Table of contents:
- Plant lighting
- To be or not to be
- What is good lighting
- How much light do plants need
- Illumination measurement
- Using a reflector
- How to make a homemade reflector for a fluorescent lamp
- Duration and quality of lighting
- Calculation of lamp power
- Part 1: Why light plants. Mysterious lumens and suites
- Part 2: Plant Lamps
- Part 3: Choosing a Lighting System
In this part we talk about calculating lamp power, practical measurement of illumination, etc.
In the previous parts, we talked about the basic concepts and the different types of lamps used to illuminate plants. This part tells about which lighting system to choose, how many lamps are needed to illuminate a particular plant, how to measure the lighting at home, and what reflectors are needed in lighting systems.
Light is one of the most important factors in successful plant maintenance. Plants use photosynthesis to “make food” for themselves. Little light - the plant is weakened and either dies of "hunger", or becomes an easy prey for pests and diseases.
To be or not to be
So you've decided to install a new lighting system for your plants. First of all, answer two questions.
- What's your budget constraint? If there is a small amount of money allocated to the entire lighting system that you ripped off the scholarship, and you need to meet it, then this article will not help you. The only advice is buy what you can. Don't waste time and energy looking. Unfortunately, lighting systems for plants or an aquarium are 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 to lament over a semi-dead gardenia, which is sorely lacking.
- Are you just going to perekantovat until spring, according to the principle "not to fat, I would live"? 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 conditions, such as aquariums.
If you have decided on the answers to these questions and decided to install a complete lighting system, then read on.
What is good lighting
Three main factors determine whether a lighting system is good or bad:
- Light intensity. There should be enough light for the plants. Weak light cannot be replaced by long daylight hours. There is not much light in indoor conditions. It is quite difficult to achieve illumination, which happens on a bright sunny day (more than 100 thousand lux).
- Lighting duration. Different plants require different daylight hours. Many processes, such as flowering, are determined by the length of daylight hours (photoperiodism). Everyone has seen the red poinsettia (Euphorbia pulcherrima) sold at Christmas and New Years. This bush grows under the window of our house in the south of Florida and every winter, without any tricks on our part, it “does everything by itself” - we have what is needed 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 a 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 spectrum, for example, compact fluorescent or metal halide, 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, the temperature or the concentration of carbon dioxide, etc. When growing aquarium plants, it often happens that under 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 their light requirements. The figures for each of the groups are fairly approximate, since many plants can feel good both in bright light and in the shade, adapting to the light level. The same plant needs a different amount of light, depending on whether it develops vegetatively, blossoms or bears fruit. From an energetic point of view, flowering is a process that wastes a lot of energy. The plant needs to grow a flower and supply it with energy, while the flower itself does not generate energy. And fruiting is an even more wasteful process. The more light, the more energy "from a light bulb" the plant can store for flowering, the more beautiful your hibiscus will be, the more flowers there will be on the jasmine bush.
Below are some plants that prefer certain lighting conditions. Illumination levels are expressed in lux. Lumens and suites have already been discussed in the first part. Here I will only repeat that the suites characterize how "light" the plants are, and the lumens characterize the lamps with which you illuminate these plants.
- Bright light. These plants include those that grow in nature in an open place - most trees, palms, succulents, bougainvillea, gardenia, hibiscus, ixora, jasmine, plumeria, tunbergia, crotons, roses. These plants prefer a high level of illumination - at least 15-20 thousand lux, and some plants require 50 thousand or more lux for successful flowering. Most variegated plants require high illumination, otherwise the leaves may "return" to a uniform color.
- Moderate light. These plants include underbrush plants - bromeliads, begonias, ficus, philodendron, caladium, chlorophytum, brugmanzia, brunfelsia, clerodendrum, crossandra, medinilla, pandorea, rutia, barleria, tibukhina. The desired illumination level 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 dragon tree in the darkest corner. It's just that some plants can grow (rather exist) in low light. If you're not chasing growth rates, they'll do well in low light. Basically, these are plants of the lower tier - hamedorea, whitefeldia, anthurium, diphenbachia, philodendron, spathiphyllum, echinanthus. From 5 to 10 thousand lux is enough for them.
The illumination levels given are approximate enough to serve as a starting point for choosing a lighting system. I emphasize once again that these figures are for the full growth and flowering of a plant, and not for "wintering", when you can get by with a lower level of illumination.
So now you know how much light your plant needs and you want to check if it is getting everything it is supposed to. 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're in luck (pictured on the left). If there is no light meter, then do not despair. The light meter of the camera is the same light meter, but instead of the illumination it gives out the shutter speed, i.e. time to open the camera shutter. The lower the illumination, the longer the time. It's simple.
If you have an external exposure meter, place it where you are measuring the illumination, so that the photosensitive element is perpendicular to the direction of the incident light on the surface.
If you are using a camera, place a sheet of white matte paper perpendicular to the direction of the incident light (do not use glossy paper - it will give incorrect results). Select the frame size so that the leaf fills the entire frame. You don't need to focus on it. Select the film sensitivity - 100 units (modern digital cameras allow you to "simulate" the film sensitivity). Use the shutter speed and aperture values to determine the illumination in the table. If you set the film sensitivity to 200 units, then the tabular values must be halved, if the value is set to 50 units, then the values are doubled. Moving to the next higher f-number also doubles the values. In this way, you can roughly estimate the level of illumination where your plants are standing.
|Aperture||Excerpt||Illumination (Lx) for film 100 units|
|External exposure meter||Camera when aiming at a sheet of paper|
Using a reflector
If you use a fluorescent lamp without a reflector, then you reduce the useful light several times. As you can easily understand, only the light that is directed downward hits the plants. The light that is directed upwards is useless. The light that dazzles your eyes when you look at an open lamp is also useless. A good reflector directs the blinding light down onto the plants. The results of modeling 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 the plants, and not everything around.
Most fixtures sold in hardware stores do not have a reflector or have something that should not be called a reflector. Special lighting systems for 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, is of no fundamental importance - any "good" shape, 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 within 10-15%. The figure shows a cross-section of a reflector. It can be seen that its height should be such that all rays above the boundary (ray 1 in the figure) are intercepted by a reflector - in this case, the lamp will not blind the eyes.
Given the direction of the reflected boundary ray (for example, downward or at an angle), it is possible to construct a perpendicular to the surface of the reflector at the point of reflection (point 1 in the figure), which divides the angle between the incident and reflected rays in half - the law of reflection. In the same way, the perpendicular is determined at the remaining 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 be sent back into the lamp. This will keep the lamp warm.
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 efficiency will be practically the same as for a "mirror" reflector. Be sure to punch holes on top of the reflector for ventilation.
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 the poinsettia mentioned above bloom) can be found in the specialized literature. For most plants, the above figure is sufficient.
The quality of lighting has already been discussed more than once. One of the illustrations is a photograph of plants grown under illumination with a mercury lamp (a picture from an old book, at that time 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 the plants in a way that makes them pleasant to look at. In this sense, a sodium lamp 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.
Calculation of lamp power
So we come 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 illumination level on the surface. Find the luminous flux in lumens (by multiplying the illumination in lux by the surface area in meters). The 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 are using a reflector. Without it, the losses increase several times. Having determined the luminous flux of the lamps, you can find their total power, knowing that fluorescent lamps give about 65 lumens per watt of power.
As an example, let's estimate how many lamps are required for lighting for a 0.5 × 1 meter shelf. Illuminated surface area: 0.5 × 1 = 0.5 sq. M. Let's say we need to illuminate plants that prefer moderate light (15,000 Lx). It will be difficult to illuminate the entire surface with this 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.
In 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 55 W compact fluorescent lamps with reflector provide the right amount of light. If you want to install ordinary 40 W tubes, then three pieces or even four of them will be required, since tubes placed close to each other begin to mutually shield, and the efficiency of the lighting system decreases. Try to use modern compact fluorescent lamps instead of conventional, mostly outdated, tubes. If you do not use a reflector, then in this scheme you will have to take three or four times as many lamps.
Calculation of the number of fluorescent lamps
- Choose 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 gas discharge lamps, the calculation is similar. A special luminaire with a sodium lamp with a power of 250 W provides an average illumination level of 15 thousand lux on an area of 1 sq. M.
If the lighting parameters of the luminaire are known, then it is quite simple to calculate the illumination. For example, from the figure on the left you can see that the luminaire (OSRAM Floraset, 80W) illuminates a circle about a meter in diameter at a distance of just under half a meter from the lamp. The maximum illumination value is 4600 Lx. Illumination to the edge falls off rather quickly, so such a lamp can only be used for plants that do not need a lot of light.
The figure on the left shows the luminous intensity curve (same light as above). To find the illuminance at a distance from the luminaire, divide the luminous intensity by the square of the distance. For example, at a distance of half a meter under the lamp, the illumination value will be 750 / (0.5 × 0.5) = 3000 Lx.
A very important point - the lamps should not overheat. As the temperature rises, their light output drops sharply. The reflector should have cooling holes. If many fluorescent lamps are used, a cooling fan such as a computer should be used. High-power gas discharge luminaires usually have a built-in fan.
This series of articles has covered various issues of plant lighting. Many issues remained unaffected, for example, the choice of the optimal electrical circuit for switching on lamps, which is an important point. For those who are interested in this issue, it is better to 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 toptropicals.com team for permission to publish the article on our resource.