Lights & Plants Plant growth, harvest, potency and even the time to flower are all dependent on the light they receive. Light quality, intensity and duration are all important. The following is a brief introduction to plants, light, lumens and PAR; Light is a plants food, nutrients are only building blocks for the plant cells but it is light that provides the energy ~ so how does it work? When light falls onto leaves it triggers the process of photosynthesis, which in simple terms is the process of turning light, which is radiant energy, into chemical energy. The amazing process of photosynthesis, turning light energy into chemical energy, is one of nature's wonders. This energy transfer happens inside the plants cell structures called chloroplasts. The basic components of chloroplasts are individual membranous sacs which contain fats, proteins and pigments (stay with us ~ it's worth knowing!) Pigments & Chemical Energy Pigments play an important part. They absorb light in the photosynthesis process of turning light energy into chemical energy. Chlorophyll, for example, is an important pigment which absorbs red and blue wavelengths. There are different types of pigment and each absorbs different wavelengths of light. The light absorbed by the pigment causes a reaction, which produces chemical energy (it makes electrons out of the light, and the electrons use their charges to make sugar energy for the plant) For those interested in the current theory of photosynthesis, it works something like this ~ The chemical energy produced by the chlorophyll (pigment) from light is sufficient to split the water molecules apart. This provides units of hydrogen (H) and hydroxide (OH). The hydroxide combines with carbon dioxide, which is absorbed from the air, to produce carbohydrates, which provides the energy for plant growth. (and you thought there was nothing going on in your plants ~ for more details visit your library!) So to summarise ~ light falls on the leaves which convert it into electrons and the plant uses these electrons (electricity) to make energy as sugar. Light & Light Measurement Light, its intensity, quality, its colour, spectrum, wavelength are therefore all-important factors, but how do we measure light and what are the most important components? Light is measured in photons (which we, or at least I, do not really understand). Light actually hits objects, just like a spray of water, and the sun emits lots of light photons ~ to give you an idea of how many; the sun hits our body with over 12,000,000,000,000,000,000,000,000 photons every second and a plant needs about 20 photons to make a finished molecule of sugar. So our scientists can count the number of photons hitting the plant and even predict how much of this energy will convert into flowers or fruits. Each industry has its own way of measuring light. Photographers use a light meter, the lighting industry uses lumens or lux and the gardening industry uses PAR. All are only measurements; the actual light coming from a lamp or the sun does not change ~ only the methods we use to measure are different. Light Spectrums & Colours Light from the sun is ideal but it's not the same as artificial light, where output quality, spectrum etc varies upon the type of lamp and how it is used. Many growers think that more lumens = better growth / yields, when in fact artificial light, even at its best in a HID or HPS lamp in not so good in terms of colours. Much of the light from the bulb is not used by the plant, mainly because it is not in the 400 to 700 nw (nanowave) spectrum, and plants can only see and use light in this range. Light quality and its colours are as important as lumens. Light, as seen by plants is not a single colour but separate bands of active colours and the plant senses each colour-band of light as a separate signal. Each band of colour has a different effect on plants and the following are only a few of the functions which each band of light promotes. Blue Light (350 – 500 NW) powers chlorophyll production, powers cell actively, energies the stomata movement and makes the plant follow light. Green / Yellow Light (500 – 650 nw) ~ not much action from these bands of light. Red Light (600 – 700 NW) makes sugar from CO2, powers chloroplast production, signals light and dark times among other functions. Strong blue and red light photons (as above) are also needed for good carbon dioxide uptake. If you are wondering about NW, it is also referred to as nanometers NM. Most people use Kelvin as a reference to color Temp instead of using NW or NM. In my next post i will delve into spectrum and specific lighting a little more. The PAR scale measures all these coloured photons between 400nw & 700nw, the critical range for plants, as this is only range that plants can use light. If it is not in this range then it's wasted light. PAR For growers PAR is all-important ~ and as important as lumens! PAR stands for > Photosynthetic Active Radiation. Photosynthetic, the light sensed by a leaf pigment. Active, the light that causes the leaf pigment to become active for making energy Radiation, another word for light & photon energy PAR is a measurement scale used internationally as a metric light measurement and is becoming more and more relevant to growing and greenhouse light measurement. Why is it important to you? PAR is the measure of light that a plant actually senses and uses, and it is the light the plant sees and can use that is more important then the actual output lumen of the grow lamp! * A large HID lamp may give out loads of lumens, but if it's too far away from your plant most are wasted (remember light intensity diminishes with distance) In addition the light a plant can use from these lamps is limited because the plant cannot see or use it because it is in the wrong spectrum. So the main value of the PAR measurement is that it is the only measure that takes into account the actual light and light colours that the plant uses to energise its pigments and generate sugar energy, and it's the sugar that makes your plants grow and produce such sweet fruits! PAR and Fluorescent Lamps In the past fluorescent lamps were always known to have excellent 'daylight' colour output but not the same photon power as HID / HPS lamps. The spectrum from fluorescents was ideal for propagation/seedlings but not for real time growing, because they were small watt versions and did not have the lumen / photon output. (i.e. an average household fluorescent tube is only about 35 watts. Nice spectrum but low light output! ) HID and HPS lamps have large lumen / photon output but are poor on colours omitted, but these lamps were the best available lamps at the time. However they do generate lots of heat and can be expensive to operate. They also need separate ballasts, control contacts and systems. CFL Lamps (CFL = Compact Fluorescent Lamps) The development of high-output compact fluorescent means you can now get the correct colour spectrum, always associated with fluorescents, but with much higher light output. This means that CFL's are now capable of much, much higher lumen output with all the benefits of the ideal, spectrum output. Photon strength is still not as strong as HID Lamps (although with our new reflectors we are getting there) but because these new lamps generate much less heat they can be placed just inches of the leaves, and this is a very, very important factor when using grow lamps Light Intensity Light intensity diminishes the further it has to travel. This is the same for HID, CFL or your normal household lamp. If you hold a light meter up close to any lamp and then slowly lower the meter, even a few inches, you will see the light measurement reduces dramatically. (If you can borrow, beg, steal or get access to a light meter please try this ~ you will be extremely surprised at the rate at which the light intensity reduces over a short distance) Light from an HID or HPS lamp reduces by half for every foot it is away from the plants. So if your lamps are 2' or 3' above your plants much of the light is wasted. The problem with HID lamps is they are so hot you cannot place them close to your plants and much of the light, and your money, is wasted. One benefit of using high-out put CFL's is that they do not generate as much heat and can be kept almost on top of the plants producing the exact 100% PAR light, with no loss of intensity. So if you position these new lamps close to the leaves you get the benefit of 100% PAR light in the correct 400nw to 700nw range, giving the plant the correct light colours and light quality. So now that we know all this useless stuff, lets grow some bomb ass shit.
You should always include what color spectrum they are in kelvins. NW aren't on light bulb packages...
The diagrams above shows the full range of light and where each type of lighting system falls within that range. Artificial lights produce just a slice of the full range. This leads to much discussion and experimentation to determine which, or which combination of lighting is best for a particular crop. Lets establish a reference point to work from, examine several types of lighting and put this information to practical use. Reference point: For most of the daylight hours, the outside daylight peak is centered on 5500 degrees Kelvin (refer to the above chart). Metal Halide: These lights emit a light on the bluish side of the spectrum. They are considered a grow light and it is considered that they produce a more stalky vegetative type of growth in plants. These lights are commonly used throughout all phases of plant growth and produce excellent results. Agro Sun Halide: Agro Sun is a hybrid halide bulb that generates extra red light for flower and fruit production. Sodium Vapor: Sodium vapor lighting is way down in the red. There is some indication that the spectrum produced by these lights promote flowering. Thanks to ik3002 for extra info!
Okay I have a question about CFL lights for growing. When you guys say you are using CFLs, are you just using the same compact flourescent bulbs that you can put into regular old ceiling light sockets (or whatever the fuck their called) or is there like a special type of CFL made solely for growing plants?
hey NJ! you can veg under daylight CFL bulbs and you can flower under warm CFL's veg is blue-ish and flower is red-ish I used 22 watt daylight cfls in clamp lights from home depot (1 per plant) for veg all the time until now
Thanks green. I couldnt be any more of a noob at growing but I really want to start soon. Edit: Sorry, one more question. On google I'm getting to different types of CFLs. Thete the one that is like two vertical tubes connected at the tops, and one that is a continuous spirling tube, which one is better for growing?
Excellent thread!!! Quick question if possible. I am currently growing using a total of 14 18W CFL emitting a total of around 1500W (energy saving bulbs). I have found t5 Arcadia Plant Pro Lights (I like the name) promising excellent results. http://www.arcadia-uk.info/product.p...10&id=4&lan=en The spectrum is: I take it that they would be good for veg and then switch back to my white ones during flowering. They are a bit expensive. It would cost me £200 to change them all, but should that mean an extra 0.5 ounce per plant, and I have 5, then it could be worth going for.
DT When you calculate CFL or Flouro wattage you use the actual wattage of the bulb. on the packaging they usually say "like 125w" this is the incandescent wattage. when you compare the usable lumen's, the wattage will come out to whatever the bulb is rated for. I hope this makes sense? i shouldn't talk when I'm blitzt. As for your other question on switching out, it would help to know exactly what you are using to give an answer. I used 10 28w t5ho, 5 warm Phillips silhouette(2500k) t5ho, and 5 cool (Osram)Sylvania Pentron t5ho(5500k) for veg and I loved it. Not to mention the plants did too. these costs under 10 Euros too, and they have a really good light spectrum.
There are Four types of lights that are right for growing marijuana: FLUORESCENT LIGHTS Fluorescent bulbs are well suited for growing marijuana in the seedling & vegetative stage of growth and are also recommended for clones and mother plants. Many growers prefer the use of fluorescents during vegetative growth because of their slower pace of growth and better root development. Fluorescents fixtures such as a shoplight can be purchased at any home improvement store for under $20. When using fluorescent bulbs try and find ones with a highest color temperature in the blue range of the spectrum (6500K). These are typically your "cool white" or "full spectrum" bulbs. Keep the tops of your plants just inches away from the fluorescent lights for best development. Fluorescent bulbs are not good for flowering marijuana plants. They simply do not put out the lumens needed to develop meaningful buds. The light from a fluorescent bulb will not penetrate the canopy. They are also comparatively inefficient to HID (High Intensity Discharge) lights such as high pressure sodium or metal halide. 10 x 40W fluorescent bulbs consume the same amount of energy as a 400 watt HID but produce far less luminosity. HIGH PRESSURE SODIUM LIGHTS These are the best lights to use in flowering marijuana. They come in 250W, 400W, 430W, 600W & 1000W. The higher the wattage the larger an area the light will covert and the larger the bud development will be. These light bulbs require a ballast as they do not fit into regular sized light sockets. HPS are very efficient and put out more light with less energy than any other type of illumination available to indoor growers. HPS lighting is expensive and can range from $100 - $500 dollars in price. HPS lights, as all HID lights produce considerably more heat than standard fluorescents so attention has to be paid to not have the canopy oif your garden too close to the ligh bulb. METAL HALIDE LIGHTS Another kind of HID lighting is metal halide. These bulbs are recommended for vegetative growth because they operate in the blue end of the color temperature. Metal halide bulbs can also be used for flowering although the bud development will not be as big as with HPS lights. Edit: LED's And yet another kind of lighting is LED's. Although I have done a considerable amount of research in this area, a small thread, like this, could not really touch on this type of lighting. WATTAGE -- COVERAGE For best plant developement especially during the flowering stage your garden will need approximately (Lets just say at least) 50 watts of HID lighting per square foot of illuminated area. Below is a general guide for choosing the right wattage for your garden. A 250 watt HID will illuminate a 2' x 2' garden. A 400 watt HID will illuminate a 3' x 3' garden. A 600 watt HID will illuminate a 4' x 4' garden. A 1000 watt HID will illuminate a 5' x 5' garden. Each wattage has a limited range beyond which you do not have good light penetration. These numbers assume you have a good reflector around your bulb as well as reflective walls painted white or covered with reflective mylar. You can increase the figures a bit if using multiple bulbs. You can also increase coverage using a light mover. A growspace that's 2 x 10 feet would require 1000 watts if you go by the 50 WPSF guideline. However a 1000W bulb only covers an area about 5 feet across meaning the edges of your garden will be dark. A better choice in this case would be three 400W or two 600W.
LED Parts LEDs come in all shapes and sizes, but the 3mm T-1 or 5mm T-1¾ are probably the most common. The die is an itty bitty cube of semiconductor, the composition of which determines the color of the light given off. It sits in the bottom of the die cup, which has reflective sides to reflect the light emitted by the die toward the dome end of the LED. The epoxy body is shaped to act as an inclusion lens and focus the light into a beam. The distance from the die cup to the domed end of the lens determines how tightly focused is the resulting beam of light. Some LEDs have flat or even concave ends to dispurse the light into a wide beam. LED Color Visible LEDs <table align="right" cellpadding="2"> \t<tbody><tr> \t\t<th>Wavelength nm</th> \t\t<th>Color Name</th> \t\t<th>Color Sample</th> \t</tr> \t<tr> \t\t<td align="center">over 1100</td> \t\t<td align="center">Infrared</td> \t\t<td bgcolor="#000000"> </td> \t</tr> \t<tr> \t\t<td align="center">770-1100</td> \t\t<td align="center">Longwave NIR</td> \t\t<td bgcolor="#000000"> </td> \t</tr> \t<tr> \t\t<td align="center">770-700</td> \t\t<td align="center">Shortwave NIR</td> \t\t<td bgcolor="#000000"> </td> \t</tr> \t<tr> \t\t<td align="center">700-640</td> \t\t<td align="center">Red</td> \t\t<td bgcolor="#ff0000"> </td> \t</tr> \t<tr> \t\t<td align="center">640-625</td> \t\t<td align="center">Orange-Red</td> \t\t<td bgcolor="#ff6000"> </td> \t</tr> \t<tr> \t\t<td align="center">625-615</td> \t\t<td align="center">Orange</td> \t\t<td bgcolor="#ffa500"> </td> \t</tr> \t<tr> \t\t<td align="center">615-600</td> \t\t<td align="center">Amber</td> \t\t<td bgcolor="#ffd000"> </td> \t</tr> \t<tr> \t\t<td align="center">600-585</td> \t\t<td align="center">Yellow</td> \t\t<td bgcolor="#ffff00"> </td> \t</tr> \t<tr> \t\t<td align="center">585-555</td> \t\t<td align="center">Yellow-Green</td> \t\t<td bgcolor="#aadd22"> </td> \t</tr> \t<tr> \t\t<td align="center">555-520</td> \t\t<td align="center">Green</td> \t\t<td bgcolor="#00ff00"> </td> \t</tr> \t<tr> \t\t<td align="center">520-480</td> \t\t<td align="center">Blue-Green</td> \t\t<td bgcolor="#00a0a0"> </td> \t</tr> \t<tr> \t\t<td align="center">480-450</td> \t\t<td align="center">Blue</td> \t\t<td bgcolor="#0000ff"> </td> \t</tr> \t<tr> \t\t<td align="center">450-430</td> \t\t<td align="center">Indigo</td> \t\t<td bgcolor="#7b00b2"> </td> \t</tr> \t<tr> \t\t<td align="center">430-395</td> \t\t<td align="center">Violet</td> \t\t<td bgcolor="#ee82ee"> </td> \t</tr> \t<tr> \t\t<td align="center">395-320</td> \t\t<td align="center">UV-A</td> \t\t<td bgcolor="#000000"> </td> \t</tr> \t<tr> \t\t<td align="center">320-280</td> \t\t<td align="center">UV-B</td> \t\t<td bgcolor="#000000"> </td> \t</tr> \t<tr> \t\t<td align="center">280-100</td> \t\t<td align="center">UV-C</td> \t\t<td bgcolor="#000000"> </td> \t</tr> </tbody></table>LED colors are often given in "nm", or nanometers, which is the wavelength of the light. The wavelength given is the wavelength of the peak output - LEDs are not perfectly monochromatic, but rather produce wavelengths over a small region of the spectrum. The graph on the left shows color vs. intensity for a typical green LED - the peak is at about 565 nm, but it is emitting light over a range of about 520 nm to 610 nm. Spectral line half-width is the width of this curve at 50% intensity (0.5 on the Y-axis) - for this LED, it is about 30 nm - and is a measure of the "purity" (monochromaticity) of the color. Notice the temperature given in the upper right corner of the graph - LEDs emit slightly different colors at different temperatures. They also emit different colors at different currents, especially white LEDs which depend on phosphors to change the colored light of the die to white light. Infrared LEDs The infrared band can be divided into Near Infrared (NIR) and Far Infrared (IR). Far infrared is the thermal infrared used to detect hot objects or see heat leaks in buildings, and is way beyond the range of LEDs. (NIR can be further divided into two bands, longwave and shortwave NIR, based on how film and CCD cameras react, which I'll get into elsewhere, elsewhen, and elsewhy.) Infrared LEDs are sometimes called IREDs (Infra Red Emitting Diodes). Ultraviolet LEDs Ultraviolet light is divided into three bands: UV-A, which is fairly innocuous; UV-B, which causes sunburns; and UV-C, which kills things. Most UV-B and all UV-C from the sun is filtered out by the ozone layer, so we get very little of it naturally. LEDs emit UV-A. 400 nm is a pretty common wavelength for UV LEDs. This is right on the border between the violet and ultraviolet, so a significant portion of the light emitted is visible. For this reason 400 nm UV LEDs are sometimes rated in millicandela, even though as much as half of their energy is invisible. LEDs with lower wavelengths, such as 380nm, are usually not rated in millicandela, but in milliwatts. DO NOT STARE INTO AN ULTRAVIOLET LED. White LEDs White light is a mixture of all the colors. Color Temperature is a measure of the relative amounts of red or blue - higher color temperatures have more blue. <table align="center"> \t<tbody><tr> \t\t<th>Color Temperature</th> \t\t<th>Example</th> \t</tr> \t<tr> \t\t<td>2000</td> \t\t<td>Gaslight</td> \t</tr> \t<tr> \t\t<td>2470</td> \t\t<td>15 watt incandescent bulb</td> \t</tr> \t<tr> \t\t<td>2565</td> \t\t<td>60 watt incandescent bulb</td> \t</tr> \t<tr> \t\t<td>2665</td> \t\t<td>100 watt incandescent bulb</td> \t</tr> \t<tr> \t\t<td>2755</td> \t\t<td>500 watt incandescent bulb</td> \t</tr> \t<tr> \t\t<td>2900</td> \t\t<td>500 watt Krypton bulb</td> \t</tr> \t<tr> \t\t<td>3100</td> \t\t<td>Projector type filament bulb</td> \t</tr> \t<tr> \t\t<td>3250</td> \t\t<td>Photo Flood</td> \t</tr> \t<tr> \t\t<td>3400</td> \t\t<td>Halogen</td> \t</tr> \t<tr> \t\t<td>3900</td> \t\t<td>Carbon arc</td> \t</tr> \t<tr> \t\t<td>4200</td> \t\t<td>Moonlight</td> \t</tr> \t<tr> \t\t<td>4700</td> \t\t<td>Industrial smog</td> \t</tr> \t<tr> \t\t<td>5100</td> \t\t<td>Hazy weather</td> \t</tr> \t<tr> \t\t<td>5500</td> \t\t<td>Sun 30 above horizon</td> \t</tr> \t<tr> \t\t<td>6100</td> \t\t<td>Sun 50 above horizon</td> \t</tr> \t<tr> \t\t<td>6700</td> \t\t<td>Electronic Flash</td> \t</tr> \t<tr> \t\t<td>7400</td> \t\t<td>Overcast sky</td> \t</tr> \t<tr> \t\t<td>8300</td> \t\t<td>Foggy weather</td> \t</tr> \t<tr> \t\t<td>30,000</td> \t\t<td>Blue sky</td> \t</tr> </tbody></table> Remember that this is a measure of color, not brightness, so don't freak out because moonlight is "hotter" than a carbon arc! It just means that the color is bluer, that's all. White LEDs have a color temperature, but monochromatic LEDs do not. LED Brightness The total power radiated as light is radiant power or radiant flux, and is measured in watts. How bright the object appears, however, will depend on two additional factors: how much radiant flux is emitted toward the observer; and how sensitive the observer is to the wavelength(s) of the light. To quantify the first, we must introduce the concept of the steradian, a solid (3-D) angle. Think of a cone with the apex at the emitter. If the radiant flux of a source is radiated uniformly in all directions, the radiant intensity will be simply the total radiant flux divided by 12.57 (4π) steradians, the solid angle of a complete sphere. In the case of LEDs, the radiant flux is usually concentrated into a beam, however, so the radiant intensity will be the radiant flux divided by the solid angle of the beam. Beam angles are usually expressed in degrees, while radiant intensity is usually expressed in mW/sr, making a conversion from beam angle to steradians necessary: sr = 2 π (1 - cos(θ/2)) where sr is the solid angle in steradians, and θ is the beam angle. <form> </form> Luminous flux and luminous intensity are measurements like radiant power and radiant intensity, only adjusted for the sensitivity of the human eye. Radiant power of a wavelength of 555 nm is multiplied by a factor of 1, but light of higher and lower wavelengths are multiplied by lower factors, until infrared and ultraviolet wavelengths are reached, when the radiant power is multiplied by zero. Luminous flux is measured in lumen, while luminous intensity is measured in lumen per steradian, also called a candela. The relationship between luminous flux, luminous intensity, and beam angle means is that focussing a given LED into a tighter beam (decreasing the beam angle) will increase its luminous intensity (brightness) without actually increasing the luminous flux (amount of light) it puts out. Keep this in mind when buying LEDs for illuminating purposes - a 2000 mcd 30 LED puts out just as much light as am 8000 mcd LED with a 15 viewing angle. (The angle is half in both width and height, so the beam is four times as bright.) This is one of the reasons that ultra-bright LEDs are often "water clear", to keep the light going in one direction and not diffuse it all over the place. The brightness of LEDs is measured in millicandela (mcd), or thousandths of a candela. Indicator LEDs are typically in the 50 mcd range; "ultra-bright" LEDs can reach 15,000 mcd, or higher (the 617 nm Luxeon Star (part number LXHL-NH94) can reach 825,000 mcd). By way of comparison, a typical 100 watt incandescent bulb puts out around 1700 lumen - if that light is radiated equally in all directions, it will have a brightness of around 135,000 mcd. Focused into a 20 beam, it will have a brightness of around 18,000,000 mcd. Confused yet? Just in case you're not, here is an excerpt from my college physics textbook, University Physics by Sears, Francis, et al. (6th ed. with corrections. Reading, MA: Addison-Wesley Publishing Company, 1982. 727-728.) 38-11 Illumination \t We have defined the intensity of light and other electromagnetic radiation as power per unit area, measured in watts per square meter. Similarly, the total rate of radiation of energy from any of the sources of light discussed in Sec. 38-2 is called the radiant power or radiant flux, measured in watts. These quantities are not adequate to measure the visual sensation of brightness, however, for two reasons: First, not all the radiation from a source lies in the visible spectrum; and ordinary incandescent light bulb radiates more energy in the infrared than in the visible spectrum. Second, the eye is not equally sensitive to all wavelengths; a bulb emitting 1 watt of yellow light appears brighter than one emitting one watt of blue light. The quantity analogous to radiant power, but compensated to include the above effects, is called luminous flux denoted by F. The unit of luminous flux is the lumen, abbreviated lm, defined as that quantity of light emitted by 1/60 cm² surface area of pure platinum at its melting temperature (about 1770C), within a solid angle of 1 steradian (1 sr). As an example, the total light output (luminous flux) of a 40-watt incandescent light bulb is about 500 lm, while that of a 40-watt fluorescent tube is about 2300 lm. When luminous flux strikes a surface, the surface is said to be illuminated. The intensity of illumination, analogous to the intensity of electromagnetic radiation (which is power per unit area) is the luminous flux per unit area, called the illuminance, denoted by E. The unit of illuminance is the lumen per square meter, also called the lux: \t 1 lux = 1 lm/m² \tAn older unit, the lumen per square foot, or foot-candle, has become obsolete. If luminous flux F falls at a normal incidence on an area A, the illuminance E is given by \t E = F ÷ A \tMost light sources do not radiate equally in all directions; it is useful to have a quantity that describes the intensity of a source in a specific direction, without using any specific distance from the source. We place the source at the center of an imaginary sphere of radius R. A small area A of the sphere subtends a solid angle [omega] given by [omega]=A÷R². If the luminous flux passing through this area is F, we define the luminous intensity I in the direction of the area as \t I = F ÷ [omega] \tThe unit of luminous intensity is one lumen per steradian, also called one candela, abbreviated cd: \t 1 cd = 1 lm/sr \tThe term "luminous intensity" is somewhat misleading. The usual usage of intensity connotes power per unit area, and the intensity of radiation from a point source decreases as the square of distance. Luminous intensity, however, is flux per unit solid angle, not per unit area, and the luminous intensity of a source in a particular direction does not decrease with increasing distance. \t EXAMPLE: A certain 100-watt bulb emits a total luminous flux of 1200 lm, distributed uniformly over a hemisphere. Find the illuminance and the luminous intensity at a distance of 1 m, and at 5 m. SOLUTION: The area of a half-sphere of radius 1 m is \t (2[pi])(1 m)² = 6.28 m² \tThe illuminance at 1 m is \t E = 1200 lm ÷ 6.28 m² = 191 lm/m² = 191 lux \tSimilarly the illuminance at 5 m is \t E = 1200 lm ÷ 157 m² = 7.64 lm/m² = 7.64 lux \tThis is smaller by a factor of 5² than the illuminance at 1 m, and illustrates the inverse-square law for illuminance from a point source. The solid angle subtended by a hemisphere is 2[pi] sr. The luminous intensity is \t I = 1200 lm ÷ 2[pi] sr = 191 lm/sr = 191 cd. \tThe luminous intensity does not depend on distance. There. Now, isn't that better? Would you like an asprin? LED Brightness - IR and UV LEDs Question: How bright is an IR LED? Answer: 0 mcd. Editorial Comment: Duh. Since candela and lumen are units that are adjusted to compensate for the varying sensitivity of the human eye to different wavelengths, and IR and UV are totally invisible (by definition) to the human eye, all IR and UV LEDs are automatically zero lumens and zero mcd. These units of measure, used for visible-light LEDs, can't be used for UV and IR LEDs (despite the "3000 mcd IR LED" currently on eBay). IR and UV LEDs are measured in watts for radiant flux and watts/steradian for radiant intensity. A fairly typical "bright" IR LED will put out about 27 mW/sr, though they go up to 250 mW/sr or so. Signaling LEDs, like for TV remotes, are considerably less powerful. HOWEVER - keep in mind that LEDs are not perfectly monochromatic. If their peak output is close to the visible spectrum, then their bandwidth may overlap the visible spectrum enough to be visible as a dim cherry-red light. Furthermore, some people can see further into the red region than can others, seeing as deep-red colors that to others are invisible infrared. While it would be possible to give such an LED a rating in millicandella, it would be misleading. This dim red glow, by the way, is often claimed - wrongly - to differentiate good illumination IR LEDs from much dimmer IR LEDs. Which LED is better for such a purpose is totally dependant on the wavelength at which the receiver is most sensitive. Using LEDs As a rule of thumb, different color LEDs require different forward voltages to operate - red LEDs take the least, and as the color moves up the color spectrum toward blue, the voltage requirement increases. Typically, a red LED requires about 2 volts, while blue LEDs require around 4 volts. Typical LEDs, however, require 20 to 30 mA of current, regardless of their voltage requirements. The table on the left shows how much current a typical red LED will draw at various voltages. Notice that this LED draws no current under 1.7 volts; the LED is "off". Between 1.7 volts and about 1.95 volts, the "dynamic resistance", the ratio of voltage to current, decreases to 4 ohms. Above 1.95 volts, the LED is fully "on", and dynamic resistance remains constant. Dynamic resistance differs from resistance in that the curve isn't linear. Just remember that this non-linear relationship between voltage and current means that Ohm's Law doesn't work for LEDs. Notice how steep the slope is - almost vertical. LEDs have a much more vertical slope than do normal diodes (but not as bad as laser diodes).This means that a tiny increase in voltage can produce a large increase current, and lots of smoke. In the above-mentioned LED, 2 volts is required to drive the LED properly, but as little as 2.04 volts could destroy it. To keep the current down to a reasonable level, a series resistor must be included in the circuit. The formula for calculating the value of the series resistor is: R[SUB]series[/SUB] = (V - V[SUB]f[/SUB]) / I[SUB]f[/SUB] where R[SUB]series[/SUB] is the resistor value in ohms, V is the supply voltage, V[SUB]f[/SUB] is the voltage drop across the LED, and I[SUB]f[/SUB] is the current the LED should see. For example, the above LED would run very nicely off 12 volts with a 500 ohm series resistor. Since 500 ohms is an odd value, you could do almost as well with a 470 ohm resistor, which would let the LED draw 21 mA. You can use a single resistor to control the current to a series of LEDs, in which case V[SUB]f[/SUB] is the total voltage drop across all the LEDs. You can sometimes get away with using a single resistor to control the current to a group of LEDs in parallel, but it's not generally a good idea - if there is any variation in the LEDs, they won't each draw the same current, resulting in differences in brightness - or in smoke. Is A Series Resistor Really Necessary? In a word, no. However, neither is a seat belt. Both are "cheap insurance" against disaster. A series resistor is not necessary if the voltage can be regulated to match the LEDs V[SUB]f[/SUB]. One way to do this is to match a battery to the LEDs. If your LED's V[SUB]f[/SUB] is 1.2 volts, you can string ten of them (10 x 1.2v = 12v) in series and power them from a 12 volt battery with no series resistor. However, you must be sure that the battery is capable of supplying the expected voltage - not only do batteries often supply a bit more than the rated voltage (a "12v" car battery for example, reaches 13.8v at full charge), but different types of batteries have different internal resistance, which results in different voltage "sag" under different load conditions. A friend of mine gave a good example of this problem in an email a while ago. My "Tri-Star Phazer" LED flashlight has 1 ohm resistors in series with each of its three diodes, both for the "operating voltage range" issue that you mentioned, and so that nickle-metal-hydride or NiCad batteries can be used in it. The original design had no resistors. The designer found that the voltage from four normal alkaline "C" batteries, even though their no-load voltage is a full 1.5 volts as opposed to only 1.2 volts from NiCads and NiMH's, will decrease under load to the safe operating voltage of the LED's, because of the alkaline battery's higher internal resistance. But when he tried loading the flashlight with NiMH or NiCad batteries, all three LED's smoked nearly instantly. The rechargeable batteries put out less no-load voltage, but their internal resistance is so low that under the load of the three LED's, the batteries don't "sag" nearly as much as the alkalines will, resulting in a higher voltage across the LED's and their quick destruction. So, he added the resistors. Here is a small table giving typical internal resistances of different types of battery. Notice how the alkaline AA battery has five times the internal resistance of the NiMH AA battery, and how the alkaline D battery has eleven times the internal resistance of the NiCad D battery. <table align="center" border="0" cellpadding="5" cellspacing="0" width="50%"> \t<tbody><tr align="center" valign="middle"> \t\t<th>Battery Type</th> \t\t<th>Internal Resistance (Ohms)</th> \t</tr> \t<tr align="center" bgcolor="pink" valign="middle"> \t\t<td>9V Zinc-Carbon</td> \t\t<td>35</td> \t</tr> \t<tr align="center" bgcolor="pink" valign="middle"> \t\t<td>9V Lithium</td> \t\t<td>16-18</td> \t</tr> \t<tr align="center" bgcolor="pink" valign="middle"> \t\t<td>9V Alkaline</td> \t\t<td>1-2</td> \t</tr> \t<tr align="center" bgcolor="skyblue" valign="middle"> \t\t<td>AA Alkaline</td> \t\t<td>0.15</td> \t</tr> \t<tr align="center" bgcolor="skyblue" valign="middle"> \t\t<td>AA NiMH</td> \t\t<td>0.03</td> \t</tr> \t<tr align="center" bgcolor="lightgreen" valign="middle"> \t\t<td>D Alkaline</td> \t\t<td>0.10</td> \t</tr> \t<tr align="center" bgcolor="lightgreen" valign="middle"> \t\t<td>D NiCad</td> \t\t<td>0.009</td> \t</tr> \t<tr align="center" bgcolor="lightgreen" valign="middle"> \t\t<td>D Lead-Acid</td> \t\t<td>0.006</td> \t</tr> \t<tr align="center" valign="middle"> \t\t<td colspan="2">Note: internal resistances shown above are at full charge and room temperature.</td> \t</tr> </tbody></table> Also, as the battery discharges, the voltage will drop significantly. Because of the steep voltage/current curve (see the graph under "Using LEDs" above), small changes in voltage will result in large changes in current. Adding resistance to the circuit will help stabilize the voltage across the LED. In a sense, an LED and resistor in series act as a voltage regulator. In series with a resistor, an LED will see the entire voltage drop across the circuit if it is not conducting. As soon as it starts to conduct, however, its resistance drops to almost nothing - just a few ohms. The voltage drop across the resistor rises, and the voltage drop across the LED remains almost fixed. The voltage drop across the LED cannot drop, as the LED would turn back off, which would raise the voltage drop across it and turn it back on again. Instead, the voltage drop across the LED remains just above the threshhold voltage even as the supply voltage rises. Any further increase in supply voltage increases the voltage drop across the resistor, but not the LED. Look what happens when the voltage, supplied to a 150 ohm resistor in series with an LED ( , varies from 4.5v to 5.5v. <table align="center" border="0" cellpadding="5" cellspacing="0"> \t<tbody><tr align="center" valign="middle"> \t\t<th>Voltage</th> \t\t<th>V[SUB]e[/SUB]</th> \t\t<th>I</th> \t\t<th>V[SUB]series[/SUB]</th> \t\t<th>V[SUB]led[/SUB]</th> \t</tr> \t<tr align="center" valign="middle"> \t\t<td>4.50</td> \t\t<td>2.60</td> \t\t<td>0.017</td> \t\t<td>2.52</td> \t\t<td>1.98</td> \t</tr> \t<tr align="center" valign="middle"> \t\t<td>4.60</td> \t\t<td>2.70</td> \t\t<td>0.017</td> \t\t<td>2.62</td> \t\t<td>1.98</td> \t</tr> \t<tr align="center" valign="middle"> \t\t<td>4.70</td> \t\t<td>2.80</td> \t\t<td>0.018</td> \t\t<td>2.72</td> \t\t<td>1.98</td> \t</tr> \t<tr align="center" valign="middle"> \t\t<td>4.80</td> \t\t<td>2.90</td> \t\t<td>0.019</td> \t\t<td>2.81</td> \t\t<td>1.99</td> \t</tr> \t<tr align="center" valign="middle"> \t\t<td>4.90</td> \t\t<td>3.00</td> \t\t<td>0.019</td> \t\t<td>2.91</td> \t\t<td>1.99</td> \t</tr> \t<tr align="center" valign="middle"> \t\t<td>5.00</td> \t\t<td>3.10</td> \t\t<td>0.020</td> \t\t<td>3.01</td> \t\t<td>1.99</td> \t</tr> \t<tr align="center" valign="middle"> \t\t<td>5.10</td> \t\t<td>3.20</td> \t\t<td>0.021</td> \t\t<td>3.11</td> \t\t<td>1.99</td> \t</tr> \t<tr align="center" valign="middle"> \t\t<td>5.10</td> \t\t<td>3.20</td> \t\t<td>0.021</td> \t\t<td>3.20</td> \t\t<td>2.00</td> \t</tr> \t<tr align="center" valign="middle"> \t\t<td>5.30</td> \t\t<td>3.40</td> \t\t<td>0.022</td> \t\t<td>3.30</td> \t\t<td>2.00</td> \t</tr> \t<tr align="center" valign="middle"> \t\t<td>5.40</td> \t\t<td>3.50</td> \t\t<td>0.023</td> \t\t<td>3.40</td> \t\t<td>2.00</td> \t</tr> \t<tr align="center" valign="middle"> \t\t<td>5.50</td> \t\t<td>3.60</td> \t\t<td>0.023</td> \t\t<td>3.49</td> \t\t<td>2.01</td> \t</tr> </tbody></table> You can see how flat the V[SUB]led[/SUB] curve is - it varies only 0.03 volts even as the supply voltage varies by 1.0 volt. Even with this small rise in V[SUB]led[/SUB], I[SUB]led[/SUB] increases 6mA. The LED in question has a threshold voltage (V[SUB]threshold[/SUB]) of 1.9v, above which it has a dynamic resistance (R[SUB]dynamic[/SUB]) of 4.55 ohms, and draws 20 mA at 2.0v. (This is an example of a real LED, see the graph under "Using LEDs" above) The supply voltage is 5v, and R[SUB]series[/SUB] is 150 ohms. Here are the formulae: V[SUB]e[/SUB] = V[SUB]supply[/SUB] - V[SUB]threshold[/SUB] I = V[SUB]e[/SUB] / (R[SUB]series[/SUB] + R[SUB]dynamic[/SUB]) V[SUB]series[/SUB] = R[SUB]series[/SUB] / (R[SUB]series[/SUB] + R[SUB]dynamic[/SUB]) * V[SUB]e[/SUB] V[SUB]led[/SUB] = V[SUB]supply[/SUB] - V[SUB]series[/SUB] V[SUB]e[/SUB] is the voltage above the threshhold, I is the current through the circuit, V[SUB]series[/SUB] is the voltage drop across the resistor, and V[SUB]led[/SUB] is the voltage drop across the LED. The only time it is really worth driving an LED without a series resistor is when you need absolute maximum efficiency - a series resistor wastes power (P = I<sup>2</sup>R) - and variations in brightness can be tolerated. There are other ways to control the current through an LED, however. A voltage regulator will do the job nicely, but perhaps a current regulator, such as this one, is better: LEDs driven by a simple current regulator A current-regulated switching power supply is the best of both worlds - constant current to keep the LEDs happy, and pretty good efficiency as well. Driving LEDs with AC The first, and most obvious question is: why? But we'll skip that one, assuming that you've got a reason. There are several factors to consider. One is that the LED will only conduct during that portion of the positive half of the cycle during which the voltage is above the threshold voltage of the LED. This means that the LED is conducting less than half the time, which will effect brightness. Second, even when the LED is conducting, the average voltage will be far less than the peak voltage. The average voltage of the positive half of a sine wave is only 64% of the peak voltage. (Think "area under the curve.") Brightness is therefore further reduced. This is what I mean. The X axis is time, the Y axis is voltage. The blue line is the supply voltage; the red line is the LED threshold. In this case, the peak voltage is 5 volts, and the threshold is 1.2 volts (typical for a red LED). The "effective voltage" (my term), is the voltage that is above the threshold voltage, the voltage that actually lights the LED; the rest of the voltage does nothing, either because it is under the threshold, or it's of the wrong polarity. Effective voltage is shown in the graph by the gray areas. The light gray area is the average effective voltage for an AC supply voltage; here, 1.04 volts. The dark gray area is the average effective voltage for a DC supply, 3.8 volts, that the AC supply voltage misses. The light gray area is a mere 27% of the area of both gray areas combined. If the LED had a threshold voltage of zero (wouldn't that be nice?) the effective AC voltage would still be only 32% of the effective DC voltage. As threshold voltage rises, the "duty cycle" goes down from there. The effective voltage is the (V-V[SUB]t[/SUB]) term from the formula given above, and can replace it for calculating the value of the desired resistor. You could increase the effective AC voltage toward the theoretical maximum of 32% of the effective DC voltage by increasing the supply voltage - this makes the threshold voltage a smaller portion of the peak voltage, so the LED turns on sooner in the cycle and stays on longer. But you have to avoid using a peak voltage greater than the reverse voltage that the LED can tolerate - typically only 5 volts. Remember that when the LED isn't conducting, all the voltage drop will be across the LED. You can sidestep this problem by including a separate diode - silicon diodes can withstand far more reverse voltage than LEDs can, although an additional diode will impose a second threshold voltage. Incorporating a full-wave bridge rectifier would let you drive the LED with both halves of the cycle, increasing the maximum possible effective voltage to 64% of that of DC, at the price of two additional threshold voltages. Some white LEDs require forward voltages (typically 3.5 or 4 volts) very close to their maximum reverse voltage (typically 5 volts), so the LED will only be on for a very small fraction of the cycle, making it very dim. For example, an LED requiring 3.5 volts driven on 5 volts AC would only see an effective AC voltage of only 0.25 volts, only 17% of the effective DC voltage of 1.5 volts. To compensate for the low average effective voltage, we'd want to drive the LED pretty hard to get the average current up to 20 mA. If the effective voltage is only 0.25 volts, then the resistor should be 13 ohms, and the peak current will be 120 mA. Can the LED stand a peak of 120 mA? Probably not. One possible solution is two LEDs in reverse-parallel, that is, one polarized to light during the positive half of the cycle, and the other polarized to light during the negative half. Right off the bat, this doubles the light output, since we're now using both halves of the cycle. Furthermore, since the only reverse voltage each LED will see is the forward voltage drop of the other LED, you can drive them with just about any voltage you want, so the "duty cycle" can approach 64% pretty closely. Using square-wave AC instead of sine-wave AC would let you reach almost 100%, either by using two reverse-parallel LEDs, or using one LED driven at twice the normal current for half the cycle. Thanks to Bill over at Gizmology for this info. He is the foremost in LED lighting.
can i harm my plants if i have 2 much light?....EX= I have a T5 high output 8x24....going 2 put it on 1 plant that is a week old.....Flouro does not produce alot of heat so i am not worried about burning them ...just concerned that it might be 2 much light...don't sound like a problem but don't wanna be disappointed
yes, you can give plants too much light. it would be radiation poison. but in your case, not a problem. drop that light 2-3 inches away and that girl will love you forever, or until you chop her head off.
Great guide. Is it possible to use too much light on seedlings? I have 6 seedlings growing under 7 26W CFL's and they have been yellowing. Is it best to start of with only a couple bulbs and work your way up?
generally with cfl's the more the better. at the moment i have 4 seedlings in rockwool under 2x200w cfl's and they could take more. i keep them typically about 2 inches away, even though they do get a little warm, i'm not really worried about it. i think the yellowing might have something to do with the ph, your soil type or a combination of both. if you are referring to the first petals (the name escapes me) that appear, they usually turn yellow and die. the cfl man i have followed in this forum is Kamel, Kamel's CFL Guide - Grasscity.com Forums. he has alot of cfl grow info in his journal.
sorry for not getting back to this. i have the hps lighting chart but not the mh. you or anyone is welcome to post it in here if you have it. i think this one is kind of cool too.
Does hight of the plant matter? What I mean is, you say that 250W HPS is good for 2'x2' grow space, but does the height of the plant matter? I've read places that, not specific to our favorite plant, the taller the plant, the more wattage you need. So for a 2'x2' grow space with three 4' tall plants, would I need more along the lines of 400W HPS?