An illumination device, comprising: a lamp housing having a tube shape and defining a geometric central axis: a first light emitting element comprising: a first transparent substrate having a first surface, and a first side surface perpendicular to said first surface and arranged parallel to said central axis; a first plurality of light-emitting diode chips disposed on said first surface, a first wavelength conversion layer directly contacting said light-emitting diode chips; and a second light emitting element comprising: a second transparent substrate having a second surface, a second side surface perpendicular to said second surface; a second plurality of light-emitting diode chips disposed on said second surface; and a second wavelength conversion layer directly contacting said light-emitting diode chips; wherein said first transparent substrate is transparent to light from said first light-emitting diode chip, wherein said second transparent substrate is transparent to light from said second light-emitting diode chip, and wherein said first and second light emitting elements are disposed on a supporting structure, wherein said first side surface and said second side surface are arranged to point toward said geometric central. a lamp housing having a tube shape and defining a geometric central axis: a first light emitting element comprising: a first transparent substrate having a first surface, and a first side surface perpendicular to said first surface and arranged parallel to said central axis; a first plurality of light-emitting diode chips disposed on said first surface, a first wavelength conversion layer directly contacting said light-emitting diode chips; and a second light emitting element comprising: a second transparent substrate having a second surface, a second side surface perpendicular to said second surface; a second plurality of light-emitting diode chips disposed on said second surface; and a second wavelength conversion layer directly contacting said light-emitting diode chips; a first light emitting element comprising: a first transparent substrate having a first surface, and a first side surface perpendicular to said first surface and arranged parallel to said central axis; a first plurality of light-emitting diode chips disposed on said first surface, a first wavelength conversion layer directly contacting said light-emitting diode chips; and a first transparent substrate having a first surface, and a first side surface perpendicular to said first surface and arranged parallel to said central axis; a first plurality of light-emitting diode chips disposed on said first surface, a first wavelength conversion layer directly contacting said light-emitting diode chips; and a second light emitting element comprising: a second transparent substrate having a second surface, a second side surface perpendicular to said second surface; a second plurality of light-emitting diode chips disposed on said second surface; and a second wavelength conversion layer directly contacting said light-emitting diode chips; a second transparent substrate having a second surface, a second side surface perpendicular to said second surface; a second plurality of light-emitting diode chips disposed on said second surface; and a second wavelength conversion layer directly contacting said light-emitting diode chips; wherein said first transparent substrate is transparent to light from said first light-emitting diode chip, wherein said second transparent substrate is transparent to light from said second light-emitting diode chip, and wherein said first and second light emitting elements are disposed on a supporting structure, wherein said first side surface and said second side surface are arranged to point toward said geometric central.