2510332 - February 2009
Projection Optical System Design Considerations
Regardless of the architecture, system design consists of an illumination system and a projection system. In some cases, these systems can be treated independently. In others, particularly those designs having field lenses that are in the path of both the illumination and the projection systems, there is obvious interaction that must be accounted for. The following paragraphs address the design considerations and the components of each system.
Illumination-System Components and Design Parameters
The simple function of the illumination system is to collect as much useable light as possible from a light source and put it on the device active area (mirror array). The components typically used to do this are: lamp, reflector, color wheel, integrator, relay and folding optics (including field lenses, if any), and, possibly, a TIR prism.
3.1.1 Lamp Lamp selection depends on several factors:
Projector size/weight/noise goals. Most lamps are rather inefficient at converting electrical energy into visible light. This means there will be a thermal load on the projector from the lamp. In most cases, this load is the highest load in the system. The amount of power (heat) that can be dissipated by the projector is determined by the number and size of the fans used to cool it, given projector size, weight, and noise requirements. The most efficient lamp, in terms of lumens per watt output collected into the available etendue of the device, is the parameter to optimize.
Life. Lamp life requirements vary by application. Consumer applications require very long lamp life to overcome consumer resistance to replacing a high-cost lamp. Lamp life of 5000 to 10000 hours, or more, in the product is required for these applications. Portable projectors, however, typically see relatively infrequent usage. Lamp life of 2000 hours in these products can exceed the products’ useful lifetime, given the duty cycle. One must be careful to understand what a lamp manufacturer may mean by lamp life, versus what a product requirement for lamp life means. Typically, product lamp life means no less than 50% of initial product brightness output after the specified lifetime under certain duty cycle requirements with few, if any failures. Manufacturers typically specify catalog lamp life in terms of 50% of a sample lot still running after a specified period of time, usually under ideal thermal conditions. These are very different requirements, and both parties must understand what is meant or needed.
Spectral content. The output spectrum of the lamp must be balanced into useable color space in the projector by the color-wheel. The efficiency of this conversion can vary greatly from lamp to lamp. TI has a color-wheel design/modeling tool that can compare the relative efficiencies of lamps after color-wheel conversion. The results often are not obvious. As a rule of thumb, the closer the CCT of the lamp is to the desired whitepoint of the projector, the more efficient the lamp/color-wheel combination will be.
Commutation and arc stability. Whether the lamp is ac or dc, and whether it has some means of preventing arc jump or arc flicker, can be important factors in the application.
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