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.

See also: ciesky

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:

ParameterDescription
specReturns the spectral power distribution of a Tregenza hemisphere with 145 patches, row-wise
lamSpecifies the wavelengths, 360:5:830
LReturns the according luminance distribution of the Tregenza hemisphere
x and yReturns the according chromaticity coordinates x and y
rgbReturns a colour matrix 145 \times 3
typeDefines the CIE sky type 1 – 16. Multiple sky types can be submitted as vector.
sunazIs the sun azimuth angle in ° (0 – 360). The azimuth angle starts in north position and goes clockwise.
sunelIs the sun eleavation angle in ° (0–90).
'target'Specifies the target unit (optional)
’Lz’ for zenith luminance
’Eh’ for horizontal illuminance (default)
target valueAbsolute value of specified target: horizontal illuminance E_{\text{h}} or zenith luminance L_{\text{Z}}, default: Eh = 10000
'mode'Enables to specifies the luminance determination mode. (optional)
mode value’center’ for luminance detmerination for the patch center
’mean’ for mean luminance of the 4 patchcorner luminances as suggested by
Tregenza

Examples

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

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

See also: plotspecrange

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)

See also: plottregenza

Result:

References

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.

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