2510332 - February 2009
Projection Optical System Architectures
Optical systems for single-panel projection applications can be grouped into two main architectures by describing the conditions at the device. Each type has unique advantages or disadvantages that determine suitability for a given application, depending on the most critical performance parameters for that application. Pros and cons of architectures, along with distinguishing performance characteristics for certain applications, are discussed in general terms.
Because DMD devices are reflective, the illumination and projection paths to the device share the same space in front of the device. The architectures described below are typical ways to separate these paths in that space. Since the mirror hinges are along the diagonal of the mirror, the mirrors rotate about an axis that is oriented 45 degrees to the array dimensions, and steer light in a plane compounded by this axis of rotation. Therefore, for any given location of a projection pupil relative to the device, there exists only one axis for the incident illumination path to the on-state mirrors, as determined by Snell’s Law of reflection. This is the basis for many possible embodiments in detail, all of which must consider the axis of rotation of the mirrors for proper performance.
In general, the device tilt angle sets the maximum useful numerical aperture of the optical system at the device. This prevents overlap of the on- and flat-state pupils for contrast control. This rule of thumb can be “stretched”, depending on performance tradeoffs allowed, but it is a good place to start. How to stretch this rule will be discussed in terms of performance parameters later in this application report.
Telecentric systems are defined by locating the exit pupil of the illumination system (entrance pupil of the projection lens) at or near infinity from the device surface. The chief rays of every bundle incident on every mirror then are essentially parallel to each other. For the illumination system, this provides uniform angles of incidence across the entire field, creating uniform black levels for the dark field. Typically, the illumination axis is separated from the projection axis by an angle just larger than twice the device tilt angle. The projection axis then is typically perpendicular to the device. If a prism is used to separate the paths (see Figure 2) the telecentric condition also produces uniform distribution of angles of incidence across the antireflection (AR) coated surfaces to avoid spatial nonuniformities in display brightness due to coating-performance variation with angle of incidence.
In the example of the total-internal-reflectance (TIR) prism embodiment, the illumination is separated from the projection path by choosing the angle of the TIR-prism face to be at the critical angle for the illumination path. The uniform angles of incidence and reflection prevent critical angle failure (TIR failure) in the projection or illumination paths through the prism.
One common embodiment of the TIR design is the so-called “Reverse TIR” or RTIR design. This is based on US patent 5309188, which employs a right-angle prism as the TIR prism. The TIR path occurs in the projection path, not the illumination path, which gives it the name “Reverse TIR.” There are several advantages to this architecture, as well as few disadvantages. Contact TI for additional information on the relative merits of this design.
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