1. Field of the Invention
The present invention relates to solid-state imagers. More particularly, the present invention relates to solid state imagers mounted onto color separation prisms, methods for attaching such solid-state imagers to color separation prisms, and cameras employing same.
2. The Prior Art
In the art of electronic imaging, it is necessary to resolve the incoming image into a minimum of three primary colors. The principles are well understood, and are adequately described in the book xe2x80x9cThe Reproduction of Colourxe2x80x9d by R. W. G. Hunt, published by Fountain Press, 1995. Historically, three principal methods have been employed, originally with photo-sensitive vacuum tube sensors (vidicons, image orthicons, etc.), and more recently with silicon CCD imaging chips.
In the first method, the sensor is sequentially exposed to the image, first through a first filter, then through a second filter, then through a third filter. A separate image is captured through each filter, thus forming the basis of a color image. The advantages of the sequential method are that only one image sensor is required, and the three images are automatically aligned with each other. Disadvantages of the sequential method are the cost and complexity of the mechanism used to sequentially place the color filters in the image path, that at least three times the exposure time is required, compared with that of a single sensor, that color artifacts are produced by moving objects in action images, and that synchronization with a flash lamp is problematic.
In the second method, each picture element (pixel) of the sensor is provided with a separate color filter. The filters are arranged in a mosaic pattern, and a separate image is captured from the subset of pixels covered by each color of filter, thus forming the basis of a color image. The advantages of the mosaic method are that only one image sensor is required, and the three images are automatically aligned with each other, and that all three images are captured at the same time. Disadvantages of the mosaic method are that three to four times the exposure time is required, compared with that of a single sensor, that color artifacts are produced due to details in the image aliasing against the periodic mosaic filter array, that the sensor is two to three times the area of a single-color sensor of the same resolution, and that a complex process is required to apply the filter mosaic.
The last two disadvantages significantly decrease the manufacturing yield of the sensors, thus increasing their cost.
In the third method, a color-separation prism is used to split the incoming image into three separate images, one in each wavelength range. Three separate sensors are used, each capturing the image from one wavelength range, thus forming the basis of a color image. The advantages of the prism method are that all the incoming light in each wavelength band is available to the corresponding sensor, thus shortening exposure time by typically a factor of four compared with either of the other methods, that the full resolution of each individual sensor contributes to the final resolution, thus allowing the use of smaller sensors for a given final resolution and thereby improving the yield of the sensors and greatly decreasing aliasing artifacts, that a simple process can be used to fabricate the sensor, since no special filters must be added to it, and that all three images are captured at the same time, making flash synchronization easy. Disadvantages of the prism method are the expense of the prism, and the cost of the critical alignment of the sensors with each other. Thus the prism method appears to be the preferred method of color separation for electronic imaging systems, if its cost could be reduced to a level comparable with that of the other methods.
Many methods have been developed for attaching solid-state imagers to color-separation prisms and other optical components. Numerous such methods have been described in the patent literature.
U.S. Pat. No. 4,268,119 to Hartmann teaches the use of foamed gaskets to space the detectors from the prism faces.
U.S. Pat. No. 4,323,918 to Bendell teaches the use of a hollow spacer between the prism face and the imaging device.
U.S. Pat. No. 4,418,284 to Ogawa et al. discloses a method for bonding a color filter to a solid state image sensor using an ultraviolet light curable adhesive. The bonding step is carried out in an oxygen atmosphere to prevent curing of adhesive squeezed out of the space between the bonded members.
U.S. Pat. No. 4,507,679 to Bendell discloses a television camera that employs a mounting technique also involving the use of spacers between the prism face and the imaging device.
U.S. Pat. No. 4,622,580 to Levine teaches bonding a thinned-substrate solid-state imager chip to a prism face having an interconnect metallization pattern formed thereon. Bonding wires for making electrical connections to the imager chip are disposed between bonding pads on the back side of the imager chip and corresponding bonding pads on the prism face.
U.S. Pat. No. 4,591,901 to Andrevski teaches the use of a metal bracket to hold imaging devices behind the output faces of a prism.
U.S. Pat. No. 4,623,916 to Levine teaches a metal bracket which positions individual imaging devices at the output ports of a prism, in which the positions of the imaging devices are secured by means of metal solder.
U.S. Pat. No. 4,789,891 to Kanayama and Tanaka teaches the adhesive attachment of solid image pickup elements to the faces of a prism by means of tapered spacers.
U.S. Pat. No. 4,916,529 to Yamamoto and Ohmuro teaches the mounting of imagers to the output faces of a prism by means of a solder layer, which also serves to space the imager away from the prism face.
U.S. Pat. No. 5,042,913 to Yamamoto also teaches the spacing of imagers from the output faces of a prism by means of metal parts, adhered to the glass prism faces by means of solder.
U.S. Pat. No. 5,340,420 to Ozimek et al. discloses a method for bonding a color separation filter to an image sensor. Wires for making electrical connections to the image sensor are wire bonded to bonding pads on the image sensor and on a required carrier for the image sensor.
U.S. Pat. No. 5,570,231 to Mogamiya teaches the positioning of imagers opposite the output faces of a prism and spaced therefrom by means of a precision mechanical clamping arrangement.
U.S. Pat. No. 4,507,679 to Bendell shows CCD image planes spaced a substantial distance behind the output face of a prism.
Japanese patent application 58-63279 shows a metal spacer similar to that of Yamamoto.
Japanese patent application 61-135279 shows metal spacers separating imaging elements from the output faces of a prism.
In all the prior art except for U.S. Pat. No. 4,622,580, the imaging array has been treated as a separate component, usually packaged in an electronic enclosure with a glass window. Three of these packaged imaging array devices are mounted opposite the output faces of a color-separation prism by one of the means described in the prior art.
There are several problems inherent in the prior-art approaches to the mounting of imagers. First, the output faces of the prism and the front and back surfaces of the glass windows generate reflections that degrade image quality. Second, the windowed packages are expensive. Third, the attachment procedures mentioned are complex and costly. The method disclosed in U.S. Pat. No. 4,622,580 requires that electrical connections to the imager be made from the face of the imager opposite that containing the imaging array.
The present invention teaches a method for mounting solid-state array image sensor integrated circuits directly to the output faces of a color-separating prism. The method of the present invention includes the steps of: arranging three solid-state array image sensor integrated circuits behind and in close proximity to the output faces of a color-separating prism having substantially equal optical path lengths for the three paths, the three solid-state array image sensor integrated circuits each having a solid-state array image sensor and bonding pads for electrical connections disposed on a same face thereof; aligning the three sensors such that the images traversing the three paths are coincident within a pixel dimension of the image sensors; filling the space between each output face of the prism and the front surface of the corresponding solid-state array image sensor integrated circuit with index-matched adhesive; and causing the index-matched adhesive to become rigid while maintaining the alignment of the three solid-state array image sensor integrated circuits.
According to one embodiment of the invention, each output face of the color-separation prism has a step incorporated therein to provide space for the electrical connections to be made from the solid-state array image-sensor integrated circuits to the circuit boards upon which they are mounted.
The method of the present invention is simpler than the prior-art methods, eliminates undesired reflections from glass surfaces between the prism and the surface of the imaging arrays, and eliminates the cost of a separate package for each imaging array. The method involves using an index-matched adhesive to adhere the imaging array directly to the prism face, thereby eliminating any air gap between the prism face and the surface of the imaging array. An essential aspect of the invention is to fabricate the prism in such a way that the effective optical path length from the input face to the three output faces is well matched. The index of refraction of optical glass changes with wavelength. If the color-separating prism is fabricated in such a way that the physical distances traveled by light rays from the input face to the three output faces is equal, the three color images will not be simultaneously in focus on the three output faces. The reason is that the effective optical path length is the physical path length divided by the index of refraction at the wavelength corresponding to the color light traveling the particular path. Prisms fabricated in this manner are known in the art.
In addition, the method of the present invention avoids the use of fragile thin substrates and eliminates the need for employing a wiring pattern on the prism faces. The assembly of the imagers onto the prism is therefore made simpler.