Indoor cultivation has changed the way that growers cultivate anything. Indoor greenhouses allow for full control over the growing medium, light availability, water, humidity, and temperature. This means that hobby gardeners and botanists can cultivate all year round without the limitation of seasons.

One of the biggest aspects of indoor cultivation is lighting. As indoor growing has become increasingly popular, lighting has become more of a concern. How does one create the same spectrum of natural light with artificial lighting? The lighting industry has developed technology for all kinds of lighting scenarios, including different colours and spectrums.

Let’s break down the different kinds of lighting and how they are best applied in indoor greenhouses. We also talk about the different times that different spectrums may be used, and the different ways this can affect yield.

What are HPS, LED and LEP lights?

• HPS lights emit light by sending a pulse of high-voltage energy through a pressurized quartz tube filled with vaporized sodium, along with other elements such as xenon and mercury.

As the gases heat, they emit light. Sodium produces an intensely yellow-orange light; this can be mediated by xenon and mercury, both of which emit light in the blue end of the visible spectrum. The final result is whiter light.

• LED (light-emitting diode) lights are semiconductors, which allow electrical energy to pass with little resistance in one direction and overwhelming resistance in the other, across a “p-n junction”.

On one side of the junction is a material that has been treated with extra electrons, while on the other side is material that has been treated to lack electrons. When voltage is applied, the extra electrons move across the junction to fill the “electron holes” on the other side.

This causes light to be emitted, the colour of which varies according to the material used. Phosphides and nitrides of gallium, aluminium, zinc and silicon are the most commonly-used materials.

• LEP (light-emitting plasma) lights work quite similarly to HPS lights, but instead of passing a high-voltage discharge through a gas-filled quartz chamber, electrical energy is directed through a magnetron to be converted into a radio frequency field. It is then passed through the chamber. Similar mixtures of gases are used for HPS and LEP lights.

The efficacy of different lighting systems

The efficiency (or “luminous efficacy”) of lighting systems can be expressed in a basic equation: output (lumens) divided by total units of power consumption (Watts). High-efficiency lighting is generally accepted to output at least 90 lumens per Watt (lm/W), with some bulbs achieving 150 lm/W or even higher. Increasingly, grow lights are also rated in terms of PAR (photosynthetically active radiation) output, measured in µmol/s (the number of photons per second falling on 1m² of plant matter).

Mmol/s is increasingly seen as superior across the board for horticultural lights, as it is a measure of light usable by plants. Lm/W is a measure of what is visible to the human eye. Thus, it is advisable to get to know how to calculate PAR output measurement. A guide on the concept of PAR can be found here.

Currently, the highest-performing HPS bulbs produce around 150 lm/W. For example, GE Lucalox 600W bulbs produce 90,000 initial lumens (“initial lumens” refers to mean output over the first 100 hours), while Lumatek’s high-par 600W HPS lamps produce 92,000. Although previously seldom the case, newer HPS lights are now beginning to include PAR output ratings. For example, Lumatek’s 600W bulbs have a PAR output of 1,030 µmol/s.

For LED grow lights, lm/W is generally considered less meaningful than the PAR output. This is due to the fact that red-blue systems generally have much lower overall lm/W ratio – as they only provide the most necessary bands for photosynthesis, therefore expenditure of “unnecessary” light energy can be avoided.

However, newer LED lights use multiple bands to create a more complex and full spectrum of light. This is thought to be beneficial to plant growth, and modern LEDS often incorporate white along with reds and purples of the visible spectrum. These new full-spectrum LEDs may or may not list lm/W rating. However, it is generally accepted that the most reputable vendors include both lm/W and the PAR output.

For example, Kind LED, based in California, advertises the PAR values at different spots in a grow room for all of its products. Under a single light, the PAR value can change considerably from the space directly below the light, to the perimeters of the light’s reach. With Kind LED’s LumiGrow light, PAR is measured at a total value of 1856 µmol/s, with the highest figure being read directly in the centre of the square metre measured.

Theoretically, LEP’s luminous efficacy should exceed a HPS source by 15-20%, as no power is wasted on heating electrodes. In practice, light source efficacy of LEP (also known as “HEP” (high-efficiency plasma) or even simply “plasma”) grow lights range from 80 – 100 lm/W.

The light spectrum and mimicking daylight

Although improvements have been made in HPS lighting technology, they are still generally considered poorer in their replication of daylight than other modern lighting systems. Without the addition of xenon or mercury, sodium vapour gives off an intensely reddish-yellow light.

However, new “full-spectrum” bulbs now exist, and often consist of a dual-arc system comprising both a metal halide and an HPS component. For example, the Hortilux Super Blue HPS/MH lamp comprises a 600W HPS and a 400W metal halide arc, giving 110,000 initial lumens and a far more accurate representation of daylight than HPS alone.

LEDs are probably the best contenders in terms of spectrum at this stage, with newer full-spectrum models offering up to 11 bands of light wavelengths most needed by plants. For example, the US-made Platinum P450 274W LED grow light (which is listed as equivalent to a 600W HPS) offers 11 bands ranging from ultraviolet to infrared.

LEPs are often sold as full-spectrum and the closest approximation to daylight of any indoor light, but the spectrum is limited by the composition of the gases inside the chamber. Early LEP technology took decades to get off the ground for the precise reason that their colour rendering was so poor, with little coverage of the red areas of the spectrum.

Now, fine-tuning of variables such as the gases used, the coatings (such as metal halide salts) and even the pressure of the chamber can create far more precise replications of daylight. They also have the advantage of emitting some light in the UV spectrum. But the jury is still out on whether they are truly superior to “full-spectrum” LED systems.

How lighting can affect yields

As light is one of the two most basic foods for any plant (water being the other), it naturally has a big impact on final yield. Annual flowering species follow very strict patterns of temperature, day and light hours, and light spectrum.

As the season changes from summer to autumn, the light spectrum changes, as do the other factors that affect flowering. For this reason, to achieve maximum yields, annual flowering plants should be exposed to different lights at different stages during their growth cycle.

HPS lighting is best used during the flowering stage of a plant’s growth cycle. This is because the spectrum of light emitted is conducive to flower production. LED lighting is also ideal for the flowering stage as they produce a lot of lumens. For most annual flowering species, the summer initiates flowering, a period of the year that is rich with sunlight. LEP lighting technology is still miles behind HPS and LED, and does not produce yields as big as other lighting systems.

Typically, growers begin a grow cycle with fewer lights. This ensure that seedlings and young plants are not burned by excess light and heat. Growers can increase the amount of light globes as the cultivation process progresses. This also ensures that there is sufficient light once plant size has increased.

The cost of lighting

Aside from the upfront cost of purchasing lights, growers must also consider the cost of electricity. The question of LED vs HPS lights can also be affected by what continent you are living on. In Europe, electricity is more expensive than in the US. This tends to lead to gardeners in the US growing under 1000W bulbs, whereas their European counterparts lean towards 600W or even 400W bulbs.

LED and HPS lights, while emitting the most lumens, are also the most expensive to run. For this reason, many growers will not use LED and HPS in the early stages of plant growth. Fluorescent lights suffice during early growth as seedlings and young plants don’t use as much light as fully matured plants. Another way to decrease the cost of lighting is, as mentioned earlier, to start with fewer lights and increase as the plants mature.

Growers who combine different lighting systems create more complexities in their grow rooms, more room for error, and many more variables to contend with. This is something important to consider when choosing lighting. For “no fuss”, one lighting system should be chosen. Once knowledge increases of different lighting systems and their effects, they can be combined.

The results of our Twitter poll: LED vs HPS

We ran a twitter poll to understand how our viewers used different lights. Check out the results below!

1. Have you ever grown using LED lights? 

• yes – 36%
• no – 39%
• no, but I would – 21%
• no, and I wouldn’t – 4%[AK1] 

2. Have you ever grown using HPS lights? 

• yes – 73%
• no – 18%
• no, but I would – 5%
• no, and I wouldn’t – 4%

3. Have you ever grown using a combination of LED and HPS?

• yes – 11%
• no – 74%
• no, but I would – 11%
• No, and I wouldn’t – 4%

4. Were you satisfied with the results from LED?

• yes, very – 43%
• yes – 21%
• yes, but could have been better – 21%
• no – 15%

5. Were you satisfied with the results from HPS? 

• yes, very – 32%
• yes – 32%
• yes, but could have been better – 21%
• no – 15%

6. If you were going to buy a new lighting system, what factor is most important to you? 

• yield – 40%
• cost – 20%
• electricity usage – 25%
• other – 15%

7. If you were going to buy a new lighting system, what factor is least important to you?

• yield – 22%
• cost – 11%
• electricity usage – 50%
• other – 17%

8. If LED cost the same as HPS, would you choose LED?

• yes – 68%
• no – 23%
• don’t know – 9%

What has your experience been with lighting systems and which one do you prefer? We would love to hear from you in the comments below.

Source: https://sensiseeds.com/en/blog/lighting-systems-led-vs-hps-lights/