Additionnal Interior Lighting Power

I'm working on a LEED-CI offices project. I'd like to apply additonnal interior lighting power as specified in ASHRAE 90.1-2004 section 9.6.2b: Lighting to be installed to meet the requirements of visual display terminals. So I assumed that a computer is a visual display and the user needs more lighting for his work. So we are using a T8 luminaires under desk shelf. To have additional power accepted, the luminaire must meet specific requierements. ASHRAE specify a maximum luminance of 80cd/ft² at 65 degrees. What the ft² refer to? Area of the desk, the floor, the luminaire? It is unclear...