As discussed in our previous blog post Why We Use PAR and PPFD for Growing Houseplants, artificial light can be deceptively bright (or dim) to our eyes. As it turns out, light fixtures don't provide an even level of light coverage to all areas they illuminate. This is not always a negative, as home growers tend to grow a "hodgepodge" of different plants, often with many different lighting requirements. Understanding these dimmer and brighter areas will allow you to make good use of the lighting microclimates produced by your fixtures. In this article we will discuss how many lights you will need to properly illuminate a grow space on a 4-foot shelf.
For the purposes of this article, we looked at these Barrina Full Spectrum LEDs. They have good specs: they're full spectrum, very affordable, have a reflector, and are about the brightness you'd want to grow plants under (i.e. they wont fry your houseplants like a cannabis growlight would). Barrina's marketing materials also included a pretty handy lighting diagram showing PPFD at different distances from the light.
We had a few goals in mind when we started looking at the Barrina lights. First, we wanted to determine if the marketed PPFD was accurate. Second, we wanted to map out the different brightness values produced by a single fixture. Lastly, we wanted to produce clear graphs plotting the brightness values at different distances and with different orientations and quantities of lights.
Because our first goal was to determine whether the marketed PPFD values were accurate, we set up an experiment to measure the brightness as it was illustrated in the Barrina marketing materials. This image can be found below:
Figure 1. Barrina PPFD Illustration (Source: Barrina marketing materials)
We set up our light exactly like the illustration; leveled at 20" above the ground. To better facilitate accurate measuring, we put a 1" grid underneath and marked 0,0 with a sticker (illustrated by the blue dots in figure 1). Using our Apogee PAR meter, we took measurements every 4" going down the length of the fixture and perpendicular to the fixture. We then repeated the experiment at 12 in, 8 in, and 4 in distances from the fixture. The results of our experiment can be found in Figures 2 and 3 below.
Distance from Fixture (meter placed at center of the fixture, directly underneath: 0,0)
Measured PPFD in umol/m2/s
Advertised PPFD in umol/m2/s
Figure 2. Barrina Lights Measured at 0,0
Figure 3. Barrina Bar Light Intensity Distribution
Our results favored using a height of 8" or 12", for most even light distribution. Measurements taken at 4" showed that the light was too intense and narrow for growing plants under, and at 20" there was all around too little light for growing most plants. Therefore, as we continued with our experiment we looked only at the 8" and 12" heights.
Figure 4. One light slice plot
Figure 5. Two-light slice plot at 12 inches between lights
Figure 6. Two-light slice plot at 16 inches between fixtures
Figure 6 looks at two lights, 16 inches apart. At a height of 8 inches, this setup isn't optimal, since there is a fairly big dip in the middle. At a 12" height, you have a wonderfully even light distribution in the center and at the edges. At 115umol/m2/s (which is achieved at a distance of 12" from the fixture along the middle), you can grow brighter-growing Paphiopedilum, maybe enough for Phragmipedium, some smaller Dendrobiums, miniature Cattleya. The light at the ends of the fixture would be enough for Phalaenopsis, Jewel Orchids, Maudiae Paphiopedilum, and not much else.
Figure 7 Three-light slice plot with 10 inches between fixtures
It should be noted that these heights are all distance from your plant canopy, and not distance from the bottom of your shelf. A height of 8-12" above your plant canopy leaves plenty of room for spikes to grow, but might not be enough for those very tall-spike growing Phrags, Paphs, and the like.