Humanity began experimenting with fire and oil lamps more than two millennia ago to create engaging presentations of light and shadow. Today, video projectors are the tool of choice for creating a visual presentation on a grand scale. Whether commercial cinema, home theater, or the transformation of landmark structures, a projector’s ability to generate the bright and colorful imagery we see today starts with a light source that has dramatically evolved over the last 200 years.
The early years
The discovery in the 1820s of generating a bright white light by directing a flame of burning hydrogen and oxygen at a cylindrical block of calcium oxide (i.e., quicklime) to the point of incandescence ushered in the “Limelight Era.” This ingenious discovery enabled practical outdoor illumination for special events and proved popular in projection and stage lighting applications for theatrical presentations until the turn of the century.
Lamp-based illumination began as a source of a film projector’s output almost as soon as the incandescent light bulb was introduced in the 1870s – essentially in lockstep with the spread of electrical power transmission. Early commercial cinemas transitioned to the impressive brightness of carbon arc lamp illumination in the early 1900s, and that lighting technology remained in use until the late 1960s. A limitation of the carbon arc lamp’s use in early projection systems was that the carbon rod electrodes required replacement after less than 30 minutes of use. This characteristic influenced the length of film reels, captured at the famous 24 frames-per-second frame rate. It also promoted using two projectors to streamline the transition from reel to reel.
15 kW xenon short-arc lamp used in IMAX film projectors
The discovery in the mid-1940s of the xenon discharge and its bright white light, composed of a nearly continuous and even spectral distribution of wavelengths similar to sunlight, sparked interest in its commercial development and applications. Before the end of 1950, the xenon arc lamp saw its first public demonstration as a projection light source. The xenon lamp proved superior to the carbon arc lamp by producing illumination with a high color rendering index (CRI) close to 100, resulting in accurate color representation. Another practical advantage of xenon arc lamps compared to the carbon arc lamps they replaced was their consistent and flicker-free output, which required much less adjustment and maintenance over its significantly extended lifespan (measured in the hundreds of hours).
The quality of visible illumination the xenon arc lamp provides has made it the gold standard for commercial projection systems for decades. Although relatively inefficient in converting input power to visible light, the design and output of a xenon arc lamp can be scaled to support all sizes of commercial projectors in use today – the most powerful of which require liquid cooling to manage the intense heat generated during operation. However, a downside of xenon lamps as a modern projection light source is a relatively short lifespan – about 500 to 2500 hours, depending on the lamp’s size and use case. As a consumable for long-term operation, xenon lamp modules can cost upwards of several thousand dollars apiece to replace.
An end-view of a 15 kW IMAX lamp showing the liquid-cooling ports
The ultra-high performance (UHP) lamp modules used in today’s consumer projection systems are a form of high-pressure mercury-vapor lamp technology. While costing a fraction of the price of xenon lamp systems and offering improved efficiency, they produce a less consistent spectral output with significant peaks in blue and greenish-yellow. The lifespan of a UHP lamp can reach 2000-4000 hours at full brightness, with some designs claiming up to 15000 hours in reduced power (reduced brightness) operation. The relatively small amount of mercury in mercury-vapor lamps (approximately 15mg to 50mg) is a toxic waste that necessitates proper handling and disposal.
UHP projection lamp module
Breaking the blue barrier
The introduction of the light-emitting diode (LED) and the visible laser diode in the early 1960s sparked the dreams of projection display designers, promising an efficient solid-state light source with high color purity. However, one color proved stubbornly problematic. Fast forward several decades to the early 1990s, and a dedicated and committed engineer named Shuji Nakamura finally solved the primary challenge of manufacturing semiconductor materials with the appropriate characteristics to generate blue light with ample efficiency and longevity. Nakamura’s perseverance led to the first blue LED demonstration in 1992, and four years later, researchers applied similar techniques to introduce the first low-power blue laser diode, opening up a new era in projector light sources.
The first display credited with using laser technology for the projected image was the Mitsubishi LaserVue TV (model L65-A90), introduced in 2008. The LaserVue used red, green, and blue (RGB) lasers as a light source, with a Texas Instruments DLP (digital light processing) DMD (digital micromirror device) chip providing pixel control. This rear-projection HDTV wowed reviewers with its highly saturated depiction of color and good picture contrast.
Mistubishi LaserVue TV
Two years later, in 2010, Casio introduced a hybrid laser, phosphor, and LED-illuminated front projector, the XJ-A130, regarded as among the first laser-based video projectors to reach consumers. The XJ-A130’s DLP-based light engine featured a red LED and a blue laser emitter that stimulated phosphor materials on a segmented glass wheel to produce a green primary color. The company highlighted the projector’s compact design and the lack of a mercury-laced lamp module.
Casio XJ-A130 hybrid laser projector
Solid-state future and pitfalls
The primary benefits of LEDs and lasers as light sources for modern projection systems are longevity, color purity, and reduced time to maximum output, i.e., fast startup. Many projection light engines based upon these solid-state light sources claim 20000 hours or more of operation until they reach an undesirable reduction in light output. However, these lighting technologies also have technological quirks and associated costs that limit their universal acceptance – at least for now.
The efficiency benefits offered by LED lighting are currently achieved at relatively low power levels. Feeding the average LED too much power causes a condition described as “droop,” where the device becomes increasingly inefficient as power input exceeds a certain threshold. The exact cause of LED droop is highly debated among competing researchers, and there is still no universally agreed-upon answer. This has led to lighting designs using many lower-powered LEDs to achieve the desired output while maintaining good energy efficiency. High-powered LED designs do exist, but they sacrifice efficiency and expel more waste heat – reducing their advantage over lamp-based sources.
The laser light source used in projection has been carefully implemented through years of iterations to avoid associated artifacts that affect image quality. One of these artifacts is “speckle,” which occurs when the highly-collimated beam of laser light interferes with itself, resulting in a shimmering, grainy pattern. Laser speckle is usually seen when the light interacts with a rough or uneven surface, and it can be minimized through several techniques, including optimized screen materials, optical diffusers, and modulation techniques that blur the artifacts to make them less noticeable.
An example of a liquid crystal-based speckle reducer
The continuing popularity of projectors that excite phosphor materials with blue lasers (fluorescence) or in combination with LEDs as a light source is to minimize cost, offer increased longevity, and reduce the potential generation of speckle artifacts by mixing spectrally different but similarly appearing colors – or, to avoid laser speckle in more easily noticeable colors such as red and green. The fact that lasers in modern consumer and commercial projectors are lamp replacements providing illumination for pixel modulators like DLP chips, LCoS microdisplays, or LCD panels offers additional opportunities to integrate speckle reduction strategies.
Show me the colors
The red, green, and blue color primaries of ultra-high-definition television (UHDTV), defined in the International Telecommunication Union (ITU) standards known as BT.2020 and BT.2100, are essentially laser-like sources of light with very narrow spectral bands – each, effectively, a single wavelength of monochromatic color. Compared to the inefficiencies of using color filters with lamps or the relatively limited brightness of LEDs, lasers properly tuned for these primary colors are an obvious choice for projection systems that wish to accurately represent the color pallet available in modern video formats like HDR10 and Dolby Vision.
Dolby Cinema is an example of today’s state of the art in commercial video projection. Launched in 2014, Dolby partnered with Christie Digital to design and implement an RGB laser dual-projection system that offered superior brightness and wide color gamut coverage that exceeds 95% of the color space defined in BT.2020. The laser light sources in these projectors are rated for 50000 hours of operation and feature multi-wavelength primary colors that minimize potential speckle artifacts when used with high-gain screens that are especially useful with 3D presentations. Experiencing the incredibly rich color and superb picture contrast of a Dolby Cinema presentation (or commercial equivalent) should be on every cinephile’s bucket list.
The efficient future
A recent demonstration of a prototype laser DLP projector from Barco and Hisense incorporated light phase modulators to improve contrast by dynamically concentrating the projector’s light source into image highlights and reducing it in darker portions of the picture. This “local dimming for projectors” was visually impressive. It could enable them to recreate a commercial-grade cinema experience at home and improve performance in less-than-ideal viewing environments.
Increasing efficiency is a goal for all projection systems – LEDs and lasers will dominate projector lighting for the foreseeable future. LED droop will eventually be solved, allowing them to drive even higher light output levels with less waste heat – with potential cost savings over laser illumination. Consumer RGB laser projectors are quickly maturing, ditching phosphors to deliver color gamut coverage comparable to their commercial counterparts and remaining the light source of choice for today’s premium video content.
Robert is a technologist with over 20 years of experience testing and evaluating consumer electronics devices, primarily focusing on commercial and home theater equipment.
Robert's expertise as an audio-visual professional derives from testing and reviewing hundreds of related products, managing a successful AV test lab, and maintaining continuous education and certifications through organizations such as CEDIA, the Imaging Science Foundation (ISF), and THX.
More recently, Robert has specialized in analyzing audio and video display systems, offering comprehensive feedback, and implementing corrective measures per industry standards. He aims to deliver an experience that reflects the artists' intent and provides coworkers and the public with clear, insightful product information.
