Relativistic beaming can be measured in terms of the Doppler factor, 
, which relates the intensity in the observer's frame to the intensity in the rest frame, of radiation originating from material travelling at a significant fraction of the speed of light.
for a spherical component, where 
and 
have their usual definitions in relativity and refer to the motion of the radiating material (e.g. Blandford and Konigl 1979). 
is the angle the material motion makes to the line of sight and 
( 
) is the spectral index of the emission.
Another relativistic effect, apparent superluminal motion, can be observed when a source of radiation is moving at a substantial fraction of the speed of light and in a direction close to the observer's line of sight,
where 
is the apparent speed relative to the speed of light. Usually from the observation of the apparent speeds of components in VLBI jets some limits on the intrinsic speed and angle to the line of sight can be derived. For instance the minimum intrinsic speed of a component with can be found by differentiating the above equation and is 
. The maximum angle to the line of sight that the motion of the component can make is found by assuming that 
, the maximum possible, and is 
.
These limits on and can be used to loosely constrain the Doppler factor.
However, the measurement of radio core brightness temperatures with VLBI observations can provide a direct estimate of the Doppler factor. The observed radio core brightness temperature can be written as [Murphy 1993]
where 
is the observed flux density (in Jy) from a radio core at the frequency 
(in GHz) and a and b are the semi-major and semi-minor axes of the core component (in mas). 
is the brightness temperature in the observer's frame, in units of 10 
K. If the radio core appears bright due to beamed emission then 
, as above, and 
. A factor of (1+z) also needs to be included to account for the effects of cosmological red shift [Lang 1986], thus
Since the rest frame brightness temperature is limited to the nominal value of 10 K [Kellermann & Pauliny-Toth 1969] it follows that if 
then 
. The Doppler factor can be directly estimated from the three observables T, z, and .