The importance of measuring light

When a grower uses supplemental lighting in their greenhouses, the goal is to help their plants — annuals, perennials, produce and others — grow as healthily and as efficiently as possible. With the right amount of supplemental light, a grower can expand their growing season, growing 24/7 or, in some cases, speed up production time.

There, however, is a catch: Using supplemental lighting is not as simple as buying fixtures on the internet, installing them in the greenhouse and then reaping the rewards a few weeks later. With lighting, growers need to be mindful of how much photosynethically active radiation (PAR) is actually being provided by the fixtures. To do that, they need to measure their light, most commonly with a quantum light meter and/or light sensors.

“You're not going to be able to calculate your daily light integral (DLI) [with a handheld quantum light meter], but it's a good way of going into your greenhouse and seeing what the light intensity is when it's bright outside or what the light intensity is when it's cloudy,” says Dr. Roberto Lopez, an assistant professor and extension specialist at Michigan State University specializing in controlled environment specialty crop production. “Obviously, it's going to vary from day-to-day and from one season to the other. But at least it's going to give you kind of a snapshot of what your light levels are and whether you should be using supplemental lighting or whether you should be using your shade curtains.”

Light measurement options

Light meters are not a new technology — although Lopez says they have become more widely used by greenhouse growers over the past few years — and there are a number of options available to purchase, ranging in cost from a few hundred dollars to over $1,000. In Lopez’s opinion, there is one type of light meter growers should invest in because it measures the wavelengths of light that are utilized for photosynthesis.

“Having a light meter is important, but having a quantum meter is even more important,” he says. “A quantum meter is basically measuring the light that a plant perceives in the PAR range (400 to 700 nm) and that’s what we're most concerned with. If you have a foot candle meter, it is basically just measuring the light that we can see as humans and that's not what we are concerned with in this particular situation.”

Additionally, Lopez says that you need to do your homework and ensure the light meter you purchase is calibrated with the type of lights you are using in order to give you the most accurate readings possible. For example: If a grower has LED fixtures, but the meter is not calibrated for use with LEDs, their readings are going to be incorrect. The cost is higher for quantum sensors, but if growers want accurate readings, the extra cost is necessary, Lopez says.

There are also other ways to measure light in the greenhouse. One option is light sensors that can be paired with light meters to measure exact wavelengths and light intensity. According to Dave Johnson, a senior product manager at Li-Cor, there is also software growers can purchase to track different light measurements over time and give their readings context over months and years.

“Ultimately, in a greenhouse, you are trying to grow your plants most effectively, most efficiently as far as energy is concerned, as far as water is concerned,” Johnson says. “And so what you’re looking at, potentially, is figuring out that best way to do that with the light sources in your greenhouse.”

Best practices

Although Lopez says the cost of a light meter can be intimidating for some growers, he says it is a must for growers who have invested in supplemental lighting. It can also help inform future decisions about whether or not to add more lighting.

“Sometimes it may be under [what you expect],” Lopez says. “Sometimes it may be over. Over isn't necessarily a bad thing, but being able to go in at night when those lights are on and take your own measurements is quite important. Being able to determine what light intensity your fixtures are putting out, then you can use that to calculate your supplemental DLI and determine whether that light intensity is sufficient or if you’ll need to add more lights.”

Johnson says that having the ability to properly measure light is important for smaller greenhouse operations even though they may only have a few light fixtures in the greenhouse.

“I think it’s still important [for smaller growers],” he says. “The main thing driving growth is light. If you don't know what your light level is or don't have some kind of control on your light, you’re kind of at a disadvantage.”

Once a light meter is purchased, both Lopez and Johnson say the key is to actually use it. Lopez recommends taking measurements “as often as you can” — several times a week at minimum, especially if the greenhouse is not fitted with an automated environmental control system that manages the lights — and recording the readings. From there, a grower can determine if the lights are aiding plant growth or wasting energy, and what the next step should be.

“Use it,” Lopez says. “Don’t just have it on your desk.”