This invention relates to a printing system, and more particularly, t o a line printing system which is capable of simultaneously transferring all pixel information of one raster line or one text line through use of a vertical cavity surface emitting laser (VCSEL) array as a light bar print head without using an imaging optical element. 
A light bar is an array of individual light emitting devices such as light emitting diode (LED) or electroluminescent (EL) edge emitters.  For simplicity hereinafter, "light emitting devices" are called "light sources".  Typically, a light bar array is utilized to produce an image on a photosensitive medium such as a xerographic photoreceptor used in a xerographic printer.  In this kind of application, there is a need for a full width array of light sources, one per picture element or pixel, so that an array of light beams can be formed in such a manner that where they strike a photoreceptor, they generate a single line.  Usually, this generated line on a photoreceptor of a scanning printing system is called a scan line.  However, in this application since the line is not scanned and each individual light source is responsible to generate one pixel of the line on the photoreceptor, hereinafter, "the generated line on the photoreceptor" will be called "line of pixels". 
Each light source is individually addressed.  Therefore, by applying a certain voltage selectively to the light sources, the light sources emit light beams to selectively discharge the photoreceptor in order to generate line-by-line a latent image on the moving photoreceptor. 
Conventional light bar printing systems require imaging optical elements to be positioned between the photosensitive medium and the light source array.  Since the output beams of the light sources diverge very fast, there is a need to focus the light from the array sources onto the line of pixels on the surface of the photoreceptor by the imaging optical elements. 
A conventional imaging optical element is a Selfoc lens array.  A Selfoc lens array is an array of micro-lenses which will be placed between the light bar and the photoreceptor.  Each micro-lens receives multiple light beams from multiple light sources and focuses each light beam from each light source onto one spot on the photoreceptor. 
Referring to FIG.  1, there is shown a tangential or the fast scan view of an optical printing system 10 which utilizes a Selfoc lens and referring to FIG.  2, there is shown a sagittal or cross-scan view of the optical printing system 10.  Referring to both FIGS.  1 and 2, a light bar 12 emits a plurality of light beams 14.  A Selfoc lens 16, focuses each individual light beam onto an individual spot on the photoreceptor 18. 
Typically, a Selfoc lens exhibits chromatic aberration problems which surface when used with a broad band emitter such as a EL edge emitter.  In addition, a Selfoc lens is a significant contributor to output non-uniformity, short depth of focus, pixel placement errors and generally poor image quality. 
Non-uniformity is caused by the fact that each micro-lens of a Selfoc lens array is an individual optical element and due to the manufacturing tolerances, each lens transmits the light beam in a different manner.  Therefore, the light beam exiting each lens can have a different intensity causing an intensity non-uniformity over a line of pixels or it can be slightly deflected from the intended path causing a pixel placement error. 
Also, due to the limitations and tolerances of the micro-lenses, the depth of focus of a Selfoc lens is very small.  Depth of focus is the tolerance in which either the light source, the Selfoc lens or photoreceptor can have a positional error with respect to the other two without losing the focus.  In other words, depth of focus is the tolerance of the spot size (i.e. spot size .+-.10%) to the positional errors of the optical elements.  It is desirable to improve the depth of focus in order to maintain the focus on the photoreceptor while having positional errors between the optical elements. 
In addition, some light sources emit light beams which have an elliptical cross section.  This type of light beam is not suitable for printing systems using light bars since the spot created buy each light beam on the photoreceptor will be elliptical instead of circular and therefore, the pixel created by the elliptical spot will have an elliptical shape. 
Considering the aforementioned problems, it is an object of this invention to eliminate the imaging optical element (typically a Selfoc lens) and provide generally circular pixels on a photoreceptor.