Plant photobiology guide

Wavelengths and their effects on crops

Photosynthetically active radiation (PAR) covers wavelengths from 400 to 700 nm. This is the spectral range that plants use for photosynthesis. But not all wavelengths have the same effect on growth, morphology or harvest quality.

A fixed-spectrum grow light provides a constant light composition from seedling to harvest. It makes it possible to reproduce specific effects on plants, based on validated studies and tests.

A professional dynamic spectrum, on the other hand, allows the light composition to be adjusted to achieve precise objectives: controlling plant stretch, stimulating natural defences or activating secondary metabolism.

However, a dynamic spectrum is only truly relevant if the manufacturer has carried out rigorous trials and can explain precisely how to use it, the conditions of application and the expected effects.

Not to be confused with a basic dynamic spectrum, which allows the light to be changed without control over its impact on the plant, for lack of data, tests and clear recommendations from the manufacturer.

PPF (Photosynthetic Photon Flux)

Definition: PPF measures the total amount of light useful for photosynthesis emitted by a light.

Unit: µmol/s (micromoles per second)

Formula: Power × photon efficacy = PPF
600 W × 2.9 = 1,740 µmol/s

It is the total light output your grow light can emit, like the ‘horsepower’ of an engine.

PPFD (Photosynthetic Photon Flux Density)

Definition: PPFD measures the amount of light received by plants over a given area.

Unit: µmol/m²/s (micromoles per m² per second)

In plain terms: it is the light actually received by the plant, depending on the distance and the light distribution.

Example

A 600 W grow light with a photon efficacy of 2.9 emits 1,740 µmol/s (PPF).

• At 20 cm from the light, plants will receive more than 1,000 µmol/m²/s (PPFD)
• At 40 cm from the light, plants will receive around 600 µmol/m²/s (PPFD)

Each time the distance between the light and the plants is doubled, the light intensity (PPFD) is divided by 4.

Replacing conventional HPS lighting with high-efficiency LEDs is the most immediate way to reduce costs. Using high-efficiency LED chips, combined with an optimised design that allows the grow light to be placed closer to the plants, can reduce energy consumption by up to 60% while maintaining the same PPFD, or even increasing it.

Definition: energy efficiency is a light’s ability to convert electricity into light.

Unit: µmol/J (micromoles per joule)

The more we use latest-generation, high-efficiency LED chips, the more light the grow light emits.

Examples

Low-efficiency conventional technology
600 W HPS × 2.2 µmol/J = 1,320 µmol/s (PPF)

General-purpose LED, low efficiency
720 W LED × 2.4 µmol/J = 1,728 µmol/s (PPF)

High-performance horticultural LED
600 W LED × 2.9 µmol/J = 1,740 µmol/s (PPF)

A 600 W LED grow light with an efficacy of 2.9 µmol/J will use around 20% less energy to emit the same amount of light as a 720 W LED grow light with an efficacy of 2.4 µmol/J.

High photon efficacy depends on the use of latest-generation LED technologies, involving high-quality and therefore more expensive components.

Beware of low-cost grow lights that claim both high power and high photon efficacy: it is technically inconsistent to offer more power and more efficacy for a lower cost. A strategy based solely on price often results in lower LED quality and efficacy, leading to higher energy consumption in order to offer a cheaper grow light.

In our tests, we found that many grow lights on the market overstate their actual efficacy, sometimes by up to +30% compared with the data stated on their data sheets, for marketing purposes.

Several warning signs help identify these products: abnormally low prices, private-label brands, LED components not specifically designed for horticulture, and no documented agronomic tests.

Investing in a quality grow light that has been tested and validated avoids low-cost solutions that are often unreliable and unprofitable, which can lead to 20 to 30% higher consumption and heat output, at the expense of your profitability and the quality of your production.