differences—such as slots, holes, or metal wires—whose dimensions are related to the thickness of the test piece in specific ways. The IQI image on the radiographic film serves as permanent evidence that the radiographic examination was performed under appropriate conditions; however, the indication provided by the IQI does not represent the actual size of natural defects that could be detected in the test piece. Rather, it is a relative measure of sensitivity. 2. What are the classifications of IQIs? Currently, there are three main types of IQIs used in industrial radiography: wire-type, hole-type, and slot-type. Among these, the wire-type IQI is the most widely used and is adopted by most major industrial countries. In addition, some countries still use flat-hole-type IQIs—for example, the United States. The United Kingdom and France, on the other hand, employ stepped-hole-type IQIs. There is also a dual-wire-type IQI, which is not used to measure radiographic sensitivity but rather to assess radiographic unsharpness. Category 1: Wire-Type IQIs Based on the variation in wire diameter, wire-type IQIs can be further classified into arithmetic-series, geometric-series, constant-diameter, and single-wire types. Globally, the geometric-series IQI is currently the most prevalent. A wire-type IQI consists of a set of seven numbered metal wires connected together. All wires in each IQI must be made of the same material and embedded in a low-absorption matrix—such as an engineering-plastic housing—to ensure that their presence does not compromise image quality. The wires themselves should be arranged in parallel. The IQI marking comprises the designation of the largest-diameter wire, the wire material, and the standard code. The largest-diameter designation is placed on the side corresponding to the largest-diameter wire and simultaneously indicates the IQI number. According to the wire-diameter ratio, IQIs are divided into four models. IQIs can also be categorized by material: steel IQIs, aluminum IQIs, titanium IQIs, copper IQIs, and so on, represented by the codes FE, AL, TI, and CU, respectively. During radiography, the IQI material should match that of the test piece; if an IQI made of the same material is unavailable, one made of a material with a lower atomic number may be used as a substitute. Category 2: Flat-Hole-Type IQIs Flat-hole-type IQIs are manufactured by drilling small holes of specified dimensions into a uniformly thick plate. This type of IQI is recognized in U.S. ASME codes, MIL standards, European EN 1435, and Chinese GB 3323 standards. There are three options for selecting the IQI thickness T: 1%, 2%, or 4% of the exposure thickness. The corresponding hole diameters are IT, 2T, and 4T. IQIs come in rectangular and circular forms, with the latter being suitable for thicker workpieces. The sensitivity level of a flat-hole-type IQI is expressed as “thickness–hole type.” For example, 2-1T means that a 1T-diameter hole on a 2% thickness IQI can be detected; 1-2T means that a 2T-diameter hole on a 1% thickness IQI can be detected. Following this notation, each thickness of flat-hole-type IQI can display three sensitivity levels, and three different thicknesses can collectively show nine sensitivity levels: 1-1T, 1-2T, 1-4T, 2-1T, 2-2T, 2-4T, 4-1T, 4-2T, and 4-4T. In actual radiographic inspections, five of these sensitivity levels are typically used: 1-1T, 1-2T, 2-1T, 2-2T, and 2-4T. Flat-hole-type IQIs should be made from materials belonging to the same group or grade as the material being radiographed, or alternatively from materials with lower radiographic absorption characteristics than the test piece. Category 3: Slot-Type IQIs General-purpose wedge-shaped test blocks are available in two versions: stainless steel and aluminum, with the choice based on the most stringent materials actually used in practice. 3. How to Use an IQI Regardless of the type of IQI used, its placement directly affects the sensitivity reading indicated by the IQI. Therefore, when positioning the IQI, it is generally placed in areas within the radiographic field where sensitivity is relatively low—for example, on the surface of the workpiece far from the film or in regions with greater exposure thickness. If the unfavorable area can achieve the specified sensitivity, it is generally assumed that the favorable area will achieve even higher sensitivity. In certain special cases, the IQI cannot be placed on the source-side surface; in such instances, a comparative test must be conducted. The procedure involves fabricating a short test specimen with the same material, diameter, and wall thickness as the test piece, placing one IQI on both the inner and outer surfaces of the test area, adding an “F” mark to the IQI on the film side, and then performing radiography under the same exposure conditions as the test piece. On the resulting radiograph, the IQI index or relative sensitivity achieved by the source-side IQI is used to determine the corresponding IQI index or relative sensitivity that the film-side IQI should attain. When using a double-wall single-image IQI on the film side, an “F” mark must be added to indicate that the IQI is positioned on the film side. For flat-hole-type IQIs, placement requires positioning them at least 5 mm away from the edge of the weld being inspected, on the base metal surface, and under the IQI a shim of a specific thickness must be placed—approximately equal to the total excess height of the weld being inspected. The purpose of this shim is to ensure that the density of the inspected area is no less than 15% of the IQI’s density range, while the shim’s dimensions should exceed those of the IQI itself, so that at least three of the IQI’s contour lines are clearly visible in the radiograph. 4. Have You Ever Made These IQI Mistakes? As a nondestructive-testing professional new to the field, what are the most common mistakes made when using IQIs? A. Selection Errors When performing radiographic inspections on steel materials, failing to use a steel IQI and instead mistakenly choosing an aluminum IQI; selecting the wrong IQI for different thicknesses—for example, using a 1-6# IQI when a 6-13# model is required; using the wrong standard—for instance, applying an ASME-standard IQI when NB/T47013 specifies a particular marking system; or marking the IQI in a way that does not comply with the relevant standards. B. Placement Errors This is the most common mistake made by inspectors and also the easiest to overlook during the inspection process. Placing the IQI in the center of the radiograph (for non-tubular components)—the correct method is to position it at the 1/4 mark of the film or in an area of lower sensitivity. Reversing the orientation of the IQI’s thick and thin wires is another frequent error among beginners: the thick wire should be placed on the film’s inner side, while the thin wire goes on the outer side—not the other way around. Placing the IQI on the film side is also a common convenience-based mistake made during on-site inspections. According to standards, most IQIs should be placed on the source side; only in a few special cases are they placed on the film side, such as in double-wall single-image projections. When an IQI is placed on the film side, a lead letter “F” should be appropriately affixed to the IQI as a marker. The “F” mark must appear on the radiograph alongside the IQI’s own markings, and this fact must be noted in the inspection report. All content in this article has been collected from the internet for technical exchange and sharing purposes only. If any infringement occurs, please contact us, and we will promptly remove the offending material.

In radiographic testing, the image quality indicator is one of the commonly used and important inspection tools.

Unassuming in appearance—so much so that it often goes unnoticed in a corner—it looks at first glance like just a few metal wires or metal sheets. Yet it plays the role of the “unheralded master,” serving as an essential testing accessory for quality control and inspection.

This calls for a profound question: Do you still remember whether you’ve ever made mistakes simply because you weren’t familiar with it?

1. What is a image quality indicator?

A quality indicator, also known as a radiographic image quality comparator, is primarily used to inspect and quantitatively evaluate the image quality of radiographic films; it is also referred to by various other names, such as IQI or penetrameter.

The material requirements for image quality indicators generally stipulate that they be fabricated from materials with the same composition as the test workpiece or with radiographic absorption characteristics similar to those of the workpiece. Image quality indicators typically incorporate artificial structures with thickness variations, such as slots, holes, and metal wires, whose dimensions are quantitatively related to the thickness of the test workpiece.

The image of the IQI on the radiographic film serves as permanent evidence that the radiographic examination was performed under appropriate conditions; however, the indication values provided by the IQI do not represent the actual size of natural defects that may be present in the test piece—rather, they constitute a relative measure of sensitivity.

2. What are the classifications of image quality indicators?

Classification of image quality indicators: Currently, three types of image quality indicators are used in industrial radiography: wire-type, hole-type, and slot-type. Among these, the wire-type is the most widely used, and it is adopted by all major industrial countries.

In addition, some countries still use flat-hole image quality indicators, such as the United States. The United Kingdom and France, on the other hand, employ stepped-hole image quality indicators.

In addition, there is a dual-wire type image quality indicator, which is not used to measure radiographic sensitivity but rather to measure radiographic unsharpness.

Category 1: Wire-Type Image Quality Indicators

According to the variation pattern of metal wire diameter, wire-type image quality indicators are classified into arithmetic progression, geometric progression, constant-diameter, and single-wire types. Currently, the geometric progression type is the most widely used worldwide.

The wire-type image quality indicator consists of groups, each comprising seven numbered metal wires connected to one another. All the wires in each indicator must be made of the same material and embedded in a low-absorption matrix—such as an engineering-plastic housing—to ensure that their presence does not adversely affect image quality. The wires of the indicator shall be arranged parallel to one another.
The image quality indicator consists of the wire number corresponding to the maximum diameter, the wire material, and the standard designation. The wire number for the maximum diameter shall be placed on the side of the line with the maximum diameter, and this wire number also serves as the image quality indicator number. Based on the ratio of wire diameters, image quality indicators are classified into four models.

Image quality indicators can be classified according to the material used, including steel image quality indicators, aluminum image quality indicators, titanium image quality indicators, copper image quality indicators, and so on, which are represented by the codes FE, AL, TI, and CU, respectively.

The material of the image quality indicator used during radiographic inspection shall be the same as that of the test piece. When a matching-material image quality indicator is unavailable, an indicator made of a material with a low atomic number may be used as a substitute.

Category 2: Flat-Plate Hole-Type Image Quality Indicator

Flat-hole image quality indicators are manufactured by drilling small holes of specified dimensions in a flat plate of uniform thickness. Such indicators are specified in the American ASME Code, MIL standards, the European EN 1435 standard, and the Chinese GB 3323 standard.

There are three options for selecting the penetrameter thickness T: 1%, 2%, and 4% of the radiographic thickness. The corresponding diameters of the drilled through-holes are IT, 2T, and 4T, respectively. Penetrameters are available in rectangular and circular forms, with the latter being suitable for thick workpieces. The sensitivity level of a flat-hole penetrameter is denoted as “thickness × hole type.”

For example, 2-1T means that a 1T-diameter hole on a penetrameter with a thickness of 2% can be detected; 1-2T means that a 2T-diameter hole on a penetrameter with a thickness of 1% can be detected.

According to the above representation method, a flat-panel hole-type IQI of each thickness can display three sensitivity levels; therefore, a flat-panel hole-type IQI with three different thicknesses can display nine sensitivity levels, namely: 1-1T, 1-2T, 1-4T, 2-1, 2-2T, 2-4T, 4-1T, 4-2T, and 4-4T.

In practical radiographic testing, five sensitivity levels are typically used: 1-1T, 1-2T, 2-1T, 2-2T, and 2-4T.
The flat-hole image quality indicator shall be fabricated from material of the same group or class as the material being radiographed; alternatively, it may be fabricated from material of a group or class with lower X-ray absorption characteristics than the material being radiographed.

Category Three: Groove Type

Setting up and using universal test wedges—available in two versions: stainless steel and aluminum; the choice should be based on the most stringent material requirements of the specific application.

3. How to use the image quality indicator

Regardless of the type of image quality indicator used, its placement directly affects the indicated sensitivity value.

Therefore, when positioning the IQI, it is generally placed in areas within the radiographic exposure field that exhibit lower sensitivity, such as the workpiece surface far from the film or regions with greater thickness. If the unfavorable areas can achieve the specified sensitivity, it is generally considered that the favorable areas will even more easily meet the requirement.

In certain special circumstances, when the image quality indicator cannot be placed on the source-side surface, a comparative test shall be conducted.

The procedure is as follows: fabricate a short test specimen with the same material, diameter, and wall thickness as the workpiece under inspection; place one image quality indicator on each of the inner and outer surfaces at the area to be inspected; affix an “F” mark on the image quality indicator on the film side; then perform radiographic exposure using the same exposure parameters as those applied to the workpiece.

On the obtained radiographic film, the corresponding density index or relative sensitivity required on the film side of the densitometer shall be determined based on the specified density index or relative sensitivity achieved by the densitometer on the source side.

When the double-wall single-image IQI is placed on the film side, an “F” shall be added to indicate that the IQI is positioned on the film side.

The flat-hole image quality indicator shall be placed on the base metal surface at a distance of at least 5 mm from the edge of the weld being inspected, and a shim of specified thickness shall be positioned beneath the indicator. The shim thickness should be approximately equal to the total excess height of the weld being inspected, with the aim of ensuring that the density in the inspection area is no less than 15% of the density range specified for the image quality indicator. The dimensions of the shim shall exceed those of the image quality indicator, so that at least three outline lines of the indicator can be clearly discerned in the radiographic image.

4. Have you ever made mistakes like those with the image quality meter?

What are the common misuses of image quality indicators among nondestructive testing practitioners who are new to the field?

Option A is incorrectly selected

During radiographic testing of steel materials, iron-based image quality indicators are often not used, and aluminum-based ones are mistakenly employed instead. Furthermore, different image quality indicators should be selected according to the material thickness; for example, models 6–13 should be used, yet models 1–6 are erroneously substituted. There are also instances of incorrect standard application—for instance, when spray-marking as specified in NB/T 47013 is required, ASME-type image quality indicators are improperly used—and of marking on the image quality indicator in a manner that does not comply with the relevant standards.

B Placement Error

This is the area where inspectors are most prone to making mistakes—and also the one they are most likely to overlook during the inspection process.
Place the image quality indicator in the center of the film (for non-tubular objects); the accurate method is to position it at the 1/4 mark on the film or in a region with lower sensitivity.
Placing the penetrameter’s coarse wire on the inside and the fine wire on the outside is a common mistake made by beginners; the coarse wire should be positioned on the film’s inner side, while the fine wire should be on the outer side—never the other way around.

Placing the image quality indicator on the film side is also a common and convenient but incorrect practice in field inspections.

According to the standard requirements, most image quality indicators should be placed on the source side; only in a few special cases are they placed on the film side, such as in double-wall single-image projection.

When the IQI is placed on the film side, a lead letter “F” shall be appropriately positioned on the IQI as a marker. The image of the “F” marker shall appear on the radiograph simultaneously with the IQI’s own markings, and this shall be noted in the inspection report.

All content in this article is collected from the internet and is intended solely for technical exchange and sharing. If any infringement occurs, please contact us, and we will promptly remove the offending material.

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