Radiant energy – Photocells; circuits and apparatus – With circuit for evaluating a web – strand – strip – or sheet
Reexamination Certificate
1999-05-04
2001-02-13
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
With circuit for evaluating a web, strand, strip, or sheet
C250S216000, C250S234000
Reexamination Certificate
active
06188078
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to calibration of optical instruments. In particular, the present invention relates to calibration of a reflecting optical device, such as a pointing mirror, using an optical calibration apparatus. The calibration apparatus includes a light source for projecting a beam of light at the reflecting optical device, a plurality of facet mirrors pointed at predetermined angles, and a detector for indicating a properly registered beam. The reflecting optical device steers the beam at the facet mirrors during calibration.
BACKGROUND OF THE INVENTION
Many monitoring, measuring and input devices utilize scan mirrors to optically scan a field of view. For example, weather satellites, such as the GOES satellite, incorporate scan mirrors for scanning weather patterns over earth. In the GOES satellite, the scan mirror reflects to a detector light received from a portion of the atmosphere at which the mirror is directed. The precise pointing direction of the scan mirror is important, as the detected radiation represents atmospheric data, such as cloud and precipitation data, collected from a precise portion of the earth's atmosphere. This pointing direction is then used to correlate the atmospheric data collected with the underlying geography of the earth for depiction of weather conditions on weather maps. Other instruments which may have scan mirrors include semiconductor wafer scanners and photocopying machines. In each of these applications, it is necessary to accurately track and control the position of the scan mirror.
In the case of satellite and semiconductor wafer scanning mirrors, the scan mirror may operate in a vacuum and under extremes of temperature. In the case of a scan mirror located in a photocopying machine, the scan mirror may be subject to large temperature fluctuations when the photocopier goes from a resting state to a state of continuous copying.
The characteristics of the scan mirror may change over temperature and pressure extremes. Therefore, it is desirable to have a device to measure the accuracy of and calibrate a scan mirror over a wide range of temperatures and pressures. It would further be desirable to have a device for measuring scan mirror angles with high accuracy, which itself requires setup and manual manipulation only one time.
Prior art devices, such as theodolites, exist for measuring angles. However, theodolites are limited in their angle measuring accuracy.
SUMMARY OF THE INVENTION
According to the present invention, a method and apparatus measure the accuracy of and optionally calibrate a scan mirror. Both the method and apparatus may operate over a wide range of environmental conditions. The environmental conditions may include variations in pressure from a vacuum to several atmospheres. Similarly, large variations in temperature may be accommodated.
According to the method, in a projecting step, a laser beam is projected at a scan mirror. Then, in a commanding step, the scan mirror is commanded to reflect the laser beam successively at a first and a second facet mirror, where the first and second facet mirrors have known angles of reflection. In a reflecting step, the laser beam is reflected substantially back onto itself by each facet mirror. Then, in a determining step, an angle between the first and the second facet mirrors is determined at the scan mirror. Then an error is calculated in a calculating step, based on the determined angle and the known angles.
Prior to the projecting step, an offset of the laser beam may be measured and used in the error calculation. Also, the commanding step may include commanding the scan mirror to move until the reflected laser beam is centered. This occurs when a null or near null value is detected by a detector receiving the reflected laser beam.
The apparatus includes a laser source, a plurality of facet mirrors and a detector. The laser source projects a beam onto a reflective surface of a rotatable scan mirror, which directs the beam to each of the plurality of facet mirrors. Each facet mirror is positioned at a known angle. Each facet mirror in turn reflects the beam from the reflective surface of the scan mirror substantially back onto itself (autocollimation). The angle detector then receives the reflected beam and measures a value related to a return angle of the beam.
The apparatus may further include a processor, coupled to the angle detector and the scan mirror. The processor may command the scan mirror to point the beam at a predetermined facet and may command the scan mirror to move until the angle detector reads a null or near null value. The optical instrument under test typically includes an encoder coupled to the scan mirror for measuring position values corresponding to positions of the scan mirror. The position values may be used to determine angles for comparison with the known angles of the facet mirrors.
To achieve environmental stability, the laser source, facet mirrors and the detector are typically mounted to a housing made of a material with a low coefficient of thermal expansion, such as Invar. Moreover, the facet mirrors in a preferred embodiment are mounted to the housing using a three ball kinematic mount. The three ball mounting technique permits precise angular positioning of each facet mirror, even under relatively lax manufacturing tolerances of each facet mirror and the housing itself.
REFERENCES:
patent: 5671359 (1997-09-01), Godlewski et al.
patent: 5778016 (1998-07-01), Sucha et al.
patent: 5821526 (1998-10-01), Krishna
patent: 5841574 (1998-11-01), Willey
patent: 5864132 (1999-01-01), Holcombe
Bell, Jr. Raymond Mack
Perkins Patrick E.
Stubbs David Mark
Le Que T.
Lockheed Martin Missiles & Space Company
Swidler Berlin Shereff & Friedman, LLP
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