In order to measure the temperature of the object in the case of non-diffuse emission within a finite solid angle, a new concept for radiation thermometry, the apparent emissivity, has been presented firstly after a proper mathematical transformation of the radiation measurement equation and its characteristics have also been investigated. The results indicated that although the apparent emissivity is complex in form, it is only the function of the wavelength for one measurement even if the object is non-diffuse. So the temperature of the object in the case of non-diffuse emission within a finite solid angle can be solved by modeling the apparent emissivity. Then the equations for monochromatic and wavelength radiation measurement were deduced. At the same time, the measurement of temperature for a finite area was investigated, and it can be found that if the temperatures of the finite area are the same, they can also be determined by modeling the apparent emissivity.