I think this is how I would piece it all together. There aren’t any hits for the entire expression (Projected in-service maintained illuminance (level), Projected maintained illuminance (level) in service) but then again there aren’t any for your Spanish expression either.

The “Em” in your text is “maintained illuminance (level)” – which, as you can see by the following definition, is an average, so I don't think you need to say "average" on top of it.

Em is the Maintained Illuminance Level (specification level) and is the Design illuminance level, below which the lighting should not be allowed to fall during the period to planned maintenance.
http://www.lighting4sport.com/sportslighting.asp?sport=sn07

Maintained illuminance (Em, unit: lux). The average illuminance over the reference surface at the time maintenance has to be carried out by replacing lamps and/or cleaning the equipment and room surfaces.
http://www.dstan.mod.uk/data/00/025/25000100.pdf

I did find these two refs. that, combined, point to a possible translation of “minimum maintained illuminance (level)” for the "iluminancia media en servicio", bearing in mind the "average" in the first definition, which also states that it is a minimum value (as explicitly stated in the second definition), and the "in-service" of the second definition. I think I would play it safe and use one of the translations suggested above, but I thought I would throw this out here for you anyway in case you decide it fits your text. :-)

MAINTAINED ILLUMINANCE: The average illuminance over the reference area at the end of the complete maintenance cycle, and is the minimum value to which the illuminance is allowed to fall.
http://www.mbm.net.au/uss/lighting_glossary.html

Minimum Maintained Illuminance Level. The lowest ***in-service*** values of a lighting
system designed with the proper light loss factor.
http://64.233.183.104/search?q=cache:ABnAkrMjYYQJ:www.extran...

--------------------------------------------------
Note added at 9 hrs (2007-12-13 07:14:55 GMT)
--------------------------------------------------

Sorry, I meant to shrink that last link before posting it. Here it is: http://tinyurl.com/yqa8dj