How to measure grow light intensity

In plant growth, sufficient and even light is crucial, especially for indoor growers where grow lights are often the only source of light. However, many people may not realise that relying solely on the feel of light intensity is not enough to ensure optimal growth conditions. 

This article explores how to scientifically measure light intensity to help you use grow lights more effectively, boosting plant health and yield.

1.Why is Light Intensity So Important?

Grow lights must provide a specific intensity for plants to perform effective photosynthesis. If light intensity is too low, plants may show slow growth or yellowing leaves. If too high, it can cause leaf burn or stunted growth.

2.Common Light Terms and Their Comparison

When measuring grow light intensity, you’ll encounter several common light terms that have subtle yet important differences. Understanding these terms helps you more accurately assess the impact of light on plants.

1. PAR

PAR refers to the light wavelengths that plants can absorb and use for photosynthesis, typically between 400-700 nanometers (nm). In grow lights, PAR is the main measure of light intensity, as it specifically considers the spectrum of light needed for plant photosynthesis.

2. PPFD

PPFD refers to the number of PAR photons hitting a unit area per second. It gives a more accurate representation of the light energy that plants actually receive. It is usually measured in µmol/m²/s.

3. PPF

PPF is the total amount of Photosynthetically Active Radiation (PAR) emitted by a light source per second, typically measured in µmol/s. Unlike PPFD, which measures light received by plants, PPF measures the total output of the light source.

4. Lumens

Lumens refer to the total visible light output from a light source, typically used in home lighting, but it doesn't effectively reflect plant light needs.

5. Lux

Lux and Lumens are often confused. They measure light intensity, but for plants, PPFD is more important.

Lux measures the brightness per unit area, more related to human eye perception. For plant growth, we care more about PPFD since plants need specific light wavelengths (PAR).

3. Common Plant PPFD Requirements

Cannabis requires 18 hours of light during vegetative growth, which can be reduced to 12 hours during flowering. Supplementing with 60W UV+IR LED lights can improve cannabis flavour and THC content.

3. PPFD Measuring Device Guide

1). Using a Smartphone App

If you don't want to invest in expensive devices, you can use smartphone light measurement apps. Some free apps, like Lux Light Meter or Spectroid, can measure light intensity.

Pros:

• Low cost, many apps are free.
• Easy to use, no extra equipment needed.

Cons:

• Accuracy is lower, suitable for hobbyists, not professional needs.
• May not accurately measure PPFD, more for lux measurements.

2). Using a Professional PPFD Meter

For higher precision, a professional PPFD meter like the Apogee MQ-500 is the best choice. These meters accurately measure light intensity plants can use, providing precise PPFD data to evaluate if the light conditions meet plant needs.

Pros:

• High accuracy, measures different wavelengths of PAR.
• Ideal for fine-tuned growing environments, especially indoor.

Cons:

• Higher cost, suitable for professionals with larger budgets.

3). Product Selection Table

4. Simple Three-Step Light Measurement

How to Measure Grow Light Intensity: Using Apogee MQ-500

Using the Apogee MQ-500 to measure grow light intensity is easy and effective. This sensor is designed specifically for measuring light that plants can use (PAR/PPFD). It helps ensure the light intensity is right for your plants. Here are the steps:

1). Preparation

Before starting, make sure:

• Battery Check: Ensure the Apogee MQ-500 has enough power. If low, change the battery or recharge.
• Measurement Location: Identify the light source and the plant area to measure. Position the sensor where it represents the main light exposure to the plants.

2). Turn on the Apogee MQ-500 Sensor

• Press the power button to turn on the sensor. The display will show the light intensity in PPFD (µmol/m²/s).

3). Set Measurement Mode

• The MQ-500 is set to PPFD measurement mode by default, so it is ready to measure the light intensity plants are receiving. If it's in another mode, switch to PPFD mode.

4). Start Measuring Light Intensity

• Measurement Location: Place the probe above the plants in the light zone. Ideal height is 15-30 cm above the plant canopy.
• Stable Position: Keep the sensor still during measurement to avoid influencing results. Measure with the light turned on, ensuring no interference (e.g., windows or reflective surfaces).

5). Record and Analyse Data

• Measure in multiple locations for even light distribution. Record readings to assess if light intensity is uniform.

6). Adjust Light and Measure Again

• If Light is Too Low: If the PPFD is below plant needs, raise the light height, add more lights, or switch to higher-efficiency grow lights.
• If Light is Too High: High PPFD can cause light burn. Raise the light or lower its power.

7). Turn Off Apogee MQ-500

• Once measurements are complete, turn off the sensor to save battery. Hold the power button for a few seconds to shut it off.

5. Conclusion

By using the right measurement tools (like a PPFD meter), you can measure light intensity accurately, adjust light conditions to suit your plants, and boost plant health and yield.

FAQ

1. Can I use a smartphone app to measure light intensity?
While smartphone apps can measure light, their accuracy is much lower than professional PAR meters. For precise measurements, it's best to use a dedicated device.

2. What’s the difference between PAR and Lux?
PAR is specifically for plant growth, whereas Lux measures visible light for human eyes, which doesn’t reflect what plants need.

3. How do I know if my light is suitable for plants?
Use a PPFD meter to check if the light intensity falls within the required range for your plants.