Optimal Lighting Orientation for LED Bar Lights

by Kelly McCracken

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

Percent difference

4 in 299 - -

8 in

150 220 32%
12 in 99 132.1 25%
20 in 57 69.3 18%

Figure 2. Barrina Lights Measured at 0,0

Our first goal was to determine if the Barrina marketing materials' advertised PPDF values were accurate. We found that the measured value varied from 18-32% less than the advertised value. Unfortunate, but the results we got showed that there is still ample light to grow plants under. (You can find a detailed guide to what we grow under what PPFD in our article Target PPFD for Orchids and Houseplants.) We did email Barrina asking for an explanation why our results varied so much from theirs, and they simply stated that their experiment was done under laboratory conditions and offered no other explanation.  
The second goal of the experiment was to determine the bright and dim spots produced by a single fixture. These results are plotted in Figure 3. 

Figure 3. Barrina Bar Light Intensity Distribution

There were two significant findings after mapping out the light intensities. First, at a height of 4" below the fixture, the light provided is very intense for a narrow strip. Very little light reaches outside of this strip. Second, no matter the height, the intensity at the edges of the fixture was significantly less than the intensity at the center of the fixture. The intensity peaked between (10,-10) at all heights. 

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 is a slice plot of one light, with 8" and 12" heights. The graph plots the light distribution of a single fixture, as if you are looking down the length of the light. The solid lines are the measurements taken at the middle of the fixture, and the dotted lines are the measurements taken at the end of the fixture. The placement of the fixtures are marked with the grey line in all subsequent figures. 

Figure 4. One light slice plot

Looking at the light plotted like this gives us a new perspective, a perpendicular orientation. It clearly shows the sharp peak and subsequent drop off of the light's brightness. With a 4 foot Barrina LED light at 8" height, you would have a space of approximately 6 x 20" of even light, at about 140umol/m2/s. 140umol/m2/s is enough light to grow some miniature Cattleya, some Paphs, brighter growing Phalaenopsis, and others. The space outside of this 8 x 20" area would be much darker, but the area would be suitable to growing maudiae Paphiopedilum, Phalaenopsis and other low-light plants. A home grower can definitely make a single-light setup work, but for more even lighting distribution as well as overall brighter light, more lights must be added. This is what we have done below. 

Figure 5. Two-light slice plot at 12 inches between lights 

Figure 5 examines what the lighting distribution would be if the lights were on a shelf, placed 12" apart. As you can see, this double-light configuration really increases the amount of evenly lighted perpendicular space that a home grower can take advantage of. While previously, we only had a few inches of the brightest light, now we now have a width of 12 inches, doubling that of a single light setup. The overall brightness of the center region has also increased. At a distance of 8 inches, the lights emit a total of 200umol/m2/s, allowing for brighter growing plants. With these levels you could grow strap-leafed Paphiopedilum, some Dendrobium species, miniature Cattleya, and many other medium-bright plants. 

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. 

In order to grow very bright-light plants such as Cattleya, Ascocenda/Vandas, and Brassavola (which is totally doable under artificial light) you will need to add a third light. Figure 7 examines just that. 

Figure 7 Three-light slice plot with 10 inches between fixtures

At a height of 8" from the fixture we have the brightest light. Measuring around 275umol/m2/s, you can definitely grow brighter-light loving plants like standard Cattleya, Vanda, and Brassavola. But, we are again limited in the amount of perpendicular space that the light is brightest. At the edges, we get values of about 175umol/m2/s, plenty of light for brighter-growing Paphiopedilum, Dendrobium, miniature Cattleya, many Angraecoids, and Oncidum intergeneric hybrids. This three-light setup is ideal if you are growing anything brighter-growing than Paphiopedilum and Phalaenopsis.

 

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. 


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