A high brightness LED spot lighting device has been under examination. The device has one multichip LED module which nominal electric power is 15 W. A 3D simulation model has been created and simulated with Comsol Multiphysics software. The temperatures of the LED chip junction, the LED module and the heatsink and have been simulated using a finite-element-method (FEM) software. Simulations have been validated with measurements. Main heat flow paths and the associated thermal resistances in a stationary condition have been resolved. Simulations have been made for the case of external natural and forced external heat convection. Time dependent simulations resolved the time constants of the lamp. The time constants were calculated also by using the thermal resistances and heat capacities of the lamp. Use of thermal grease between the LED module and the heatsink reduces chip temperature. This has also been simulated. Photometric characteristics of the light device, especially luminous flux versus input electrical power and lamp temperature, have been measured. One LED spot light device with a defected LED module was found in photometric measurements and IR-imaging.