Grow-lamps for plants: a discussion about HPS and LED 

There is a long-standing controversy about which plant grow-lamp should be chosen, HPS or LED? Today, Lao Ma analyzes the advantages and disadvantages of these two grow-lamps for plant from different perspectives.

Grow-lamps are different from ordinary illumination lamps. As a product engineer in a lamp manufacturer, let me show you by comparing the following aspects:

1. Luminous efficiency and heat dissipation

The luminous efficiency of HPS grow-lamps is generally between 1.3-1.9µmol/J, while the luminous efficiency of LED grow-lamps is up to 1.9-3.0µmol/J. Higher luminous efficiency means more light output and less heat dissipation. For example, when both of them consume 300W electric energy at the same time, the electric energy will either be converted into 300W luminous energy or heat energy. According to the conservation of energy, the more luminous  energy there is, the less heat energy there will be, while the more heat energy there is, the less luminous energy there will be. The HPS grow-lamps is just in this latter category. Another  result caused by this is that in two rooms that are using lamps with the same electric power, the energy consumption of the air conditioner in the room that is using LED lamps will be less than the one using HPS ones.

2. Spectrum

Spectrum is just the so-called light formula which describes the ratio of visible light and invisible  light in different wavelength bands. In simple terms, it describes how many percentages are shared by all the light emitted from a lamp, such as red, blue, green light, etc. The advantage of LED lamp is that the percentage of light in different wavelength bands can be configured according to the grower's planting requirements to achieve different effects: larger or thicker leaves, longer  or shorter distance between nodes, earlier or later blossom, more or less branches and so on. However, the spectrum of HPS is hard to be customized, and most of it is located in the areas of  green or yellow light which can hardly be absorbed by plants. (Check figure 1 for reference)

3. Photosynthetic photon flux density and light distribution

 In simple terms, the photosynthetic photon flux density(PPFD)refers to the amount of photons  passing through each square meter every second at a certain height. It shows that the PPFD of  a grow-lamp is highly related to its mounting height.  Light distribution refers to how the light of a grow-lamp is emitted and distributed on the surface  it illuminates after it is turned on.  The lighting characteristic of HPS lamp is that the light will be emitted from the center to the  surroundings areas in a spherical shape without specific directions. Therefore, many HPS lamps  equip with a reflector to reflect the light emitting to the useless space back to the useful areas.  However, the reflection efficiency is very low, with a light loss of at least 30-60%. Moreover, since  the HPS is a concentrated point light source, its light distribution on the surface will be divergent  from the center to the surrounding (check figure 2 for reference). The more light is distributed in  the center, the less light will be distributed in the surrounding areas with the distance increasing  between them. This uneven light distribution is not conducive to the growth of plants. Too much  light focused on the center can form hot spots which will burn the plants easily. While the  insufficient light in the surrounding areas will cause excessive growth (high and thin) which fails  to meet the requirements of planting.

 LED lamps are designed to distribute light evenly in the area emitted (check figure 3 for reference)  to make all plants grow uniformly.  In addition, compared with HPS, LED is a kind of cold light source, so the distance from the lamp  to the plant corolla can be appropriately reduced. Since the radiation intensity is inversely  proportional to the square of the distance, the reduction in the distance to the light source can  greatly affect the light radiation intensity absorbed by plants, which make it particularly important  to control the distance between lamps and plants.

4. Service life and light decay curve

The service life of HPS lamp is around 20,000 hours, but light decay begins after around 3,000 hours, and the light decay is faster in early stage. The service life of LED is usually more than 100,000 hours, and the LM90 lamp beads tested for 35,000 hours show that the light decay is no more than 10% within 35,000 hours. The light decay directly affects yield during the process of growth, since 95% of the weight of plants come from light energy. In addition, it will not only need to buy additional HPS bulbs, but also consume a high labor cost when HPS lamps are replaced.

That is all for this article. In general, with the continuous improvement of LED technology, the advantages of LED grow-lamps will become more and more prominent. But not all LED grow-lamps are better than all HPS ones, how to measure the quality of LED lamps? Please  stay tuned for the next article "How to comprehensively measure the quality of LED grow-lamps".

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