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.

 

[i] Illuminating Engineering Society. 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. New York: IES; 2020.

[ii] Illuminating Engineering Society. ANSI/IES LS-8-20, Lighting Science: Vision – Perceptions and Performance. New York: IES; 2020.

[iii] Fotios, S., & Yao, Q. The association between correlated colour temperature and scotopic/photopic ratio. Light Res Technol. 2009;51(5):803-13. DOI: 10.1177/1477153518779637.

[iv] Illuminating Engineering Society. ANSI/IES RP-10-20, Recommended Practice: Lighting Common Applications. New York: IES; 2020.

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