What is an LED grow light? What is the difference between and regular LED light and an LED grow light?
The answer is that plants require a much higher light intensity to grow plants effectively than humans need for vison. The minimum light intensity required to grow plants is at least 30 times greater than required for human lighting in an office environment for example.
High light intensity is required to drive photosynthesis which generates plant growth. Photosynthesis occurs when photons of light reach the plant leaves and causes a reaction which generates plant growth. The measure of light intensity for plant growth is called Photosynthetically Active Radiation or PAR and includes photons of wavelengths from 400nm to 700nm or from deep blue to deep red. It is about the same range of wavelengths as the visual range for humans.
LED grow lights are designed to withstand the high humidity in a grow room and will be able to withstand condensed water dripping on the grow light. The LEDs are often protected with a silicone or acrylic coating and the connectors and cables are Ingress Protection (IP) rated so that they can operate in damp conditions.
What is an LED?
A light-emitting diode (LED) is a semiconductor device that emits light of a certain wavelength (color). A die, the part of the LED that emits light, is encased in plastic or ceramic housing. The housing may incorporate one or many dies. When LED is forward-biased or switched on, electrons are able to recombine with holes within the device, releasing energy in the form of photons. This effect is called electroluminescence.
Types of LED packaging
LEDs are packaged in a few popular formats.LED COB
Chip-on-Board or "COB" refers to the mounting of a bare LED chip in direct contact with a substrate (such as silicon carbide or sapphire) to produce LED arrays. These arrays can have hundreds of individual LEDs packaged into a small square or circle and mounted on an aluminium or ceramic base. They typically range from 20 to 150 watts and have a wide range of colour temperature options depending on the phosphorous coating specified.
COB LEDs have a number of advantages over older LED technologies, such as Surface Mounted Device ("SMD") LED. Most notably, COB technology allows for a much higher packing density of the LED array, or what light engineers refer to as improved "lumen density". This can be useful if you want a small sized light fixture or to use with lenses or reflectors to direct the light in a narrow beam.
Both reflectors and lenses can be used to direct the light from the LED and reduce light ‘overspill’ which is waste. Sometimes a combination of both lens and reflector are used.
In general terms the wider the light spread the lower the hanging height but the spread is less uniform. A narrower light beam means the hanging height must be relatively high but the light spread is more uniform.
Reflector is an optical element that regulates the light spread from the fixture by reflection. There are different reflective surfaces such as- a mirror reflection, a diffuse reflection, and a mixed reflection.
Reflector types include conic reflectors of four basic geometries – elliptical, zonal, hyperbolic, and parabolic.
The light emitted from LEDs can be directed with a lens in narrow or wide beams. Wide angled lenses are typically used to spread light widely and allow for a low hanging height i.e. a small distance from the LED to the plant canopy. Narrow angle lenses are used when the light source will be hung high over the target area.
The secondary lens is not part of the LED package but will be mounted over the LEDs. The lenses will narrow the beam of light and improve the system efficiency and will generally be more efficient at doing so than the primary lens. However they are more bulky and expensive. The lenses can be made of acrylic or glass.
The lens also provides protection for the LED and can facilitate sealing the lluminaire against dirt and moisture.
SMD stands for surface mounted diode and these LEDs are small and typically run at less than half a watt each and therefore many of them are needed for a high wattage light fixture. They are usually arranged on aluminium boards in arrays to spread the light source and allow for efficient heat dissipation.
SMDs spread out over long LED bars or rectangular plates also allow for more even light distribution over the plant canopy and reduce the required hanging height and potential for hot spots compared to COB LED light fixtures.
A primary lens is mounted directly onto the LED and is part of the LED package.Primary lens will improve system efficiency by narrowing the beam of light so that more light is directed onto the plant canopy. However the lens is not perfect and will absorb some of the light but is a low cost lens solution.
The most common LED type used to generate white light involves coating LEDs of one color (mostly blue LEDs made of InGaN) with phosphor to form white light. These LEDs are called phosphor-based white LEDs. The “blue” photons emitted by High-brightness LED either passes through the phosphor layer without alteration, or they are converted to the “yellow” photons in the phosphor layer . The combination of “blue” and “yellow” photons leads to white light.
The “blue” photons emitted by High-brightness LED either passes through the phosphor layer without alteration, or they are converted to the “yellow” photons in the phosphor layer . The combination of “blue” and “yellow” photons leads to white light
White LEDs - Colour temperature
Color temperature or CCT of a light source is the temperature of an ideal black-body radiator (solid object with certain properties heated up to point of incandescence) that radiates light of comparable hue to that of the light source, and its temperature is expressed in Kelvins (K). As a black body gets hotter, wavelength of light emits progress through a sequence of colors from red to blue
Increasing the quantity of phosphor coating on the blue led leads to more phosphorescence and increased the ‘yellow’ to blue ratio. This results in a ‘warmer’ or more orange light with a lower correlated color temperature (CCT) of 3000K, for example
Decreasing the phosphor coating has the opposite effect and the proportion of blue photons emitted is greater and the light is a ‘cooler’ light spectrum with a lower correlated color temperature (CCT) of 5000K, for example.
Higher CRI white LEDs 'smoothen' the spectrum curve so that there is a more even spread of wavelengths across the PAR range. However it does not increase the Photosynthetic efficiency of the light spectrum.
Deep red LEDs
Although more expensive per watt the deep red LEDs of about wavelength 660nm are very electrically efficient and are added to grow lights to increase the system efficiency.
Typically a small number of 660nm reds are used in most full spectrum LED grow lights and the spike at this wavelength is visible on the grow light spectrum graph
UVA and Far red LEDs
Some grow lights will also include UVA LEDs at about 380nm and Far Red LEDs of wavelength 730nm. These are not in the standard PAR range so the additional UVA or Far red light is not sensed with a PAR sensor. However these wavelengths do generate photosynthesis and contribute to growth and yield.
LEDs run on direct current (DC) but mains power is Alternating Current (AC). In order to power LEDs we need a device to convert the mains AC power supply to DC and we call it an LED driver.
The LED driver will also provide electrical protection to the LEDs to prevent damage by power surges, over heating etc.
There are type main types of LED driver:
Constant Current (CC) - LEDs are mostly in serial connections and the LED driver delivers a precise current value. Ideal for dimming.
Constant Voltage (CV) - LEds are mostly in parallel connection, ideal for decorative strips which can be cut to the required length. Not recommended for dimming.
LED driver characteristics
Rated Current/Voltage - predefined output current or voltage to service the number of LEDs it will power and how hard they will be driven.
Rated Power - Output power of the driver. Output Voltage x Amps = rated power
Efficiency - The ratio between the output power and input power in %.
Higher quality drivers tend to be higher efficiency, run cooler and last longer.
LED driver efficiency
LEd drivers will range from about 80% to 95% efficiency. The efficiency is a measurement of the power output/power input and indicates how much power will be lost. For example a 90% efficiency driver loses 10% of the power it consumes to heat. To have an efficient and effective grow light it must have a good LED driver with high efficiency. An LED driver is considered efficient if it is 90% efficient or greater.
LED driver reliability
The most common failure of an LED light is not the LEDs themselves but the LED driver. The drivers tend to fail due to high operating temperatures which dry out the capacitors in the circuits and the driver will stop working. A good quality and high efficiency LED driver is necessary for a grow light to be reliable.
There are two main methods of dimming LED lights: Pulse Width Modulation (PWM) and analogue.
PWM: Unlike traditional lighting such as incandescent, this method of LED dimming doesn’t rely on voltage to influence the level of brightness. Instead, an ‘on and off’ cycle is used. This cycle operates in a matter of milliseconds, so to the naked eye, you won’t actually notice them turning on and off.
For instance, if you have dimmed your lights to 30%, they will cycle ‘on’ for 30% of the time and ‘off’ for the remaining 70%. This creates an optimal dimming effect without having to increase or decrease voltage reaching the light.
Analogue: A more straight forward approach to dimming. Analogue relies on controlling the current to either dim or brighten the lights. Lowering the current will dim the lights, and at the opposite end, increasing the current will create a brighter light.
Other LED driver functions
Most good LED drivers will also have other safety features such as:
- Overcurrent protection
- Overtemperature protection
- Short circuit protection
Characteristics of LED grow lights
The light output from all types of lighting fixtures reduces over time. Grow lights are used from about 12 to 20 hours per day. Using an average of 15 hours per day the running hours per year will be 365 x 15 = 5,475 hours. Typically grow lights should be replaced after reducing to 80% of their original output. This means replacing Metal Halide bulbs after 6 months and High Pressure Sodium bulbs after 1 year. LEDs do not have to be replaced until around 4 years of use
Incandescent and HID bulbs emit light in all directions and therefore light has to be reflected back towards the target plant canopy. All of the light will not be reflected back and will be lost in the form of heat.
LEDs emit light in one direction only and therefore not as much light must be reflected and therefore have an efficiency advantage.
Spectrum can be tuned
LED grow lights can use different white LEDs ranging from warm white to cool white, in effect adjusting the percentage of blue in the spectrum. Deep red, UVA and Far Red LEDs can also be added to change and ‘broaden’ the spectrum as needed by the grower.
There are two benefits to higher efficiency:
- Reduced electricity consumption - reduced running costs
- Reduced heat output
LEDs are much more efficient than any other lighting technology and allow growers to reduce running costs and heat levels in the grow area. The payback period for high efficiency LEDs is less than 2 years.
The reduced heat output of LEDs also allows growers to increase the light intensity in the grow areas for the same or less electrical footprint and maintain the room temperature.
Both LEDs and LED drivers will last longer the cooler they run. To maximise the LED driver lifespan you can locate it in a cool area outside of the grow tent and hang it up so the airflow around it is optimised.
The LED bars or panels have heatsinks on the back of them. Ensure there is enough space above and around the heatsink to allow air to move freely and the heat to dissipate. If you are running extraction fans than you can direct airflow across the top of the LED grow light to increase air flow and minimise the LED temperature.
There are no moving parts on the latest generation of LED light fixtures but you can keep the heatsink and fixtures clean to maintain the efficiency of the heatsink and keep the operating temperatures down. A lot of LED lights do not have lenses or covers over the LEDs and they may attract first or moisture. However a lot of LEDs have either an acrylic or silicone coating. In this case you can clean the LED surface. Turn off the light and clean gently with a damp cloth. Do not use cleaning products as they may damage the LED surface.
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