The invention provides an illumination apparatus (10) and a method of assembling the illumination apparatus. The illumination apparatus comprises a light source (101) having a plurality of LED arrays, wherein at least two of the plurality of LED arrays have different lumen degradations as a function of junction temperature of the respective LED arrays; and a heat dissipation unit (102) configured to be capable of dissipating heat generated by the light source, wherein the heat dissipation unit is mounted on a first surface of the light source in such a way that there is a gap between the first surface and the heat dissipation unit when the light source is not in operation, and the gap is narrowed or can be deemed to disappear when the light source reaches a preset temperature, so that the heat dissipation efficiency of the heat dissipation unit is improved.