Lights and Reflectors 101

A cannabis plant with the white light in the background

In the – often illegal – grower circles, a rule of thumb has emerged, partly thanks to numerous grower reports in hemp magazines, of “one watt per gram” as the standard for efficient growing under artificial light. But one should not be too dogmatic about applying this formula. Why not? Find out in this article.

The yield depends not only on the strain and the light intensity, but also on factors such as the phenotype selected, the strain planted, the size and vitality of the cuttings or seedlings, the length of the germination and initial growth phases, the quality of the water and the supply of nutrients, to name only the most important.

Sodium vapour lamps

Since cannabis was first grown indoors 30 years ago, high pressure vapour lamps have been used to try and get the best possible yields. Over the years, new lighting systems have arrived thick and fast, such as sulphur-plasma lamps or U-shaped energy saving lighting (CFL), but none have managed to become the standard. Until very recently, sodium vapour lamps had no real competition, even though it now appears that the new generation of LEDs will soon be overtaking them. But the purchase costs, at an average €500 per square metre, are still several times higher than an equally efficient gas discharge lamp.

LEDs, which have been on the market for seven or eight years, are very inefficient, despite promises to the contrary. In grower circles, these lamps are also referred to as “disco LEDs”, because humans perceive their spectrum as tri-coloured (i.e. red, white, blue) and because of their poor yields. The light from really efficient LEDs is perceived by humans as an orange similar to that of a sodium vapour lamp. But until now, there have been very few suppliers of really effective LED lamps, and they are now certainly poised to take over the future of indoor growing. As soon as the production volumes and number of vendors go up, the prices will come down, which is why most cannabis farmers, both legal and illegal, are for now still growing their plants under sodium vapour lamps.

The choice of the right reflector

With LED lights, unlike with traditional lights, no additional reflectors are necessary. When selecting them, it is hugely important how much of the expensive high pressure vapour light is actually falling on the plants, or whether some of this valuable light is falling uselessly on the walls, sides or even the ceiling of the growing space instead of on the top buds. Almost all reflector manufacturers promise the best lighting and the highest yields. First of all: There are no really bad reflectors, because without any kind of metal lampshade, a large proportion of the light would be lost anyway.

Each model has specific ranges in which it will work well, very well or not very well. Whether you’re looking at an Adjust a Wing or a simple add-on reflector, it is not just the reflectivity or the heat dissipation that matters. The shape of the reflector and of the growing area are critical. In order to illuminate each individual growing area as ideally as possible, the two components need to work together. Because, unlike many (square) growing areas, the ideal lighting area of sodium vapour lighting is rectangular (80×130 cm for 400 watts and 90 x 150 cm for 600 watts). The reflectors described here have all been culled from grower reports of their use in practice published in hemp magazines, and the results have been compiled without claiming to be exhaustive*:

Standard Reflector Hood

In use for over 20 years with a highly-polished or hammer-blow dimpled surface. In large areas with many lights, any light that is scattered can be used by the surrounding plants. This effect more or less compensates for any less than ideal illumination. Only moderately suitable for small or warm areas without the scattered light effect, as the heat can build up between the lighting element and the base of the hood, creating hotspots, which can scorch the tips of plants. There are many different models of the standard reflector hood. The best ones are those models which have a slight indentation opposite the lighting element, to reduce the hot-spot effect. Highly polished reflectors provide slightly better illumination than dimpled reflector surfaces, but become dirty very quickly, which then means their positive aspect becomes a negative one.

A reflector hood against the gray background


Invented in the Australian heat to deal with the heat in the growing areas there. With an Adjust-a-Wing, the level of reflection can be controlled using adjustable wings, to suit the growing phase and the growing area. Together, the spreader (a heat shield) and the hood form a valuable combination that can be used anywhere. The spreader prevents a hotspot from building up under the light and improves the illumination around the edges. Thanks to these adjustments, the distance from the plants can be reduced by up to 25% compared to a standard hood. Optimum illumination and no hotspots make the Adjust-a-Wing models the favourite among reflectors. There are many imitations these days, some of which are almost as good as the original, but several have not even grasped the principle of heat dissipation. To get the best from the options the Adjust-a-Wing offers, the manufacturer has published a table with all the settings needed for the width of the wings and the spreader settings.


Ideal for space where it becomes too hot despite adequate extractor ventilation. As tests have shown, the light yield is barely affected by the glass cylinder. It is more effective to modify the Cooltube than to fit it as delivered. This means fitting it under a standard or even a high-end reflector instead of using the add-on reflector that is supplied. Instructions for this can be found on various forums or in trade magazines. There are major differences in the glass used and the quality of the sheet metal; any good professional shop will be able to inform you if you ask them.

CoolShade, Sputnik, Air Cool

The same principle as the Cooltube, but with a built-in reflector. The level of reflection is better than that produced by an unmodified Cooltube. However, the sheet metal in the glass casing does heat up and that is why these models offer rather less cooling (one or two degrees) than a Cooltube.

Diamond Reflector

Developed over 10 years ago in the UK. Thanks to its extreme shape and the differently finished surfaces, no hotspots build up, and the reflector spreads the light perfectly across the plants beneath it. There are two versions (D400 and D600), that cater to the differences in power of different lighting systems. There is now also an air-cooled version that has a connector for a 125 mm air pipe.

Northstar/Matrix Reflector

Due to patent rights, this reflector is sold under various names. Because of the arrangement of the internal sheet metal and its square shape, even the corners of a square homebox receive perfect illumination and no hotspots form. The heat dissipation system is also attractive, with a central opening above the lighting element, allowing the heat from the light to escape upwards, like through a chimney, without affecting the reflectivity. This avoids heat building up in the sensitive area directly above the plant shoots. The temperature in the upper area is noticeably lower than would be the case with a reflector without a heat dissipation mechanism. Especially suitable for warm areas with square growing beds.

Power Lux Reflector

Has stood the test of time and is almost as cheap as a standard reflector. Comes in the form of a kit; putting it together is child’s play. Provided you stick at the maximum distance to the plants, the reflector works well, but it creates a hotspot faster than any of the other reflectors mentioned here as soon as you move it closer to the plants.

Add-on Reflector for the Cooltube

Automatically sold with the Cooltube. Best to remove it immediately. This mini add-on reflector allows too much light to escape upwards and sideways. It is time Cooltube began collaborating with a leading reflector manufacturer, so that they can provide air-cooled tubes with really good reflective capabilities, too. Then even small home growers will stop having to cobble together their own solutions.

Growing cannabis requires a licence and is otherwise strictly forbidden. The information about the reflectors and lighting equipment provided here must not be used to grow cannabis illegally. This article is intended to inform, not encourage.

* There are many more types of reflectors than those listed here. However, the author has attempted to discuss the original version of each of the models that are most commonly used, and over time, most frequently copied.

  • Disclaimer:
    Laws and regulations regarding cannabis cultivation differ from country to country. Sensi Seeds therefore strongly advises you to check your local laws and regulations. Do not act in conflict with the law.


5 thoughts on “Lights and Reflectors 101”

  1. Well Micha, today is 7/29/2017, stated above is 10/04/2017! LOL! LED works now, look at QPAU LIGHTING!

  2. thanks for the article, I use 2 different types of lights flourecent tubes daylight spectrum and high pressure sodium lamps 600w ,I have used cool tubes before but I sold them on as without the cool tube the light is clearly much brighter ,I just use cheap reflectors and only replace the HPS bulbs after 2 years ,replacing them sooner is a waste and bad for the environment,

  3. Man, the gavitas lights are by far the best!!!! Leds suck n Mother Nature sun is always the best!!!!

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  • Michael_Knodt

    Michael Knodt

    Berlin-based Michael Knodt specialises in cannabis and hemp-related journalism. Over the years, he’s written extensively on the topic for various German and English magazines, including Vice Magazine. Michael is the face and moderator of DerMicha, the popular German YouTube channel on cannabis and its prohibition.
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