Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
1999-04-22
2001-09-25
Allen, Stephone B. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S216000, C346S106000, C359S206100
Reexamination Certificate
active
06294778
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method and apparatus for recording or reproducing a flat field image, and in particular, to an apparatus that provides a high-quality and high-resolution image substantially insensitive to variation in illumination wavelength over a broad wavelength range.
Some image printers use lasers, such as helium-neon lasers or semiconductor laser diodes to generate an image modulated light beam for recording or reproducing an image or text either by exposing a photosensitive material (e.g., a film, plate, or paper) or by photosensitive charging of an electrostatic recording device. Currently available semiconductor laser diodes are small, cost efficient, operate in the milliwatt range and can be readily modulated at rates up to 5 GHz by modulating its driving current, which is desirable for high resolution printing. In contrast, helium-neon lasers are relatively large, costly, and typical require expensive optical components to provide high-resolution printing. Thus, semiconductor laser diodes are preferred as the source of a modulated light beam for recording in high-resolution printing.
High-quality printing is achieved when optical aberrations yielding variations in focus spot size across the printed image are minimized. For example, chromatic aberrations cause different wavelengths of the light beam to focus at different points. Although semiconductor laser diodes produce predominantly monochromatic light beams, they can suffer from chromatic aberrations because of power output fluctuations and multiple longitudinal mode operation that change their output wavelengths. In such cases, the chromatic aberration can produce unwanted image variations at the photosensitive material.
SUMMARY OF THE INVENTION
In general, in one aspect, the invention features a flat field image printing apparatus. The printing apparatus includes: a light source responsive to an image modulated signal and producing an image modulated light beam; a beam deflector supported for rotary motion to deflect the image modulated light beam onto an image recording medium in response to the rotary motion; and a lens assembly configured to convert the deflected image modulated light beam into an achromatic focused beam spot which moves repeatedly in a first scan direction and over a range of scan angles to provide a highly resolved printed image across the recording medium.
In general, in another aspect, the invention features a high-speed flat field image printing apparatus. The high-speed printing apparatus includes: a plurality of light sources responsive to an image modulated signal and producing a plurality of image modulated light beams having a plurality of wavelengths; a beam deflector supported for rotary motion to deflect the plurality of image modulated light beams onto an image recording medium in response to the rotary motion; and a lens assembly configured to convert the plurality of deflected image modulated light beams into a plurality of achromatic focused beam spots which move repeatedly in a first scan direction and over a range of scan angles to provide a highly resolved printed image across the recording medium.
Embodiments of either the printing apparatus or the high-speed printing apparatus can include any of the following features. The range of scan angles can be −32° to +32°. The recording medium and the achromatic focused spot(s) can be configured to be translated relative one another in a second direction perpendicular to the first scan direction in order to print a two-dimensional image. The recording medium can include photosensitive film, plate, or paper. Alternatively, the recording medium can include an electrostatic recording device.
In addition, the lens assembly in the embodiments of either the printing apparatus or the high-speed printing apparatus can include any of the following features. The lens assembly can be configured to produce an achromatic focused beam spot in the visible and near infrared spectral wavelength range, e.g., the wavelength range of 670 nm to 780 nm. The lens assembly can include: a first lens having a negative power; a second lens having a positive power; and a third lens having a positive power, wherein the first, second and third lenses are arranged in the order from the object to the image side and satisfy the following formulae (1) to (6)
−3.6<ø
1
<−3.0 (1)
1.6<ø
2
<2.0 (2)
1.5<ø
3
<1.8 (3)
&ngr;
1
<34 (4)
44<&ngr;
2
<61 (5)
37<&ngr;
3
<51 (6)
where ø
n
is the power of the n
th
lens with respect to the total power of the lens assembly and &ngr;
n
represents the Abbe dispersion number of the nth lens. The lens assembly can include: a first lens having a negative power and a refractive index in the range of 1.64 to 1.85; a second lens having a positive power and a refractive index in the range of 1.60 to 1.75; and a third lens having a positive power and a refractive index in the range of 1.65 to 1.75, wherein the first, second and third lenses are arranged in the order from the object to the image side.
In general, in yet another embodiment, the invention features an f-theta lens assembly configured to convert an image modulated light beam into an achromatic focused beam spot which moves repeatedly in a first scan direction and over a range of scan angles to provide a highly resolved image. The lens assembly includes: a first lens having a negative power; a second lens having a positive power; and a third lens having a positive power, wherein the first, second and third lenses are arranged in the order from object to image side and satisfy the following formulae (1) to (6)
−3.6<ø
1
<−3.0 (1)
1.6<ø
2
<2.0 (2)
1.5<ø
3
<1.8 (3)
&ngr;
1
<34 (4)
44<&ngr;
2
<61 (5)
37<&ngr;
3
<51 (6)
where ø
n
is the power of the n
th
lens with respect to the total power of the lens assembly and &ngr;
n
represents the Abbe dispersion number of the n
th
lens.
Embodiments of the lens assembly can have any of the following features. The lens assembly can be configured to produce an achromatic focused beam spot in the visible and near infrared spectral wavelength range, e.g., the wavelength range of 670 nm to 780 nm. The range of scan angles can be −32° to +32°. The first lens can have a refractive index in the range of 1.64 to 1.85, the second lens can have a refractive index in the range of 1.60 to 1.75, and the third lens can have a refractive index in the range of 1.65 to 1.75.
The total optical power ø of a multi-component system is the reciprocal of its effective focal length at the primary design wavelength. The optical power ø
n
of the n
th
element of the multi component system equals the reciprocal of the focal length of the n
th
element at the primary design wavelength, divided by the total optical power ø of the multi-component system.
The Abbe dispersion number &ngr;
d
for a material is given by &ngr;
d
=(n
d
−1)/(n
f
−n
c
), where n
d
, n
F
, are n
c
are the indices of refraction for the material at 587.5618 nm, 486.1327 nm, and 656.2725 nm, respectively. The Abbe dispersion number &ngr;
n
for the nth element of a multi component system is the Abbe dispersion number &ngr;
d
for the material of the n
th
element.
Among the advantages of the invention are the following. Because of the achromatic lens assembly, a new laser diode can replace an old laser diode in a printing apparatus without any optical realignment, even though the wavelength produced by the new laser diode may differ from that of the old diode by as much as 20 nm. Typically, laser diodes have a lifetime of about 50,000 hours and need to be replaced every 5 years. Absent the achromatic property of the lens assembly, the printing apparatus would need to be refocused and realigned by a trained operator every time a laser diode is replaced. The achromatic lens assembly, however, compen
Allen Stephone B.
ECRM, Inc.
Hale and Dorr LLP
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