Having a light plan on paper is one thing, but we measure LED lighting in the actual situation to make sure that the light level calculated is achieved on a consistent basis. For our LED lighting projects, we use a spectroradiometer or a PAR (Photosynthetic Active Radiation) sensor that shows the light measurement in µmol/s/m², to take periodic measurements of the average light level in the greenhouse. This is the only accurate way to measure the amount of light that the plant uses. In contrast, a lux meter measures the light intensity as perceived by the human eye, in luminous flux/m². Our eyes are most sensitive to green light, but red and blue light are the most effective colors to maximize photosynthesis in plants so those are the ones you want to measure.
The light level in µmol/s/m² that can be achieved for a given area (m²) depends on the number of LED modules installed, the output (µmol/s) of the installed modules and nearby reflection surfaces. When we perform measurements, we measure the light level both at the position directly under the LED modules as well as the position between the LED modules. We do this to calculate a correct average light level. Figure 2 shows these two measurement positions as small green dots. The closer you are to the module, the higher the light intensity. The light intensity goes down as you go farther away, because the light coming out of the module is spread over a larger area. The measurement positions and distance between the light sensor and the LED modules, are the key factors for obtaining accurate measurements. Measurements also have to be taken at the correct height in the greenhouse to ensure that the light beam is optimal and uniform at the top of the plant. Measurements could be performed at various heights to determine an optimal uniformity. The uniformity increases if the difference between the maxima and minima of the green curves in Figure 2 decreases.