Abstract

Flash light-emitting diodes (LEDs) of modern mobile phones lack red and cyan spectral parts, however, the color gamut of their respective displays has increased in recent years. The influence of this discrepancy on the color reproduction of smart phones is investigated in this paper. Based on the CIECAM02 color appearance model, a metric is introduced to judge color reproduction of mobile phones under flash LED illumination. An evaluation method is established to compare the visual appearance of a scene under various surrounding illuminations with the reproduction of that scene. To facilitate a comparison with measurements, the evaluation method is based on the raw data of two test cameras and a Digital ColorChecker SG. To reduce the color shift between perception and reproduction, optimized flash LED spectra are presented. A single-LED and a double-LED concept with adjustable color temperature are derived from these results. Additionally, the common characteristics of flash LED spectra giving good results is investigated, identifying the spectral parts with the most influence on camera color reproduction and showing the spectral parts not contributing or even resulting in poor color reproduction. Finally the efficiency of optimized flash LED spectra is compared to standard flash LEDs.

© 2013 Optical Society of America

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References

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  1. “Technical information: xenon flash lamps,” 2005, http://sales.hamamatsu.com/assets/applications/ETD/Xe-F_TLSX9001E05.pdf .
  2. “CIE colorimetry-Part 1: standard colorimetric observers,” (2006).
  3. M. R. Pointer, “The gamut of real surface colours,” Color Res. Appl. 5, 145–155 (1980).
    [CrossRef]
  4. Digital Colorchecker SG, 2007, http://xritephoto.com/ph_product_overview.aspx?ID=938 .
  5. N. Moroney, P. J. Alessi, G. Dispoto, M. D. Fairchild, X.-F. Feng, R. W. Hunt, H. Komatsubara, C. Li, M. R. Luo, M. Mahy, T. Newman, and H. Yaguchi, “A colour appearance model for colour management systems: CIECAM02,” (International Commission on Illumination, 2004).
  6. Y. Xue, “Uniform color spaces based on CIECAM02 and IPT color difference equations,” Master’s thesis (Rochester Institute of Technology, 2008).
  7. N. Moroney and H. Zeng, “Field trials of the CIECAM02 colour appearance model,” in CIE Publ. 152:2003 Proceedings of the 25th Session of the CIE, San Diego, USA (2003).
  8. M. R. Luo, G. Cui, and C. Li, “Uniform colour spaces based on CIECAM02 colour appearance model,” Color Res. Appl. 31, 320–330 (2006).
    [CrossRef]
  9. “Multimedia systems and equipment—colour measurement and management—Part 2-1: colour management—default RGB colour space—sRGB,” , (October1999).

2006 (1)

M. R. Luo, G. Cui, and C. Li, “Uniform colour spaces based on CIECAM02 colour appearance model,” Color Res. Appl. 31, 320–330 (2006).
[CrossRef]

1980 (1)

M. R. Pointer, “The gamut of real surface colours,” Color Res. Appl. 5, 145–155 (1980).
[CrossRef]

Alessi, P. J.

N. Moroney, P. J. Alessi, G. Dispoto, M. D. Fairchild, X.-F. Feng, R. W. Hunt, H. Komatsubara, C. Li, M. R. Luo, M. Mahy, T. Newman, and H. Yaguchi, “A colour appearance model for colour management systems: CIECAM02,” (International Commission on Illumination, 2004).

Cui, G.

M. R. Luo, G. Cui, and C. Li, “Uniform colour spaces based on CIECAM02 colour appearance model,” Color Res. Appl. 31, 320–330 (2006).
[CrossRef]

Dispoto, G.

N. Moroney, P. J. Alessi, G. Dispoto, M. D. Fairchild, X.-F. Feng, R. W. Hunt, H. Komatsubara, C. Li, M. R. Luo, M. Mahy, T. Newman, and H. Yaguchi, “A colour appearance model for colour management systems: CIECAM02,” (International Commission on Illumination, 2004).

Fairchild, M. D.

N. Moroney, P. J. Alessi, G. Dispoto, M. D. Fairchild, X.-F. Feng, R. W. Hunt, H. Komatsubara, C. Li, M. R. Luo, M. Mahy, T. Newman, and H. Yaguchi, “A colour appearance model for colour management systems: CIECAM02,” (International Commission on Illumination, 2004).

Feng, X.-F.

N. Moroney, P. J. Alessi, G. Dispoto, M. D. Fairchild, X.-F. Feng, R. W. Hunt, H. Komatsubara, C. Li, M. R. Luo, M. Mahy, T. Newman, and H. Yaguchi, “A colour appearance model for colour management systems: CIECAM02,” (International Commission on Illumination, 2004).

Hunt, R. W.

N. Moroney, P. J. Alessi, G. Dispoto, M. D. Fairchild, X.-F. Feng, R. W. Hunt, H. Komatsubara, C. Li, M. R. Luo, M. Mahy, T. Newman, and H. Yaguchi, “A colour appearance model for colour management systems: CIECAM02,” (International Commission on Illumination, 2004).

Komatsubara, H.

N. Moroney, P. J. Alessi, G. Dispoto, M. D. Fairchild, X.-F. Feng, R. W. Hunt, H. Komatsubara, C. Li, M. R. Luo, M. Mahy, T. Newman, and H. Yaguchi, “A colour appearance model for colour management systems: CIECAM02,” (International Commission on Illumination, 2004).

Li, C.

M. R. Luo, G. Cui, and C. Li, “Uniform colour spaces based on CIECAM02 colour appearance model,” Color Res. Appl. 31, 320–330 (2006).
[CrossRef]

N. Moroney, P. J. Alessi, G. Dispoto, M. D. Fairchild, X.-F. Feng, R. W. Hunt, H. Komatsubara, C. Li, M. R. Luo, M. Mahy, T. Newman, and H. Yaguchi, “A colour appearance model for colour management systems: CIECAM02,” (International Commission on Illumination, 2004).

Luo, M. R.

M. R. Luo, G. Cui, and C. Li, “Uniform colour spaces based on CIECAM02 colour appearance model,” Color Res. Appl. 31, 320–330 (2006).
[CrossRef]

N. Moroney, P. J. Alessi, G. Dispoto, M. D. Fairchild, X.-F. Feng, R. W. Hunt, H. Komatsubara, C. Li, M. R. Luo, M. Mahy, T. Newman, and H. Yaguchi, “A colour appearance model for colour management systems: CIECAM02,” (International Commission on Illumination, 2004).

Mahy, M.

N. Moroney, P. J. Alessi, G. Dispoto, M. D. Fairchild, X.-F. Feng, R. W. Hunt, H. Komatsubara, C. Li, M. R. Luo, M. Mahy, T. Newman, and H. Yaguchi, “A colour appearance model for colour management systems: CIECAM02,” (International Commission on Illumination, 2004).

Moroney, N.

N. Moroney, P. J. Alessi, G. Dispoto, M. D. Fairchild, X.-F. Feng, R. W. Hunt, H. Komatsubara, C. Li, M. R. Luo, M. Mahy, T. Newman, and H. Yaguchi, “A colour appearance model for colour management systems: CIECAM02,” (International Commission on Illumination, 2004).

N. Moroney and H. Zeng, “Field trials of the CIECAM02 colour appearance model,” in CIE Publ. 152:2003 Proceedings of the 25th Session of the CIE, San Diego, USA (2003).

Newman, T.

N. Moroney, P. J. Alessi, G. Dispoto, M. D. Fairchild, X.-F. Feng, R. W. Hunt, H. Komatsubara, C. Li, M. R. Luo, M. Mahy, T. Newman, and H. Yaguchi, “A colour appearance model for colour management systems: CIECAM02,” (International Commission on Illumination, 2004).

Pointer, M. R.

M. R. Pointer, “The gamut of real surface colours,” Color Res. Appl. 5, 145–155 (1980).
[CrossRef]

Xue, Y.

Y. Xue, “Uniform color spaces based on CIECAM02 and IPT color difference equations,” Master’s thesis (Rochester Institute of Technology, 2008).

Yaguchi, H.

N. Moroney, P. J. Alessi, G. Dispoto, M. D. Fairchild, X.-F. Feng, R. W. Hunt, H. Komatsubara, C. Li, M. R. Luo, M. Mahy, T. Newman, and H. Yaguchi, “A colour appearance model for colour management systems: CIECAM02,” (International Commission on Illumination, 2004).

Zeng, H.

N. Moroney and H. Zeng, “Field trials of the CIECAM02 colour appearance model,” in CIE Publ. 152:2003 Proceedings of the 25th Session of the CIE, San Diego, USA (2003).

Color Res. Appl. (2)

M. R. Pointer, “The gamut of real surface colours,” Color Res. Appl. 5, 145–155 (1980).
[CrossRef]

M. R. Luo, G. Cui, and C. Li, “Uniform colour spaces based on CIECAM02 colour appearance model,” Color Res. Appl. 31, 320–330 (2006).
[CrossRef]

Other (7)

“Multimedia systems and equipment—colour measurement and management—Part 2-1: colour management—default RGB colour space—sRGB,” , (October1999).

“Technical information: xenon flash lamps,” 2005, http://sales.hamamatsu.com/assets/applications/ETD/Xe-F_TLSX9001E05.pdf .

“CIE colorimetry-Part 1: standard colorimetric observers,” (2006).

Digital Colorchecker SG, 2007, http://xritephoto.com/ph_product_overview.aspx?ID=938 .

N. Moroney, P. J. Alessi, G. Dispoto, M. D. Fairchild, X.-F. Feng, R. W. Hunt, H. Komatsubara, C. Li, M. R. Luo, M. Mahy, T. Newman, and H. Yaguchi, “A colour appearance model for colour management systems: CIECAM02,” (International Commission on Illumination, 2004).

Y. Xue, “Uniform color spaces based on CIECAM02 and IPT color difference equations,” Master’s thesis (Rochester Institute of Technology, 2008).

N. Moroney and H. Zeng, “Field trials of the CIECAM02 colour appearance model,” in CIE Publ. 152:2003 Proceedings of the 25th Session of the CIE, San Diego, USA (2003).

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Figures (26)

Fig. 1.
Fig. 1.

Illumination used in mobile phone flash photography. Typical LED flash and xenon flash spectrum [1].

Fig. 2.
Fig. 2.

Measured camera color filters (RGB) compared to the color matching functions (xyz) [2].

Fig. 3.
Fig. 3.

Method to evaluate optimal flash spectra. The direct perception of a scene (top left frame) under ambient light, is compared to the reproduction on an ideal monitor (right frame), created by a camera measurement (bottom left frame) under flash illumination of that scene. The CIE color appearance model (CIECAM02) is used to take account of the different chromatic adaptation.

Fig. 4.
Fig. 4.

Surface colors measured by Pointer [3] under various illuminations and the color coordinates of the Digital ColorChecker SG und D65, shown in the CIE Diagram from 1931.

Fig. 5.
Fig. 5.

Noise illustration of the raw data of two test cameras. Color patches red (G4) and yellow (H4) of the Digital ColorChecker SG in their respective camera color space for test camera 1.

Fig. 6.
Fig. 6.

Noise illustration of the raw data of two test cameras. Color patches red (G4) and yellow (H4) of the Digital ColorChecker SG in their respective camera color space for test camera 2.

Fig. 7.
Fig. 7.

Examples of post processing. Result of the processed raw data of G4 and H4 of mobile phone 1.

Fig. 8.
Fig. 8.

Examples of post processing. Contrast enhancement algorithm of the gray patch of mobile phone 2 (F4).

Fig. 9.
Fig. 9.

Example of a white balance calculation shown in CAM02-UCS. Test colors reproduced by the camera are red points, and directly seen test colors are black points. The color shift of some test colors (black circles) has been reduced by optimizing the matrix.

Fig. 10.
Fig. 10.

Evaluation method used to judge good color reproduction. The LED spectrum generator creates 800 different but realistic LED spectra with the same color coordinates. Depending on a set of test colors and the camera color filters the perceived color difference is calculated, taking various ambient illuminations into account.

Fig. 11.
Fig. 11.

Color difference mCD for scenes illuminated by 800 different LED spectra with identical D55 white point under different Tamb for ramb=1.

Fig. 12.
Fig. 12.

Color difference mCD for scenes illuminated by D55 LEDs under different ratios ramb for Tamb=5000K.

Fig. 13.
Fig. 13.

Mean color difference m¯CD for scenes illuminated by LEDs with distinct color temperatures and averaged over various Tamb and ramb.

Fig. 14.
Fig. 14.

Standard and optimized flash LED spectrum with a color temperature of 6500 K.

Fig. 15.
Fig. 15.

Definition of the color coordinates for the double-LED concept. Resulting spectra for the best combination of a mixture of 2800 and 8500 K LEDs.

Fig. 16.
Fig. 16.

How the color coordinates for the two-LED concept are defined. Spectra of two mixed LEDs.

Fig. 17.
Fig. 17.

Relative spectral intensity in the range from 420 to 460 nm (Q420460) for 2800 K.

Fig. 18.
Fig. 18.

Relative spectral intensity in the range from 420 to 460 nm (Q420460) for 5000 K.

Fig. 19.
Fig. 19.

Relative spectral intensity in the range from 420 to 460 nm (Q420460) for 8500 K.

Fig. 20.
Fig. 20.

Relative spectral intensity in the range from 510 to 570 nm (Q510570) for 2800 K.

Fig. 21.
Fig. 21.

Relative spectral intensity in the range from 510 to 570 nm (Q510570) for 5000 K.

Fig. 22.
Fig. 22.

Relative spectral intensity in the range from 510 to 570 nm (Q510570) for 8500 K.

Fig. 23.
Fig. 23.

Relative spectral intensity in the range from 580 to 620 nm (Q580620) for 2800 K.

Fig. 24.
Fig. 24.

Relative spectral intensity in the range from 580 to 620 nm (Q580620) for 5000 K.

Fig. 25.
Fig. 25.

Relative spectral intensity in the range from 580 to 620 nm (Q580620) for 8500 K.

Fig. 26.
Fig. 26.

Measured camera color filters (RGB) compared to the color matching functions (xyz) [2] and the found boundaries.

Tables (3)

Tables Icon

Table 1. Color Differences for Spectra in Fig. 16 with No Ambient Light Added (ramb)

Tables Icon

Table 2. Relative Spectral Intensity of Good Spectra with CCT for Test Camera 1

Tables Icon

Table 3. Relative Spectral Intensity of Good Spectra with CCT for Test Camera 2

Equations (8)

Equations on this page are rendered with MathJax. Learn more.

ΔEucs=ΔJucs2+Δaucs2+Δbucs2.
{XYZ}n,direct={x¯(λ)y¯(λ)z¯(λ)}·qe,n(λ)·Ee,amb(λ)dλ,
{RGB}n,raw={r(λ)g(λ)b(λ)}·qe,n(λ)·(Ee,amb(λ)+Ee,flash(λ))dλ.
minM|(XYZ)directM3×10·(RGBR2G2B2RGGBRB1)rawT|.
mCD=n=184ΔEucs.
mCD*=16n=16mCD(Tflash).
Qlu=λlλusflash(λ)dλ380780sflash(λ)dλ,
PsensPel=sflash(λ)·(r(λ)+2·g(λ)+b(λ))dλ4·sflash(λ)dλ·ηwp,

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