26.06.2013

Why laser is just another part of the image quality puzzle - part two

Lesezeit 4 Min.

In this installment, we’ll take a look at how étendue is impacting image quality. In addition, we’ll see how laser technology has a direct impact on the way light is transmitted, thus creating better contrast and uniformity.

 

Étendue

Étendue describes how big or compact a light source or bundle of light is. It is a combination of the surface and the angle at which light is emitted. The most specific thing about étendue (also the biggest challenge for optical designers) is that étendue can only grow. Applying this to our case above where we were thinking of combining multiple lamps to achieve a higher light input, we are confronted with the –relatively huge étendue of lamps: the arc where the light is formed is several mm long; furthermore the light is emitted in all possible angles. Combining more lamps only makes the system étendue even bigger. So if you want to make use of all this light, you will have to dimension your projector accordingly: a big light path, bigger chips, bigger angles, ... all too expensive. The nice thing about a laser light source is that the étendue is tiny: the surface of one beam is micrometer scale and thanks to the collimation the angular distribution is almost zero. This makes it easier to stack and scale lasers and accumulate a huge (but still low étendue) light source. Net result: the possibility of higher brightness, and better image quality!

Stray light

Lasers are not only about more raw power and pumping more light into the projector light engine; the higher collimation (i.e. a lower étendue, our word of the day) also has a direct positive effect on efficiently transporting light and an indirect contribution to better contrast and uniformity. In any projection system you want to control the light flowing through the system as tightly as possible: light bouncing off surfaces and going places where you don't want it, never has a positive impact on the image you are trying to project. Looking at digital cinema, the image-generating component (the chip) always has a certain light acceptance angle: any light in the system outside that cone is stray light. Stray light is out of your control and exits the lens on top of to the actual image you are trying to create. Added to a very bright image, the impact will be small. Added to dark scenes, it will ruin your contrast ratio. Existing cinema set-ups operate at a contrast of around 2000:1. The geometry of the lamp light distribution plays an important limiting role here. When taken into account in the system design, laser light sources and their more collimated nature can boost contrast ratio. Do note that other components (such as the chip) are still impacting and an infinite contrast ratio will never be possible.

The power of uniformity

Another important part of the image quality puzzle is uniformity. Whereas brightness and contrast ratio are local parameters (measured in one location on the screen, sometimes averaged out); uniformity is a global parameter. It quantifies how well the image quality is maintained across the entire screen, from center to edges. A good uniformity is achieved (among other things) by illuminating the surface of the imaging chip with a nicely filled and rectangular-shaped bundle of light. As mentioned above, the angles of this light cone affect the contrast ratio; the shape and fill of the cone impact uniformity. In the case of lamp-illuminated projectors, at the very start of the light emission, we start from an ‛arc-shaped’ source of light: between the anode and cathode of the lamp, a small 'bridge of light' is where the actual light generation happens. To achieve perfect uniformity, the optical components between the lamp and imaging chip (reflector, light rod, ...) have to reshape this arc (long, thin) into the rectangular shape of the chip. If you are not willing to compromise the size (and cost) of the set-up, this is a very hard thing to do. That is why the current cinema DCI spec for uniformity takes realistic values around 80%. This is where the laser comes in. Light generation no longer starts from an arc between two electrodes, but as a stable collimated bundle out of an aperture. This makes the life of the optical designer easier: by opening up the bundle (uncollimating) or adding up several bundles (here our good friend étendue again) it is possible to fill that rectangular aperture that the imaging chip is built for. The net result is not only a better uniformity and better image quality; but ‒ once again ‒ a more efficient light use since you don't have to start applying compensation tricks for inherent lamp non-uniformity. Do note that ‒ just like in the case of contrast ratio ‒ other parameters, like the lens, also impact the on-screen uniformity. Even parameters outside of the projector’s control, like screen gain, impact the perceived uniformity. So yes, laser can positively impact uniformity; but it will not magically lead to 100% perfect uniformity. This is true of any laser-illuminated projector.

In the last installment of our image quality triptych tomorrow, we'll explain yet another interesting metric: color gamut. Don't miss out!

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