What is the difference of filter photometer and uv-vis spectrophotometer in terms of absorbance reading factor?
There is none. Absorbance is the same relative value at a given wavelength for either type of instrument.
A filter photometer is a colorimeter in which the length of light is selected by the use of appropriate glass filters. A UV-Vis Spectrophotometer may use filters or a variable scanning sensor. In all cases spectrometry readings of absorbance units go from 0 to 1 based on a relative absorbance by a standard filter at a given wavelength.
Thus, the “absorbance reading” is the same, regardless of the specific instrument mechanism. See also the Beer-Lambert Law used for the calculation: http://chemguide.co.uk/analysis/uvvisible/beerlambert.html