Skip to content NREL Buildings Research National Renewable Energy Laboratory (NREL)
Field Test Best Practices: A Resource for Practical Residential Building Science

Main menu


Photometers, also referred to as light meters or lux meters, are used to measure light intensity inside a building. Hand held photometers are often used to spot check the light levels in a room or on a particular surface. Photometers can also be installed as a part as a lighting control system to automatically dim the lights based on the amount of daylight.

Photo of a handheld lux meter. Two pieces connected by a chord include the sensor and a digital display.Photo shows a buildings researcher standing with a photo meter pointed at a ceiling to measure light.

A handheld lux meter
Tilt u, Wikipedia Commons

A researcher checks the quantity of daylight and the
accuracy of the lighting sensors
NREL/PIX #05171

Photometers can be used in a variety of ways in a field test. Photometers could be installed as a means of detecting occupancy. Photometers could be installed in several rooms in the house and when lights are turned on, occupancy can be inferred. For that application, the light meters should be installed on a wall without direct sunlight, changes in natural light will not trigger the light sensor.

Another application for light meters in a field test is for verification of a lighting control system. Sophisticated lighting control systems use photometers to adjust room lighting levels as the natural light entering the room changes.  To verify that the lighting control system is working as intended, the photometers for the field test should be installed by the researchers next to the photometers used for control. If the appropriate lighting levels (as prescribed by the lighting control system) are being maintained, the control system is working as intended.

Photo shows a closeup of a light sensor.Photo shows a closeup of a field-test grade photo meter.

Light sensor for daylighting in NREL's
Research Support Facility
Dennis Schroeder

Field-test grade photometer

The handheld photometers generally do not produce an analog output. They could be used for a quick verification of a lighting control system, but would not be suitable for occupancy detection, which would require a long-term installation and an output signal. There are other options for photometers that send an analog signal that can be read by a data logger. The photo above, right, shows a photometric sensor that uses a silicon photodiode that produces a μA signal that is proportional to the incident light intensity.

For more information about lighting control using photometers, see:

Guglielmetti , R.; Scheib, J.; Pless, S. D.; Torcellini , P.; Petro, R. Energy Use Intensity and its Influence on the Integrated Daylighting Design of a Large Net Zero Energy Building: Preprint. National Renewable Energy Lab, March 2011. NREL/CP-5500-49103.