PARbars: Cheap, Easy to Build Ceptometers for Continuous Measurement of Light Interception in Plant Canopies

Our handmade ceptometers allow us to continuously measure the transmission of light through plant canopies. We can then use these measurements to estimate canopy properties such as leaf area index and plant area index. PARbars are much lower cost than commercially available ceptometers.

This allows us to purchase them in large numbers, and deploy them in the field for extended periods of time. To construct a continuously logging ceptometer, begin by drilling one four millimeter diameter hole, 20 millimeters from each end of a white 1200 by 30 by 4.5 millimeter acrylic diffuser bar, then drilling and tapping threaded holes 20 millimeters from each end of a section of aluminum u-bar. Then drill and tap threaded holes into the base of the aluminum u-bar, to suit the mounting hardware.

Next, straighten two 1.25 meter lengths of 1.25 millimeter diameter bare copper wires, one at a time, by securing one end to a vise, and the other end within the grips of a hand drill, then turning on the drill at low speed. Use a fine tip permanent marker to mark the intended locations of the photodiodes along the edge of the diffuser, beginning with the first photodiode position at 13.5 centimeters from one end of the diffuser, and the other positions located every two centimeters between the first diode and the far end of the diffuser. Center one photodiode on the diffuser bar with its electrical connection tabs pointing toward the sides of the bar, and place the wire underneath one of the tabs to mark the position of the first copper wire on the diffuser.

Mark the position of the wire at the center and at the opposite end of the bar in the same manner, before using cyanoacrylate glue to secure the first straightened copper wire to the diffuser along the marks. Use the glue to secure 50 photodiodes facedown along the diffuser at the marked 20 millimeter intervals, taking care that the diodes are in the center of the diffuser and arranged all in the same orientation, such that the large tab sits on the copper wire, and the small tab sits opposite the wire. All of our photodiodes must be arranged in the same orientation due to their polarity.

We strongly advise checking the orientation of all of the photodiodes prior to gluing and soldering. Place the second copper wire such that it sits under each of the smaller tabs of the photodiodes, and secure the wire to the diffuser with more cyanoacrylate glue. Using a solder flux pen, wet both tabs of one photodiode, and the adjacent and underlying wires with the flux.

Use a fine tipped soldering iron set to 350 to 400 degrees Celsius temperature to solder each tab of the diode to the underlying copper wires. Shine a light onto each photodiode to test the solder connections, and use a multimeter to check for a voltage signal for each diode across the wires. Optionally, solder a 1.5 ohm low temperature coefficient precision resistor in parallel across the copper wires, and solder the male end of a waterproof direct current connector to the ends of the copper wires.

Use glue lined heat shrink tubing to seal the connections, and apply a bead of silicone sealant to the surface of the diffuser near the edge to create a continuous silicone barrier around the circuitry on the diffuser. Inspect the bead closely to ensure that no air gaps remain between the silicone and the diffuser bar. Once the sealant has cured, fill the well with epoxy resin.

Allow the resin to harden overnight, before using a razor blade to remove the silicone sealant. Use M4 bolts to bolt the diffuser to the pre-threaded aluminum u-bar, and use masking tape to secure the diffuser to the aluminum along its whole length. Fill the void inside the ceptometer with polyurethane foam filler.

Allow the filler to set overnight, and remove the masking tape, then solder the female end of the direct current connector to a length of two conductor cable, and seal the connections with glue lined heat shrink. To calibrate the photosynthetically active radiation ceptometer, or PARbar, against a quantum sensor, connect both sensors to a data logger, and set the sensors outside in full sun on a level plane. Record the outputs of both of the sensors across a period during which the solar radiation varies widely.

Determine the calibration factor for the PARbar as the slope of a linear regression of PAR, reported from the quantum sensor versus the raw voltage output. To infer the effective plant area index, install one PARbar above the canopy, taking care that it is not shaded by any light-absorbing elements within the canopy. Install another below all of the light-absorbing elements, for which the absorbance will be measured.

In the field, both PARbars should be aligned at a 45 degree angle to the planting rows. Confirm that the upper PARbar does not shade the lower PARbar, and level both PARbars with a spirit level. Then connect the PARbars to the data logger and convert the differential voltage output to PAR, using the calibration factor previously determined for each PARbar.

The differential voltage output of a PARbar is linearly proportional to the PAR output from a quantum sensor. In this representative experiment, PARbars were deployed in wheat canopies and logged every 20 seconds across the development of the plants. These data illustrate a typical diurnal time course of the canopy light environment, collected using a PARbar on a clear, sunny day.

Note the bias can be introduced by taking instantaneous ceptometry measurements at various times of day. For example, the wheat plots used for the collection of this data had a real planting orientation due north/south, with the transmission of light to the lower canopy peaking at 12:30. If an instantaneous measurement were to be taken at this point, the effective plant area index would be underestimated.

Whereas if a measurement were taken in the morning or afternoon, it may be overestimated. Weather-proof PARbars can also be deployed in the field for long time periods. For example, to monitor how the canopy light environment changes as plants develop.

PARbars can be installed in the field for extended periods of time. This allows researchers to monitor canopy development in ways not possible previously using expensive commercial equipment.