Silver halide has been widely used as a light sensitive component in photographic compositions and elements. Because silver halide is intrinsically sensitive only to blue light, it has often been desirable to impart to silver halide sensitivity to other wavelengths of radiation. This has generally been accomplished through the use of one or more spectral sensitizing dyes, such as cyanine dyes. The dye is adsorbed to the surface of the silver halide. The dye absorbs light or radiation of a certain wavelength. The energy thus absorbed by the dye is transferred to the silver halide to form a latent image exposure from which a visible image can be developed during photographic processing.
Cyanine dyes have been used to sensitize silver halide to various portions of the spectrum, such as red, green, and blue, as well as invisible radiation, such as infrared, depending on the radiation source to which a photographic element is intended to be exposed. In recent years, diode lasers that emit infrared radiation have become increasingly popular as exposure sources for a number of applications, such as for making prints from computer assisted tomography scanners, various graphic arts products that are exposed by diode lasers, and infrared sensitive false color-sensitized photographic materials as described in Simpson et al. U.S. Pat. No. 4,619,892.
While traditional infrared recording films designed for infrared radiation given off or reflected by various objects (e.g., aerial photography film for detection of vegetation growth) require fairly broad sensitivity in the region of about 700 nm to 900 nm, infrared laser diodes emit radiation at one specific wavelength, which is often longer than 800 nm and may be as deep as 850 nm or even deeper.
With the increasing popularity of infrared laser diodes emitting at specific wavelengths in the deep infrared (e.g., up to about 900 nm) as exposure sources for photographic elements, it is desirable to provide silver halide emulsions offering high peak sensitivity to infrared radiation at wavelengths where laser diodes emit.