Frequently Asked Questions

Indoor Grow Lights

Light is one of the most vital factors to consider when growing plants. It is very straightforward, no light means no development. The general rule is that the more light that your plants have contact with, the quicker they will grow and more they will produce. The benefit of an indoor grow light is that it allows you to grow any plant, anywhere, anytime. However, to get the very finest out of your indoor grow light and growing, it is helpful to be aware of the basic principles of light.


Anyone who has seen a rainbow will recognise that sunlight is made up of different colours. They can be differentiated by their wavelength, which is measured using nanometres or nm.

The human eye is at it’s most sensitive to light around the centre of the spectrum, between 500 and 600nm. Plants use wavelengths between 400 and 700nm useful for turning light into energy, formally known as photosynthesis. This area is referred to as Photosynthetically Active Radiation, commonly known as the abbreviation PAR.

While the human eye discovers light at 555nm – as a result more useful, plants find two separate areas of the spectrum most useful. Those areas being the blue area between 400-460nm and the also the red area between 580-700nm.

(Diagram reprinted with permission from Gavita Holland

Measuring Light

The term ‘lumens’ is the measurement of light intensity and is frequently used to define the output from artificial lights. This is handy for lights to help us see in the dark however becomes particularly poor when measuring horticultural grow lights because lumens are measured according to what the eye is sensitive to. Therefore using lumens is not an accurate representation of the properties of a lamp that are useful to plants.

Light-measuring devices commonly use ‘lux’, which is quite simply the measurement of how many lumens fall on each square metre of surface. So the illumination of 50,000 lux is 50,000 lumens falling on each square metre. Lux measurements are handy for measuring intensity from grow lamps, be used to help determine the accurate height to position the light above the plants or to check for lamp deprivation. However lux is still not a good measurement for determining the quality of light, nor how good it is for growing plants.

Professional growers and light manufacturers have moved ahead of measuring light in lumens and lux to photon counting in the PAR area. Without going into too much detail; A photon is a particle of light. There are two types of photons; A blue photon and a red photon. A blue photon has a short wavelength and does not have as much energy as a red photon, which has a long wavelength. The plant however is only interested in the number of photons, as it does not use the energy in the photon for photosynthesis. A plant needs 8 to 10 photons to bind one molecule of CO2. Therefore a blue 600w light produces less photons than a red 600w light and is also less capable for use in photosynthesis, however more than just red colours in the spectrum is needed to grow a plant healthily.

A quantum meter is a device made to measure photons. The entire amount of photons is known as the photosynthetic photon flux (also identified as PPF) from a lamp can be measured with a quantum meter to give you precise data on the amount of photons coming from your lamp.

If you are interested in the scientific facts; photons are counted in micromoles (µmol). One µmol is 602214150000000000 photons. The unit used for PPF is micromole/second (µmol/s), and a good 600W HPS lamp will emit 1100 µmol/s.

In conclusion, if you have two lights with one of the lights having a higher lumen output, it is not an indication that it is better for growing plants. The most constructive light output data is plant useable light measured in micromoles.

Day Length

The length of time that your plant collects light in a 24-hour instalment is called the ‘photoperiod’ or ‘day length’. Some plants use it as a signal to know when to produce leaves, flowers or fruit. Using this tool, indoor flowers can adjust the photoperiod using timers to control their lights and their plants at the same time. Long day lengths of 16 to 24 hours are generally used for vegetative growth and short cycles of light of 12 hours and are used for flowering or fruiting.

Light and Environment

All lights on the market produce heat as well as they give off light. Therefore when growing indoors, the heat generated by the grow lights needs to be removed using an extractor fan to keep the growing environment for the plants comfortable. If you position the light too close to your plants, the heat from the lamp may possibly burn them. Therefore it is essential to suitably hang your growing lights.

Types of Grow Lights

There are two categories of lights regularly used for growing plants. These are fluorescent and high intensity discharge (HID).

Fluorescent Grow Lights

Fluorescent grow lights are much more suitable for propagation and vegetative growth. They have good colour rendering properties and produce less heat than HID grow lights, which allows them to be positioned closer to your plants to get the most out of their output.

There are two types of fluorescent light used for growing. The first being Compact Fluorescent Lamps (CFL) and T5 lamps. CFL’s are large ‘energy saving’ lamps and have the electronics to alight the lamp at their base. These can be screwed into a reflector or easily hung vertically above the crop. T5’s are the most efficient fluorescent tube light and need separate electronics to ignite the lamps. These electronics are built inside T5 reflectors. Due to the size of the T5 tubes, they generate a very uniform level of light over a larger area in contrast to CFL’s.

Compact Fluorescents are available in 150W and 250W, as well as a propagation lighting system utilising 2 x 55W PL lamps. T5’s are available as 2ft 24W lamps and 4ft 54W lamps. These are available individually (without a reflector) or as an inclusive lighting system with two, four or eight lamps.

Fluorescent lights are great for seedlings and cuttings, as plants at this stage do not need intense light. Both CFL and T5 are low intensity, so they need to be placed close to plants to be successful at promoting growth. If you are using fluorescent lights above propagators, make sure not to place them too close, as this will cause the internal propagator temperature to get too high.

The amount of light emitted from a fluorescent lamp can vary, depending on the lamp colour. Fluorescent lamps are available in a range of colour variations differentiated by the Kelvin colour temperature scale.

The Kelvin scale has become industry standard for differentiating commercial and domestic lighting, however is rarely referred to in horticultural lighting other than for fluorescent lamps.

Fluorescent lamps that have a high Kelvin, are most commonly used for propagation or vegetative growth; these usually come in around 6400K. Lamps with a low Kelvin are used for flowering and usually emit light around 2700K.

A combination of both (1 x 27K for every 3 x 64K) is a good method for vegetative growth, and the reverse (3 x 27K for every 1 x 64K) fruiting or flowering.

Top Tips:

  • Due to their lower light output, fluorescent grow lights should not be used instead of HID lights during the flowering stage. However, they are effective as supplementary lighting when hung between your plants with CFL lamp hangers, or as side lighting with T5’s.
  • All of our fluorescent lights can be plugged into a timer to control the photoperiod. A relay or contactor is unnecessary.

HID Grow Lights

HID (High Intensity Discharge) lighting is the most effective way to transfer electricity into light and is the most common form of horticultural grow light.

HID grow lights are offered in a large choice of wattages; the most common are 250W, 400W, 600W and 10000W – with the 600W being the most popular option. A HID grow light is made up of three parts:

  • The first part is the ballast that contains the necessary electronics to ignite and run the lamp.
  • The next part is the reflector, which holds the lamp firmly in position and reflects light down to the plants.
  • The final part is the lamp itself that can be either a High Pressure Sodium (HPS) or Metal Halide (MH).


A ballast or power pack as it can also be identified as, is at the very heart of a HID lighting system. There are 2 types of ballast available; your average everyday electromagnetic ballast and also the more recent electronic or ‘digital’ ballast. Both which deliver a surge of electricity at a high voltage to ignite the HID lamp. After ignition, the ballast then regulates the electricity being transported to the lamp for safe operation. Electronic ballasts are around 3-4% more efficient than electromagnetic ballasts, as they run much gentler and can have dimming functions to influence the lamp power.


HID reflectors come in a range of different shapes and sizes, however they are all designed to do one job. That job is to reflect as much light as possible down onto your plants. The most effective reflectors are about 95% efficient, meaning that of the 100% light being reflected from the lamp itself, only 95% of it is emitted by direct light from the lamp or reflected light being caught from the reflector. However, even the best reflectors on the market will only have roughly a 5% loss. A reflector should help to create a standardised spread of light whilst also avoiding, commonly known as ‘hot spots’ of intense light. Some reflectors are sealed using a glass plate or tube and are air-cooled using an extractor fan to reduce any further heat from being released from the lamp and keeping grow room temperatures down.


Metal Halide lights produce a lot of light in the blue spectrum. This is because the blue colour of the light promotes leafy green plant growth and keeps your plants short and compact. Metal Halide lamps produce a broad spectrum of light and also a small amount of UV that can help to improve the quality of your produce. However, Metal Halide lights are not as capable as High Pressure Sodium lights, producing roughly 30-40% less micromoles.

High Pressure Sodium lamps give off mainly orange-red light. This type of light is best for fruiting and flowering plants, however it can also be used for vegetative plants with good results. High Pressure Sodium lamps are the most effective grow light currently on offer and produce the very best yields. Standard High Pressure Sodium lamps are slightly ineffective in the blue spectrum. In addition this, you can use fluorescent lights or use a ratio of three High Pressure Sodium lamps to one Metal Halide lamp in the flowering stage. Some High Pressure Sodium lamps are also known as ‘Dual Spectrum’ or also meaning that they have an improved output in the blue spectrum. Dual Spectrum lamps are also touted as being an all-in-one vegetable and flower lamp, however their spectrum is still much more suited to flowering plants. Best results will be accomplished using a Metal Halide lamp for vegetative growth and High Pressure Sodium lamp for flowering.

Top Tips:

  • If you want to supplement your High Pressure Sodium lamp with extra CFL’s, use the 6400K ‘Cool White’ lamp as these will provide the blue light that your High Pressure Sodium lamp is missing.
  • Switching from a High Pressure Sodium lamp to a Metal Halide lamp in the last 1-2 weeks of the plants life cycle can significantly develop the quality of your produce by enhancing essential oil production.

Relays and Contactors

When a ballast starts up, it draws a great amount of electricity for a split second so that it ignites the lamp. This spike of high voltage is enough to burn out a standard plug-in timer, so to automatically turn on and off a HID light, therefore you need to use a relay or a contactor. These devices use the timer as a signal, and can draw the power through a 13-amp plug, rather than through the timer. One 13-amp relay can switch 1 x 1000W, 3 x 600W, or 4 x 400W HID lighting systems. Contactors are more heavy duty than relays and are largely used for indoor gardens with 4 or more 600W lights.

Other Types of Grow Lights

New technologies currently trying to improve horticultural lighting are Light Emitting Diodes (LED’s) and Light Emitting Plasma (LEP).

Unfortunately, there has been a flood of cheap LED’s that have been launched in the hydroponics market over the years that have all over-promised and under-delivered. Many companies claim that their LED’s are more efficient than High Pressure Sodium lights, however currently there are still no LED units that can produce yields that are as good as.

On the other hand, LED’s are a very promising area for supplementing light to change or ‘steer’ the plants growth. The benefit of LED’s is that they can be tuned to a specific wavelength to activate a certain response or improve a particular growing stage.

The regular LED grow lights on the market that have a combination of blue and red LED’s state that they are tuned to match the two PAR peaks and drive targeted photosynthesis, yet they are missing all of the other wavelengths (colours) that plants also need. Interestingly, the most capable LED’s to come to market during 2012 are the ‘full spectrum’ LED’s that are designed to produce light across the whole PAR spectrum. However, the diode wattage and unit cost still means that LED’s are not able to replace High Pressure Sodium lamps.


Light that is emitting plasma has come to market for the purpose of ‘solar simulation’. They produce a very broad spectrum of light, including UV’s, which are close to natural sunlight.

LEP units can be used on their own for vegetative growth, but are lacking in the red spectrum to produce good yield of fruits or flowers. They are highly recommended for supplementary light, particularly for adding UV light to improve quality that is missing from most High Pressure Sodium lamp. The market leaders in LEP technology is Gavita Holland and we are sure that there will be some great further developments in LEP lighting systems over the coming years.

HID Lighting – Helpful Tips

What lighting should I use in my grow room?

Your choice of grow light should be decided upon by the size of your growing area. Each type of HID grow light is suitable for a specified area. For guidelines on the type and number of lights you can put in your growing are see below.

Lamp Area Coverage:

  • 250 Watt = 0.25m2 – 0.5m2
  • 400 Watt = 0.5m2 – 1m2
  • 600 Watt = 1m2 – 1.5m2
  • 1000 Watt = 1.5m2 – 2m2

Hanging Height

Due to the heat that is produced from HID lights, you should hang your lighting system in accordance to size. The following is the suggested distance between the lamp and the plant canopy.

  • 250W = 30 – 40cm
  • 400W = 40 – 60cm
  • 600W = 50 – 70cm
  • 1000W = 75 – 100cm

Top Tips:

  • Metal Halide lamps release more heat than High Pressure Sodium lamps, therefore mounting heights will need to be adjusted depending on your chosen lamp.
  • Use the back of your hand as a guide; if it is too hot for your hand to cope with, then it is definitely too hot for your plants.

How Much Will it Cost?

To get the correct operating cost per hour for light, take the lights combined wattage and divide it by 1000 to get the kilowatts used. Then multiply that number by the amount that your electric company charges per kilowatts an hour (usually found at the top of your electricity bill).

HID lights will use a little more than the stated wattage; a typical 600W system will use 640-660W. But to find out the precise usage you can purchase a plug in power meter for less than £10 and these will give you a digital reading of the actual power consumption.

Example: A grower with 1 x 600W HPS lighting systems finds his power consumption is 655W per light.

  • (power consumption wattage  / 1000) x electricity cost per kilowatt an hour = Operating cost per hour.
  • 655 / 1000 = 0.655
  • Electricity cost = 12.5p per kWh
  • 0.655 x 12.5 = 8.2p

This grower example shown above is paying 8.2p for every hour his 600W light is on. That equates to £1.48 per day for vegetative growth (18 hours) and £0.98p per day for flowering growth (12 hours).

How do you provide a more uniform level of lighting?

If you find that your reflector is not throwing enough light out to the edges of your room, or is creating hot spots then we recommend that you look at investing in a better reflector to improve uniformity. If you are using a high wattage HID light such as a 1000W then you can use an overhead moveable track, also recognised as a light mover, to move your light back and forth to create more uniform growth. These systems are highly worth investing in for high value hydroponic crops because a more uniform lighting pattern can help optimise yield.

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