During photosynthesis, plants use energy in the region of the electromagnetic spectrum from 400-700 nm. The radiation in this range, referred to as Photosynthetically Active Radia- tion (PAR), can be measured in energy units (watts m-2) or as Photosynthetic Photon Flux Density (PPFD) which has units of quanta (photons) per unit time per unit surface area. The scaled units most commonly used are micromoles of quanta per second per square meter (µmol s-1 m-2). Measuring PAR within a plant canopy can be very difficult because of the non-uniformity of the light field. When PAR is measured with a small diameter quantum sensor such as the LI-190SA Quantum Sensor, intensity can vary 10-fold between sunflecks and shadows, requiring a large number of readings to get an accurate average. The LI-191SA Line Quantum Sensor reduces the number of individual readings required because it effectively averages PPFD over its one- meter length. One person can quickly make plant canopy PPFD measurements in many plots in a short period of time. Rather than using multiple detectors linearly arranged over its one-meter length, the LI-191SA uses a one-meter-long quartz rod under a diffuser to conduct light to a single, high- quality quantum sensor whose response is shown in the graph on Page 3. There are two advantages of this design. First, the sensor has a very good quantum response, unlike sensors using inex- pensive gallium arsenide detectors with only an approxima- tion of the ideal quantum response. Second, the single quantum sensor is much easier to keep in calibration than multiple (up to 80) individual gallium arsenide detectors.