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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 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

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