A measurement of reflected light off a surface. To work out foot lamberts, take the light output of your projector and divide by your screen size measured in square feet then multiply by the screen gain. Note that projector lamps dim over their life span so it's a good idea to add around 25-30% to the ANSI Lumens requirement to allow for this.
In a well set up home cinema room, with no ambient light an reflectance of 10-22 foot lamberts is ideal, however in most home cinema rooms which double up as the family sitting room a higher output will be required as it is difficult to achieve complete darkness. In this situation aim for around 30 foot lamberts which will be fine for watching films in relative darkness.
Business projectors generally need to be brighter as ambient light is rarely as easy to control as in the home. Aim for 60 foot lamberts or more if possible. A fuller explanation is available in our article Choosing the Right Surface