A flow cytometer having an extended light source configured to provide light that forms an object, and a wavelength division multiplexer WDM, the WDM comprising: a collimating optical element configured to capture light from the extended light source and project a magnified image of the object as a first light beam; and a first focusing optical element configured to focus the first light beam to a size smaller than the object of the extended light source to a first semiconductor detector, wherein the WDM is adapted for separating a beam of light emitted from the extended light source into multiple colored bands, wherein the collimating optical element is configured to magnify an image of the object to produce an image of substantially the same size as the effective size of said collimating optical element; wherein the WDM further comprises at least one dichroic filter located between said collimating optical element and said image, wherein said dichroic filter is configured to separate the collimated beam of light into two branches of distinctive colors; wherein the focusing optical element is located in one of said branches, wherein the focusing optical element is configured to focus the beam of light in said branch to a spot having a diameter of less than 1.0 mm; and wherein the WDM further comprises an image relay optical element located near the image produced by said collimating optical element in the other branch, wherein said image relay optical element is configured to produce an image of said collimating optical element at substantially unit magnification, and wherein another focusing optical element is located in said other branch and is configured to focus the beam of light in said other branch into a spot having a diameter of less than 1.0 mm. an extended light source configured to provide light that forms an object, and a wavelength division multiplexer WDM, the WDM comprising: a collimating optical element configured to capture light from the extended light source and project a magnified image of the object as a first light beam; and a first focusing optical element configured to focus the first light beam to a size smaller than the object of the extended light source to a first semiconductor detector, wherein the WDM is adapted for separating a beam of light emitted from the extended light source into multiple colored bands, wherein the collimating optical element is configured to magnify an image of the object to produce an image of substantially the same size as the effective size of said collimating optical element; wherein the WDM further comprises at least one dichroic filter located between said collimating optical element and said image, wherein said dichroic filter is configured to separate the collimated beam of light into two branches of distinctive colors; wherein the focusing optical element is located in one of said branches, wherein the focusing optical element is configured to focus the beam of light in said branch to a spot having a diameter of less than 1.0 mm; and wherein the WDM further comprises an image relay optical element located near the image produced by said collimating optical element in the other branch, wherein said image relay optical element is configured to produce an image of said collimating optical element at substantially unit magnification, and wherein another focusing optical element is located in said other branch and is configured to focus the beam of light in said other branch into a spot having a diameter of less than 1.0 mm.