Alliance for Lighting Information

Rayleigh Scatter Index (RSI) Data for Sources

by David M. Keith

Rayleigh Scatter Index (RSI) is a simple metric that provides a first estimate for the reflectance of the atmosphere under clear sky conditions.

The RSI evaluation at each wavelength can be summed over the entire spectrum or used as SPDs of the radiation "reflected" by the sky. In all cases the resulting values are only suitable for comparisons and the absolute values should not be considered as having merit in themselves, because the constant used in these RSI calculations is chosen strictly for convenience and has not other numerical basis. However, using the analysis basis of "all other things being equal", by starting with "equal amounts" and treating them all the same, the values do have merit for comparisons.

The following tables shows the evaluations for different sources' SPDs as photopic lumens (Pho), as scotopic lumens (Sco) and as melanoptic units (mel) and then the RSI summation overall, followed by the results of the "reflected SPDs" evaluated again with photopic (RSIph) or scotopic (RSIsc) or melanoptic (RSImel) sensitivity functions. The sources are also described in colormetric terms here.

Source				Pho	Sco	Mel	RSI     RSIph   RSIsc   RSImel

Equal Energy			100	235	0.14	4.4	5.4	19.6	1.5
Blackbody 6500K (sunlight)	100	255	0.16	4.6	5.5	21.4	1.7
CIE D65 (daylight)		100	257	0.15	3.8	5.5	21.4	1.6
CIE Illum A			100	147	0.07	2.3	4.9	10.9	0.6
HPS	lowest among 4 SPDs	100	 66	0.02	1.2	4.4	 4.5	0.2
	highest among 4 SPDs	100	 70	0.03	1.2	4.5	 4.8	0.2
MH	lowest among 4 SPDs	100	155	0.08	2.3	5.0	11.9	0.8
	highest among 4 SPDs	100	184	0.10	2.5	5.2	14.4	1.0
Fluorescent 3000K		100	131	0.06	1.7	4.9	 9.5	0.5
Fluorescent 3500K		100	151	0.07	1.8	5.0	11.4	0.7
Fluorescent 4100K		100	169	0.08	2.0	5.1	13.0	0.8
Fluorescent 6500K		100	227	0.12	2.5	5.4	18.6	1.2

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