TM-24 Adjustments to Illuminance Criteria

This task or application qualifies for TM-24 consideration, which allows adjustment to illuminance levels based on the S/P value of the source using the Equivalent Visual Efficiency (EVE) method. Much more detailed information is provided in ANSI/IES TM-24-20, Technical Memorandum: An Optional Method for Adjusting the Recommended Illuminance for Visually Demanding Tasks within IES Illuminance Categories P through Y Based on Light Source Spectrum.^[i] The following information is excerpted rom Sections 4.4 and 4.5 in that document.

Using the EVE method:

The EVE method can be applied to the IES Illuminance Determination System (refer to ANSI/IES LS-8-20, Lighting Science: Vision – Perceptions and Performance^[ii]). The S/P-based EVE multipliers for IES Recommended Illuminance Targets are determined with a reference S/P value of 1.4—approximately that of standard incandescent light sources. That is, the recommendations can be adjusted when a light source has an S/P value different from 1.4 by the values in Table 4-1, which presents EVE-modified Illuminance Targets for Categories P through Y. These values may be applied subject to the constraints described under Restrictions, below. When implementing the EVE method, the S/P value for the specific light source being considered should be used to determine an appropriate modified illuminance target.

Table 4-1. EVE-Modified Target Illuminances (Lux) for IES Categories P through Y

Light Source S/P Value	EVE Multiplier	Illuminance Category
		P	Q	R	S	T	U	V	W	X	Y
0.60	1.97	590	790	980	1,480	1,970	2,950	3,940	5,910	9,850	19,700
0.70	1.74	520	700	870	1,310	1,740	2,610	3,480	5,220	8,710	17,410
0.80	1.56	470	630	780	1,170	1,560	2,350	3,130	4,690	7,820	15,650
0.90	1.42	430	570	710	1,070	1,420	2,140	2,850	4,270	7,120	14,240
1.00	1.31	390	520	650	980	1,310	1,960	2,620	3,930	6,540	13,090
1.10	1.21	360	490	610	910	1,210	1,820	2,430	3,640	6,060	12,130
1.20	1.13	340	450	570	850	1,130	1,700	2,260	3,390	5,660	11,310
1.30	1.06	320	420	530	800	1,060	1,590	2,120	3,180	5,310	10,610
1.40	1.00	300	400	500	750	1,000	1,500	2,000	3,000	5,000	10,000
1.50	0.95	280	380	470	710	950	1,420	1,890	2,840	4,730	9,460
1.60	0.90	270	360	450	670	900	1,350	1,800	2,700	4,490	8,990
1.70	0.86	260	340	430	640	860	1,280	1,710	2,570	4,280	8,560
1.80	0.82	250	330	410	610	820	1,230	1,640	2,450	4,090	8,180
1.90	0.78	230	310	390	590	780	1,170	1,570	2,350	3,920	7,830
2.00	0.75	230	300	380	560	750	1,130	1,500	2,260	3,760	7,520
2.10	0.72	220	290	360	540	720	1,080	1,450	2,170	3,610	7,230
2.20	0.70	210	280	350	520	700	1,040	1,390	2,090	3,480	6,970
2.30	0.67	200	270	340	500	670	1,010	1,340	2,020	3,360	6,720
2.40	0.65	200	260	320	490	650	970	1,300	1,950	3,250	6,500
2.50	0.63	200	250	310	470	630	940	1,260	1,890	3,140	6,290

Table note: For restrictions associated with applying these modified values, see Restrictions, below.

 

The application of the EVE calculation to the Illuminance Determination System has three general results:

·       When lighting specifications utilize light sources with S/P values of 1.3 through 1.5, there will be small changes in visual efficiency that are within IES acceptable tolerance in lighting calculations.

·       If light sources with S/P values less than 1.3 are used in interior lighting applications that include visually demanding tasks, there could be a negative effect on the occupants’ visual efficiency if the illuminance levels are not increased. The results of not increasing the illuminance levels could manifest itself in decreased visual performance. However, increasing light levels could have a different negative affect, which is added glare. The designer is therefore cautioned to carefully consider all of the design criteria of the application prior to using low-S/P light sources (that is, 1.3 or less) for interior lighting applications with visually demanding tasks.

·       If light sources with S/P values greater than 1.5 are used in interior lighting applications that include visually demanding tasks, then illuminance levels can be reduced by the EVE Multiplier to maintain visual efficiency relative to the baseline. How this may influence the lit environment and building occupants in other ways is discussed in Section 5 of ANSI/IES TM-24-20.

It should be noted that correlated color temperature (CCT) and S/P should not be used interchangeably. For example, a commercial light source with a CCT of 3000 K might have an S/P value from as low as 1 to as high as approximately 2.25.[iii] This difference would have large consequences when implementing the EVE method to adjust illuminance targets. With theoretical SPDs, representing future possibilities (e.g., laser diodes), the discrepancy and consequences grow even larger. A 4000-K light source might have an S/P value as low as 0.78 or as high as 5.0.

Restrictions: 

The EVE method should only be considered for application when all of these conditions apply:

• The lighting is for an interior application, with photopic viewing conditions
• The visual tasks are covered by IES Illuminance Categories P through Y for the Recommended Illuminance Target (refer to Table A-2 in ANSI/IES RP-10-20, Recommended Practice: Lighting Common Applications^[iv])
• The activity is a visually demanding task, as defined in Section 2.5 of ANSI/IES TM-24-20.
• The task is performed under an unobstructed full field of view, with relatively uniform lighting (luminance ratios do not exceed the Default Luminance Ratio Recommendations; refer to Table A-4 in ANSI/IES RP-10-20)
• The lighting is consistent temporally, allowing for complete adaptation
• The task background luminance is 50 cd/m² or greater
• The population is known to have generally good ocular health (typically those under the age of 65)

The EVE method should not be used for the development of energy policy or energy efficiency programs for any lighting applications.