Comparative Colorimetric Simulation and Evaluation of Digital Cameras Using Spectroscopy Data (original) (raw)
Testing and optimizing the spectral response of digital cameras
Imaging Science journal, 2005
A colorimetric analysis and design method for cameras is presented which enables a camera to be used as an imaging colorimeter, capable of capturing an accurate record of the Commission Internationale de l'Éclairage (CIE) coordinates of the imaged objects. The spectral response characteristics of some of the component observers in the 1964 CIE Standard Observer average are first analysed and compared. CIE colour coordinates are calculated using both the Standard Observer average and individual observer CMFs for each of 32 test colours. The personal coordinates for each observer are then linked to differences in their visual response CMFs at the spectral level. A colorimetric testing and optimization method for cameras is then reported, which allows the colour capture properties of imaging systems to be analysed, controlled and modified. The method is based on imaging spectrally defined colour charts and is used to demonstrate that even minor differences in the chosen RGB filter/sensor characteristics substantially affect the numeric accuracy of the colour-defining information in images. Colour imaging error is quantified and minimized, first by establishing the camera RGB to CIE XYZ relationship, and then by adjusting the spectral response of the sensors. Under controlled lighting and exposure conditions, the design method enables output pixel-colour definitions that are a close analogue of the measured CIE XYZ tristimulus values for the imaged surface colours.
Practical camera characterization for colour measurement
The Imaging Science Journal, 2001
This paper reviews the major issues involved in the use of digital cameras to derive the CIE X, Y and Z tristimulus values of objects in real scenes. Both practical and theoretical investigations have been carried out to gain experience in this specialized field of imaging that is finding application in, for example, machine vision, product quality assessment by panel observation and digital archiving of art objects. The practicalities of camera characterization described include: lighting-spectral power and uniformity; test target-choice and number of colours; camera signal processing-linear or gamma corrected; colour analysis-filter transmittance and infrared filtration; characterization method-linear or higher order; quality measure-CIELAB, CMC, CIE94 colour difference; quality statistic-mean, median, etc. The potential impact of these parameters is discussed by the use of a computer model and from practical experience. It is shown, as might be expected, that the choice of colour separation filtration is the most sensitive variable. If a 'colour' camera is used, then it needs to be carefully selected; a more adaptable choice, however, may be a monochrome camera with external filters. In addition, the illumination uniformity of the test target used for characterization is shown to be important: that it is never perfectly uniform must be considered in the characterization process. With careful selection of system components, a median value of less than 1.0 CIELAB colour difference can be obtained between independently measured colorimetry and that calculated from the camera output.
Characterization of a digital camera as an absolute tristimulus colorimeter
2003
An algorithm is proposed for the spectral and colorimetric characterization of digital still cameras (DSC) which allows them to be used as tele-colorimeters with CIE-XYZ color output, in cd/m 2 . The spectral characterization consists in the calculation of the color-matching functions from the previously measured spectral sensitivities. The colorimetric characterization consists in transforming the raw RGB digital data into absolute tristimulus values CIE-XYZ (in cd/m 2 ) under variable and unknown spectroradiometric conditions. Thus, in the first stage, a gray balance was applied over the raw RGB digital data to convert them into RGB relative colorimetric values. In the second stage, an algorithm of luminance adaptation versus lens aperture was inserted in the basic colorimetric profile. Capturing the ColorChecker chart under different light sources, and comparing the estimated XYZ data according to the developed color model in relation to the measured XYZ data (in cd/m 2 ) using a telespectroradiometer, we verified that the proposed characterization model may be broken down into two portions. Firstly, there is the basic colorimetric profile in combination with the new luminance adaptation algorithm. Secondly, there is the linear correction term due only to the mismatch of the color matching functions of the camera. Although the linear color correction term works relatively well, despite the imposed initial conditions (unknown spectral content of the scene), the separation of the proposed characterization model into two portions (raw and corrected performance) would allow the future comparison of various commercial cameras.
A strategy toward spectral and colorimetric color reproduction using ordinary digital cameras
Color Research & Application, 2018
In this work, a methodology is introduced to use ordinary digital RGB cameras for the purpose of spectral and colorimetric color reproduction. First, it is attempted to recover the spectral reflectance from RGB camera response using different approaches, among which, it is shown that weighted nonlinear regression as performed better than other approaches. After analyzing the results, it is realized that although spectrally the results are satisfactory, there is still a significant colorimetric error left, that should be addressed. Therefore, in the second part of the article, different linear and nonlinear matrix transforms are used to change the RGB camera response to CIEXYZ tristimulus values under a specific condition. It is concluded that colorimetric error of the recovery can be reduced significantly when a separate path is used for colorimetric color reproduction.
Estimation of the device gamut of a digital camera in raw performance using optimal color-stimuli
Using a spectroradiometric model of capture for a digital camera based on the mathematical description of the empirical opto-electronic conversion spectral functions (OECSF), the capture of MacAdam or optimal spectra with fixed illumination level is simulated. This model of capture allows to change freely the f-number of the zoom-lens and/or the photosite integration time of the electronic shutter of the camera, regardless of the spectral composition of the stimulus. If we follow the procedure employed by MacAdam in 1935 working with the CIE-1931 XYZ standard observer, these color-stimuli are arranged in decreasing pyramidal form as the luminance factor increases for any chromaticity diagram (CIE-xy, UCS-u'v' or CIE-L*a*b*). These loci are often called MacAdam limits or Rösch color solid. On the other hand, transforming the simulated RGB digital output levels of the optimal colors to XYZ data through the raw colorimetric profile with luminance adaptation of our digital image capture device, the corresponding MacAdam loci for each luminance factor are smaller than those of the colorimetric standard observer. This systematic desaturation of the optimal color-stimuli shows that our color device, in raw performance, desaturates in general the real color-stimuli, so this result justifies the additional use in digital photography of color correction algorithms, more or less complex, in order to reach the colorimetric status of color reproduction.
Camera characterization for color research
Color Research & Application, 2002
We introduce a new method for estimating camera sensitivity functions from spectral power input and camera response data. We also show how the procedure can be extended to deal with camera non-linearities. Linearization is an important part of camera characterization and we argue that it is best to jointly fit the linearization and the sensor response functions. We compare our method with a number of others, both on synthetic data and for the characterization of a real camera. All data used in this study is available on-line at http://www.cs.sfu.ca/\~colour/data.
Imaging colorimetry using a digital camera
In this work, we investigate the use of a digital camera for colorimetry. Our system consists of a measurement device and a corresponding calibration mapping. The goal is to design a system that will accurately assess the color of a sample. We develop two colorimetry systems by applying model-based and regression-based techniques. For both systems, the measurement device is formed by a digital camera and a set of filters. The term multi-exposure refers to the multiple snapshots taken by the camera along with filters. The calibration mapping which consists of matrices then takes these filtered camera RGB outputs, and returns the CIE XYZ tristimulus values under several pre-selected illumination conditions. For the model-based system, a model for the measurement device is employed; and our objective is to find the optimal filters and the corresponding calibration sets that minimize a cost function which accounts for errors in L*a*b* space, system robustness, and filter smoothness. For the regression-based system, no modeling technique is applied to the measurement device. The objective is simply to find the optimal calibration matrices that minimize the total least squared errors of a given color set in CIE XYZ coordinates under several pre-selected illumination conditions. We apply both types of colorimetry systems to two specific tasks: general purpose measurement of color samples and colorimetry of human teeth. We present experimental results for both applications. Finally, in order to measure the parameters for these systems and evaluate their performance, we had to develop special instrumentation. We will briefly describe this effort as well.
Concerning the calculation of the color gamut in a digital camera
Color Research & Application, 2006
Several methods to determine the color gamut of any digital camera are shown. Since an input device is additive, its color triangle was obtained from their spectral sensitivities and it was compared with the theoretical sensors of Ives-Abney-Yule and MacAdam. On the other hand, the RGB digital data of the optimal or MacAdam colors were simulated to transform them into XYZ data according to the colorimetric profile of the digital camera. From this, the MacAdam limits associated to the digital camera are compared with the corresponding ones of the CIE-1931 XYZ standard observer, resulting that our color device has much smaller MacAdam loci than those of the colorimetric standard observer. Taking this into account, we have estimated the reduction of discernible colors by the digital camera applying a chromatic discrimination model and a packing algorithm to obtain color discrimination ellipses. Calculating the relative decrement of distinguishable colors by the digital camera in comparison with the colorimetric standard observer at different luminance factors of the optimal colors, we have found that the camera distinguishes considerably fewer very dark than very light ones, but relatively much more colors with middle lightness (Y between 40 and 70, or L* between 69.5 and 87.0). This behavior is due to the short dynamic range of the digital camera response.
Digital Camera Characterization for Color Measurements
The use of spectrophotometers for color measurements on printed substrates is widely spread among paper producers as well as within the printing industry. Spectrophotometer measurements are precise, but timeconsuming procedures and faster methods are desirable. Colorimetrically calibrated flatbed scanners have been proved to provide a fast and fairly accurate alternative to spectrophotometers. Moreover, the rapid development of digital cameras has made it possible to transfer successfully implemented methods for color calibration of flatbed scanners to a camera-based system.
Color measurements with a consumer digital camera using spectral estimation techniques
Image Analysis, 2005
The use of spectrophotometers for color measurements on printed substrates is widely spread among paper producers as well as within the printing industry. Spectrophotometer measurements are precise, but timeconsuming procedures and faster methods are desirable. The rapid development of digital cameras has opened the possibility to use consumer digital cameras as substitutes for spectrophotometers for certain applications such as production control. Two methods for estimating the reflectance properties of objects from camera RGB measurements using linear estimation techniques combined with linear and non-linear constraints are presented. In the experiments, we have investigated if these techniques can be used to measure the reflectance properties of flat objects such as printed pages of paper. Reflectances were converted to CIELAB color values, and the minimization of color errors were evaluated with CIE color difference formulas. Our experiments show that a consumer digital camera can be used as a fast and inexpensive alternative to spectrophotometers for color measurements on printed substrates.
On Accurate Color Calibration for Digital Cameras
Journal of Imaging Science and Technology, 2014
Conversion from raw camera coordinates to a standard color space is developed here as a direct product of two independent maps, one for chromaticity coordinates of hue and saturation, the other for brightness. The principal focus is on the former. While the main approach is a target-based mapping, comparison with a more conventional approach shows that the structure of chromaticity for point reflection spectra at the capture gamut boundary influences the map far into the interior. Failure to account properly for this systematic effect is the largest source of error in standard color mappings. Restriction to chromatic mappings defines a class of corresponding chromatic metamers. Failure of a camera to satisfy the Luther condition produces two distinct classes of chromatic metamer errors: essential and adventitious.
Application of a colour reproduction index to digital camera characterization
The Imaging Science Journal, 2001
This paper examines the use of a colour reproduction index as a quality metric for the characterization of ten digital cameras in an attempt to achieve colorimetrically exact reproduction. This essentially extends the use of the index further into the world of digital imaging. The successful application of an observer response model to data linking acceptability of colour prints with the colour reproduction index is demonstrated, and a linear relation between the index and the median value of CIELAB colour difference is established for standard lighting conditions common to both image acquisition and viewing stages. This defines the quartile values of acceptability in terms of the index scale. A camera rendering index, based on the concepts of the CIE colour rendering index, is also shown to give values that are linearly related to the median value of CIELAB colour difference, again under identical illumination conditions.
IEICE Transactions on Information and Systems, 2010
Acquisition of accurate colors is important in the modern era of widespread exchange of electronic multimedia. The variety of device-dependent color spaces causes troubles with accurate color reproduction. In this paper we present the outlines of accomplished digital camera system with device-independent output formed from tristimulus XYZ values. The outstanding accuracy and fidelity of acquired color is achieved in our system by employing an embedded color calibration system based on emissive device generating reference calibration colors with user-defined spectral distribution and chromaticity coordinates. The system was tested by calibrating the camera using 24 reference colors spectrally reproduced from 24 color patches of the Macbeth Chart. The average color difference (CIEDE2000) has been found to be ΔE = 0.83, which is an outstanding result compared to commercially available digital cameras.
Determining colour gamuts of digital cameras and scanners
Color Research & Application, 2002
The determination of the colour gamuts of colour reproduction media is an important aspect both of understanding them in isolation and of using them in the context of a colour reproduction system. While this is well understood for output colour reproduction media, a solution for input media is not to be had in a simple way. To this end, the present paper proposes a method which is based on simulating the responses of an input medium to given spectral power distributions. The gamut of an input medium is then determined on the basis of having a set of spectra that cover the majority of all possible spectra, knowing a medium's quantised responses to them and then determining a boundary beyond which the medium does not produce variation in its responses.
Total Colorimetric Evaluation of a Set of Color Sensors for a Variety of Illuminants and Objects
Optical Review, 2001
Recently, a new colorimetric evaluation model for a set of color sensors was proposed by the author (N. Shimano: J. Inst. Image Electron. Eng. Jpn. 29 (2000) 506 and 517.), and the quality was shown to be defined by the ratio of the statistical mean energy of color stimuli at the retina (SMECS) to the captured energy of the SMECS by the sensors. It was also shown that the quality depends on the illuminants and objects. The experimental results agreed quite well with the proposed model. However, an extension of the model is needed to evaluate the full colorimetric quality of an image acquisition device for a variety of illuminants and objects. In this paper the extended model called the total colorimetric quality model and experimental results for sensors under such a variety are presented.
Choice of Colour Space in Illuminance Measurement with a Low-cost Digital Camera
ICSIPA, 2015
Poor colour rendering characteristic of a low cost camera results in poor colour recognition, especially with images taken under non-white illumination. A simplified colour calibration process for a digital web cam to be used for indoor illuminance measurement, enabling automatic, real-time, simultaneous measurement of illuminance of an indoor space at multi-locations based on its video image is proposed. Lab test results on 12 colours yielded 88% accuracy, which can be further improved in future work
Today, many attempts have been made to use the digital color cameras for color measurement applications. The gold of this work is a comparison between the measure of CMCCON02 color inconstancy index obtained by the spectrophotometric data and the data from a characterized digital color camera. According to the results, the square of correlation coefficient of 0.8 was obtained between the color inconstancy indices acquired of the spectrophotometric data and the camera RGB data. The attained differences between two methods could be decreased by improving camera characterization process. Besides, it is observed the spectral power distribution of the applied illuminant lead to the important effects on the results of colorimetric characterization of the camera.