TWO SENSOR QUANTITATIVE LOW-LIGHT
REFERENCE TO RELATED APPLICATION
 This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/317,923 filed Sep. 10, 2001.
 This invention relates to digital imaging, specifically quantitative imaging for computer analysis of digital images.
 The prior art has evolved several methods of acquiring color images with solid-state cameras. For example, in the so-called "mosaic color" method, one of a red, green, or blue primary color filter is applied directly to each one of the pixels of a solid-state image sensor, giving each pixel a red, green, or blue spectral absorption characteristic. This method is attractive in many cases because of its relatively low cost and high image acquisition speed characteristics. However, the mosaic color method's light sensitivity and spatial resolution characteristics are reduced by the filters. The filters' fixed wavelength characteristics also restrict the ability to image specific color bands.
 The "3-chip color" prior art method splits an input light beam into three sub-beams; passes each sub-beam through a distinct color filter (i.e. red, green, or blue); and couples the output of each filter to one of three monochrome image sensors. The 3-chip color method offers high image acquisition speed and high spatial resolution, but at a relatively high cost, since three image sensors (typically the single most expensive component in a solid-state camera) are required. The 3-chip color method also restricts the ability to image specific color bands, since the filters again have fixed wavelength characteristics.
 Another prior art technique is to place a filter wheel or electrically tunable color filter in the light path of a monochrome image sensor. This method offers high spatial resolution, relatively low cost, and flexible selection of color bandwidths. However, image acquisition speed is significantly reduced, since a separate image must be acquired for each filter wheel position and a minimum of three images (i.e. red, green, and blue) must be acquired to produce a full color image. This method has the added disadvantage of reduced sensitivity if an electrically tunable color filter is used, since such filters attenuate a significant amount of the input light.
 A fourth prior art solid-state camera color image acquisition method uses two image sensors: one monochrome image sensor and one mosaic color image sensor. This method has been used in tube type cameras as disclosed in U.S. Pat. No. 3,934,266 Shinozaki et al. U.S. Pat. No. 4,166,280 Poole discloses a similar method using a lower resolution color solid-state sensor in combination with a higher resolution monochrome tube sensor to generate the luminance signal. U.S. Pat. Nos. 4,281,339 Morishita et al; 4,746,972 Takanashi et al; 4,823,186 Muramatsu; 4,876,591 Muramatsu; 5,379,069 Tani; and, 5,852,502 Beckett further exemplify use of a monochrome solid-state sensor in com
bination with at least one lower resolution color sensor. In general, these prior art techniques maximize the spatial resolution of the luminance or monochrome signal relative to the chrominance or color signal. However, in order to achieve higher spatial resolution with the same optical interface, one must reduce sensitivity to light and photometric resolution or signal-to-noise ratio. Such reduction may be acceptable in qualitative imaging devices such as mass consumer market cameras which rely on the human eye to assess image quality, but is unacceptable in quantitative imaging devices used for computerized digital image analysis. The human eye has relatively good spatial resolution, but relatively poor photometric resolution; whereas in quantitative imaging (so-called "machine vision") applications, light sensitivity and photometric resolution are of primary importance, particularly under low-light conditions.
SUMMARY OF INVENTION
 In accordance with the invention, a quantitative color image is produced by providing first and second light sub-beams representative of an imaged object, such that the first sub-beam's light intensity exceeding the second subbeam's light intensity. Preferably, the ratio of the first sub-beam's light intensity to that of the second sub-beam is between about 70:30 and 80:20. The first sub-beam is processed at a relatively high sensitivity to produce a first plurality of monochrome image pixels representative of the imaged object. The second sub-beam is processed at lower sensitivity to produce a second plurality of color image pixels representative of the imaged object.
 The first sub-beam is preferably processed at maximal signal-to-noise ratio so that the monochrome image pixels are maximally representative of the imaged object. Advantageously, the first sub-beam can be processed selectably and independently of the processing of the second sub-beam.
BRIEF DESCRIPTION OF DRAWINGS
 FIG. 1 is a block diagram of the optical front end and associated electronics of a solid-state camera quantitative color image acquisition system in accordance with the invention.
 FIGS. 2a and 2b schematically depict coupling of a monochrome image sensor pixel to a group of color image sensor pixels in a primary (FIG. 2a) and in a complementary (FIG. 2b) quantitative color image acquisition system in accordance with the invention.
 Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense.
 FIG. 1 schematically illustrates a solid-state camera quantitative color image acquisition system in accordance with the invention. Light passing through lens 10 is initially processed through infrared (IR) cutoff filter 11 to