CRI is the ability of the light source to produce natural colors in objects.  CRI is measured on a scale from 0 - 100 where 100 is best.  The higher the CRI the more natural people and objects will appear when lighted.  Incandescent and halogen bulbs as well as natural daylight have a 100 CRI.   

Initial lumens in lighting calculations is the total light output at the very beginning of the lamps life before depreciation (light losses) occur.

Lamp lumen depreciation is common for all gas discharge lamps. As the lamps age through use they emit less light. This characteristic must be accommodated for when lighting design professionals predict maintained light levels.   

Lumen maintenance is the ability of a lamp to retain its initial light output over time.  Greater lumen maintenance means a lamp will remain brighter longer.  The opposite of lumen maintenance is lumen depreciation. 

Mean lumens is light output typically evaluated at 40% of rated lamp life.   

With some manufacturers of high pressure sodium (HPS) lamps the mean lumens is rated at 50% of their useable life, yet other manufacturers rate HPS lamps at 40% of their useable life. The three types of metal halide (MH), mercury vapor (HgV), and low pressure sodium (LPS) lamps have their mean lumen rating established at 40% of their useable life.

Fluorescent and compact fluorescent (CFL) lamps apply a slightly different term called ’design lumens’ which is essentially the same thing as mean lumens. Most manufacturers rate the different types of fluorescent lamps at 40% of the lamp’s useable life.