Reconstruction of Daylight Spectral Power Distribution Based on Correlated Color Temperature: A Comparative Study between the CIE Approach and Localized Procedures in Assessing Non-image Forming Effects

Figures & data

Fig. 1. Schematic representation of the CIE (2018b) reconstruction procedure approach to reconstructing daylight spectral power distribution ($S\left(\lambda \right)$) as a function of either chromaticity coordinates ($x,y$) or correlated color temperature ($T_{cp}$).

Table 1. Compilation of the reconstruction methods of daylight

Publication	Location(s)	Measuring procedure	Length of record	Number of SPDs	Daylight locus	$S_i\left(\lambda \right)$^a with $\left|i\right|$ equal
Judd et al. (1964) CIE (2018b)	Ottawa, Canada ^Budde, unpublished (Henderson 1977)	Daylight and skylight, northern sky (global spectral irradiance)	Mar – Apr 1963	99	y = −3.000x^2+ 2.870x – 0.275	3
	Rochester, USA ^Condit and Grum (1964)	Daylight (global spectral irradiance)	Jun-Jul 1962	249
	Enfield, UK ^Henderson and Hodgkiss (1963)	Daylight and skylight, northern sky (global spectral irradiance)	Apr 1961 – Mar 1962	274
Sastri and Das (1968)	New Delhi, India	Skylight, northern sky (global spectral irradiance)	1964– 1965	187	n/a	3
Dixon (1978)	Coburg, Australia	Daylight (global spectral irradiance)	Sep 1975 – Mar 1976	113	y = 2,2210x^2 − 0.4888x + 0.2553	5
	Bendigo, Australia	Skylight (global spectral irradiance)	Oct 1976	123	y = −0.96000x^2+ 1.58500x – 0.08054	4
Kobayashi et al. (1996)	Atsugi, Japan	Skylight, northern sky (spectral irradiance E(e, λ, 45°)	Jan -Mar 1991	220	y = −1.8500x^2+ 2.1010x – 0.1520	3
Kobayashi et al. (1997)	Nagaoka, Japan	Skylight, northern sky (spectral irradiance E(e, λ, 35, 55°)	Nov 1990 – Dec 1996	847	y = −1.469x^2+ 1.887x – 0.120	3
Hernández-Andrés et al. (2001a)	Granada, Spain	Skylight (spectral irradiance E(e, λ, 3°)	Feb – Aug 1998	1567	y = −2.77935x^2+ 2.72203x – 0.24770	3 or 6
Hernández-Andrés et al. (2001b)	Granada, Spain	Daylight (global spectral irradiance on horizontal plane)	Feb 1996 – Feb 1998	2600	y = −1.09234x^2+ 1.55320x – 0.05188	3 or 6
Chain (2004)	Vaulx-en-Velin, France	Skylight: diffuse, spectral irradiance E(e, λ, 1°)	n/a	7732	y = −3.2877x^2+ 2.9668x – 0.27683	3 or 5
	Vaulx-en-Velin, France	Skylight: diffuse, spectral irradiance E(e, λ, 1°)	n/a	7732	y = 781.4x^4– 910.05x^3+ 391.62x^2– 72.718x + 5.1294	3 or 5
Rusnák (2014)	Bratislava, Slovakia	Skylight: diffuse, spectral irradiance E(e, λ, 11°)	2013	12,905	y = −2.576x^2+ 2.401x −0.166	3
	Bratislava, Slovakia	Global spectral irradiance	2013	89	y = −2.220x^2+ 2.166x – 0.132	3
Diakite et al. (2018)	Berlin, Germany	Skylight: diffuse, spectral irradiance E(e, λ, 10°)	Jan – Dec 2016 for modeling and Jan – Dec 2015 for validation	23,085 (2016) and 22,419 (2015)	y = −1.2848x^2+ 1.7519x – 0.0937	3

^a$S_i\left(\lambda \right)$: components, functions of wavelength,$\lambda$

Table 2. Summary of the CIE and locally adjusted reconstruction approaches

	CIE	BLN1	BLN2
$x_D$	$x_{D,CIE}$ from CIE (2018b), Equations(4) $x_{D,CIE}=\frac{-4,6070\cdot {10}^9}{{T_{cp}}^3}+\frac{2,9678\cdot {10}^6}{{T_{cp}}^2}+\frac{0,09911\cdot {10}^3}{T_{cp}}+0,244063$(4) (4(4) $x_{D,CIE}=\frac{-4,6070\cdot {10}^9}{{T_{cp}}^3}+\frac{2,9678\cdot {10}^6}{{T_{cp}}^2}+\frac{0,09911\cdot {10}^3}{T_{cp}}+0,244063$(4) ) and (5(5) $x_{D,CIE}=\frac{-2,0064\cdot {10}^9}{{T_{cp}}^3}+\frac{1,9018\cdot {10}^6}{{T_{cp}}^2}+\frac{0,24748\cdot {10}^3}{T_{cp}}+0,237040$(5) )	$x_{D,BLN}$ from Equation(10) $x_{D,BLN}=\frac{-4.5036\cdot {10}^9}{{T_{cp}}^3}+\frac{2.5225\cdot {10}^6}{{T_{cp}}^2}+\frac{0.22336\cdot {10}^3}{T_{cp}}+0.23535$(10) (10(10) $x_{D,BLN}=\frac{-4.5036\cdot {10}^9}{{T_{cp}}^3}+\frac{2.5225\cdot {10}^6}{{T_{cp}}^2}+\frac{0.22336\cdot {10}^3}{T_{cp}}+0.23535$(10) )	$x_{D,BLN}$ from Equation(10) $x_{D,BLN}=\frac{-4.5036\cdot {10}^9}{{T_{cp}}^3}+\frac{2.5225\cdot {10}^6}{{T_{cp}}^2}+\frac{0.22336\cdot {10}^3}{T_{cp}}+0.23535$(10) (10(10) $x_{D,BLN}=\frac{-4.5036\cdot {10}^9}{{T_{cp}}^3}+\frac{2.5225\cdot {10}^6}{{T_{cp}}^2}+\frac{0.22336\cdot {10}^3}{T_{cp}}+0.23535$(10) )
Daylight locus $y_D$	$y_{D,CIE}$ from CIE (2018b), Equation(6) $y_{D,CIE}=-3.000\cdot {x_D}^2+2.870\cdot x_D-0.275$(6) (6(6) $y_{D,CIE}=-3.000\cdot {x_D}^2+2.870\cdot x_D-0.275$(6) )	$y_{D,BLN}$ from Equation(11) ${\mathrm{y}}_{\mathrm{D},\mathrm{B}\mathrm{L}\mathrm{N}}=-2.0795x_{\mathrm{D},\mathrm{B}\mathrm{L}\mathrm{N}}^2+2.2376x_{\mathrm{D},\mathrm{B}\mathrm{L}\mathrm{N}}-0.16974$(11) (11(11) ${\mathrm{y}}_{\mathrm{D},\mathrm{B}\mathrm{L}\mathrm{N}}=-2.0795x_{\mathrm{D},\mathrm{B}\mathrm{L}\mathrm{N}}^2+2.2376x_{\mathrm{D},\mathrm{B}\mathrm{L}\mathrm{N}}-0.16974$(11) )	$y_{D,BLN}$ from Equation(11) ${\mathrm{y}}_{\mathrm{D},\mathrm{B}\mathrm{L}\mathrm{N}}=-2.0795x_{\mathrm{D},\mathrm{B}\mathrm{L}\mathrm{N}}^2+2.2376x_{\mathrm{D},\mathrm{B}\mathrm{L}\mathrm{N}}-0.16974$(11) (11(11) ${\mathrm{y}}_{\mathrm{D},\mathrm{B}\mathrm{L}\mathrm{N}}=-2.0795x_{\mathrm{D},\mathrm{B}\mathrm{L}\mathrm{N}}^2+2.2376x_{\mathrm{D},\mathrm{B}\mathrm{L}\mathrm{N}}-0.16974$(11) )
Components $S_0$, $S_1$, $S_2$	$S_{0,CIE}$, $S_{1,CIE}$, $S_{2,CIE}$ from CIE (2018b), Equation(7) $S_{CIE}\left(\lambda \right)=S_{0,CIE}\left(\lambda \right)+M_{1,CIE}\cdot S_{1,CIE}\left(\lambda \right)+M_{2,CIE}\cdot S_{2,CIE}\left(\lambda \right)$(7) (7(7) $S_{CIE}\left(\lambda \right)=S_{0,CIE}\left(\lambda \right)+M_{1,CIE}\cdot S_{1,CIE}\left(\lambda \right)+M_{2,CIE}\cdot S_{2,CIE}\left(\lambda \right)$(7) )	$S_{0,CIE}$, $S_{1,CIE}$, $S_{2,CIE}$ from CIE (2018b), Equation(7) $S_{CIE}\left(\lambda \right)=S_{0,CIE}\left(\lambda \right)+M_{1,CIE}\cdot S_{1,CIE}\left(\lambda \right)+M_{2,CIE}\cdot S_{2,CIE}\left(\lambda \right)$(7) (7(7) $S_{CIE}\left(\lambda \right)=S_{0,CIE}\left(\lambda \right)+M_{1,CIE}\cdot S_{1,CIE}\left(\lambda \right)+M_{2,CIE}\cdot S_{2,CIE}\left(\lambda \right)$(7) )	$S_{0,BLN}$, $S_{1,BLN}$, $S_{2,BLN}$ from Equation(12) $S_{BLN}\left(\lambda \right)=S_{0,BLN}\left(\lambda \right)+M_{1,BLN}\cdot S_{1,BLN}\left(\lambda \right)+M_{2,BLN}\cdot S_{2,BLN}\left(\lambda \right)$(12) (12(11) ${\mathrm{y}}_{\mathrm{D},\mathrm{B}\mathrm{L}\mathrm{N}}=-2.0795x_{\mathrm{D},\mathrm{B}\mathrm{L}\mathrm{N}}^2+2.2376x_{\mathrm{D},\mathrm{B}\mathrm{L}\mathrm{N}}-0.16974$(11) )
Factors $M_1$ and $M_2$	$M_{1,CIE}$ and $M_{2,CIE}$ from CIE (2018b), Equations(8) $M_{1,CIE}=\frac{-1,3515-1,7703x_D+5,9114y_D}{0,0241+0,2562x_D-0,7341y_D}$(8) (8(8) $M_{1,CIE}=\frac{-1,3515-1,7703x_D+5,9114y_D}{0,0241+0,2562x_D-0,7341y_D}$(8) and 9(9) $M_{2,CIE}=\frac{0,0300-31,4424x_D+30,0717y_D}{0,0241+0,2562x_D-0,7341y_D}$(9) )	$M_{1,CIE}$ and $M_{2,CIE}$ from CIE (2018b), Equations(8) $M_{1,CIE}=\frac{-1,3515-1,7703x_D+5,9114y_D}{0,0241+0,2562x_D-0,7341y_D}$(8) (8(8) $M_{1,CIE}=\frac{-1,3515-1,7703x_D+5,9114y_D}{0,0241+0,2562x_D-0,7341y_D}$(8) and 9(9) $M_{2,CIE}=\frac{0,0300-31,4424x_D+30,0717y_D}{0,0241+0,2562x_D-0,7341y_D}$(9) )	$M_{1,BLN}$ and $M_{2,BLN}$ from Equations$M_{1,BLN}=\frac{5.0767-13.8483\cdot x_D-1.9943\cdot y_D}{0.0157+0.0469\cdot x_D-0.3791\cdot y_D}(13)$ (13$M_{1,BLN}=\frac{5.0767-13.8483\cdot x_D-1.9943\cdot y_D}{0.0157+0.0469\cdot x_D-0.3791\cdot y_D}(13)$ and 14$M_{2,BLN}=\frac{1.3617+61.3011\cdot x_D-62.6447\cdot y_D}{0.0157+0.0469\cdot x_D-0.3791\cdot y_D}(14)$)

Table 3. Percentage of colorimetrically “accurate” ($GFC\ge 0.9950$), “good” ($GFC\ge 0.9990$), and “exact” ($GFC\ge 0.9999$) reconstructions depending on the reconstruction method

Classification	$GFC$	CIE	BLN1	BLN2
Accurate	$GFC\ge 0.9950$	95.97%	96.44%	99.72%
Good	$GFC\ge 0.9990$	0.004%	0.01%	67.74%
Exact	$GFC\ge 0.9999$	-	-	2.58%

Table 4. $\mathrm{M}\mathrm{e}\mathrm{a}\mathrm{n}\left(\mathrm{G}\mathrm{F}\mathrm{C}\right)$ values for three wavelength ranges and the complete wavelength range depending on the selected reconstruction method, color coded as follows: $GFC<0.9950$, $GFC\ge 0.9950$, $GFC\ge 0.9990$, $GFC\ge 0.9999$

Wavelength range	CIE method	BLN1 method	BLN2 method
$380nm\le \lambda \le 780nm$	0.99699	0.99714	0.99911
$380nm\le \lambda \le 490nm$	0.99806	0.99812	0.99975
$490nm<\lambda \le 585nm$	0.99970	0.99972	0.99988
$585nm<\lambda \le 780nm$	0.99389	0.99411	0.99780

Table 5. Mean (GFC) for sky conditions

Sky condition	CIE method	BLN1 method	BLN2 method
Overcast sky	0.99676	0.99692	0.99918
Intermediate sky	0.99708	0.99721	0.99915
Clear sky	0.99733	0.99743	0.99899

Table 6. $\mathrm{M}\mathrm{e}\mathrm{a}\mathrm{n}\left(\mathrm{G}\mathrm{F}\mathrm{C}\right)$ for summer and winter measurements

Season	CIE method	BLN1 method	BLN2 method
Summer (Jun, Jul, Aug)	0.99675	0.99689	0.99910
Winter (Dec, Jan, Feb)	0.99703	0.99726	0.99864

Fig. 2. Daylight measuring site at Technische Universität Berlin.

Fig. 3. Chromaticities of daylight in Berlin in the CIE 1931 $x,y$ chromaticity diagram compared to the CIE daylight locus. The diagram on the left shows the modeling data set from the 597,998 SPD measurements gathered in 2016 (left); the one on the right shows the validation data set from the 538,205 SPD measurements gathered in 2015 according to the procedure laid out by the CIE (2018b), limiting the $x_D$ from 0.25 to 0.38 (right).

Fig. 4. Sky patches as defined in Tregenza (1987, 2004) and the CIE (1994). Patches shaded in gray were used for the accuracy analysis and sensitivity study; the patches used to examine atmospheric influences are circled in black.

Fig. 5. Diagram of the approach to deriving the Berlin (BLN) daylight chromaticities $x_{D,BLN},y_{D,BLN}.$

Fig. 6. Non-linear relation between chromaticities of daylight in Berlin (left) and the correlation between the daylight chromaticity coordinate $x_D$ and the inverse correlated color temperature $T_{cp}$ (MK⁻¹) (right).

Fig. 7. Loci of the chromaticities of daylight obtained for different sites along with the CIE daylight locus and the Berlin daylight locus: Granada, Spain (Hernández-Andrés et al. 2001a, 2001b) (upper left); Vaulx-en-Velin, France (Chain 2004) (upper right); Bratislava, Slovakia (Rusnák 2014) (middle left); Berlin, Germany (Diakite et al. 2018) (middle right); Coburg and Bendigo, Australia (Dixon 1978) (lower left); Atsugi and Nagaoka, Japan (Kobayashi et al. 1997, 1996) (lower right).

Fig. 8. Diagram of the procedure of deriving the Berlin components and Berlin weighting factors for SPD reconstruction.

Fig. 9. Components of the daylight distribution S₀(λ), S₁(λ), S₂(λ), CIE (left); Berlin (right).

Fig. 10. Diagram of the reconstruction procedure.

Fig. 11. Diagram of the approach to determine the Goodness-of-Fit Coefficient ($GFC$) of the CIE reconstruction procedure and locally adjusted procedures (BLN₁ and BLN₂).

Table 7. Mean absolute percentage error $\mathrm{M}\mathrm{A}\mathrm{P}{\mathrm{E}}_{\alpha }$ [%] for α-opic quantities

$\mathrm{M}\mathrm{A}\mathrm{P}{\mathrm{E}}_{\alpha }$	Melanopic			S-cone-opic			M-cone-opic			L-cone-opic			Rhodopic
$T_{cp}$ range	CIE	BLN1	BLN2	CIE	BLN1	BLN2	CIE	BLN1	BLN2	CIE	BLN1	BLN2	CIE	BLN1	BLN2
4000 $<T_{cp}\le$ 5000	0.7	0.5	0.3	1.6	2.8	2.9	0.1	0.1	0.1	0.1	0.1	0.1	0.4	0.3	0.2
5000 $<T_{cp}\le$ 6667	0.5	0.4	0.4	1.2	0.6	0.7	0.1	0.1	0.1	0.1	0.1	0.1	0.3	0.3	0.3
6667 $<T_{cp}\le$ 10,000	0.4	0.3	0.3	1.1	0.8	0.9	0.2	0.2	0.2	0.1	0.1	0.1	0.3	0.3	0.3
10,000$<T_{cp}\le$ 12,500	0.3	0.3	0.3	0.7	0.7	0.7	0.2	0.2	0.2	0.1	0.1	0.1	0.3	0.3	0.3
12,500 $<T_{cp}\le$ 16,667	0.4	0.4	0.4	0.6	0.5	0.5	0.3	0.2	0.2	0.2	0.1	0.1	0.4	0.4	0.4
16,667 $<T_{cp}\le$ 25,000	0.5	0.5	0.4	0.6	0.6	0.5	0.3	0.3	0.2	0.3	0.1	0.1	0.4	0.5	0.4
25,000 $<T_{cp}\le$ 50,000	0.6	0.4	0.6	0.5	1.0	0.7	0.3	0.2	0.2	0.3	0.1	0.1	0.4	0.4	0.5
50,000 $<T_{cp}\le$ 1,000,000	0.5	0.3	1.1	0.4	1.7	1.4	0.3	0.2	0.2	0.4	0.1	0.1	0.3	0.3	1.0

Table 8. Root mean square percentage error $\mathrm{R}\mathrm{M}\mathrm{S}\mathrm{P}{\mathrm{E}}_{\alpha }$ [%] for α-opic quantities

$\mathrm{R}\mathrm{M}\mathrm{S}\mathrm{P}{\mathrm{E}}_{\alpha }$	Melanopic			S-cone-opic			M-cone-opic			L-cone-opic			Rhodopic
$T_{cp}$ range	CIE	BLN1	BLN2	CIE	BLN1	BLN2	CIE	BLN1	BLN2	CIE	BLN1	BLN2	CIE	BLN1	BLN2
4000 $<T_{cp}\le$ 5000	0.8	0.7	0.4	2.3	3.2	3.2	0.1	0.1	0.1	0.1	0.1	0.1	0.5	0.4	0.3
5000 $<T_{cp}\le$ 6667	0.6	0.5	0.5	1.3	0.9	0.9	0.1	0.1	0.1	0.1	0.1	0.1	0.4	0.3	0.3
6667 $<T_{cp}\le$ 10,000	0.5	0.4	0.4	1.3	1.1	1.2	0.2	0.2	0.2	0.1	0.1	0.1	0.4	0.4	0.4
10,000 $<T_{cp}\le$ 12,500	0.4	0.4	0.4	0.9	0.9	0.9	0.3	0.3	0.3	0.2	0.1	0.1	0.4	0.4	0.4
12,500 $<T_{cp}\le$ 16,667	0.5	0.5	0.5	0.8	0.7	0.7	0.3	0.3	0.3	0.2	0.1	0.1	0.4	0.5	0.5
16,667 $<T_{cp}\le$ 25,000	0.6	0.6	0.6	0.8	0.7	0.6	0.4	0.3	0.3	0.3	0.1	0.1	0.5	0.5	0.5
25,000 $<T_{cp}\le$ 50,000	0.7	0.5	0.7	0.6	1.0	0.8	0.3	0.3	0.2	0.4	0.1	0.1	0.5	0.5	0.7
50,000 $<T_{cp}\le$ 1,000,000	0.6	0.4	1.2	0.5	1.7	1.4	0.4	0.2	0.3	0.4	0.1	0.1	0.4	0.4	1.0

Table 9. Mean percentage error $\mathrm{M}\mathrm{P}{\mathrm{E}}_{\alpha }$ [%] for the α-opic quantities

$\mathrm{M}\mathrm{P}{\mathrm{E}}_{\alpha }$	Melanopic			S-cone-opic			M-cone-opic			L-cone-opic			Rhodopic
$T_{cp}$ range	CIE	BLN1	BLN2	CIE	BLN1	BLN2	CIE	BLN1	BLN2	CIE	BLN1	BLN2	CIE	BLN1	BLN2
4000 $<T_{cp}\le$ 5000	0.7	−0.5	−0.1	1.0	−2,4	−2,5	0.1	0.0	0.1	0.1	−0.1	−0.1	0.4	−0.3	0.1
5000$<T_{cp}\le$ 6667	0.4	−0.1	−0.2	1.1	−0.5	−0.5	0.0	0.1	0.0	0.1	0.0	−0.1	0.2	0.0	0.0
6667 $<T_{cp}\le$ 10,000	0.3	0.1	0.0	0.5	−0.3	−0.4	0.0	0.0	0.0	0.0	0.0	0.0	0.2	0.1	0.1
10,000 $<T_{cp}\le$ 12,500	−0.1	−0.1	0.1	−0.4	−0.3	−0.4	0.1	0.0	0.0	−0.1	−0.1	0.0	0.0	−0.1	0.1
12,500 $<T_{cp}\le$ 16,667	−0.2	−0.2	0.2	−0.5	0.0	−0.1	0.2	−0.1	0.0	−0.2	0.0	0.0	−0.1	−0.2	0.2
16,667 $<T_{cp}\le$ 25,000	−0.4	−0.3	0.3	−0.5	0.4	0.2	0.2	−0.1	0.0	−0.3	0.0	0.0	−0.2	−0.3	0.3
25,000 $<T_{cp}\le$ 50,000	−0.5	−0.2	0.6	−0.5	0.9	0.7	0.3	−0.1	0.1	−0.3	0.0	0.0	−0.3	−0.2	0.5
50,000 $<T_{cp}\le$ 1,000,000	−0.5	0.2	1.1	−0.4	1.7	1.4	0.3	0.0	0.2	−0.4	0.0	0.1	−0.2	0.1	1.0

Fig. 12. Diagram of the approaches for the sensitivity study.

Fig. 13. Normalized measured spectral irradiance (-, gray, $S_{Me}\left(\lambda \right)$) with the best achieved $max\left(\mathrm{G}\mathrm{F}{\mathrm{C}}_{\mathrm{B}\mathrm{L}\mathrm{N}2}\right)=0.99998$ conducted on 10 September 2015, at 14:58:13 CEST, with a solar altitude of 37.17°, for sky patch 114, with a $T_{cp}$ equal to 6016 K (overcast sky) compared to the corresponding reconstructions performed using: the CIE method (-, $S_{CIE,Re}\left(\lambda \right)$); the BLN₁ method, using $x_{D,BLN},y_{D,BLN}$(--,$S_{BLN1,Re}\left(\lambda \right)$); and the BLN₂ method, using $x_{D,BLN},y_{D,BLN}$ and components of the daylight distribution $S_{0,BLN}$, $S_{1,BLN}$, and $S_{2,BLN}$ (:, $S_{BLN2,Re}\left(\lambda \right)$).

Fig. 14. Goodness-of-fit of the reconstructions as expressed by $GFC\ge 0.9950$, as a function of the inverse correlated color temperature $T_{cp}$. Upper left, calculated using the CIE method; upper right, the BLN₁ method, using $x_{D,BLN},y_{D,BLN}$; and lower left, the BLN₂ method, using $x_{D,BLN},y_{D,BLN}$ and the components $S_{0,BLN}$, $S_{1,BLN}$, and $S_{2,BLN}$. The numerical data of the percentages of colorimetrically accurate reconstructions can be found in the supplemental material.

Fig. 15. Percentage of colorimetrically “accurate” ($GFC\ge 0.9950$) reconstructions according to sky conditions for each of the three methods. Upper left, the CIE method; upper right, the BLN₁ method ($x_{D,BLN},y_{D,BLN}$); and lower left, the BLN₂ method ($x_{D,BLN},y_{D,BLN}$ and components $S_{0,BLN}$, $S_{1,BLN}$, and $S_{2,BLN}$). The values above the bars indicate the percentage of colorimetrically “accurate” ($GFC\ge 0.9950$) reconstructions.

Fig. 16. Percentage of colorimetrically “accurate” ($GFC\ge 0.9950$) reconstructions according to seven sky patches (north and zenith direction, nos. 45, 73, 96, 118, 132, 142, 145). Upper left, the CIE method; upper right, the BLN₁ method (using $x_{D,BLN},y_{D,BLN}$); and lower left, the BLN₂ method (using $x_{D,BLN},y_{D,BLN}$ and components $S_{0,BLN}$, $S_{1,BLN}$, and $S_{2,BLN}$). The values above the bars indicate the percentage of colorimetrically “accurate” ($GFC\ge 0.9950$) reconstructions.

Fig. 17. An overall (left) and detailed (right) display of a normalized measured spectral irradiance (-, gray, $S_{Me}\left(\lambda \right)$) retrieved on 30 March 2015, at 17:29:01 CEST, with a solar altitude of 18.09°, for sky patch 75 with $T_{cp}=76,294K$ as well as the corresponding reconstructions performed using the CIE method (-, $S_{CIE,Re}\left(\lambda \right)$); the BLN₁ method, using $x_{D,BLN},y_{D,BLN}$ (--,$S_{BLN1,Re}\left(\lambda \right)$); and the BLN₂ method, using $x_{D,BLN},y_{D,BLN}$ and components of the daylight distribution $S_{0,BLN}$, $S_{1,BLN}$, and $S_{2,BLN}$ (:, $S_{BLN2,Re}\left(\lambda \right)$).

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