Lamps, LEDs and Lasers
Since the invention of cinema, the main light source used in projection has been a lamp of some form or another.These days the majority of projectors use a bulb - either UHP or xenon - but in recent years there have been other forms of lighting used. We have seen projectors that use LEDs, we have seen projectors that use lasers and even projectors that use a hybrid light source that combines both LED and laser. In our previous article we took you through the various projector technologies, now we'll take you through the different light sources, explaining their benefits and weaknesses.
UHPThe most common light source currently used by a projector is a UHP (Ultra High Performance) Lamp which has certain advantages and disadvantages. The main advantages of UHP lamps are that they can be very bright and they are relatively inexpensive to replace. The downside to UHP lamps is that they struggle to produce fully saturated reds and their colour consistency will change over time. As with any lamp based light source they dim over time but are usually optimal for up to 2,000 to 3,000 hours. Although they are bright, they also produce a lot of heat and thus require fans for cooling which can be noisy. A projector that uses a UHP lamp also can't just be turned on and off instantly, it requires a period of warming up and cooling down for optimal performance.Lamps remain the primary light source for projectors but dim over time and have a limited lifespan.
XenonThe most common alternative to a UHP lamp is a xenon lamp which is often used in professional projectors and also has certain advantages and disadvantages. The big advantage is that the white light produced by a xenon bulb is better saturated in the three primary colours, resulting in a more consistent performance. They also produce much less heat than UHP bulbs and so require less cooling but again care needs to be taken when turning the projector on and off. The big disadvantages of xenon lamps are that they are very expensive and their optimal lifespan is only around 1,000 to 2,000 hours. Due to the short optimal lifespan and expense of xenon lamps, they are rarely used these days in domestic projectors and when they are it’s only on high-end models.
LEDThere have been attempts to use LEDs as a light source for DLP projectors and although this has met with a certain degree of success their implementation has been limited. The use of red, green and blue LEDs does result in a more accurate and consistent colour performance and their lifespan is much longer - at least 10,000 hours. Aside from their consistency, they also don't dim as much as a regular lamp will, so that consistency tends to last for the active lifespan of the projector. The LEDs can be instantly turned on or off, so there's no warming up or cooling down period and thus no heat to dissipate. The ability to quickly turn the LEDs on and off also eliminates the need for a colour wheel, thus reducing the rainbow effect - although it doesn't completely eliminate the problem. Although LEDs don't produce as much heat as a lamp will, they do need to be kept at a constant temperature which has resulted in many early LED projectors being quite expensive. There have been cheaper alternatives such as the Optoma HD91 but LED projectors always suffered from limited brightness and have fallen out of favour.LED light sources have increased the effective lifespan but often struggle with brightness.
LED-Laser HybridIn an attempt to utilise the advantages of LEDs but boost the brightness, an LED-Laser Hybrid light source was developed that used red and blue LEDs and a blue laser bounced off a phosphor coated surface to produce the green light. Since this is the largest part of the visible spectrum it increases the brightness of the projector whilst also retaining all the advantages of an LED light source. So again an LED-Laser hybrid can be instantly turned on or off, meaning there's no warming up or cooling down period and far less heat is generated and it has a long life span with a more consistent performance over that life. The ability to quickly turn the LEDs and laser on and off also eliminates the need for a colour wheel on a DLP projector, thus reducing the rainbow effect - although it doesn't completely eliminate the problem. The other issue is that with the LED-Laser Hybrid projectors we reviewed there was far too much green in the image and we were unable to reduce this effectively using the calibration controls included on the projectors themselves. For various reasons there are very few LED-Laser Hybrid projectors available these days outside the professional market, where their brightness and long lifespan are useful.
LaserThe latest light source drops the LEDs entirely and just uses Lasers. The new Epson LS10000 is the first domestic projector to include a laser light source and it uses two blue lasers. The first blue laser creates the colour blue and the other blue laser is bounced off a phosphor to create red and green. This results in images that are much brighter than LED projectors but also retain many of the advantages listed for LED and LED-Laser Hybrid light sources such as fast on and off times, a much longer lifespan than bulbs and far greater consistency over that lifespan. The use of lasers also results in less heat being produced, so there is less fan noise, and on the sample that we have reviewed to date, the colour accuracy was very good. It is likely that over the next few years, lasers will become the primary light source in mid to high end projectors and also be used extensively in commercial cinemas.Lasers could be the future of projection with bright and accurate images that have a long lifespan.
What Next?Although there are new light sources being develop the reality is that the majority of projectors still use a UHP bulb and until alternatives become cheaper and easier to implement, the situation is unlikely to change. However even after deciding which projector technology and light source is most appropriate, you still need something to project the image onto and in our next article, we'll explain which projector screen is best for you.
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