In 2013, the Illuminating Engineering Society published TM-24-13, An Optional Method for Adjusting the Recommended Illuminance for Visually Demanding Tasks Within IES Illuminance Categories P through Y Based on Light Source Spectrum. This technical memorandum allows the use of spectrum in determining illuminance recommendations for visually demanding tasks, generally in the realm of commercial and industrial interior lighting. Such tasks require the ability to discern detail for efficient performance and include most reading tasks, as well as other critical vision tasks typically found in office, inspection, repair, manufacturing, education, and medical settings.

Specifically, the memorandum states that, for visually demanding tasks that fall under IES Categories P through Y, the illuminance recommendations in the IES Handbook can be reduced according to specific formulas when the light sources utilized in the design have S/P ratios greater than 1.4. TM-24-13 provides specific guidelines, formulas, and tables for the applications, along with examples.

In approving TM-24-13, the IES Board of Directors included a position statement, PS-06-13, which makes it clear that the methods described are optional and that the document should not be used in developing energy policy or energy efficiency programs.