This chapter illustrates various types of instruments that can aid in the energy audit survey. The primary focus of the energy audit is on the quantity of light but there are many qualitative issues that the auditor may have to note depending on the application. Light meters used to measure lighting levels in the home, office, restaurant, school, etc. differ from conventional photographic meters in that light meters relate to the way the human eye sees light, while photographic exposure meters relate to the way film “sees” light. Data loggers can be used to monitor and record building and system conditions, unattended, on a 24-hour, around-the-clock basis. Carbon dioxide is exhaled by building occupants and can be used to provide a more accurate accounting of occupancy periods and patterns than just relying on posted business hours or interviews with building personnel. Carbon dioxide can also be used to evaluate ventilation rates in the area being monitored.