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How to measure light output.

There are several measurement types that get used to describe energy output. Most common are LUMENS, LUX, PPF, PPFD. They don’t all measure light output the same way.

LUMENS and LUX measure visible light only. Because plants use some light that is invisible to humans, LUMENS and LUX don’t always account for all the light available to plants. 

PPF and PPFD measure all the light plants use, including visible light and invisible light, making them the most accurate measurements for horticultural lighting.

LUMENS and PPF are measurements taken at the fixture.  They measure raw output. They are good measurements to use when comparing power draw vs. output to determine how efficient a fixture is.

LUX and PPFD are measurements taken at a distance from the fixture, accounting for light falloff and reflection.  They are highly accurate and useful measurements because they indicate exactly how much light a plant’s leaves are receiving.

PPFD measurements are captured by lighting manufacturers using special equipment in a lab environment. Light manufacturers include PPFD as part of their published specifications for their lights, usually.

The best way to present PPFD measurements is in a chart that illustrates the intensity and coverage of the light at a stated distance from the canopy. To create these charts, a tester hangs a light above a graph that represents a coverage area. With the light turned on, a quantum sensor takes spot PPFD readings at the intersections of the graph. The data is compiled into a PPFD chart that makes it easy to visualize the light’s performance.

PPFD chart taken 18″ over canopy.
PPFD chart taken 24″ over canopy.

A good PPFD chart includes these features:

• Readings expressed in micro moles per square metre per second (μmol/m2/s)

• Distance the readings were taken from the light. For example, 12″ from light, 24″ from light.

• Multiple readings taken in a grid pattern over a wide coverage area, not just a single reading.

• Indicates if the readings were taken with or without reflection at the sides to reflect back light and boost the light levels. It’s better if the readings are taken without reflection because reflection could be used as a way of exaggerating the capabilities of a light.

Some manufacturers publish only a single PPFD measurement, not a chart that includes multiple readings. This can be misleading if the measurement was taken directly under the fixture where light is strongest.  If a single PPFD reading is given, it should be an average of several readings. For example, an average PPFD over a 4′ x 4′ coverage area.

The amount of PPFD required for cannabis depends on its stage of growth and the strain. The recommended light levels listed below will work for most strains. Keep in mind that hybrid strains with more southern genetics (sativa) require more light than northern varieties (indica). The measurements can be modified up or down 10-20% depending on the strain.

Also consider the depth of the canopy. As the charts above demonstrate, light becomes less intense the further it has to travel. All buds should receive adequate illumination, not just the ones at the top. If the canopy is a foot deep, consider if the amount of light reaching the lower half is adequate.

Below are the recommended PPFD levels for each stage of growth: 

Seedlings, clones: 100 PPFD 

Vegetative, moderate growth: 310 PPFD

Vegetative, heavy growth: 620 PPFD 

Flowering, moderate growth: 620 PPFD 

Flowering, heavy growth: 925 PPFD

PPFD is sometimes alternatively expressed as μmol/m2/s.

LUX is another useful measurement because although it only measures visible light, it can be measured using a commercially available handheld device. This gives LUX a distinct convenience advantage over PPFD which can only be measured in a lab. A LUX metre is a fantastic tool for taking spot measurements that every grower should own. 

Below are the recommended Lux readings for each growth stage:

Seedlings, clones – minimum range: 5,000 LUX

Seedlings, clones – maximum range: 7,000 LUX

Vegetative – minimum range: 15,000 LUX

Vegetative – mid range: 40,000 LUX

Vegetative – maximum range: 70,000 LUX

Flowering – minimum range: 35,000 LUX

Flowering – mid range: 60,000 LUX

Flowering – maximum range: 85,000 LUX