RGB Calibration Research Articles

Minimum error adaptive RGB calibration in a context of colorimetric uncertainty for cultural heritage preservation

In this paper, two aims are sought. First, the path to a minimum-error colour calibration and its feasibility for non-invasive cultural heritage conservation is explored. Using RGB information as temporal cues of the conservation state of the pieces under monitoring, an adaptive calibration function, working in syntony with a colour calibration chart destined to general usage, has been proposed. Since the context involves citizen participation, the pictures to calibrate are crowdsourced and the cameras of origin unknown; therefore, the requirements of the work context assume images that imply a heavy colorimetric uncertainty whilst maintaining their structural content. Therefore,a differentiable, multidimensional, adaptive transfer curve that offers the best trade-off between calibration error (staying in minimum perceivable colour differences for tangible materials) and calculation requirements is refined. The conception of this system offers flexible possibilities of an effective cultural heritage conservation without heavy costs on money and human effort or relying on external solutions, while standing on par in quality with state-of-art technologies, achieving therefore a balance between generalist usage and specific solutions. Secondly, a neural network-based final process performs the final tuning of the calibration functions, and its conclusions shed light on how a calibration process works together with the colour chart on an algebraic level. The verification of this phenomenon opens possibilities of different paths on how to proceed with colour calibrations depending on the conditions of acquisition and the content of interest, so the minimum error can be always achieved regardless of the case. The ideas presented here are an expansion and conclusion of the developments and results previously published by the same authors.

A novel color calibration method of multi-spectral camera based on normalized RGB color model

True-color image of Mars is important for mineral analysis. The working light source of the Mars exploration multi-spectral camera is different from the laboratory calibration light source, so that the commonly used RGB calibration method has a large color difference in correcting the on-orbit images. After analyzing the influence of light source lightness and relative spectral distribution on the camera output values, their influences are considered separately in the calibration. A new color calibration method in the luminance-independent chromaticity space is proposed. The RGB color components of multi-spectral camera are normalized and converted to chromaticity values, and the polynomial calibration model in chromaticity space is compared with the RGB method. The experimental results showed that the average value of CIE DE2000 color differences after color calibration was 4.32, which was 0.96 less than that of RGB method. When the calibration matrix was used to correct the outdoor scene images, the corrected color difference gain because of the change of imaging light source was reduced by 0.54 compared to the RGB method. The color calibration method and the calibration data in this paper provide theoretical support for the color correction of the Mars multi-spectral camera on-orbit images.

SU‐E‐T‐746: The Use of Radiochromic Film Analyzed with Three Channel Dosimetry as a Secondary Patient‐Specific QA Tool for Small SBRT Fields

Introduction:As diagnostic techniques become more sensitive and targeting methods grow in accuracy, target volumes continue to shrink and SBRT becomes more prevalent. Due to this fact, patient‐specific QA must also enhance resolution and accuracy in order to verify dose delivery in these volumes. It has been suggested that when measuring small fields at least two separate detectors be used to verify delivered dose. Therefore, we have instituted a secondary patient QA verification for small (<3cm) SBRT fields using Gafchromic EBT2 film.Methods:Films were cross‐calibrated using a Farmer chamber in plastic water at reference conditions as defined by TG‐51. Films were scanned, and an RGB calibration curve was created according to best practices published by Ashland, Inc. Four SBRT cases were evaluated both with the Scandidos Delta4 and with EBT2 films sandwiched in plastic water. Raw values obtained from the film were converted to dose using an in‐house algorithm employing all three color channels to increase accuracy and dosimetric range. Gamma and dose profile comparisons to Eclipse dose calculations were obtained using RIT and compared to values obtained with the Delta4.Results:Film gamma pass rates at 2% and 2mm were similar to those obtained with the Delta4. However, dose difference histograms showed better absolute dose agreement, with the average mean film dose agreeing with calculation to 0.3% and the Delta4 only agreeing to 3.1% across the cases. Additionally, films provided more resolution than the Delta4 and thus their dose profiles better succeeded in diagnosing dose calculation inaccuracies.Conclusion:We believe that the implementation of secondary patient QA using EBT2 film analyzed with all three color channels is an invaluable tool for evaluation of small SBRT fields. Furthermore, we have shown that this method can sometimes provide a more detailed and faithful reproduction of plan dose than the Delta4.

Novel calibration and color adaptation schemes in three-fringe RGB photoelasticity

Isochromatic demodulation in digital photoelasticity using RGB calibration is a two step process. The first step involves the construction of a look-up table (LUT) from a calibration experiment. In the second step, isochromatic data is demodulated by matching the colors of an analysis image with the colors existing in the LUT. As actual test and calibration experiment tint conditions vary due to different sources, color adaptation techniques for modifying an existing primary LUT are employed. However, the primary LUT is still generated from bending experiments. In this paper, RGB demodulation based on a theoretically constructed LUT has been attempted to exploit the advantages of color adaptation schemes. Thereby, the experimental mode of LUT generation and some uncertainties therein can be minimized. Additionally, a new color adaptation algorithm is proposed using quadratic Lagrangian interpolation polynomials, which is numerically better than the two-point linear interpolations available in the literature. The new calibration and color adaptation schemes are validated and applied to demodulate fringe orders in live models and stress frozen slices.

Efficacy of photoelasticity in developing whole-field imaging sensors

The paper investigates the efficacy of developing a whole-field imaging sensor based on the principles of photoelasticity. A flatbed sensor has been designed that produces colored fringes under white light when load is applied. These fringes can be analyzed using conventional photoelastic techniques; however, for whole-field sensing applications the loading has to be on the plane rather than the conventional in-plane photoelasticity. This requires some new strategies to be developed to analyze the out of plane loading situations. Further, for such applications low modulus materials are to be used for appreciable in-plane deformation under vertical load and for pronounced photoelastic effect. The paper discusses efficacy of both RGB calibration and phase shifting techniques in sensing applications and proposes a neural network based approach in an expert system environment that can extract load information from photoelastic images in real time. The characteristics of fringe patterns obtained under vertical and shear forces have been studied and the results obtained under these conditions are discussed with their limitations. Finally, a case study has been conducted to analyze a foot image and conclusions drawn from this have been reported together with future research directions.

Calibrating the Yule–Nielsen Modified Spectral Neugebauer Model with Ink Spreading Curves Derived from Digitized RGB Calibration Patch Images

The Yule–Nielsen modified spectral Neugebauer model (YNSN) enhanced for accounting for ink spreading in the different ink superposition conditions requires a spectrophotometer to measure the reflectances of halftone calibration patches in order to compute the ink spreading curves mapping nominal ink surface coverage to effective ink surface coverage. Instead, as a first step towards the "on the fly characterization of printers, we try to deduce the ink spreading curves from digitized RGB images of halftone calibration patches. By applying the Yule–Nielsen broadband formula to the average RGB intensities, we deduce the effective dot surface coverages of halftone calibration patches. We then establish the ink spreading curves mapping nominal dot surface coverages to effective dot surface coverages. By weighting the contributions of the different ink spreading curves according to the ratios of colorant surface coverages, we predict according to the YNSN model the reflectance spectra of the test patches. We compare the prediction accuracy of the YNSN model calibrated by digitized RGB intensities of the calibration patches with the prediction accuracy of the same model calibrated by the spectral reflectances of these calibration patches. The decrease in spectral prediction accuracy due to RGB model calibration is about 30% for ink jet prints and 10% for thermal transfer prints.

Isochromatic Demodulation by Fringe Scanning

Abstract: RGB calibration is the fastest isochromatic demodulation technique, as it does not require unwrapping as in the case of phase‐shifting method. The technique is based on constructing a look up table (LUT) of fringe order against RGB triplet from digital images and decoding the test image from the LUT. Research has shown that the technique, though very fast, is limited to fringe orders up to three with the conventional white light. The colours tend to merge beyond that and make it difficult to obtain a unique value of RGB triplet. Changes in fringe gradient caused by stretching/bunching of fringes in test model further add to errors. Special light sources with narrow‐band spectral response are required with fringe tracking algorithms to demodulate higher fringe orders. The calibration technique is also sensitive to geometric and chromatic variations. This paper presents a cost‐effective alternative solution to conventional RGB technique using a flatbed scanner. The system is capable of demodulating higher fringe order and incorporates information from other colour spaces. It does not require separate light sources and cameras, and is found to be insensitive to geometric and chromatic variations. Curve fitting technique has been proposed to determine accurate fringe orders.

Improved method for isochromatic demodulation by RGB calibration.

The red-blue-green (RGB) calibration technique consists in constructing an a priori calibration table of the isochromatic retardation versus the triplet of RGB values obtained with a RGB CCD camera. In this way a lookup table (LUT) is built in which the entry is the corresponding RGB triplet and the output is the given retardation. This calibration (a radiometric quantity) depends on the geometric and chromatic parameters of the setup. Once the calibration is performed, the isochromatic retardation at a given point of the sample is computed as the one that minimizes the Euclidean distance between the measured RGB triplet and the triplets stored in the LUT. We present an enhanced RGB calibration algorithm for isochromatic fringe pattern demodulation. We have improved the standard demodulation algorithm used in RGB calibration by changing the Euclidean cost function to a regularized one in which the fidelity term corresponds to the Euclidean distance between RGB triplets; the regularizing term forces piecewise continuity for the isochromatic retardation. Additionally we have implemented a selective search in the RGB calibration LUT. We have tested the algorithm with simulated as well as real photoelastic data with good results.

RGB calibration for color image analysis in machine vision

A color calibration method for correcting the variations in RGB color values caused by vision system components was developed and tested in this study. The calibration scheme concentrated on comprehensively estimating and removing the RGB errors without specifying error sources and their effects. The algorithm for color calibration was based upon the use of a standardized color chart and developed as a preprocessing tool for color image analysis. According to the theory of image formation, RGB errors in color images were categorized into multiplicative and additive errors. Multiplicative and additive errors contained various error sources-gray-level shift, a variation in amplification and quantization in camera electronics or frame grabber, the change of color temperature of illumination with time, and related factors. The RGB errors of arbitrary colors in an image were estimated from the RGB errors of standard colors contained in the image. The color calibration method also contained an algorithm for correcting the nonuniformity of illumination in the scene. The algorithm was tested under two different conditions-uniform and nonuniform illumination in the scene. The RGB errors of arbitrary colors in test images were almost completely removed after color calibration. The maximum residual error was seven gray levels under uniform illumination and 12 gray levels under nonuniform illumination. Most residual RGB errors were caused by residual nonuniformity of illumination in images, The test results showed that the developed method was effective in correcting the variations in RGB color values caused by vision system components.