ASTM E3168 - 20

    Standard Practice for Determining Low-Contrast Visual Acuity of Radiographic Interpreters

    Active Standard ASTM E3168 | Developed by Subcommittee: E07.02

    Book of Standards Volume: 03.04


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    Significance and Use

    4.1 This practice is used to evaluate the ability of a radiographic interpreter to discriminate low contrast slit images in a radiographic interpretation environment. A radiographic viewer, as described in Specification E1390, and a viewing environment, as described in Guide E94, are strongly recommended. The minimum acceptable test score in any given application depends on the requirements of the application. Using parties should develop and maintain records of their test results to guide the establishment of acceptable test scores for their applications. (See Note 1.)

    Note 1: During round robin testing with experienced radiographic interpreters, 76 % of the interpreters achieved a score of 85 % or higher, and 95 % achieved a score of 80 % or higher. The average score was 90.7 %, and the standard deviation was 6.7 %. In a second study from 2017, with both certified radiographers and uncertified personnel, the average and standard deviation among certified radiographers was 90.4 ± 4.0 % and among uncertified personnel was 88.4 ± 4.9 %. It was found that on each test page there are 3 or 4 images where the average score for each was less than 80 % correct and the remainder of the images all individually scored greater than 80 % on average. A limited number of the general public was examined, and the average score among these was 75.0 ± 3.3 %.

    4.2 Administration of the Test 

    4.2.1 The test procedure described in this practice is intended to determine the ability of a radiographic interpreter to detect low contrast images in a low light level environment. Appropriate dark adaptation time should be permitted. A minimum of 1 min is recommended; however, longer dark adaptation times may be required by some users.

    4.2.2 The test should be administered by a test administrator. The individual being tested shall not know the identification of the plate or orientation prior to the test.

    4.2.3 The interpretation of each of the 25 image areas on a plate is recorded on an answer sheet, Fig. 2, by drawing a line corresponding to the location and orientation of the slit image in that image area. Where no line image is detected, a circle should be drawn on the answer sheet in the area corresponding to the image area in which no slit image was detected. An example score sheet is given in Fig. 3, illustrating typical line locations and orientations and illustrating the method for marking answers. The markings shown in the sample score sheet are not taken from any of the actual test plates; however, they illustrate typical distributions of slit images. Fig. 2 of this standard may be photocopied to provide answer sheets, or the using organization may generate their own suitable answer sheet. In any case, the answer sheet must have provisions for recording both the location and orientation of the indication in each of the 25 image locations.

    FIG. 2 Visual Acuity Test Score Sheet

    Visual Acuity Test Score Sheet Visual Acuity Test Score Sheet

    FIG. 3 Example of Completed Visual Acuity Test Score Sheet

    Example of Completed Visual Acuity Test Score SheetExample of Completed Visual Acuity Test Score Sheet

    4.2.4 The order in which the indications are marked is not important. The reader may mark the indications in order, or may mark the easier images and return to the more difficult images.

    4.2.5 Once the score sheet is completed, the test administrator shall determine the identity and orientation of the plate that was read and score the answers using the appropriate answer key.

    1. Scope

    1.1 This practice details the procedure for determining the low-contrast visual acuity of a radiographic interpreter by evaluating the ability of the individual to detect linear images of varying radiographic noise, contrast, and sharpness. No statement is made regarding the applicability of these images to evaluate the competence of a radiographic interpreter. There is no correlation between these images of slit phantoms and the ability to detect cracks or other linear features in an actual radiographic examination. The test procedure follows from work performed by the National Institute of Standards and Technology presented in NBS Technical Note 1143, issued June 1981.

    1.2 The visual acuity test set consists of five individual plates, each containing a series of radiographic images of 0.5 in. (12.7 mm) long slits in thin metal shims. The original radiographs used to prepare the illustrations were generated using various absorbers, geometric parameters (unsharpness, slit widths), and source parameters (kV, mA, time) to produce images of varying noise, contrast, and sharpness. Each radiographic image has a background density of 1.8 ± 0.15. The images are viewed in a radiographic interpretation environment as used for the evaluation of production radiographic films, for example, illuminators and background lighting as described in Guide E94 and Specification E1390, and without optical magnification.

    1.3 Each visual acuity test plate consists of 25 individual image areas. The images are arranged in 5 rows and 5 columns as shown in Fig. 1. Each image area is 2 in. x 2 in. (51 mm x 51 mm). All identification is on the back side of the plate. Each plate can be viewed from any of the four orientations (that is, it can be viewed with any of the four edges “up” on the illuminator). Since there are five different plates in the set, this makes for a total of 20 different patterns that can be viewed. The identification of which of the five plates and which of the four orientations were viewed in any given test can be determined from the designation on the back side.

    FIG. 1 Layout of Visual Acuity Test Plate

    Layout of Visual Acuity Test PlateLayout of Visual Acuity Test Plate

    1.4 Within the image areas, the slit image may appear in any of five locations, that is, in any of the four corners of the image area, or near the center. No more than one slit image will appear in any one image area. The slit image may be horizontal, vertical, slant left, or slant right. Several of the plates include one or more image areas in which there is no slit image.

    1.5 Use of this standard requires procurement of the adjunct test plates.

    1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

    1.7 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.


    2. Referenced Documents (purchase separately) The documents listed below are referenced within the subject standard but are not provided as part of the standard.

    ASTM Standards

    E94 Guide for Radiographic Examination Using Industrial Radiographic Film

    E1316 Terminology for Nondestructive Examinations

    E1390 Specification for Illuminators Used for Viewing Industrial Radiographs


    ICS Code

    ICS Number Code 19.100 (Non-destructive testing)

    Referencing This Standard
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    DOI: 10.1520/E3168-20

    Citation Format

    ASTM E3168-20, Standard Practice for Determining Low-Contrast Visual Acuity of Radiographic Interpreters, ASTM International, West Conshohocken, PA, 2020, www.astm.org

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