Finding the Right Grow Lights: Lux, Lumens, and PAR

Indoor gardeners and growers need to provide their plants with an alternative to natural sunlight.

13:09 29 September 2020

They do this by purchasing specialized grow lights. Choosing the right lights can be somewhat of a hassle, though, because not all manufacturers use the same measurements of light intensity.

 

Some manufacturers measure light intensity in lux, while others use lumens. Growers also need to understand PAR ratings if they want to ensure optimal growth during each stage of their plants’ life cycles. Consumers don’t need a degree in electrical engineering to understand the differences between these different systems of measurements, though. They can read on to find out everything they need to know about measuring light intensity.

 

Why It Matters

 

All three of the metrics mentioned above tell growers something different about the quality of the light produced by a bulb, lamp, or ballast. Not all manufacturers offer all three measurements, though, so novice growers can quickly wind up feeling like they’re forced to compare apples to oranges to figure out what products to buy.

 

Growers often need help with understanding what lux is and what lumens are. It’s important to realize that these measurements define how light is perceived by the human eye. It’s equally important to understand PAR since this measurement tells consumers how plants will interact with the light.

 

What Is Lux?

 

Lux is a unit for measuring light intensity, sometimes referred to as illumination or luminance. One lux is equal to the amount of light a single candle throws on a one square meter surface one meter away. That doesn’t offer much context for growers, though, given that most Americans don’t use candles anymore. Here are a few comparisons that could help:

 

• Direct sunlight provides 32,000 to 100,000 lux.
  
  
• Ambient daylight provides 10,000 to 25,000 lux.
  
  
• On an overcast day, the sun provides around 1,000 lux.
  
  
• During sunrise and sunset, the sun produces around 400 lux.
  
  
• On the night of a full moon, the moonlight is equal to around one lux.
  
  

What Are Lumens?

 

A lumen is a unit of measurement of the quanta of light, or its total amount, produced by a light source. Lumens are also referred to by engineers as luminous flux. With lumens, there’s no regard for the size of the space to be lit. It only measures the amount of light produced by a light source that can be perceived by the human eye. Here are a few examples of lumen output by lighting type:

 

• A 400W Metal Halide (MH) light produces around 38,000 lumens
  
  
• A 200W LED array produces around 20,000 lumens.
  
  
• A 150W high pressure sodium (HPS) lamp produces around 12,000 lumens.
  
  
• A 100W incandescent bulb produces just 1,700 lumens.
  
  
• A 32W fluorescent tube light produces around 1,600 lumens.
  
  

Relationship between Lumens and Lux

 

While lumens measure light output, lux measures how many lumens will be required to illuminate a given area. One lumen spread over one square meter is equal to one lux. That means the same 1,000 lumens would produce different levels of lux when spread over different areas. Spread over a one square meter area, they would produce 1,000 lux. If 1,000 lumens were used to illuminate ten square meters, they would produce only 100 lux.

 

What that means for growers is that lighting larger areas to the same lux level requires more lumens. This goal is usually achieved by increasing the number of light fixtures. Growers can also compare lights with intensity measured in both these two units by using the formula lux = lumens/m2 or by plugging the information into a lux to lumens calculator.

 

What Is PAR?

 

While both lux and lumens measure only visible light, PAR is a measurement just for plants. It’s short for photo synthetically active radiation and refers to the light range used by plants for photosynthesis. While the visible light spectrum covers a range of wavelengths from 380 to 700 nanometers, PAR wavelengths range from 400 to 700 nanometers.

 

When grow light manufacturers offer information about the PAR output of their lamps, they use PAR meters to figure out how many photons are being emitted within this range of wavelengths. When growers select lights, they should make sure they are providing their plants with sufficient PAR. There are a few other metrics they’ll need to know to accomplish this goal since PAR measurements don’t actually tell buyers how much quality light will reach their plants’ canopies.

 

Photosynthetic Photon Flux

 

Photosynthetic photon flux (PPF) is a metric that expresses the number of PAR-range photons emitted by a light fixture each second. There is some fluctuation in PPF from moment to moment. It can’t be perceived by the human eye, but it can affect plant growth. Currently, LED lights provide the most consistent output.

 

PPF Density

 

PPF density, or PPFD, measures how much usable light reaches the plants each second. Since light quality declines as growers move the fixtures further from the canopy, PPFD varies based on heat output. LED lights radiate the least heat, which means they can be placed close to the canopy. This, in turn, means that growers can anticipate higher levels of PPFD.

 

What about Watts?

 

There’s a reason this article didn’t begin with a discussion of wattage. Watts are a measure of how much power a light fixture consumes. The amount of power required to produce a certain number of lumens is referred to as a measure of luminous efficacy and is expressed in lumens per watt (LPW). While there’s no question that power draw is a concern for most growers, it won’t affect how much light reaches their plants.

 

The Bottom Line

 

Choosing grow lights can feel like a daunting task, especially to those who don’t have any background in electrical engineering. It’s important to compare apples to apples, so growers should convert lux to lumens, or vice versa, when only one of these measurements is offered by the lighting manufacturer to get a more accurate idea of what to expect. They should also ensure that the lights are designed to provide an optimal PAR spectrum and that they have all the features growers need to ensure a smooth season.