Indoor lighting for tropical plants

Why plants need illumination

Indoor plants are very unlucky: they have to grow in "caves," and everybody knows that plants don't grow in the caves. The luckiest plants win sunny windowsills, but even there they dwell rather like in underbrush under tall trees, where the sun illuminate them only early in the morning or in the evening, and its light is diffused by foliage.

Usually domestic plants are in desperate lack of illumination not only in winter, but also in summer. No light - no growth, no flowering. So, plants need extra light to compensate the lack of illumination in the "room-cave" conditions.

Before you will start any practical actions to help your plants, you need first to decide whether you are going to set an additional illumination, or provide full lamplight. In the case of just additional illumination, rather inexpensive luminescent lamps will do, and you don't have to think about their spectrum.

Such lamps need to be set above the plants, about 8-10" from the plant. Tropical plants need light 12-14 hours per day for a full-value development. In this case they are both develop and flourish. Therefore, you need to turn on the additional illumination two hours before the daybreak, and turn it off several hours after the nightfall.

Providing full lamplight to the plants, we also need to take into consideration a spectrum of the illumination. It is not enough to use ordinary lamps. If your plants are not exposed to the daylight, then you need to use lamps with a special spectrum, intended for illumination of plants and/or aquariums.

While providing an additional illumination or a full lamplight of the plants, it is very convenient to use a timer relay. Most handy is a dual-mode one. Such a relay let the lighting system to be switched on for a few hours first in the morning, and then in the evening.

Just try to provide your plants with an additional illumination, and you will notice by yourselves how better they develop when they have enough light.


Mysterious lumen and lux


Lumen is often confused with lux, so we need to distinguish the units of measurement of the luminous flux (lumens), and of the illuminance (lux).

Electric power of the lamp is measured in watts, and luminous flux ("power of light") in lumens (lm). The more lumens, the more light gives a lamp. This is like a water-hose - the more we open a tap, the more water will spurts out. The luminous flux characterizes a source of light, whereas illuminance - the lighted surface. Taking the same example, you need to know how much water goes out of the hose to one or another place. On this factor depends how long you need to water the plants in the garden.

Illuminance is measured in lux (lx) or in footcandles (fc). The source of the light with the luminous flux of 1 lm, evenly illuminating the surface of 1sq.m (about 10 sq. ft), creates the illuminance of 1 lx. 1 lm over 1 sq.ft creates 1 fc. 1 fc equals to approximately 10 lx.

Simple enough.

Useful rules

Inverse square law The illuminance on the surface is inversely proportional to square of the distance from the lamp to surface. If an initial distance between a lamp and a plant was 1 m (approx 3 ft), and you moved it up to the height of 2 m (approx 6 ft), then the distance increased twice as large, whereas the illuminance dropped down four times as large. You have to remember it when you'll project your system for plant illumination.
Illuminance of a surface depends on the value of the angle under which the surface is illuminated. For example, the sun in the summer noon, when it is high in the sky, creates the illuminance several times higher than in the winter day, when it hangs low on the horizon. So if you use searchlight-type lamp for plants illumination, try to direct its light perpendicularly to the plants.

Spectrum and color

The color (and spectrum) of lamp radiation can be characterized by its color temperature (CCT - Correlated Color Temperature). But what is the connection between color and temperature? We know that things heated high enough begin to shine. The higher the temperature, the brighter is the shining. If we'll heat a piece of metal, its color will change from cherry-red to almost white. So the color temperature of the lamp is simply the temperature, at which the color of the heated metal is the closest to the color of the lamp. It is measured in Kelvin degrees (K).

Another parameter of the lamp is a color rendering index (CRI). It shows how close are the color of the illuminated objects to the true colors, and has a range from 1 to 100. For example, sodium lamps have low color rendering. All objects placed under them seem to have one color. New models of the luminescent lamps have high CRI. Try to use lamps with higher CRI, and your plants will look more attractive. Both these parameters are usually indicated in the marking of luminescent lamps. For example, /735 means CRI=70-75, CCT=3500 K (lamp of the warm white color); /960 means CRI=90, CCT=6000 K (daylight lamp).

CCT(K) Lamp Color
2000 Low-pressure sodium lamp (used for street lighting), CRI<10 Orange (sunrise or sunset)
2500 High-pressure sodium lamp without coating (HPS), CRI=20-25 Yellow

Incandescent (filament) lamp, CRI=100, CCT=3000K
Luminescent lamp (warm-white), CRI=70-80
Halogen lamp, CRI=100, CCT=3500K

4000-4500 Luminescent lamp (cool-white), CRI=70-90
Metal-halide (MH) lamp, CRI=70-80
5000 Mercury lamp with coating, CRI=30-50 Light blue (sky at noon)

Luminescent daylight lamp, CRI=70-90
Metal-halide lamp, CRI=70
Mercury lamp, CRI=15

The sky in the cloudy day


Absorption spectrum of chlorophyll (absorption vs. wavelength in nm)
Photosynthesis is the process of conversion of the energy of light into the chemical energy which is used by plants. In the process of photosynthesis, the plant absorbs carbon dioxide and evolves oxygen. The light which is necessary to energize the chain of chemical reactions, is absorbed by various pigments, principally by chlorophyll. This pigment absorbs the light in the red and blue regions of the spectrum.
Apart from photosynthesis, there are other processes in plants which are wavelength-sensitive. This means that using selection of the light spectrum, or light and dark periods of variable length, it is possible to speed up or slow down the development of the plant, shorten its vegetative season, etc.

For example, pigments with maximum absorption in the red region of the spectrum are responsible for a development of the root system, fruit ripening, plant flowering. In order to stimulate these processes, in greenhouses use sodium lamps that have most of their radiation in the red region of the spectrum. Pigments with the peak of absorption in the blue region are responsible for the development of the leaves, growth of the plant, etc. Plants that had deficiency in blue light, for example, those grown under the filament lamp, are higher because they loft up in order to get more "blue light". The pigment, which is responsible for the orientation of the plant to light, is also sensitive to blue rays.

Here follows the important conclusion: the lamp that is intended for plants illumination, must have both red and blue rays in its spectrum. If we'll look again on the absorption spectrum of chlorophyll, the main pigment responsible for photosynthesis, we'll see that such a light is the most "edible" for a plant. After millions of years of evolution, plants has "learned" how to utilize every bit of available daylight, either enriched with blue rays at noon, or with red rays at dawn or sunset.

Many lamp manufacturers offer lamps with spectrum already optimized for plants. They are better for the plants than common luminescent lamps, which we use for interior lighting. Such a lamp makes sense to use, if you need to change the old one. Having equal power, the special lamp gives more "healthy" light for the plants. But if you set the new system for plants lighting, then do not use these specialized lamps, which far more expensive than the common ones. Better set up a more powerful lamp with high color rendering index (with /9... marking). Its spectrum has all necessary components, and it will give much more light than the special lamp.

Lamps for plant illumination

There are two types of lamps suitable for plant illumination: filament lamps (incandescent and halogen), and gas-discharge lamps which generate light due to electric discharge in a mixture of gases.

Filament lamps can be plugged directly into wall outlet.

Gas-discharge lamps need special starting equipment, also called ballast. They can't be plugged into a wall outlet despite the fact that some of them looks similar to incandescent lamps.

Filament lamps

Apart from ordinary incandescent lamps this category include some other varieties:

Halogen lamp with built-in reflector

Halogen lamps: their bulbs are filled with a mixture of halogen gases, which increase the lamp's brightness and life time. Don't confuse such lamps with a gas-discharge, metal-halide lamps. New lamps contain the mixture of krypton and xenon gases. Due to this mixture, the brightness of the spiral became much higher (for example, xenon flashlights bulbs)

Neodymium lamps: their bulbs are made from glass with admixture of neodymium (Chromalux Neodym, Eurostar Neodymium). Such a glass absorbs yellow and green parts of the spectrum, and the lighted objects seems visually brighter. Actually, the lamp does not give more light than the ordinary one.

There is no sense to use filament lamps for plant illumination. They are not suitable for two reasons: their spectrum lacks blue rays, and they have little efficiency (only 17-25 lm/W). When turned on, all filament lamps became very hot, therefore they must not to be set close to the plants, otherwise they will get burns. Placement of such lamps farther than 1 m (3 ft) from plants is almost useless. However, incandescent or halogen lamp can be used along with the luminescent lamp, which has little red light in its spectrum. Such a combination has good enough spectrum. Nevertheless, it is better to use sodium lamp instead of it - combination of HPS and MH lamp is the best for plants.

Lately, special lamps for plant illumination came into the market, for example, plant light with built-in reflector. But despite the reflector, which turns these lamps into small floodlight, they are still the same filament lamps with the same low efficiency.

General purpose luminescent lamps

An example of lighting system using luminescent lamps

Lamps of this type are well known, they are the standard sources of light in buildings. Such lamps are more suitable for plant illumination than the incandescent. Their advantages are high luminous efficiency (50-70 lm/W), low heat, and long life. However, their spectrum is far from optimal for plant lighting.

Nevertheless, if you have enough light, the spectrum is not so important. To turn a luminescent lamp on, you will need a special ballast. There are two types of ballasts: electromagnetic and electronic. The second one is much better, because the lamp does not flicker when switched on. Electronic ballast increases the lamp life and amount light. Some electronic ballasts even allow to regulate the lamp output.

The lamp power depends on its length. Longer lamps give more light. We advise to use as long and powerful lamps as possible, because the have higher luminous efficiency. In other words, two lamps of 36 W are better than four lamps of 18 W. Lamps should be mounted not higher than 1.5-2ft above plants. An optimal use of the luminescent lamps is illumination of shelves with plants which are of approximately equal height.

Special purpose luminescent lamps

The only difference of these lamps from the general purpose lamps is an inner coating on the glass tube. Due to this coating, the spectrum of these lamps is closer to the spectrum which is needed to the plants. Such lamps are produced by OSRAM-Sylvania, Philips, GE, and some others.

Compact luminescent lamps

These lamps are made either with, or without built-in ballast. The lamps with built-in ballast has smaller size and easier to use - they can be twisted into a standard socket. Unfortunately, such lamps are made to replace incandescent lamps for indoor illumination, so their spectrum is not optimal for plants.

Best of all is to use luminescent lamps for illumination of several plants, standing side by side. In order to receive enough light, their power has to be at least 20 W (analog of 100 W incandescent lamp), and the distance to the plants has to be no more than 1 ft).

You can buy high power compact luminescent lamps (36 to 55 W). They have higher efficiency (20%-30% higher than of ordenary luminescent lamps), long life, excellent color rendering (CRI>90), and wide spectrum, which include red and blue rays necessary for the plants. One more important feature is their compact size, which allows effectively use them together with reflector. These lamps are the optimal choice for plants illumination in the case of low-power lighting system (up to 150-200 W of total power). The shortcomings are their higher cost and necessity to use electronic ballast for high-power lamps.

Gas-discharge lamps

Gas-discharge lamps, for a present, are the brightest source of light. The high luminous efficiency allows you to illuminate a big area of growing plants with a single lamp. However, these lamps require special ballasts for their work. It should be noted that it makes sense to use such lamps only when you need a lot of light. If the required total power is less than 150-200 W, then the best solution is the use of compact luminescent lamps.

There are three types of gas-discharge lamps used for plants lighting: mercury, sodium, and metal-halide.

Mercury lamps

Of all types of gas-discharge lamps, mercury are the oldest one.Common variety (without inner coating) have low color rendering index, and in their light everything looks deadly blue. Mostly they're used for security lighting. Modern lamps have a coating which improves their spectral characteristics. The luminous efficiency of mercury lamps is low. If you intend to install a new system for illumination, then it is better to avoid mercury lamps.

High-pressure sodium lamps

High-pressure sodium lamps (HPS) is one of the most effective, in respect to luminous efficiency, light sources. The spectrum of these lamps is the most favorable for the plant pigments which are sensitive to rays of the red spectrum region, and responsible for roots formation and florescence.

HPS lamps give a plenty of light, so a single high-power luminary (250 W or more) can be enough to illuminate a big area. Such a lamp is the best for additional illumination of conservatories and big plant collections. However, in order to make the combined spectrum more balanced, it is recommended to alternate them with mercury and metal-halide lamps.

Metal-halide lamps

Metal-halide lamp CDM (Philips)

The most perfect lamps for additional lighting of the plants. They have high power, ample resource, and optimal spectrum of radiation. Unfortunately, such lamps, especially with the improved spectrum, are more expensive than all other types of lamps. New lamps with ceramic burner from Philips (CDM), or OSRAM (HCI) have increased color rendering index (CRI=80-95).

Although the base of metal-halide lamp (right) is similar to base of halogen lamp (left), the former one needs special lamp-socket.


What lamp can be useful, and for what.

If you need to do something cheap and fast, then use halogen lamps or compact luminescent lamp with built-in ballast, which can be twisted into ordinary lamp-socket.

The best solution for a dozen or so close-placed small plants of about the same height (up to 2 ft) is a compact fluorescent lamp(s). For a tall single plant the searchlight-type luminaries with gas-discharge lamps with power up to 100 W can be advised.

If your plants are approximately of the same height, and grow on a shelf or a windowsill, then use long fluorescent lamps (tubes), or (even better) high-power compact lamps. Be sure to use reflectors with luminescent lamps, because they considerably increase the useful luminous flux.

If you have a big conservatory, then install ceiling luminaries with high-power (250 W or more) gas-discharge lamps.

Most of the described lamps can be bought in special store or from our online store (!!!!LINK!!!!)

The table below will help you to compare the characteristics of lamps and systems for plant illumination.

Incandescent and halogen lamp Luminescent lamp Compact luminescent lmp Discharge lamp
Cost of replacement bulb Less than $5, $10-15 - specialized $5 - common,
$10-20 - specialized
$5 - low-power, for filament lamps substitution,
$15-40 - power of 35-90 W and specialized
less than $20 - low-power,
$30-80 - medium power,
$50-150 - high-power lamps
Cost of ballas not required $5-10 - common,
$15-30 - electronic
Not needed for lamps which are twisted into lamp-socket;
$20-30 - electronic. Many high-power lamps work only with electronic ballast.
$20-50 - common
$30 -100 - electronic, capable to control switching on, etc.
Cost of lighting system (without lamp or ballast)   less than $10 - home-made reflector with lamp-sockets;
$15-40 - system with lamps and ballast
less than $20 - home-made;
$30-100 - purchased
$150-500 - complete system
Nominal service life 750 hours - incandescent lamp;
more than 2000 hours - halogen lamp
15-20 thousand hours 15-20 thousand hours 5-15 thousand hours
Real life if used daily
6 months 9-12 months 1-2 years
Heat 50 W per 1000 lm.
Almost all lamp power is heat.
Medium, 10-15 W per 1000 lm. Since the lamp is long, the heat is not concentrated in one place. In the case of large system, the use of little fan from computer (cooler) will help to solve the problem of heating. Very little heat, 5-10 W per 1000 lm, however, heat is concentrated in one place. If large lamps are used, you need a cooling system.
Lighting system power range Small lamps can be used for additional illumination and heating Small and medium size plants Groups of plants on a shelf or shelf-stand. Big groups of plants, with system's total power up 100-150 W Big groups of plants and greenhouses - overhead illumination

Lighting system selection

In the previous parts we talked about basic conceptions and different types of lamps used for plant illumination. Now we explain how to choose the best lighting system, how many lamps you need for illumination of particular plant, how to measure illumination in home conditions, and why lighting systems need reflectors.

Light is one of the most important factors of the successful plant keeping. Using photosynthesis, plants make themselves "a meal". If there is little light, the plant becomes weak, and either dies from "hunger", or becomes an easy prey to pests and diseases.

To be, or not to be?

So, you decided to install a new lighting system for your plants. First, answer these two questions:

  1. How much money you can afford to spend? If you need to confine yourself within a small sum of money, which you take from family budget, then this article will not help you. Our only advise: just buy what you can, and don't waste your efforts and time for searches. Unfortunately, lighting systems for plants aren't cheap. Sometimes a more reasonable alternative is a substitution of sun-loving plants with plants that can tolerate shade, and it will be better to have a well-groomed spathiphyllum, which doesn't require much light, than to grieve for a half-dead gardenia that longs for light and can't get it.
  2. Are you going simply to "hold out" till spring, acting on principle: better to have any additional lighting, then none at all? Then just buy the cheapest luminescent lamp from Home Depot or Wal Mart. But if you want plants to grow and even bloom under lamps, then you need to strain your powers and money for a lighting system. Especially if some of your plants all year round grow under artificial illumination, like aquarium plants.

If you already know the answers for these questions and decided in favor of full-scale lighting system, then read further.


What a good illumination is?

There are three main factors that define whether the system is good, or bad:

  • Light intensity. There has to be enough light for plants. You can not compensate weak light with more hours. There is no such thing as too much light in indoor conditions. It's rather difficult to reach a degree of illumination comparable to a bright sunny day (more than 100,000 lx)
  • Duration. Different plants require different daylight hours. Many processes, like blooming, are regulated by duration of the daylight hours (photoperiodism). You all saw Poinsettia pulcherrima (or, more precisely, Euphorbia pulcherrima), which is on the market for Christmas. It grows under our house's window on the south of Florida, and each winter, without any tricks from our part, does everything "by itself", because we have here just right conditions for red bracts formation: long dark nights and bright sunny days.
  • Illumination's quality. In the previous parts I already wrote about the fact that plant needs light both in red and blue spectral regions. And you already know that that it is not necessary to use special lamps for plants (phytolamps): if you use modern lamps with wide spectrum, like compact fluorescent, or metal-halide lamp, then you'll have "the right" spectrum.


Of course, apart from these factors, others are also important. The intensity of photosynthesis is limited to the factor that is in lack in the given moment. If illumination is low, then this factor is light. When light is abundant, then the limiting factor can be temperature, carbon dioxide concentration, etc. For example, if you cultivate aquarium plants using too strong illumination, the concentration of carbon dioxide becomes the limiting factor, and even stronger illumination will not increase the speed of photosynthesis.

How much light do plants need?

Plants can be divided in several groups on the basis of their requirements for light. Numerical value of illumination, which is necessary for each of these groups, is rather rough, since many of the plants can feel well both in the bright light and in the shade, adapting to the level of illumination. And even the same plant will need different quantity of light, depending on whether it is in vegetative, blooming or fruit-bearing period. From energy point of view, blooming is the process of "wasting" a lot of energy. Plant needs to grow a flower and supply it with energy, but the flower itself cannot produce energy. Fruiting is even more wasteful process. So the more light, the more "lamp" energy your plant is able to store up for blooming, the more beautiful will be your hibiscus, the more flowers will be on your jasmine shrub.

Below we'll list some examples of plants that one or other light conditions. Illumination levels are expressed in luxes. You already know the difference between lumens and luxes, and we'll just remind that luxes show how much light your plants get, and lumens characterize lamps which you use to illuminate them.

  • Bright light. This group include plants that naturally prefer open spaces: most of the trees, palms, succulents, bougainvillea, gardenia, jasmine, hibiscus, ixora, jasmine, plumeria, thunbergia, crotons, etc. These plants prefer a high level of illumination, at least 15-20 thousands lux, and some of them require 50 and more thousands ux for successful blooming. Most of the plants that have variegated leaves require a high illumination, otherwise their leaves can become green again.
  • Moderate light. These group unites plants of underbush: Bromeliaceae, begonias, rubber plant, philodendron, caladium, chlorophitum, brugmansia, brunfelsia, clerodendrum, crossandra, medinilla, pandorea, ruthia, barleria, tibouchina. The desirable illumination is 10-20 thousands lux.
  • Low light. The definition "shade-loving plants" is not an exactly precise one. All plants like light, including dracaena in the darkest corner of your room. It is just that some of the plants are able to grow (rather, to exist) even when the light is weak. If you don't care how fast they will grow up, then they will feel good at low level of illumination. This group include mostly lower-tier plants: chamaedorea, whitefeldia, anturium, dieffenbachia, philodendron, spathiphyllum, aeschinanthus. For them 5-10 thousands lux is quite enough.

The approximate levels of illumination listed above can serve as a starting point for the selection of lighting system. Please also have in mind, that these figures are the illumination levels necessary for full value growth and blooming. If you just want to "support" you plants from fall till spring, then a much lower level of illumination will do.

Measurement of illumination

So, now you know how much light is necessary for your plant, and you want to check whether it receive everything that it needs. Theory is a good thing, but it is better to measure real illumination of your plants.

If you have luxmeter (left photo), then you are lucky. If don't, then take your camera. Exposure meter of camera is a luxmeter, too. The only difference is that instead of units of illumination it shows the values of exposure, i.e. the time which the shutter should stay open. The less object is illuminated, the more is exposure. Simple idea, right?

If you have an external exposure meter, then place it where you want to measure illumination, with the light-sensitive element of the meter perpendicular to the direction of the falling light.

Most of the modern digital cameras show values of aperture and exposure, and make the process of measuring of illumination very easy.


If you use a camera, then put the sheet of the white matte paper (the glossy paper will distort results) at right angle to the direction of the falling light, and direct the camera on the sheet of paper. Then look into viewfinder and find the distance at which the image of the paper will cover all frame area. There is no need to focus the image. Set the camera to film sensitivity 100 ISO units (modern digital cameras allow to "imitate" a sensitivity of the film). Using the values of exposure and aperture, check the table below and find the value of illumination. If you'll set film sensitivity to 200 units, then values in the table should be cut in half, and if the sensitivity will be set to 50 units, the values should be doubled. Step to next, higher aperture, also doubles the values. This method allows you to estimate the level of illumination of your plants more or less accurately.

Aperture Exposure
Illumination (lx) for 100 ISO units film
External exposure meter Camera, aimed at sheet of paper
2.8 1/4
2.8 1/8
2.8 1/15
2.8 1/30
2.8 1/60
2.8 1/125
2.8 1/250
2.8 1/500
4.1 1/250
4.1 1/500
5.6 1/250
5.6 1/500
5.6 1/1000
8 1/250
8 1/500
8 1/1000

Using reflector

If you use luminescent lamp without reflector, then you reduce the useful light in several times. It is easy to understand, that the useful is only the light that is directed down, to your plants. The light, which is directed upward, is useless. The light that blinds your eyes when you look at the open lamp, is also useless. Good reflector directs this blinding light down to the plants. The results of luminescent lamp modeling show that, when you use reflector, the illumination in the center increases almost three times, and a spot of light on the surface becomes more concentrated, because now the lamp illuminates plants, and not all around them.
Most of the luminaries, that are for sale in the home appliances shops, do not have a reflector, or have something that is a shame to name as reflector. On the one hand, special systems for plant or aquarium illumination with reflectors are expensive. On the other hand, to make a homemade reflector is not too difficult.

How to make a homemade reflector for a luminescent lamp

Reflector form, especially for one or two lamps, is not the issue of fundamental importance. Any "good" form which provides no more than one reflection and minimal return of the light into the lamp, will have approximately the same effectiveness in the scope of 10-15%. The figure shows the cross-section of the reflector. It is clear that its height has to be such, that all the rays higher than the boundary one (the ray 1) have to be intercepted by reflector. In this case the lamp will not dazzle the eyes.

Assuming the direction of the reflected boundary ray (for example, down or under the angle), you can build a perpendicular to the surface of the reflector in the reflecting point (point 1 on the figure), which bisects the angle between falling ray and reflected ray (the law of reflection). In the same way you can define perpendicular in the remaining points (like point 2 on the figure).

For a control, it is recommended to check several more points, to avoid the situation illustrated in point 3, where the reflected ray is not going down. After that you can make either polygonal frame, or build a smooth curve, and bend the reflector using this template. We advise not to place the upper point of the reflector closely to the lamp, because in this case the rays will come back to the lamp, and it will be warming up.

You can make a reflector from aluminum foil, for example, from the kitchen one, which has sufficiently high reflection. Also, you may paint the surface of the reflector white, and its effectiveness will remain almost the same as for the mirror-like reflector. It is also necessary to make ventilation openings on the top of the reflector.

Duration and quality of illumination

Depending on plant species, they should be illuminated usually for 12-16 hours long. More specific data, as well as recommendations regarding photoperiodism (for example, how to make above mentioned poinsettia to bloom), you can find in the special literature. For most of the plants, this value is quite sufficient.

We talked a lot about a quality of illumination. Here is the illustration: the photo of the plants that were grown up using mercury and filament lamps (the snapshot is from the old book; at that time there was no any other lamps). If you are not eager to grow long and gaunt plants, then don't use filament or sodium lamp without additional illumination with luminescent or gas-discharge lamp with radiation in a blue region of the spectrum.

Photo: Tomatoes grown up under the light of different lamps. 1 - Mercury lamp without filters; 2, 3 - Mercury lamp with filters removing different parts of the spectrum; 4 - Filament lamp.
Source: Lighting for Plant Growth by Bickford and Dunn, 1972


Besides everything else, the light of specialized lamps must make plants nice to look at. In this respect, sodium lamp is not the best choice. The photo shows how plants look under sodium (to the right) and metal-halide (to the left) lamps.


Lamp power calculation

So we've arrived to the most important issue: how many lamps we have to take for plant illumination. Let's examine two schemes: the illumination with a luminescent lamp, and a gas-discharge tube.

The quantity of luminescent lamp can be determined, when average level of illumination on the surface is known. It is necessary to calculate luminous flux in lumens (you have to multiply illumination in luxes by surface area in meters). The light loss is about 30% for lamp set on the height of 30 cm (1 ft), and 50% for lamp set on the height of 60 cm (2 ft) above the plants. It is true if you use reflector. Without it light losses grow in several times. Having determined the luminous flux of the lamps, you can calculate their total power, knowing that luminescent lamps give about 65 lm per 1 W.

As an example, let's determine how many lamps one needs to illuminate a 0.5 x 1 meter (approx 1.5 x 3 ft) shelf. The area of the illuminated surface is 0.5x1=0.5 sq. m. Suppose that we have to illuminate plants preferring moderate light (15000 lx). To lighten evenly the entire surface with such a degree of illumination is hard. Therefore, we'll do a calculation assuming as a basis an average illumination 0.7 x 15000=11000 lx, and placing the plants that demand more light under the lamp, where the degree of illumination is above the average.

So we need 0.5 x 11000=5500 lm. Since the lamp on the height of 30 cm (1 ft) have the light loss about 30%, actually we need about half as much again of light, i.e. about 8250 lm. The total power of the lamps has to be about 8250/65=125 W, i.e. two compact luminescent lamps of 55 watt each, with reflector, will provide the demanded quantity of light. If you want to set the usual tubes of 40 watt each, then you will need three, or even four pieces, since tubes that are huddled together begin to screen one another, decreasing the effectiveness of the lighting system. Try to use modern compact luminescent lamps instead of the cheap and mostly out-of-date, tubes. If you don't use reflector, then the given scheme will require thrice or four times as much lamps.

Calculation of quantity of luminescent lamps
1. Choose illumination level.
2. The necessary luminous flux on the surface:
L=0.7 x A x B
(length and width in meters)
3. The necessary luminous flux of the lamps (with reflector, and losses taken into consideration):
Lamp=L x C
(C=1.5 for lamp on the height of 30 cm, C=2 for lamp on the height of 60 cm)
4. Total power of the lamps:

The calculation is similar also for gas-discharged lamps. The special luminary with 250 W sodium lamp provides an average level of illumination 15 thousands lx on the area of 1 sq. m. (10 sq ft)

If you know lighting parameters of the lamp, then to calculate the degree of illumination is quite easy. For example, the picture shows that the lamp (OSRAM 80 W) lights the circle of about one meter (3 ft) in diameter at little less than half a meter from the lamp. The maximal value of illumination is 4600 lx. The illumination at the edge is decreasing rather fast, therefore, such a lamp can be used only for plants that do not need much light.

On this figure you can see a lumious intensity curve (the same lamp as above). In order to find illumination value on the distance from the lamp, it is necessary to divide the value of the light power by square of the distance. For example, on the distance of half a meter (1.5 ft) under a lamp, the level of illumination is 750/ (0.5x0.5) =3000 lx.

Very important point: lamps must not to be overheated. With rise of temperature, their light efficiency significantly drops down. Reflector should have openings for cooling. If you use a lot of fluorescent lamps, then you should install a cooling fan, for example, computer cooler. Powerful gas-discharge lamps usually have a build-in fan.


In this article we've examined various issues of plant illumination. However, many questions remained still untainted. For example, very important is selection of the optimal electric scheme of switching on the system of lamps. Those of you who are interested in this question, are better to consult with specialists or literature.

The most rational scheme of designing the illumination system begins with a determination of the necessary level of illumination. Then you have to calculate the quantity of lamps, and their type. And only after these questions are settled, then hurry up to the store to buy the lamps.

Good luck!

Selected photos by Alena Malinovskaya, Andrew Litovkin, Julianna Neginskaya