frudawski

# specsky

The specsky function creates spectral radiance data for a Tregenza hemisphere with 145 patches for CIE sky types 3 and 12. The function utilizes the CIE method to determine a luminance distribution and a simple method by Chain et al. to derive the Correlated Colour Temperature (CCT) from the luminance distribution. The spectral radiance is determined using the CIE method given in CIE 15 Colorimetry. The Chain et al. method was reviewed in “Forecast accuracy of existing luminance-related spectral sky models and their practical implications for the assessment of the non-image-forming effectiveness of daylight” by Diakite-Kortlever and Knoop.

Chain et al. CCT from luminance formula:

CIE sky 3

T_{cp} = 6514~\textrm{K}

CIE sky 12

T_{cp} = \frac{10^6}{-132.1 + 59.77 \cdot \log_{10}(L)}~\textrm{K}

Usage:

[spec,lam,L,Tcp,x,y,rgb] = specsky(type,sunaz,sunel,'parameter',value);

Where:

Examples

Spectral daylight distribution for CIE sky 12 at \alpha = 160° and \gamma = 30°:

[spec,lam] = specsky(12,160,30);
plotspecrange(lam,spec);

Result:

Spectral daylight distribution for CIE sky 12 at \alpha = 210° and \gamma = 20° for E_h = 15000~\textrm{lx} with additional data:

[spec,lam,L,Tcp,x,y,rgb] = specsky(12,210,30,'Eh',15000);
plottregenza(Tcp,rgb)

Peter Roy Tregenza: Subdivision of the sky hemisphere for luminance measurements. In: Lighting Research and Technology, vol. 19, no. 1, pp. 13-14, 1987, (DOI: 10.1177/096032718701900103).
Chain C, Dumortier D, Fontoynont M. A: A comprehensive model of luminance, correlated colour temperature and spectral distribution of skylight: comparison with experimental data. In: Solar Energy, vol. 65, iss. 5, pp. 285-295, 1999, ISBN: 0038-092X.
CIE 15:2018: Colorimetry, 4th Edition. Commission International de l’Éclairage (CIE), Vienna Austria, 2018, ISBN: 978-3-902842-13-8 , (DOI: 10.25039/TR.015.2018).
Aicha K. Diakite-Kortlever, Martine Knoop: Forecast accuracy of existing luminance-related spectral sky models and their practical implications for the assessment of the non-image-forming effectiveness of daylight. In: Lighting Research & Technology, vol. 53, iss. 7, pp. 657-676, 2021.