PPF vs. PPFD: The Best Measurement for Grow Lights

FACTS CHECKED BY  Bob Smith

LEDs are useful for many things, including grow lights. Also, PPFD and PPF determine how much illumination plants will get from your LED lamps. But if you’re a novice in the horticulture world, there’s a high chance you haven’t heard about PPF vs. PPFD measurements. In truth, these metrics can be a bit tricky.

In this article, you will see a comparison of PPF vs. PPFD metrics to help you know the method that works best for your grow lights.

What is PPF & PPFD?

Light fixture over two plants

Light fixture over two plants

PPF and PPFD refer to the amount of light a light source generates in a location. PPF is an acronym for Photosynthetic Photon Flux, while PPFD stands for Photosynthetic Photon Flux Density.

Photons are single light particles that take on various wavelengths.  Also, they work as photosynthetic photons for photosynthesis (specifical photons with 400 nm to 700nm wavelengths). However, it’s crucial to understand what photosynthetic photons are before we jump into what these metrics mean.

But how do you measure the number of photons? That’s the job of PPF and PPFD. The significant difference between the two metrics is that PPF measures the total number of photons a light source releases. And PPFD handles the density of the photons on a specific surface.

Understanding PAR (Photosynthetic Active Radiation)

Grow light over plants

Grow light over plants

PAR refers to visible light within a 400 nm to 700 nm range.  And on the color spectrum, the range covers all wavelengths from blue to red. So, you can use a PAR meter to calculate how much PAR light a lamp generates.

How to Measure PAR

Measuring light can be tricky. And that’s because you have to account for the number of photons a single light emits per second over a specific surface area.

In addition, you can use moles when referring to the light per day from the sun and micromoles for those coming from lighting fixtures.

PPF vs. PPFD: The Differences

Let’s look closely at several factors that help differentiate these metrics.

Units of Measure

At first, it seems like most lights produce the same amount of photons. But in reality, fluctuations occur frequently.

And it doesn’t pose a problem for quality LED grow lights as these upgraded technologies produce more consistent outputs. But, accuracy is necessary for LED grow lights, so measuring is crucial.

Both metrics have different units of measure for the total number of photons. PPF measures photons in μmol/s, while PPFD calculates with μmol/s/m2

Since the values for measuring photons are large, the metrics use micromoles (μmol). Therefore, we can estimate one micromole to be 6 x 1017. Also, these formulas account for the rate of photons per second.

In short, PPF counts micromoles per second, while PPFD handles micromoles per meter squared.

Note: PPFD accounts for how many photosynthetic photons land on square meters per second.

Applications

PPFD metrics are ideal if you’re considering distance and location. Although most LED grow lights will show PPFD readings, you should watch out for the following factors before comparing values:

  • Place of the grow light (check if it covers a specific area).
  • Plant’s distance from the light source.

It’s why you see most sellers offering pictures of PPFD plots. Also, they present data lists showing the actual hanging height.  And you can get PPFD readings from multiple grow lights in a single area.

On the other hand, PPF calculates the number of PAR generated from a single grow light. Further, it’s a good measurement for comparing different lamp usage. However, you must consider that PPF can affect final PPFD readings. But, it depends on the products and the adjusted beam angles.

Measurement Methods

Since PPF measures the amount of PAR a single light emits in all directions, you’ll need more sophisticated measuring instruments. Usually, you’d need to integrate spheres to calculate PPF correctly.

These devices can capture light from all angles before rounding up the total numbers.

Contrarily, a miniature, handheld spectrometer or light meter would suffice for measuring PPFD. These devices can effortlessly gauge the number of PAR falling on a surface. Also, they’re more affordable and connect to smartphones easily.

For PPF measurement, you should place the light fixture in a sphere. That way, it’ll measure the total number of PAR light photons emitted by the institution in all directions. That’s not all. The sphere test also keeps track of all the PAR photons per second and expresses the result in uMol/s (micromoles per second). 

Although measuring and finding PPFD plots is more accessible, they can show wrong values. Hence, we recommend leaving the process to experts with more advanced measuring instruments.

Why Grow Lights use Different Metrics

Various plants under grow lights

Various plants under grow lights

Most standard lighting applications evaluate their outputs in lumens. But, you can’t get all the necessary information from lumens with grow lights.

A lumen is a measurement unit that only accounts for how bright the human eye perceives certain lights. And it works for lamps in the yellow, orange, and green spectrum.

However, what the human eye can see is irrelevant to plant growth. Hence, we use PAR (Photosynthetically Active Radiation) for growth lights. In contrast to the color spectrum perceived by humans, plants use more red and blue ranges for growth.

Therefore, high lumen output is not crucial for your plants. Instead, what they need is lights within the PAR range.

How to Measure Cost per Micromole

Let’s look at how much your grow lights will cost per micromole (not the total cost of the fixture or per watt). It’s the best way to compare the actual purchase cost of different institutions.

If a fixture can output 900 micromoles PPF for a $900 price, it’ll translate into $1 per uMol. And if it costs $5000 and outputs 2500 micromoles, it means it’s $2 per uMol.

Although it’s an effective way to measure LED fixture cost, it won’t tell you the overall price for running the fixture. So, you’ll have to look at its electrical usage efficiency in the long run.

FAQs

How can I get PPF from PPFD?

PPFD is also PPF but adds the measurement of PAR photons on a specific surface. Hence you can always get PPF while measuring PPFD.

How many PPF do I need for a grow light?

If one grow light produces about 2000µmol/s PPF, you’ll need four lamps per growing area.

What is a good PPFD for a grow light?

A good growth light for indoor plant photosynthesis should have around 500 to 700 µmol/m2. You shouldn’t go over 1000 µmol/m2 to avoid plant damage.

Rounding Up

Plants growing under lights

Plants growing under lights

PPF vs. PPFD: which is the best grow light option? First, you have to consider several things when searching for LED grow lights. Ensure you ask your manufacturers for relevant PPFD metric details and how they took the measurements. And understanding these metrics makes the process quicker.

Reliable manufacturers will show the multilevel and horizontal distance from the light source, the min/max ratio, and the average measurements. However, PPF ratings are more reliable for your plants.

Lastly, ensure you consider your LED grow light’s spectral quality. It should use blue and red more than other colors on the spectrum. 

Do you have any questions? Feel free to reach us, and we’ll be happy to help.

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