# Light Information



## camcam

*I found this today maybe it could be a sticky?  Some good info here!*

*Candela:*
The unit of luminous intensity. One candela is defined as the luminous intensity of 1/600,000 square meter of projected area of a blackbody radiator operating at the temperature of solidification of platinum under pressure of 101,325 Newtons per square meter.

*Footcandle:*
A footcandle is a measure of light intensity. A footcandle is defined as the amount of light received by 1 square foot of a surface that is 1 foot from a point source of light equivalent to one candle of a certain type. 

*End Footcandle:*
End Footcandle measurements are based on the focused light beam only. The spherical energy or surrounding light output is not captured by or reflected back to the surface of the footcandle light meter. End footcandle is the focal light beam measurement from point A to point B at one-foot distance. 

*Lumen:*
A unit of light flow or luminous flux. The lumen rating of a lamp is a measure of the total light output of the lamp. The most common measurement of light output (or luminous flux) is the lumen. Light sources are labeled with an output rating in lumens. For example, a R30 65-Watt indoor flood lamp may have a rating of 750 lumens. Similarly, a light fixture's output can be expressed in lumens. 
As lamps and fixtures age and become dirty, their lumen output decreases (i.e., lumen depreciation occurs). Most lamp ratings are based on initial lumens (i.e., when lamp is new).

*End Lumens:*
End Lumens measurements are based on a spot of light only. The spherical energy or surrounding light output is not captured by or reflected back to the surface of the lumen light meter. End lumens is the light measurement from point A to point B at one-foot distance. 

*Luminance:*
Luminous Flux (light output). This is the quantity of light that leaves the lamp, measured in lumens (lm). Lamps are rated in both initial and mean lumens.

Initial lumens indicate how much light is produced once the lamp has stabilized; for fluorescent and high-intensity discharge (HID) lamps, this is typically 100 hours. 

Mean lumens indicate the average light output over the lamp's rated life, which reflects the gradual deterioration of performance due to the rigors of continued operation; for fluorescent lamps, this is usually determined at 40% of rated life. 

*Luminous (Light Level):*
This is the amount of light measured on the work plane in the lighted space. The work plane is an imaginary horizontal, tilted or vertical line where the most important tasks in the space are performed. Measured in footcandles (fc or lux in metric), light levels are either calculated, or in existing spaces, measured with a light meter. A footcandle is actually one lumen of light density per square foot; one lux is one lumen per square meter. Like lumens, footcandles can be produced as either initial or maintained quantities.

*Work Plane:*
The level at which work is done where illuminance is specified and measured. For office applications, this is typically a horizontal plane 30 inches above the floor (e.g., desk height).

*Beam Lumens:*
The total flux in that region of space where the intensity exceeds 50 percent of the maximum intensity.

*Field Lumens:*
The total flux in that region of space where the intensity exceeds ten percent of the maximum intensity.

*Lux:*
The metric unit of measure for illuminance of a surface. One lux is equal to one lumen per square meter. One lux equals 0.0929 footcandles.

*Light Level:*
Light intensity measured on a plane at a specific location is called illuminance. Illuminance is measured in footcandles, which are workplane lumens per square foot. You can measure illuminance using a light meter located on the work surface where tasks are performed. Using simple arithmetic and manufacturers' photometric data, you can predict illuminance for a defined space. (Lux is the metric unit for illuminance, measured in lumens per square meter. To convert footcandles to lux, multiply footcandles by 10.76).

*Efficacy:*
A measure of the luminous efficiency of a radiant flux, expressed in lumens per watt as the quotient of the total luminous flux by the total flux. For daylighting, this is the quotient of visible flux incident on a surface to radiant flux on that surface. For electric sources, this is the quotient of the total luminous flux emitted by the total lamp power input.

*Efficacy of a Light Source:*
The total light output of a light source divided by the total power input. Efficacy is expressed in lumens per Watt.

*Watt:*
The unit of measuring electrical power. Watts does not relate to the light output level. It defines the rate of energy consumption by an electrical device when it is in operation. The energy cost of operating an electrical device is calculated as its wattage time in hours of use. In single-phase circuits, it is related to volts and amps by the formula: Volts x Amps x Power Factor (PF) = Watts. (Note: For AC circuits, PF must be included).

*Kilowatt Hour (kWh) Formula:*
The measure of electrical energy from which electricity billing is determined. For example, a 100-Watt bulb operated for 1000 hours would consume 100 kilowatt hours (100 Watts x 1000 hours = 100 kWh). At a billing rate of £0.10/kWh, this bulb would cost £10.00 (100 kWh x £0.10/kWh) to operate over 1000 hours.

Camcam


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## Gordian_Knot

Thank you.


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## SoFlnightlight

Excellent format and presentation. Couldn't of done it better myself(5yrsDWC)  great job


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## LassChance

Light question

Im reading lots of light info in prep for buying a grow room light.  So far,it looks like the 400 w HPS is generally thought of as OK.
OK.
Then I came to a link about the Sun Agro...then the SON Agro...and now Im wondering if those dawgs hunt?  Also, I came upon one site claiming to have a "New, improved" LED thing called the UFO. Claims to be equiv to a 400w.HPS but uses a lot less energy and is cooler to operate.  Costs more than a 400 w HPS, tho.  Can this be for real?  Ever heard of this UFO thingy?

LassChance


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## ShecallshimThor

search this site for ufo there are journals for you to read someone exp.
but some ppl think that led technology isnt there yet but close


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## angel_eyes_6923

whats the best lights to use for indoor growth and is ther anyway to make it grow fast. Need smoke bad lol


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## BuddyLuv

hps


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## Newbud

What no hello first?
Wanna smoke hit up a dealer, wanna know how to grow cannabis start reading


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## Rolling Thunder

Here's another external thread containing much useful in-depth information on bulb comparisons. It will make your head spin with all the details. - RT

*Bulbs comparison tool *
hXXp://www.gardenscure.com/420/lighting/89513-bulbs-comparison-tool-3.html


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## Rolling Thunder

*PAR:*

PAR is the abbreviation for Photosynthetically Active Radiation which is the spectral range of solar light from 400 to 700 nanometers that is needed by plants for photosynthesis. This is found from actinic UVA to infrared; 400-550nm which is the absorption bandwidth of chlorophylls a, c², and peridinin (the light-harvesting carotenoid, a pigment related to chlorophyll) and ~620-700nm which is the red absorption bandwidth of chlorophylls a and c². 

Photons at shorter wavelengths (Ultraviolet C or UVC) tend to be so energetic that they can be damaging to cells and tissues; fortunately they are mostly filtered out by the ozone layer in the stratosphere. Green light occupies the middle spectrum (550-620nm; what is mostly visible to us) and is partly why chlorophyll is green due to the reflective properties. Bulbs that emit mostly actinic light or mostly infrared will have a lower PAR, while bulbs that occupy mostly the middle spectrum (yellow-green) will produce little necessary PAR.

*PUR:*

PUR (Photosynthetically Usable Radiation) should also be considered. PUR is that fraction of PAR that is absorbed by zooxanthellae photopigments thereby stimulating photosynthesis. As noted above, PUR are those wavelengths falling between 400-550nm and 620-700nm.

(SOURCE: hXXp://www.americanaquariumproducts.com/Aquarium_Lighting.html )


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## Rolling Thunder

camcam said:
			
		

> *Lumen:*
> A unit of light flow or luminous flux. The lumen rating of a lamp is a measure of the total light output of the lamp. The most common measurement of light output (or luminous flux) is the lumen. Light sources are labeled with an output rating in lumens. For example, a R30 65-Watt indoor flood lamp may have a rating of 750 lumens. Similarly, a light fixture's output can be expressed in lumens.
> 
> As lamps and fixtures age and become dirty, their lumen output decreases (i.e., lumen depreciation occurs). Most lamp ratings are based on initial lumens (i.e., when lamp is new).
> 
> *Lux:*
> The metric unit of measure for illuminance of a surface. One lux is equal to one lumen per square meter. One lux equals 0.0929 footcandles.


 
*Lumens, Lux, PAR:*


(SEE: hXXp://www.aquabotanic.com/lightcompare.htm )

Artificial light sources are usually evaluated based on their _lumen_ output. Lumen is a measure of _flux_, or how much light energy a light source emits (per unit time). The lumen measure does not include _all_ the energy the source emits, but just the energy with wavelengths capable of affecting the human eye. Thus the lumen measure is defined in such a way as to be weighted by the (bright-adapted) human eye spectral sensitivity. If we plot this sensitivity as a function of the wavelength of the light (building the so called _photopic curve_), we see that it has an approximately bell shape, peaking up at a wavelength of around 550 nanometers (nm), the "green" region of the light spectrum, and decreasing at both longer (red) and shorter (blue) wavelengths. The consequence is that two light sources that emit the same total amount of energy can have vastly different lumen ratings, depending on how much of that energy is concentrated around the 550 nm region. 

Another quantity often quoted when talking about light output is _lux_. Lux is a measure of _illumination_, not flux. Flux refers to the light energy that leaves the source. Illumination refers to the light energy that reaches the receiving surface. Lux is equivalent to lumens/m2. Lux cannot be computed only from the know data of a light source. Additional information regarding the illumination geometry, reflectors, distances, intervening media (glass covers, water) must be taken into account. 

Other quantities used to describe light quality associated with its _visual_ characteristics are _color temperature_ and _color rendering index_ (CRI). Color temperature is defined as the temperature that a perfect electromagnetic radiator ("black body") would have to have to emit light with the same "color" as the light source in question. Higher color temperature means bluer color, lower temperature, redder color. Color temperature is expressed in degrees Kelvin (from Lord Kelvin, the 19th century physicist, and which means degrees Celsius above absolute zero). CRI measures how close to their "true color" a light source can render objects illuminated by it. A "perfect" light source would have a CRI of 100, lower values mean that the colors are shifted from their "true" hue and saturation. Many people are familiar with the color shifting that takes place when one buys clothes in a store with artificial illumination and then realizes that under natural (sun) light the colors are not quite the same. Had the store used high-CRI light bulbs that color shift would be much smaller or not noticeable at all. 

It is easy to guess from the wording in the above paragraph that these two parameters are also strongly related to the human eye response characteristics. In fact, the technical definition of the term "color" used above is directly based on psychophysics experiments performed with human subjects and standardized by the CIE (Commission Internationale d'Eclairage) about 60 years ago. In other words, color temperature and CRI are parameters entirely based on the human visual system characteristics and may carry absolutely no meaning when applied in other contexts. Laboratory experiments showed that the photosynthesis process that takes place in plants when submitted to intense light has a very different spectral response than the human eye. In fact, photosynthesis is _the least_ efficient in the region around 550 nm. Most of the light capable of inducing the photosynthesis reaction is either red or blue. 

In other words, plant leaves mostly reflect green light, while they absorb red and blue with higher efficiency. An experimental fact that confirms this statement, independent of any laboratory measurement, is the fact that many plants look green ! Portable field instruments used to quantify photosynthesis in growing plants often exploit this fact by using as light source a pair of red and blue LEDs (Light Emitting Diode) instead of a white light source. 

The curve that results from plotting photosynthesis efficiency as a function of wavelength is named "Photosynthesis Action Spectrum". It is the equivalent of the photopic curve for photosynthesis. The curve is typically double-peaked, with maxima around 420 (blue) and 670 (red) nm and a "valley" around 550 nm. The curve drops sharply below 400 nm and above 700 nm. The peaks are broad and not as pronounced as the central peak in the photopic curve. There is still significant response in the green region around 550 nm. Many plant species can show specific action spectra that differ markedly from that "average" curve. In some extreme cases there is no response at all in one of either red or blue regions. 

The important point is that photosynthesis has a much _broader_ wavelength response than the human eye, with less dependency on specific, narrow wavelength regions. Thus, light sources that look very different to us may "look" similar to a plant. Conversely, light sources that look similar to us may "look" very different to plants, all depending on their specific spectral distributions. 

In some instances we see references to "plant growth spectrum" as well. This is _not_ to be taken as equivalent to the action spectrum though. The action spectrum has a precise meaning in terms of quantity (in moles/sec/leaf surface area) of CO2 consumed by the plant subject to measurement. "Growth", on the other hand, can be defined in many different ways (height ? weight ? weight of dry plant mass ?) that can be even very species-dependent, so it hardly makes a good standard for comparison purposes. 

Based on the Photosynthesis Action Spectrum, light bulb manufacturers came up with fluorescent "plant bulbs". They basically emit most of their light in the wavelengths that are more efficient for photosynthesis, namely the red and blue ends of the visible spectrum. As expected, these light sources look dim to the human eye and consequently have poor lumen ratings. Also, their color temperature and CRI ratings have little, if any, meaning. After all, these bulbs were not designed to be "seen" by humans... 

The standard measure that quantifies the energy available for photosynthesis is "Photosynthetic Active Radiation" (aka "Photosynthetic Available Radiation") or PAR. Contrary to the lumen measure that takes into account the human eye response, PAR is an unweighted measure. It accounts with equal weight for all the output a light source emits in the wavelength range between 400 and 700 nm. PAR also differs from the lumen in the fact that it is not a direct measure of energy. It is expressed in "number of photons per second", whose relationship with "energy per second" (power) is intermediated by the spectral curve of the light source. One cannot be directly converted into the other without the spectral curve. 

The reason for expressing PAR in number of photons instead of energy units is that the photosynthesis reaction takes place when a photon is absorbed by the plant, _no matter what the photon's wavelength (or energy) is _(provided it lies in the range between 400 and 700 nm). That is, if a given number of blue photons is absorbed by a plant, the amount of photosynthesis that takes place is exactly the same as when the same number of red photons is absorbed. For convenience, number of photons is usually reported in the literature in micromole units, or microEinsteins. One microEinstein is equivalent to 6.02 1017 photons. Another important difference is that usually PAR is quoted as an illumination measure akin to lux, thus related to the receiving surface. PAR is typically reported in 
microEinstein/second/m2. 

Thus we see from the above that, to evaluate light sources for use in plant applications, we cannot in principle rely entirely on an human-based criterion, the lumen rating. Unfortunately, manufacturers provide little information in that regard. Power consumption in Watts and lumen ratings are easy to get, and for many bulbs spectral plots do exist. Many of these are not depicted in physically meaningful units though (such as Watt/nanometer), making it difficult to compare different products. PAR figures are never quoted because they depend on the detailed illumination geometry, which varies from setup to setup.


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## Rolling Thunder

Rolling Thunder said:
			
		

> Thus we see from the above that, to evaluate light sources for use in plant applications, we cannot in principle rely entirely on an human-based criterion, the lumen rating. Unfortunately, manufacturers provide little information in that regard. Power consumption in Watts and lumen ratings are easy to get, and for many bulbs spectral plots do exist. Many of these are not depicted in physically meaningful units though (such as Watt/nanometer), making it difficult to compare different products. PAR figures are never quoted because they depend on the detailed illumination geometry, which varies from setup to setup.


 
Lumen: 

Many people talk about lumen. If a lamp does not give much lumen, than the lamp is no good, or so they assert. This is not necessarily the case. We will explain why. Lumen actually is a measure of the luminous flux, which is amount of light of all wavelengths of the visible spectrum emitted by a light source. This means that lumen is a qualification that only applies to light which is visible to the human eye. Earlier we have explained that plants need blue and red light, because these colors represent the wavelengths that can energize the plants. However part of the blue and red light is less visible for humans and so cannot be properly valued in terms of lumen. That implies that lumen is not the right unit to measure the effectiveness of grow lamps for plants. Below [is] an overview of lumen per Watt for a number of light colors: 

1 Watt at 400 nm 
purple-blue - 0.27 lumen

1 Watt at 450 nm
blue - 25.90 lumen

1 Watt at 500 nm
green- 220.00 lumen

1 Watt at 550 nm
green-yellow - 679.00 lumen

1 Watt at 555 nm
yellow - 683.00 lumen

1 watt at 600 nm
orange - 430.00 lumen

1 Watt at 650 nm
orange-red - 73.00 lumen

1 Watt at 700 nm
red - 2.78 lumen

This table [above] shows that blue and red spectrum light is quite low on lumen. Still this is the light that is most suitable for plants. Or, in other words: light that is useful for plants cannot be measured in lumen, because people can't see it very well.So, contrary to what many people believe, it is not true that light which we see best is also the light that is most suitable for growing plants. The assertion 'the more light, the higher the yield' is only based on light that we can see. This may be correct for HPS lamps, as you need very powerful lamps to generate sufficient blue and red. But because HPS lamps emit primarily green, yellow and orange, it automatically follows that a tremendous amount of visible light is produced.

(SOURCE: hXXp://www.pielkenrood.net/index2.php?option=com_content&do_pdf=1&id=11 )


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## singlee

Thank you for your useful information.


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## williamraed

Awesome, after reading the whole light information I would like to say that this is one of the best and useful content. The details you have been distributed here, which is used that which type light use in indoor gardening.


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## Light

PAR Watts, Lumens, Photons, Lux and Watts

As the importance of artificial light in the plant growing industry has increased, lamp manufacturers have begun to rate lamps specifically for plant needs. This article discusses and compares the different measures of " light level" that are currently used for plant growth and hydroponic applications. Light level is one of the important variables for optimizing plant growth, others being light quality, water, carbon dioxide, nutrients and environmental factors. The appendix describes a step-by-step approach to developing a simple lighting layout using the PAR watt ratings of light sources.

In recent years, it has become increasingly cost-effective to use artificial lights for assisting plant growth. Lighting costs and lamps have become less expensive, and very efficient light sources are now available in high wattages. These developments along with the ability to preserve and transport plants and produce as well as special new products in demand today have resulted in a lucrative market for hydroponic products, that is, products grown without soil.

Artificial light can be used for plant growth in three different ways: 1. To provide all the light a plant needs to grow, 2. To supplement sunlight, especially in winter months when daylight hours are short. 3. To increase the length of the "day" in order to trigger specific growth and flowering.
PAR and Plant Response Curve

Just as humans need a balanced diet, plants need balanced, full-spectrum light for good health and optimum growth. The quality of light is as important as quantity. Plants are sensitive to a similar portion of the spectrum as is the human eye. This portion of the light spectrum is referred to as photosynthetically active radiation or PAR, namely about 400 to 700 nanometers in wavelength. Nevertheless, plant response within this region is very different from that of humans.

The human eye has a peak sensitivity in the yellow-green region, around 550 nanometers. This is the "optic yellow" color used for highly visible signs and objects. Plants, on the other hand, respond more effectively to red light and to blue light, the peak being in the red region at around 630 nanometers. The graphs below show the human eye response curve and the plant response curve. Note the vast difference in the contours.
Plant and Human eye response to light 	Plants and Humans see light differently

In the same way fat provides the most efficient calories for humans, red light provides the most efficient food for plants. However, a plant illuminated only with red or orange light will fail to develop sufficient bulk. Leafy growth (vegetative growth) and bulk also require blue light. Many other complex processes are triggered by light required from different regions of the spectrum. The correct portion of the spectrum varies from species to species. However, the quantity of light needed for plant growth and health can be measured, assuming that all portions of the spectrum are adequately covered. Light for plants cannot, however, be measured with the same standards used to measure light for humans. Some basic definitions and distinctions follow that are useful in determining appropriate ways to measure the quantity of light for hydroponic plant growth.
Measuring Light for Humans: Lumens and Lux

First, how do we measure light quantity for humans? The obvious way is based on how bright the source appears and how "well" the eye sees under the light. Since the human eye is particularly sensitive to yellow light, more weight is given to the yellow region of the spectrum and the contributions from blue and red light are largely discounted. This is the basis for rating the total amount of light emitted by a source in lumens.

The light emitted from the source is then distributed over the area to be illuminated. The illumination is measured in "lux", a measurement of how many lumens falls on each square meter of surface. An illumination of 1000 lux implies that 1000 lumens are falling on each square meter of surface. Similarly, "foot-candles" is the term for the measure of how many lumens are falling on each square foot of surface. Clearly, both lumens and lux (or foot-candles) refer specifically to human vision and not to the way plants see light. How then should the rating for plant lighting be accomplished? There are two basic approaches to develop this rating: measuring energy or counting photons.


PAR Watts for Plants

Watts is an objective measure of energy being used or emitted by a lamp each second. Energy itself is measured in joules, and 1 joule per second is called a watt. A 100 watt incandescent bulb uses up 100 joules of electrical energy every second. How much light energy is it generating? About 6 joules per second or 6 watts, but the efficiency of the lamp is only 6%, a rather dismal number. The rest of the energy is dissipated mainly as heat. Modern discharge lamps like high pressure sodium (HPS) and metal halide convert (typically) 30% to 40% of the electrical energy into light. They are significantly more efficient than incandescent bulbs.

Since plants use energy between 400 and 700 nanometers and light in this region is called Photosynthetically Active Radiation or PAR, we could measure the total amount of energy emitted per second in this region and call it PAR watts. This is an objective measure in contrast to lumens which is a subjective measure since it is based on the response of the subjects (humans). PAR watts directly indicate how much light energy is available for plants to use in photosynthesis.

The output of a 400 watt incandescent bulb is about 25 watts of light, a 400 watt metal halide bulb emits about 140 watts of light. If PAR is considered to correspond more or less to the visible region, then a 400 watt metal halide lamp provides about 140 watts of PAR. A 400 watt HPS lamps has less PAR, typically 120 to 128 watts, but because the light is yellow it is rated at higher lumens (for the human eye).

"Illumination" for plants is measured in PAR watts per square meter. There is no specific name for this unit but it is referred to as "irradiance" and written, for example, as 25 watts/square meter or 25 w/m2.
Photons

Another means of measuring light quantity for plant growth involves the understanding that light is always emitted or absorbed in discrete packets called "photons." These packets or photons are the minimum units of energy transactions involving light. For example, if a certain photosynthetic reaction occurs through absorption of one photon of light, then it is sensible to determine how many photons are falling on the plant each second. Also, since only photons in the PAR region of the spectrum are active in creating photosynthesis, it makes sense to limit the count to PAR photons. A lamp could be rated on how many actual tiny photons it is emitting each second. At present no lamp manufacturer does this rating.

Instead, plant biologists and researchers prefer to talk of the flux of photons falling each second on a surface. This is the basis of PPF PAR with PPF standing for Photosynthetic Photon Flux, a process which actually counts the number of photons falling per second on one square meter of surface. Since photons are very small, the count represents a great number of photons per second, but the number does provide a meaningful comparison.

Another measure appropriate for plant growth, called YPF PAR or Yield Photon Flux, takes into account not only the photons but also how effectively they are used by the plant. Since red light (or red photons) are used more effectively to induce a photosynthesis reaction, YPF PAR gives more weight to red photons based on the plant sensitivity curve.

Since photons are very small packets of energy, rather than referring to 1,000,000,000,000,000,000 photons, scientists conventionally use the figure "1.7 micromoles of photons" designated by the symbol "µmol." A µmol stands for 6 x 1017 photons; 1 mole stands for 6 x 1023 photons. Irradiance (or illumination) is therefore measured in watts per square meter or in micromoles (of photons) per square meter per second, abbreviated as µmol.m-2.s-1

The unit "einstein" is sometimes used to refer to one mole per square meter per second. It means that each second a 1 square meter of surface has 6 x 1023 photons falling on it. Irradiance levels for plant growth can therefore be measured in micro-einsteins or in PAR watts/sq. meter.

These three measures of photosynthetically active radiation, PAR watts per square meter, PPF PAR and YPF PAR are all legitimate, although different, ways of measuring the light output of lamps for plant growth. They do not involve the human eye response curve which is irrelevant for plants. Since plant response does "spill out" beyond the 400 nanometer and 700 nanometer boundaries, some researchers refer to the 350 &#8211; 750 nanometer region as the PAR region. Using this expanded region will lead to mildly inflated PAR ratings compared to the more conservative approach in this discussion. However, the difference is small.

Photosynthesis and Photomorphogenesis

Plants receiving insufficient light levels produce smaller, longer (as compared to wide) leaves and have lower overall weight. Plants receiving excessive amounts of light can dry up, develop extra growing points, become bleached through the destruction of chlorophyll, and display other symptoms of excessive stress. Plants are also damaged by excessive heat (infrared) radiation or extreme ultraviolet (UV) radiation.

Within the acceptable range, however, plants respond very well to light with their growth rate being proportional to irradiance levels. The relative quantum efficiency is a measure of how likely each photon is to stimulate a photosynthetic chemical reaction. The curve of relative quantum efficiency versus wavelength is called the plant photosynthetic response curve as shown earlier in this section.

It is also possible to plot a curve showing the effectiveness of energy in different regions of the spectrum in producing photosynthesis. The fact that blue photons contain more energy than red photons would need to be taken into account, and the resulting curve could be programmed into photometry spheres to directly measure "plant lumens" of light sources instead of "human lumens." This is likely to happen at some point in the future.

The main ingredient in plants that is responsible for photosynthesis is chlorophyll. Some researchers extracted chlorophyll from plants and studied its response to different wavelengths of light, believing that this response would be identical to the photosynthetic response of plants. However, it is now known that other compounds (carotenoids and phycobilins) also result in photosynthesis. The plant response curve, therefore, is a complex summation of the responses of several pigments and is somewhat different for different plants. An average is generally used which represents most plants, although individual plants may vary by as much as 25% from this curve. While HPS and incandescent lamps are fixed in their spectral output, metal halide lamps are available in a broad range of color temperatures and spectral outputs. With this in mind, the discriminating grower can choose a lamp that provides the best spectral output for his specific needs.

In addition to photosynthesis which creates material growth, several other plant actions (such as germination, flowering, etc.) are triggered by the presence or absence of light. These functions, broadly classified as photomorphogenesis, do not depend much on intensity but on the presence of certain types of light beyond threshold levels. Photomorphogenesis is controlled by receptors known as phytochrome, cryptochrome, etc., and different plant functions are triggered in response to infra red, blue or UV light.


Plants "see" light differently than human beings do. As a result, lumens, lux or footcandles should not be used to measure light for plant growth since they are measures used for human visibility. More correct measures for plants are PAR watts, PPF PAR and YPF PAR, although each in itself does not tell the whole story. In addition to quantity of light, considerations of quality are important, since plants use energy in different parts of the spectrum for critical processes.


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## Locked

Good stuff  Light.  Thanks.


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## meser74

thanks


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## widairyfarms

What is the difference between hydro bulbs and regular hid bulbs (not hps or mh/ lumens kelvin ect.)
Thanks
WDF


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## The Hemp Goddess

I do not understand your question.  I have never seen bulbs listed as "hydro" bulbs.  Also, HID just stands for high intensity discharge.  If you are not speaking of HPS or MH lights, what type of HIDs are you speaking of?


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## widairyfarms

When looking for replacement bulbs for my light I have come across light bulbs for hid saying they are for hydroponic grows. No hard data is given as the places I'm looking even on some manufacturers websites ie, lumens,kelvin, and overall life expectancy. Also the lower kelvin the better or is 2100 k the best spectrum? What are the REAL target kelvin ratings for hid bulbs both hps and mh? Life not withstanding because a yearly change will be in the cards. Thanks
WDF


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## Locked

I buy all my bulbs from 1000bulbs.com .  I would rather use cheaper bulbs and change them a little more often.


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## AluminumMonster

hamster lewis said:


> i buy all my bulbs from 1000bulbs.com .  I would rather use cheaper bulbs and change them a little more often.


+1


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## Dunbar

Light exists along a relatively narrow bandwidth of the electromagnetic spectrum, and the region of visible light is more narrow still. Yet, within that realm are an almost infinite array of hues that quite literally give color to the entire world of human experience. Light, of course, is more than color: it is energy, which travels at incredible speeds throughout the universe. From prehistoric times, humans harnessed light's power through fire, and later, through the invention of illumination devices such as candles and gas lamps


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## shahomy

Do they at least last a year?
Anyone else`s experience with the cheaper bulbs from 1000bulbs .com?
Plants grow good?
I`m looking at these 4 footers
hxxps://www.1000bulbs.com/product/152904/GE-46763.html    $3.09 ea
hxxps://www.1000bulbs.com/product/100544/EIKO-F54T5HO865.html   $2.70 ea


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## David_willis

Thanks buddy!!!!


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## Bruce111

This is a wonderful information for newbies like me.


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## Devile

angel_eyes_6923 said:


> whats the best lights to use for indoor growth and is ther anyway to make it grow fast. Need smoke bad lol



I think led lights are best, I am using Mars Hydro led and very happy with them


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## Flyby

hello all,
I think this is my first post here. I am interested in using LED lights in a small area to grow only a couple of plants, not being a seller or a heavy user. I'm classified as an A-1 Noob at this, so have a little mercy on me, and give me a good steer, as I intend to keep a small grow situation. 
thank you,
Flyby out


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## Locked

When I used to run HID lighting I would always buy the cheap HPS bulbs from 1000bulbs.com and just replace them a little more often. I couldn't see spending 50 to 100 dollars on an HPS bulb. That being said, switching to LED was the best thing I ever did.


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## Devile

Hamster Lewis said:


> When I used to run HID lighting I would always buy the cheap HPS bulbs from 1000bulbs.com and just replace them a little more often. I couldn't see spending 50 to 100 dollars on an HPS bulb. That being said, switching to LED was the best thing I ever did.



Led light is very affordable now as more and more led brand in the market


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## Alexphillips

thanx for the information i took screen short of your post.
keep doing good work


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## Jkjiv

So what type of light does this measure and what should my target number be for veg/flower?


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## WeedHopper

I think Hammy is on here. Maybe he can help. I've never used one of those.


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