1.  Field of the Invention
The present invention relates to an ink jet ink that provides an image excellent in light fastness and the like, an ink set, an ink jet recording method, an ink cartridge, a recording unit, and an ink jet recording apparatus. 
2.  Related Background Art
An ink jet recording method is a recording method involving applying a small ink droplet to any one of recording media such as plain paper and glossy media to form an image, and has become rapidly widespread owing to a reduction in its cost and an improvement in its recording speed.  With the rapid spread of a digital camera in addition to an improvement in image quality of a recorded article by this method, an ink jet printer has been required to output a recorded article comparable to silver halide photograph. 
For example, a recorded article obtained by means of an ink jet recording method must have high fastness properties so as to be comparable to silver halide photograph.  A conventional ink jet recorded article has low fastness properties as compared to silver halide photograph.  Therefore, the following problems have been posed.  When the recorded article is exposed to, for example, light, humidity, heat, and an environmental gas present in the air for a long time period, a coloring material present in the recorded article is apt to deteriorate to thereby cause the color tone change and color fading of an image.  That is, the fastness properties are low.  A number of studies have been hitherto made to solve such problems as described above. 
For example, there has been proposed the use of a coloring material having an anthrapyridone structure to improve fastness properties (see, for example, Japanese Patent Application Laid-open Nos.  2002-332419 and 2003-192930).  The use of the coloring material can improve the storage stability of ink jet ink.  However, at present, the storage stability of a recorded article obtained by means of conventional ink jet ink typified by such ink has not been comparable to that of silver halide photograph. 
In recent years, a recording medium using an inorganic substance such as alumina or silica for an ink-receiving material has been used as an ink jet recording medium in order to obtain an image comparable or superior to silver halide photograph.  However, the light fastness of a recorded article obtained by performing recording on such recording medium as described above cannot be said to be sufficient.  Such recorded article undergoes color fading in a shorter time period than that in the case of silver halide photograph when the recorded article is stored in an environment in which the recorded article is irradiated with light.