Plant lighting. Part 3: Choosing a Lighting System
- Part 1: Why illuminate plants. Mysterious Lumens and Suites
- Part 2: Lamps for lighting plants
- Part 3: Choosing a Lighting System
In this part, we talk about calculating the power of lamps, the practical measurement of illumination, etc.
In the previous parts, we talked about the basic concepts and the various types of lamps used to illuminate plants. This part tells about which lighting system to choose, how many lamps will be needed to illuminate a particular plant, how to measure illumination at home, and why reflectors are needed in lighting systems.
Light is one of the most important factors in the successful maintenance of a plant. By photosynthesis, plants "make food" for themselves. Little light - the plant is weakened and either dies of “hunger” or becomes easy prey for pests and diseases.
To be or not to be
So, you decided to install a new lighting system for your plants. First of all, answer two questions.
- What is your budget limited? If a small amount of money has been allocated for the entire lighting system, which you have torn off from the scholarship, and you need to keep within it, then this article will not help you. The only advice is to buy what you can. Do not waste your energy and time searching. Unfortunately, a lighting system for plants or for an aquarium is not cheap. Sometimes a more reasonable alternative is to replace light-loving plants with shade-tolerant ones - it is better to have a well-groomed spathiphyllum, which does not require a lot of light, than lament for the half-dead gardenia, which is sorely lacking.
- Are you going to just go out before spring, on the principle of "not fat, I would be alive"? Then just buy the simplest fluorescent lamp. If you want your plants to grow fully and even bloom under the lamps, then you need to spend energy and money on the lighting system. Especially if you grow plants that grow year round under artificial lighting, such as aquarium.
If you have decided on the answers to these questions and decided to install a full-fledged lighting system, then read on.
What is good lighting?
Three main factors determine whether a lighting system is good or bad:
- Light intensity. Light should be enough for plants. Weak light cannot be replaced with a long daylight. There is not much light in room conditions. To achieve the illumination that happens on a bright sunny day (more than 100 thousand lux) is quite difficult.
- Lighting duration. Different plants require daylight hours of varying durations. Many processes, for example, flowering, are determined by the duration of daylight hours (photoperiodism). Everyone saw the red poinsettia (Euphorbia pulcherrima), sold for Christmas and New Year. This bush grows under the window of our house in southern Florida and every year in winter, without tricks on our part, “does everything by itself” - we have what is necessary for the formation of red bracts - long dark nights and bright sunny days.
- Lighting quality. In previous articles, I touched on this issue, saying that the plant needs light in both the red and blue regions of the spectrum. As already mentioned, it is not necessary to use special phytolamps - if you use modern lamps with a wide range, for example, compact fluorescent or metal halide lamps, then your spectrum will be “correct”.
In addition to these factors, others are certainly important. The intensity of photosynthesis is limited by what is currently lacking. In low light - this is light, when there is a lot of light, then, for example, temperature or concentration of carbon dioxide, etc. When growing aquarium plants, it often happens that in strong light, the concentration of carbon dioxide in the water becomes a limiting factor and stronger light does not increase the rate of photosynthesis.
How much light do plants need
Plants can be divided into several groups according to light requirements. The figures for each of the groups are quite approximate, since many plants can feel good both in bright light and in the shade, adapting to the level of illumination. For the same plant, a different amount of light is necessary, depending on whether it develops vegetatively, blooms or bears fruit. From an energy point of view, flowering is a process that is wasting a large amount of energy. The plant needs to grow a flower and supply it with energy, despite the fact that the flower itself does not produce energy. And fruiting is an even more wasteful process. The more light, the more energy “from a light bulb” a plant can store for flowering, the more beautiful your hibiscus will be, the more flowers there will be on a jasmine bush.
Below are some plants that prefer certain lighting conditions. Light levels are expressed in lux. About lumens and suites has already been said in the first part. Here I will repeat only that suites characterize how “light” plants are, and lumens characterize the lamps with which you illuminate these plants.
- Bright light. These plants include those that naturally grow in the open - most trees, palm trees, succulents, bougainvillea, gardenia, hibiscus, ixora, jasmine, plumeria, tunbergia, crotons, roses. These plants prefer a high level of lighting - at least 15-20 thousand lux, and some plants require 50 or more thousand lux for successful flowering. Most variegated plants require high illumination, otherwise the leaves may "return" to a uniform color.
- Moderate light. These plants include undergrowth plants - bromeliads, begonias, ficus, philodendron, caladium, chlorophytum, brugmansia, brunfelsia, clerodendrum, crossander, medinilla, pandorea, rutia, barleria, tibuhin. The desired level of illumination for them is 10-20 thousand lux.
- Weak light. The concept of "shade-loving plants" is not entirely true. All plants love light, including the dracaena standing in the darkest corner. Just some plants can grow (more likely to exist) in low light. If you do not pursue the growth rate, then they will feel good in low light. Basically, these are plants of the lower tier - chamedorrhea, whitefeldia, anthurium, diphenbachia, philodendron, spathiphyllum, echinanthus. It is enough for them from 5 to 10 thousand lux.
The given light levels are approximate and can serve as a starting point for choosing a lighting system. I emphasize once again that these figures are for full-fledged growth and flowering of a plant, and not for “wintering,” when a lower level of illumination can be dispensed with.
So, now you know how much light your plant needs and want to check if it gets everything that it needs. All theoretical calculations are good, but it is better to measure the actual illumination where the plants are. If you have a light meter, then you are in luck (pictured left). If there is no light meter, then do not despair. The exposure meter of the camera is the same light meter, but instead of illumination it produces shutter speeds, i.e. the time it takes to open the camera shutter. The lower the illumination, the longer the time. Everything is simple.
If you have an external exposure meter, put it in the place where you measure the illumination, so that the photosensitive element is perpendicular to the direction of the light incident on the surface.
If you use the camera, then put a sheet of white matte paper perpendicular to the direction of the incident light (do not use glossy - it will give incorrect results). Select the frame size so that the sheet occupies the entire frame. Focusing on him is optional. Select a film sensitivity of 100 units (modern digital cameras allow you to "simulate" film sensitivity). Using the shutter speed and aperture values, determine the illumination in the table. If you set the film sensitivity value to 200 units, then the tabular values must be halved, if the value is set to 50 units, then the values are doubled. Going to the next higher aperture also doubles the value. In this way, you can roughly estimate the level of illumination where your plants are.
Illumination (Lx) for a film of 100 units
External light meter
Camera when hovering over a sheet of paper
If you use a fluorescent lamp without a reflector, then you reduce the useful light by several times. It is easy to understand that only the light that is directed downwards enters the plants. That light which is directed upwards is useless. The light that blinds your eyes when you look at the open lamp is also useless. A good reflector directs the light, blinding the eyes, down to the plants. The simulation results of a fluorescent lamp show that the illumination in the center, when using a reflector, increases almost three times, and the light spot on the surface becomes more concentrated - the lamp illuminates plants, and not everything around.
Most fixtures sold in household appliances stores do not have a reflector or what should not be called a reflector. Special systems for lighting plants or an aquarium with reflectors are very expensive. On the other hand, making a homemade reflector is not difficult.
How to make a homemade reflector for a fluorescent lamp
The shape of the reflector, especially for one or two lamps, does not matter - any “good” form, in which the number of reflections is not more than one and the return of light to the lamp is minimal, will have approximately the same efficiency in the range of 10-15%. The figure shows a cross section of the reflector. It can be seen that its height should be such that all the rays above the boundary (ray 1 in the figure) are intercepted by the reflector - in this case the lamp will not blind your eyes.
Given the direction of the reflected boundary beam (for example, down or at an angle), we can construct a perpendicular to the surface of the reflector at the reflection point (point 1 in the figure), which divides the angle between the incident and reflected beam in half - the law of reflection. In the same way, the perpendicular is determined at the other points (point 2 in the figure).
For verification, it is recommended to take a few more points so that the situation depicted at point 3 does not turn out, where the reflected beam does not go down. After that, you can either make a polygonal frame, or build a smooth curve and bend the reflector according to the template. Do not place the top of the reflector close to the lamp, as the rays will fall back into the lamp. In this case, the lamp will warm up.
The reflector can be made either from aluminum foil, for example, food, which has a fairly high reflection. You can also paint the reflector surface with white paint. Moreover, its effectiveness will be almost the same as for the "mirror" reflector. Be sure to make holes on top of the ventilation reflector.
Duration and quality of lighting
The duration of lighting is usually 12-16 hours, depending on the type of plant. More accurate data, as well as recommendations on photoperiodism (for example, how to make poinsettia mentioned above bloom) can be found in the specialized literature. For most plants, the above figure is enough.
About the quality of lighting has been said more than once. One illustration is a photograph of plants grown under lighting with a mercury lamp (a photograph from an old book, while there were practically no other lamps) and an incandescent lamp. If you do not need long and skinny plants, then do not use incandescent or sodium lamps without additional illumination with fluorescent or gas-discharge lamps with radiation in the blue region of the spectrum.
Among other things, plant lamps should illuminate plants so that they are pleasant to look at. A sodium lamp in this sense is not the best lamp for plants - the photo shows how plants look under such a lamp compared to lighting with a metal halide lamp.
Lamp power calculation
So we came to the most important thing - how many lamps to take for lighting plants. Consider two lighting schemes: fluorescent lamps and a gas discharge lamp.
The number of fluorescent lamps can be determined by knowing the average level of illumination on the surface. It is necessary to find the luminous flux in lumens (multiplying the illumination in lux by the surface area in meters). Light loss is approximately 30% for a lamp hanging at a height of 30 cm from plants, and 50% for lamps at a distance of 60 cm from plants. This is true if you use a reflector. Without it, losses increase several times. Having determined the luminous flux of the lamps, one can find their total power, knowing that fluorescent lamps give about 65 Lm per Watt of power.
For an example, let us estimate how many lamps are needed for lighting for a shelf measuring 0.5 × 1 meter. Area of the illuminated surface: 0.5 × 1 = 0.5 sq.m. Suppose that we need to illuminate plants that prefer moderate light (15,000 Lux). It will be difficult to illuminate the entire surface with such illumination, so we will make an estimate based on an average illumination of 0.7 × 15000 = 11000 Lx, placing plants that require more light under a lamp where the illumination is above average.
Total, you need 0.5x11000 = 5500 Lm. Lamps at a height of 30 cm should give about one and a half times more light (losses are 30%), i.e. about 8250 lm. The total power of the lamps should be about 8250/65 = 125 W, i.e. two compact 55 W fluorescent lamps with a reflector provide the right amount of light. If you want to put ordinary tubes at 40 watts, then they will need three pieces or even four, since the tubes placed close to each other begin to shield each other, and the efficiency of the lighting system decreases. Try to use modern compact fluorescent lamps instead of the usual, for the most part obsolete, tubes. If you do not use a reflector, then in this scheme you will have to take three or four times more lamps.
Calculation of the number of fluorescent lamps
- Select a light level.
- Required luminous flux on the surface:
L = 0.7 x A x B
(length and width in meters)
- Required luminous flux of lamps, taking into account losses (in the presence of a reflector):
Lamp = L x C
(C = 1.5 for a lamp at a height of 30 cm and C = 2 for a lamp at a height of 60 cm)
- Total lamp power:
Power = Lamp / 65
For discharge lamps, the calculation is similar. A special lamp with a 250 W sodium lamp provides an average level of illumination of 15 thousand lux on a site measuring 1 sq.m.
If the lighting parameters of the lamp are known, then it is very simple to calculate the illumination. For example, the figure on the left shows that the lamp (OSRAM Floraset, 80W) illuminates a circle with a diameter of about a meter at a distance of slightly less than half a meter from the lamp. The maximum value of illumination is 4600 Lux. Illumination to the edge decreases quite quickly, so this lamp can only be used for plants that do not need a lot of light.
The figure on the left shows the light intensity curve (the same lamp as above). To find the illumination at a distance from the lamp, it is necessary to divide the value of the light intensity by the square of the distance. For example, at a distance of half a meter under the lamp, the illumination value will be equal to 750 / (0.5 × 0.5) = 3000 Lux.
A very important point - the lamps should not overheat. With increasing temperature, their light output decreases sharply. The reflector must have openings for cooling. If you use a lot of fluorescent lamps, you should use a fan for cooling, such as a computer.High-power discharge lamps usually have a built-in fan.
In this series of articles, various aspects of plant lighting have been addressed. Many issues remained unaffected, for example, the choice of the optimal electrical circuit for turning on the lamps, which is an important point. Those who are interested in this issue will better turn to the literature or specialists.
The most rational scheme for designing a lighting system begins with determining the required level of illumination. Then you should evaluate the number of lamps and their type. And only after that - rush to the store to buy lamps.
Special thanks to the staff of the site toptropicals.com for permission to publish the article on our resource.