Illumination – Light source and modifier – Laser type
Reexamination Certificate
2000-09-07
2001-12-25
O'Shea, Sandra (Department: 2875)
Illumination
Light source and modifier
Laser type
C362S019000, C362S284000, C362S326000
Reexamination Certificate
active
06332693
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to optical illumination systems and more specifically to optical illumination systems that incorporate time-modulated light sources and recombining modulators to increase brightness. Moreover, the invention also discloses novel methods for intensifying illumination brightness by time-superimposing multiple pulsed light sources.
2. Description of the Related Art
It is well known in the art that the brightness of a light source cannot be increased by a passive optical system. Here “brightness” is used in the technical sense of optical power per unit emission etendue, where emission etendue is the product of solid angle in the emitted direction times source area measured in a cross-section perpendicular to emitted direction.
Just as a non-attenuating optical system must preserve source brightness, so it must also preserve as invariant the product of the solid angle of the illuminating light and the cross-section area of the focused illumination beam. The divergence (or convergence) of the beam can be decreased if minimum beam diameter is allowed to increase. Conversely, minimum beam diameter can be decreased i the beam is made more divergent (or convergent). However, it is only possible to make both improvements simultaneously if part of the beam is blocked, which reduces collected power. For given fixed source brightness, the light received by an illuminated object of fixed area is thus determined by the solid angle that the illuminating light occupies. A geometrically equivalent statement is that, for given fixed source brightness, the optical power projected by illuminating optics of fixed diameter is determined by the solid angle into which the optics project the light. The optical system designer must ensure that the source is powerful enough to radiate with this fixed brightness into all regions within the lens diameter, and into all directions within the output solid angle, and that the diameter and solid angle are well chosen according to the various constraints of the application. However, once the source brightness, diameter, and solid angle are fixed in this way, the designer can only increase the delivered power by minimizing absorption and scatter within the system; he cannot redesign the system to concentrate more power into the limited diameter and solid angle.
These constraints of fundamental physics significantly limit the optical designer's freedom to increase illumination intensity. For example, the following equation shows that even if an illumination system can collect all rays emitted by a source of width S, maximum possible delivered power is achieved once one chooses the source large enough that |S|≧|P &agr;|, where P is the lens diameter and &agr; the angle that the optical system projects into. When this condition is satisfied, the lens aperture is completely filled by the source, and maximum intensity is delivered within the projection angle &agr;. Unfortunately, if S is increased beyond the point needed to fill the lens, the overfilling light cannot be collected, and the extra light that is output by the larger source is therefore wasted. On the other hand, when |S|<|P &agr;|, the source is too small to fill the aperture, and illumination intensity can be increased by increasing the source size, which, for a fixed class of light source, means increasing the power consumption of the source. Image intensity is said to be power-limited in this case.
However, once |S|>|P &agr;|, further increases in source power do not increase image intensity because the additional source area is not collected within the lens aperture. Image intensity in this case is said to be brightness-limited. Loosely speaking, one might say that when the source is brightness-limited, image intensity can only be increased by increasing the brightness of the collected rays; increasing the size of the emitting region to produce ‘more rays’ does not help.
The field size or angle &agr; is often fixed by the application. To increase image intensity once the brightness limit is reached, the designer can increase the lens diameter P (or equivalently, increase the numerical aperture [NA], defined essentially as the ratio of Lens aperture radius P/2 to object distance). However, technical constraints on lens performance and/or practical constraints on cost often limit the feasibility of increasing the lens diameter. This is particularly true in projection optical systems, where the illuminated object is re-imaged by a projection lens. High quality projection lenses must not only be designed to capture the full angular and spatial extent of the light that is reflected or transmitted by the illuminated object, they must also project a high resolution image of the object using this light. Image aberrations increase as lens diameter is scaled up. Resolution requirements are particularly stringent in photolithography systems. In projection displays the optics frequently include elements for color and polarization separation/recombination whose cost scales very unfavorably with NA. Thus, in photolithography projectors or projection displays it is not easy to increase the NA of the projection system.
Of course, one requirement for maximizing brightness is that the brightest available source be chosen for the system, which essentially means using the source that produces the greatest intensity on each collected ray. It is common practice to use arc lamps in applications that demand high intensity within a limited NA or object size. It is well known that arc lamps are the brightest light sources available, with the important exception of laser sources. From the point of view of geometrical optics, a laser can be considered to be a froze point source, i.e. a source having infinitesimal extent, so that optical systems using laser sources are always power-limited and never brightness limited. Practical issues with laser sources are often cost and size, particularly as power levels rise into the 1-Watt regime and above. Compact arc lamps in the 1000-Watt range can cost several hundred dollars and might occupy ~200 cubic inches in the illuminator (plus remote power supply).
Depending on the lamp, the portion of the consumed power radiated as visible light might be 200 Watts. The cost of a laser in the 200-Watt range might be tens or hundreds of times that of the lamp, and the laser might occupy tens or hundreds of times the volume. Though the situation may change in the future, for many applications laser sources are often severely underpowered when practical constraints are enforced on cost and size. On the other hand, while practical non-laser sources can provide very high power, they do so from an extended emitting region, which means that in many applications the power they actually deliver does not reach ideal levels before a brightness-limited regime is reached.
What is needed is a way to increase the brightness of the emitting region itself. However, commercial high brightness light sources are usually engineered to generate as much energy within the emission volume as is technologically possible. For example, when an arc lamp is steadily powered above its rated level, its lifetime decreases catastrophically (i.e. drooping from hundreds or thousands of hours to a few hours). Steady output at increased power requires that the lamp must have a larger arc gap; this means that the source is increased in size but not in brightness.
Brightness can often be increased for brief intervals, but the application must permit the increased emission to accomplish its purpose before damage mechanisms in the source are initiated by the accelerated operation. The source must then be switched off for a sufficient interval to hold time-averaged power below the maximum rated level. It is known in the art that total visible light emission can be improved by pulsing a metal halide lamp, even though total power consumption is held
Dove Derek Brian
Rosenbluth Alan Edward
Yang Kei-Hsiung
International Business Machines - Corporation
Morris, Esq. Daniel P.
Neils Peggy A
O'Shea Sandra
Scully Scott Murphy & Presser
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