25 de jun de 2013

Why laser is just another part of the image quality puzzle

2 minutos de leitura

Image quality is a rather subjective metric: there is no single parameter to quantify how good an image looks. However, there are underlying parameters that contribute to image quality that are perfectly quantifiable. Some of the most important ones are brightness, contrast ratio, resolution, uniformity, color gamut, focus, ... What is important to remember is that image quality is a puzzle of all these pieces that needs to fit well in order to get an optimal end result. There is no single parameter that dominates or that can compensate for an underperforming other parameter. E.g. it serves no purpose to project at 4K resolution, if you'll be only looking at a dim 3fL image. Laser light sources can have an impact on multiple areas of the puzzle and significantly improve the total image quality.


The magic of bright projection

As mentioned previously, when we talk about laser projectors or laser projection in a cinema context; we actually mean laser-illuminated projection. In this set-up, the basic building blocks of the projector (electronics, motor, lens, ...) are still used; the laser can be considered as a white light source replacing the lamp (functionality). In this setting, it might seem strange that lasers can bring such a significant improvement to the image quality. After all: isn't the projector light engine where all the magic happens and isn't the white light just raw material that requires a lot of processing to create a crisp image? Let's see.

In its most basic description, a (digital) projector is actually nothing more than an advanced light filter: starting from the ‒ massive amount of ‒ white light that the lamp emits; it filters out all the unnecessary components to create an image on the screen. Read our online white paper to understand how this works. What is important to remember is that in the current configuration, the filtering is not really efficient: less than 10% of the light from the lamp can be used for the on-screen image. Another important fact is that the filtering happens in multiple domains: not only filtering of wavelengths to create colors or get rid of UV/IR-components; also spatial filtering to fit the light onto the small chip surface. Getting maximum on-screen brightness to enhance the image quality can be enabled in two ways: starting from more available light and/or filtering the light more efficiently after it leaves the source. Laser contributes to both parameters. Since laser light is almost perfectly monochromatic, you can ‒ in theory ‒ select the wavelengths at the start that you know you'll keep in the end. This puts a smaller constraint on the projector design (e.g. the need to remove UV/IR components) and opens up opportunities for an optimized design (e.g. coatings tailored to the specific wavelengths). This makes it possible to boost the light filtering efficiency of the projector light engine. With lasers, it is also possible to start from a higher level of white light. With lamps this is practically impossible: you could boost the power of one lamp (15kW Xenon exist), but this degrades the lifetime to a level where it is no longer practical or commercially viable. In theory, you could also combine the light of multiple lamps to go beyond the limits of what you can reach with one lamp; but here you bump into the limits of étendue. Remember this last word, étendue: it's very important in a laser context (and guaranteed to impress friends and family at parties)!

Check back tomorrow for our second installment, in which we'll explore how laser technology relates to other parts of the image quality puzzle.

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