How to Use your Lights to grow weed at Home

The lighting you choose is one of an essential factor in the production of weed; it is the engine of photosynthesis in plants. In this blog post, we will explain how the quantity and quality of lighting influence the development of our favourite plant, and which are the best bulbs for beginner growers.

How is light measured?

The most widespread measure to quantify the illumination that of a focus for marijuana is lux meter or a light meter. This should not be confused with the lumen, because although one and the other unit are directly related, they are different things. The lumens is the amount of luminous flux, i.e. the amount of light visible to the human eye which emits a light source, whether natural or artificial. The lux would come to be the “density” of light that hits a surface, so we have previously said that it was directly related:

Lux = Lumens / Square meters

Another unit to measure the amount of light is photons, which not only includes visible light but all wavelengths. Photons are invisible, individual, massless particles that are present in light, and can serve both to measure radiation and light absorption. In terms of physics, the light absorbance is known as the number of photons that a given surface receives in a unit of time.


The apparatus designed to measure the amount of light is called a luxometer and can be used to check this both indoor and outdoor. Keep in mind that these devices measure the light that we see, and does not exactly match the light used by plants to make photosynthesis.

What light do marijuana plants use?

While humans see only part of the light spectrum, between 525 and 625 nm, plants instead “see” a much wider spectrum of light, ranging from 400 to 700 nm. Another question would be to talk about what light is more efficient for the production of marijuana.

The structure responsible for the capture of light in plants is what is known in English by the abbreviations LHA (Light Harvesting System), in Spanish to this structure is called complex antenna and is found in almost all cells of the plant. This structure is composed of chlorophyll A and B molecules, which capture red-maroon and blue light, respectively. There is also a peak of light absorption around 660 nm, which is used for the synthesis of chlorophyll.

Have you ever wondered why the plants are green? It occurs because the plants absorb the entire spectrum of visible light, except this same colour. By not absorbing it and “bouncing” on the plant, our eyes see it that colour.

Plants can orient themselves towards the light source, and this is what is known as phototropism. This phenomenon occurs because they “look for” light with a wavelength around 450 nm, ie the colour blue.

light bulbs for marijuana

That said, it is clear that not all the light received by the plant is absorbed, much less used in photosynthesis, which is used takes the name PAR (Photosynthetic Active Radiation), or RFA (photosynthetically active radiation), in Spanish.

RFA is the amount of light, with a wavelength between 400 and 700 nm , that plants use for photosynthesis. The RFA is measured with a quantum sensor and is expressed in μmol / m 2 s. The Natural RFA is about 2,000 umol / m 2 • s (instantaneous measurement). More light means more photosynthesis and more plant growth.

At the moment, the devices capable of measuring PAR light, photometers, are too expensive for most growers, so, for now, we must settle for conventional luxometers.

It is for this reason that when comparing two different grow bulbs that according to their manufacturers emit the same number of lumens, in practice can give us different results. Usually, the data provided by the brands refers to total lumens and not to lumens RFA or “productive.”

How to choose the best light to cultivate

A clue to know which lamp can work best to grow is its colour temperature. This data is given by the manufacturers and their unit is Kelvin degrees. In more degrees, warmer or redder is the light emitted, and the fewer degrees, because more bluish and cold will be. The quality of light needed by plants is that whose colour temperature is between 2,000 and 6,500ºK, depending on the stage of cultivation.

Bulb Lights for marijuana

In the germination and vegetative growth phase, a cold light lamp with a high colour temperature can be of use. In the flowering stage, light with a temperature around 2,000ºK is more interesting, which favours the formation of flowers and the lengthening of the space between knots.

In the following table, we make a comparison between different models of grp light bulb.

Light bulb PAR light Visible light (lumen) Color temperature Use

  • Sunmaster Super HPS 400 780 55,000 2000 Flowering
  • Philips Son-T PIA plus 400 w 670 55,000 1950 Flowering
  • Philips Son-T PIA agro 400w 660 55,000 2.050 Mixed
  • Sylvania SHP 400 w 670 55,000 2.050 Flowering
  • Sylvania Grolux 400 w 713 58,000 2.050 Mixed

Another thing is the amount of light most suitable for growing marijuana, and this depends on the physiological state of the plant. In the growth phase, weed needs around 30,000 Lux to have rapid and vigorous development. In contrast, in the flowering cycle, raising that intensity to 60,000 lux can increase production significantly.

We must also bear in mind that an excess of light will paralyze the photosynthetic function, that is, above 100,000 lux, the plants reach the threshold of saturation or “light toxicity”, so we must not saturate the plants by placing spotlights and focuses of more, caution and knowledge first of all.

Phenological stage Maximum LUX Hours of light

  • Seedling or cutting 4.000 18
  • Vegetative 30,000 18
  • Flowering 100,000 12
    light marijuanaBut one thing is the amount of light emitted by the bulb and another is the amount that reaches the canopy of the crop. Well, to know this you have to know the law of the inverse of the square. This physical law, applicable to the foci, says that the intensity decreases with the square of the distance. That is, if we double the distance to which the focus is located, then the amount of light (lumens) that will arrive will be approximately a quarter.
  • Lumen = Initial lumen / (distance) 2
  • Example 1. The focus of 400 watts with 50000 initial lumens at 30 cm distance
  • Distance Lumens
    • 30 cm 55,000
    • 60 cm 12,500
    • 90 cm 5,555

We can see how the amount of light decreases very fast as it distances from the plants. Let’s calculate what would be the quantity in lux (not lumens!) If this lamp is 60 cm from the canopy in a DB 80 cabinet, 80 x 80 centimetres in area.

  • Emission = 12500 lumen
  • Surface = 0.8 x 0.8 m = 0.64 m 2
  • Intensity = 12,500 / 0.64 = 19,531 Lux

This configuration is suitable for the growth phase since it is below the theoretical limit of 30,000 lux.

Example 2.

  • Focus of 600 watts with 90000 initial lumens, at 30 cm distance
  • Distance Lumens
    • 30 cm 90,000
    • 60 cm 22,500
    • 90 cm 9,999
    • Intensity = 90,000 lumens / m 2 = 90,000 lux

This configuration would also be valid for the flowering phase since it is below the limit of 100,000 lux.

In summary, not only the lumens that a light bulb emits should be taken into account, but also the distance to which it is located from the canopy and the surface it must cover. We must also take into account its colour temperature and the phase of the crop in which it will be used. Knowing all these parameters, we will be able to get the most out of our indoor marijuana cultivation, both in quality and quantity.


Bud S. Daily

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