We all have long been accustomed to the warm, orange-golden lighting of our cities. This is not surprising, because the HPS lamps in the overwhelming majority of cases illuminate our streets and are one of the most effective sources of light.
The first to massively begin to use low-pressure lamps-LPL, which were replaced by more technological mercury gas-discharge lamps, and those in turn were replaced by HPSL- high-pressure sodium-vapor lamps.
What is a sodium-vapor lamp?
HPS (high pressure sodium-vapor) lamp was invented back in the 30s of the 20th century and immediately began to spread all across Europe. Such a lamp has a bright orange emission spectrum due to the resonant emission of sodium vapor.
Sodium-vapor lamps have significant light intensity in recalculation on the power consumption. That’s why they until recently were the most economical source of lighting and found wide application in street lamps for lighting motorways, streets, squares and courtyards.
But scientific and technological progress does not stand still and to replace the HPS lamps appeared a fundamentally new light source, ultra-bright LEDs.
LEDs (Light Emitting Diode) were invented in 1962, but for a long time it was’t possible to create a white-light-emitting diode with brightness sufficient to use it as a lighting device. Initially they used LEDs of three colors: red, green and blue in one crystal. Their joint glow created a white color. However, the cost of such a solution was quite high, but there was no reliability. Only in 2005 appeared white-light-emitting diodes whose brightness matched the brightness of sodium lamps. Scientists created a crystal that radiates in the invisible to the human ultraviolet range. Then, this invisible ultraviolet radiation hits the inner part of the lens, which is covered with a special phosphor-composition that is capable of glowing under the influence of UV-radiation. So it is it which radiates the light that we see. So when the ultra-bright LED was invented HPS lamps got a serious competitor as a source of outdoor lighting.
So, what are the advantages and disadvantages of LEDs and HPS lamps?
1. Brightness of light.
Here, the LEDs and HPS lamps have the same 120Lm/W. But, if the possibilities of modernization for HPS lamps are already exhausted, for LEDs it is only just the beginning and there are already LED light sources with a light output of 180 lm/W.
In addition, LED lamps are able to shine with high brightness for a maximum of half their life time -about 1 year, and then their brightness, and hence energy efficiency, fall, dropping by 40% in the second year, after which their use is generally inappropriate. Another factor that reduces the efficiency of the HPS lamp is the design of the luminaries. The HPS lamp only sends half of its radiation to the side of the illuminated object. The task of the lamp – “to catch” the second half of the light stream and reflect it in the direction we need. Over time, the reflective surface of the luminaries becomes dirty, and the reflected light stream decreases significantly, and the luminaries is cleaned – a very labor-consuming and expensive occupation. LED luminaries constructively direct the entire light stream to the illuminated object and doesn’t have such a problem.
2. Life time.
According to this indicator, the HPS lamps lose significantly to LEDs. HPS lamps serve up to 6000 to 10,000 hours, while LEDs range from 50,000 to 100,000 hours.
Luminaries based on HPS lamps are still cheaper than LED’s almost three-fold, but the situation is gradually changing, as there is a tendency to reduce the cost of LEDs and to increase their effectiveness.
4. Сolor rendering index.
This parameter, which characterizes the level of correspondence of the natural color of the body to the apparent color of this body when illuminated by this light source, is an indicator of how accurately a person can distinguish the color of an object illuminated by a light source. According to this indicator, the LED is an unconditional leader. The spectrum of its light approaches the spectrum of the sun at noon. That’s why the color rendering index of the white LED is between 80 and 90 units. This indicator is the worst of all light sources – 20-25 units for HPS lamps. Therefore, for example, a dark green machine illuminated by HPS lamps cannot be distinguished from a dark blue or a black one.
5. Stroboscopic effect.
In order for the sodium-vapor lamp to light up, a special ballast device doubles the frequency of current discharges. In this regard, the lamp has a sufficiently high ripple ratio. Because of this effect, eye fatigue is noted. In addition, the use of HPS lamps is prohibited in production as main lighting in order to avoid injury. LEDs have a stable and constant light stream.
6. Time of ignition.
The LEDs go into operating mode immediately after shutdown, maximum after 3 seconds. Whereas it takes from 6 to 15 minutes HPS lamps to start shining 100%. In addition, before switching on the lamp again, it needs to cool down; otherwise it may not turn on.
7. Function in the cold season.
HPS lamps have difficulty starting at low temperatures. In addition, the luminous flux from them also falls. LED lamps are not subject to this disadvantage at all.
8. Mechanical strength.
Although the flasks of HPS lamps are made of special glass, it is still a very fragile object, which cannot be said about LEDs that have a sufficiently high mechanical strength.
9. The possibility of dimming.
LED lamps allow dimming in the range of 10-100%, which allows additional saving of electricity and expanding their lifespan. Due to low energy consumption, CO2 emissions are reduced, which is without doubt an important factor for the preservation of the environment. In addition, the control option allows the use of LED lamps in smart street lighting systems, which allows even more flexible control of lighting and further reduces the city’s electricity costs. Also very promising and economically justified is the use of smart lighting systems for parking spaces near hypermarkets, shopping centers, parking lots and other areas with cyclical presence of people. The HPS lamps are only partially dimmable.
10. Ecological compatibility.
In the constructions of LED lamps, no harmful substances are used for human and proprietary substances. The HPS lamp contains sodium mercury amalgams and xenon for discharge ignition.
So, LEDs lead in all parameters, except for the price. This factor still hampers the massive use of LEDs, but there are already large corporate customers who use LED lighting in the project initially, knowing about the advantages of LEDs in front of HPS lamps and considering that the initial investment will easily return in a short time. More and more European cities are converting city lighting to LEDs for the same reason.
The LED lighting industry is still very young, only about 10 years, but LEDs are confidently replacing the HPS lamp in our street lighting and soon the color of night streets of our cities will change from orange to white.