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U.S. Food and Drug Administration

Monastic Color Reproduction: A software Tool for Printing and Assessing the Monk Skin Tone Scale

Catalog of Regulatory Science Tools to Help Assess New Medical Devices 

This regulatory science tool is a lab method developed to assist end users in printing accurate Monk skin tone (MST) scales with consumer-grade color printers.

Technical Description

This RST includes colorimetric characteristics and tolerances, a lab method and MATLAB code to support end users in printing an accurate Monk skin tone (MST) scale using consumer-grade color printers. This tool supports patient skin tone characterization in medical device assessment studies that include and use a printed MST scale.

This tool includes the following MATLAB functions for printing and assessing a Monk skin tone scale:

  1. a function to calculate the colorimetrical characteristics of the Monk skin tone scale,
  2. a function for defining the tolerances and grading each of the 10 patches in the Monk skin tone scale,
  3. a function to determine whether a color printer can reproduce the Monk skin tone scale, and
  4. a function to determine the correct RGB pixel values to accurately reproduce the Monk skin tone scale for a specific printer.

In order to apply this tool to functions 3 and 4 above, the user collects colorimetric measurement data following the protocol defined as part of this RST and inputs this data to the MATLAB code for assessment. The MATLAB code will then generate tabular results and 3D figures that can be examined interactively by the user. These tables and figures can then be used by the user to quantitative define the appropriate RGB pixel values necessary for accurately printing the Monk skin tone scale.

Intended Purpose 

The tool is intended for users who do not possess professional color management software, hardware, expertise, and services but still wish to create an accurate physical MST chart for visually judging and classifying a subject’s skin tone in a medical device clinical trial where consistent and comparable subgrouping is necessary.

Testing

The tool has been tested on different types of printers and paper media, including an inkjet printer, color laser printers, dye sublimation printers, glossy paper, matte paper, copy and machine paper. The tool was able to successfully determine whether a printer-paper combination was capable of reproducing the Monk skin tone scale and guide the user to create an accurate chart. Results show that an A-grade MST chart can be achieved with a USD $200 consumer-grade inkjet printer. This tool and the validation are described in Ref. [1].

Limitations

  • A colorimeter or spectrophotometer is needed for measuring the printed MST chart.
  • A light source, such as the CIE D65 illuminant, is needed both when measuring the properties of an initially printed MST chart with the colorimeter or spectrophotometer and when assessing a patient’s skin tone as part of a study.  The same light source assures the Monk skin tone scale are represented correctly.
  • A MATLAB license is necessary to execute the code.

Supporting Documentation

[1] Wei-Chung Cheng, "Monastic Color Reproduction: A Software Tool for Printing and Assessing the Monk Skin Tone Scale"  in Journal of Imaging Science and Technology,  2024,  pp 1 - 13,  https://doi.org/10.2352/J.ImagingSci.Technol.2024.68.5.050404

[2] Github Monastic Color Reproduction. https://github.com/DIDSR/mcr 

[3] U.S. Food and Drug Administration. Discussion Paper: Approach for Improving the Performance Evaluation of Pulse Oximeter Devices Taking Into Consideration Skin Pigmentation, Race, and Ethnicity. 2023, https://www.fda.gov/media/173905/download  

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