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. <EOS>