Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Patent
1988-06-20
1989-12-19
Westin, Edward P.
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
369 45, G02B 711, G11B 709
Patent
active
048884766
DESCRIPTION:
BRIEF SUMMARY
The invention concerns a scanning system that scans a surface with rays,
wherein rays emitted by a source of radiation are reflected by the surface being scanned and focused by one or more focusing elements onto a radiation detector comprised of at least two individual detectors, from the output signals of which a summation signal and at least one difference signal are constructed.
Optical scanning systems of this type, which are often called "optical pick-ups", are employed for example with compact-disk players to pick up the information stored on the disk. The usual source of radiation is a laser diode, the focusing elements are lenses, and the radiation detector is a photodetector.
The structure and function of an optical scanning system for a compact-disk player are described on pp. 209 to 215 of Electronic Components & Applications, Vol. 6, No. 4 (1984). In addition to the example described in that source, there are also other scanning systems, all of which, however, are similar in design and operate in accordance with similar principles.
FIG. 2a on page 213 of the aforesaid reference represents the path of the rays in an optical scanning system. The rays emitted by a laser diode are focused on the mirror surface of a compact disk through a prismatic ray divider, a collimator lens, and an objective lens. The rays are then reflected back to the prismatic ray divider through the objective lens and the collimator lens. The prismatic ray divider, in the capacity of a deflection unit, reflects the rays reflected from the compact disk through a cylindrical lens onto the photodetector at a right angle. The photodetector consists of four equally large square elements A, B, C, and D arrayed in a square. When the ray of light is precisely focused on the compact disk, the ray reflected from the compact disk creates a circle on the photodetector, in which case the difference signal (A+C)-(B+D) constructed from the four output signals from elements A, B, C, and D is zero and the summation signal assumes a maximum because all the light reflected from the compact disk strikes the photodetector. When the focus is slightly off, difference signal (A+C)-(B+D) assumes a value other than zero because the spot of light on the photodetector deviates from the circular and becomes an ellipse due to the astigmatic action of the cylindrical lens. From difference signal (A+C)-(B+D), which is often called a "focusing-error" signal, the focus-control loop detects whether the focal point is above or below the mirror surface of the compact disk.
The focus-control loop shifts the objective lens toward the optical axis to ensure that the focal point will always be at the mirror surface of the disk. Prerequisite of course to this fine-control process, which occurs in the micrometer range, is an equally fine adjustment of the photodetector with respect to its distance from both the prismatic ray divider and the optical axis.
The procedure for centering the photodetector on the optical axis will now be described. A semitransparent mirror is employed instead of the compact disk. A videocamera with a microscope is positioned behind the mirror. A focal point is precisely established on the semitransparent mirror. The focus is checked with the microscope and videocamera. The photodetector cannot be adjusted within the plane perpendicular to the optical axis until the focus on the surface of the mirror is precise. It will be positioned precisely at the center when difference signals (A+C)-(B+D) and (A+D)-(B+C) are zero and the summation signal is at a maximum, because all the light reflected from the semitransparent mirror will be striking it.
Whoever is adjusting the photodetector must then shift it along the plane perpendicular to the optical axis until the difference signal becomes zero. The photodetector will then be correctly positioned and can be screwed into place. This method of adjustment demands a lot of time and sensitive fingers. It is possible to displace the objective lens unintentionally during the process, in which case the f
REFERENCES:
patent: 4423495 (1983-12-01), Musha et al.
patent: 4742219 (1988-05-01), Ando
patent: 4823330 (1989-04-01), Arter et al.
Deutsche Thomson-Brandt GmbH
Fogiel Max
Westin Edward P.
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