frudawski

getskytype

The getskytype function determines the CIE general standard sky type for a given luminance distribution of a Tregenza hemisphere. The function can operate in two different modes: The ‘tregenza’ mode (Tregenza 2004) where the whole hemisphere is consideredand and the ‘kobav’ mode (Kobav et al. 2012) which only considers certain parts of the hemisphere. The Tregenza method is more accurate for full data sets but the Kobav et al. method is better suited for incomplete data sets. Both determination methods are available as indipended functions and can be used directly.

See also: tregenzaskytype, kobavskytype

[sky,RMS] = getskytype(L,sunaz,sunel,mode,parameter)

Where:

ParameterDescription
skyReturns the best fitting CIE standard general sky as in CIE S 011.
RMSReturns the Root Mean Square deviation from the best fitting CIE standard general sky. Both methods use different approaches, hence the RMS values are not comparable between the determination methods. In case of ‘kobav’ method the mean RMS from indicatrix and gradation determination is returned.
LDefines the Tregenza hemisphere luminance distribution.
sunazDefines the sun azimuth angle \alpha_S.
sunelDefines the sun elevation angle \gamma_S.
mode
(optional)
Defines the determination mode:
’tregenza’ (default) uses the determination method described in Tregenza 2004.
’kobav’ uses the determination method described in Kobav et al. 2012.
parameter
(optional)
Both determination methods allow addidtional parameters:
‘tregenza’ method
‘center’ (default) uses the patch center points for reference luminance determination.
‘mean’ uses the mean of the patch corner points (interpolated) for reference luminance determination.
‘kobav’ method:
‘cie’ (default) returns the CIE skytype according to the CIE table. Not all gradation and indicatrix group combinations result in a CIE skytype, resulting in NaN value.
‘nearest’ returns the closest match to the CIE skytype table.

Examples

Determine skytype at \alpha_S = 166\degree and \gamma_S = 33\degree with ‘tregenza’ method:

L = ciesky(12,166,33);
[sky,RMS] = getskytype(L,166,33,'tregenza')

See also: ciesky

Result:

sky = 12
RMS = 0

Determine skytype at \alpha_S = 166\degree and \gamma_S = 33\degree with ‘tregenza’ and ‘mean’ method:

L = ciesky(12,166,33);
[sky,RMS] = getskytype(L,166,33,'tregenza','mean')

See also: ciesky

Result:

sky = 12
RMS = 0.2285

Note: the differences to reference sky modell in terms of RMS are due to the different determination method ‘mean’.

Determine skytype at \alpha_S = 166\degree and \gamma_S = 33\degree with ‘kobav’ method:

L = ciesky(12,166,33);
[sky,RMS] = getskytype(L,166,33,'kobav')

See also: ciesky

Result:

sky = 12
RMS = 0.019045

References

ISO 15469:2004(E)/CIE S 011/E:2003: Spatial Distribution of Daylight - CIE Standard General Sky. Commission Internationale de l'Éclairage (CIE), Vienna Austria, 2004.

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).

Peter Roy Tregenza: Analysing sky luminance scans to obtain frequency distributions of CIE Standard General Skies. In: Lighting Research and Technology, vol. 36, no. 4, pp. 271–281, 2004, (DOI: 10.1191/1477153504li117oa).

Kobav M, Bizjak G, Dumortier D.: Characterization of sky scanner measurements based on CIE and ISO standard CIE S 011/2003. In: Lighting Research & Technology, vol. 45, no. 4, pp. 504–512, 2012, (DOI: 10.1177/1477153512458916).

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