A Deep Dive into the Science of Grow Lights

The grow light product space is plagued with false advertising and confusing terminology. This article’s goal is to teach you how to see through the marketing gimmicks and pick out the most important characteristics of grow lights based on your hydroponic setup. If you are just getting started in hydroponics I would highly recommend reading through this post first (Bright Beginnings: A Simple and Effective DIY Grow Light Setup) in which I discuss the pros and cons of natural vs artificial light. In this article, I provide plans for a simple artificial light setup that uses inexpensive 4ft LED shop lights. The setup described in that post is great for the most common hydroponically grown plants such as lettuce and herbs. If you want to upgrade your setup with purpose built grow lights to help with flowering plants or to simply offer more flexibility in the types of plants you can grow, you have come to the right place.

When you first start to look into artificial lighting you will more than likely see many products and articles that talk about lumens and using lux meters to determine the light output. Using lumens has been shown to be an ineffective measurement when used to compare grow lights. This is because lux meters are set to primarily pick up on green/yellow light which is what the human eye is most sensitive to. Whereas plants mainly require red and blue light which the lux meter is not designed to pick up on. To get a more accurate measurement for grow lights a quantum light meter must be used. These are commonly referred to as PAR meters which is an acronym for “Photosynthetically Active Radiation”. PAR describes wavelengths of light in the visible spectrum of light from 400 nanometers to 700 nanometers that contribute to photosynthesis. What a PAR meter measures is known as PPFD which stands for photosynthetic photon flux density. To put it simply PPFD is a measurement of the amount of PAR light (light in the visible spectrum that contributes to photosynthesis) that actually makes it to a specific surface each second. These meters are usually much more expensive than simple lux meters, but the manufacturers of a grow light should publish PPFD data for each light they make.

One thing that might be slightly confusing is many manufacturers will call PPFD data PAR data or a PAR map. What they are actually reporting is PPFD data because PAR simply defines a specific range of the light spectrum.

When looking at PPFD data from a manufacturer they will most likely show many different measurements in a square grid with each grid being a different length away from the light itself. You can see an example below. In this example PPFD map all of the measurements are taken from a distance of 8” from the grow light and the values cover a 4ft x 4ft grid.

In general, a grow light that has higher PPFD values is going to consume more watts, produce more heat and cost more than one with lower PPFD values, so you will want to determine the light requirements for the specific types of plants you are wanting to grow to help narrow down which grow light to choose.

To start you will first need to determine the DLI or Daily Light Intensity requirement of the plant variety. The daily light intensity is a recommendation of the total amount of PAR light a plant needs over the course of a day. The formula for calculating DLI is as follows:

$ DLI = \frac{PPFD \times (3600 \times \text{Hours Light is On})}{1,000,000} $

As you can see DLI takes into account the PPFD value along with how long that light is on over the course of a day. This is a great metric because not only can it help you determine what grow light would best fit your setup, but based on the PPFD of the grow light it can tell you how long that light will need to be on as well as how far away the light needs to be from the plant. The table at the bottom of this article lists the DLI numbers for some of the most commonly grown hydroponic plants.

Example: Determining how long a grow light should be on during the day based on a plants optimal DLI value.

Suppose you have a grow light with a PPFD of 500 µmol/m²/s at 14 inches and you want to achieve a DLI of 16. How many hours should the light be on?

To determine how long the light should be on over the course of a day we will start with the DLI formula:

$ DLI = \frac{PPFD \times (3600 \times \text{Hours Light is On})}{1,000,000} $

We can then rearrange this formula to find the required photoperiod (Hours Light is On):

$ \text{Hours Light is On} = \frac{\text{DLI} \times 1,000,000}{\text{PPFD} \times 3600} = \frac{\text{16} \times 1,000,000}{\text{500} \times 3600} = 8.9 \text{ Hours} $

For a plant that has an optimal DLI of 16 and is positioned roughly 14” from the grow light which has an average PPFD of 500 at 14”, that grow light will need to be on for approximately 8.9 hours each day.

In conclusion, understanding the different characteristics of grow lights and how they affect plant growth is essential when selecting the best grow light for your hydroponic setup. By focusing on PPFD, DLI, and the specific light requirements of your plants, you can make an informed decision and create the ideal environment for your plants to thrive.

Optimal DLI Values by Plant

Plant NameOptimal DLI (moles/m²/day)
Swiss Chard12-18
Green Beans20-30

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