Skylight Color Shifts due to Variations of Urban-Industrial Aerosol Properties: Observer Color Difference Sensitivity Compared to a Digital Camera

Abstract

Simulations of the sky dome color shifts of a cloudless sky have been performed, assuming three urban-industrial tropospheric aerosol cases. Each of these aerosols is represented by the bimodal lognormal volume distribution (Dubovik et al. 2002). A total of ten parameters are used to control the aerosol characteristics. These ten parameters are perturbed and the resulting sky color shifts are estimated. All simulations have been performed relative to the reference cases that represent the time-averaged state of each aerosol case. We have simulated the sensitivity of the theoretical CIE (Commission Internationale de l′ Eclairage) standard human observer to discriminate sky color changes, due to changes of the aerosol control parameters. A Digital color Camera Model (DCM) has also been incorporated in the simulation scheme. The DCM simulates the performance of a camera-colorimeter created specifically to measure sky color. The DCM has been used to investigate whether a color digital camera can replicate the behavior of the CIE standard observer, with respect to sky color shifts. The standard observer is most sensitive to perturbations of the parameters of the aerosol fine mode, which are particles with diameters less than 0.6 μ m. However his sensitivity is highly variable, depending on the parameter varied, on the aerosol case and on the direction of the observation. The DCM was in excellent agreement with the behavior of the CIE standard observer. The camera simulated sky color measurement accuracy was high. The results show that the camera sensitivity in discriminating color differences is much better than that of the observer.

Acknowledgments

This work was supported by funding from EUCAARI (European Integrated project on Aerosol Cloud Climate and Air Quality Interactions) Contract number: 36833.

Notes

¹In this article the “standard human observer” is an observer with normal color vision that is compatible with the CIE (Commission Internationale de l'Eclairage) standards of color especially when referring to the Just Noticeable Difference Concept (JND). CIE is the primary organization responsible for standardization of color metrics and terminology (Sharma 2003).

^aAerosol average optical thickness values at 440 nm used to estimate base cases (Dubovik et al. 2002).

^bBoth fine and coarse modes of the base cases are considered to have the same n and k.

^aSlope m, is calculated by m = (∑ _{i = 1} ⁿ O _i P _i − 1/n∑ _{i = 1} ⁿ O ⁱ ∑ _{i = 1} ⁿ P _i)/(∑ _{i = 1} ⁿ O _i P _i − 1/n∑ _{i = 1 n} O _i ∑ _{i = 1} ⁿ P _i ) (∑ _{i = 1} ⁿ O _i 2 − 1/n (∑ _{i = 1} ⁿ O _i )2). (∑ _{i = 1} ⁿ O _i ² − 1/n(∑ _{i = 1} ⁿ O _i ²)), O _i are the estimated CIE-94 color differences based on the camera measurement while P _i are the theoretically predicted color differences. N is the population of O _i or P _i.

^bCorrelation coefficient ρ is calculated by ρ = m s _o /s _o s _p . sp, s _o and s_p being the standard deviations of O _i and P _i , respectively.