Estimating the number of occupants in a building at any given time is key for determining the heating and cooling loads as well as ventilation rates necessary to maintain comfort and air quality conditions. Real-time occupancy information also provides insights into occupant behavior (e.g., when do most occupants arrive and leave? when do we turn on the air conditioning system?) and can be used in emergency evacuation procedures (e.g., if a fire occurs, how many people need to be evacuated at 10 am vs. at 3 pm? how many if we have a big conference going on?).