Just to stay alive, plants don't need a ton of light. But for vigorous growth, they need quite a bit. It's easy to look at a sunny windowsill and guess that there's probably enough light there for growing, but is there really?
We decided to use our PAR meter to find out for sure
What is PAR?
PAR stands for Photosynthetically Active Radiation. Not all colors of light can be used by plants. PAR is a measure of the amount of photons that are usable by plants. By measuring the PAR in a given location, we can know how much usable light is available for growing plants. A PAR sensor gives us a value (in mV) which when multiplied by a scaling factor (5x for our sensor) gives us how much PAR is hitting the sensor. So lets try it!
How much light is there outside?
I went out on a hot, sunny summer afternoon here in Atlanta, and in the sun we had a PAR of 1747. That's really high! A lot of plants can't use that amount of sunlight, or are even hurt by it. That's why some plants, and a lot of the leafy greens we eat (like lettuce and spinach) are shade or partial shade plants. In the shade we recorded a PAR of 139, which is appropriate for growing many salad greens and herbs.
So how much sunlight is there indoors?
I checked out the PAR on the sunniest windowsill in my house around 2pm, and the meter read 29.5. Some container plants would do just fine on that windowsill, but only if they were fully-grown. Growing plants from seed with that little light would lead to leggy (thin-stemmed) plants reaching for the light.
I also did a reading inside the nanofarm, which read 153 - plenty of light to quickly grow veggies, herbs, and salad greens. We designed the nanofarm to provide this much light because it strikes the best balance between the speed of plant growth and energy efficiency. Combine this optimal level of light with the high efficiency of LEDs and you can see how we're able to grow a full crop of lettuce in just 30 days with less than $1 of electricity.