The daylight factor model given by Charted Institute of Building Services Engineers (CIBSE) was modified in this paper to incorporate time variations with respect to zenith angle (theta(2)) and vertical height (h) of working surface above ground surface which was normalized with central height (H) of skylight dome. The modified model contains constant exponents which are determined using linear regression analysis based on hourly experimental data of inside and outside illuminance for each month of the year 2007-2008. The prediction of modified model is found in good agreement with experimental observed inside illuminance data on the basis of values of root mean square percentage error (e) and correlation coefficient (r). The annual average daylight factor values for big and small dome skylight rooms are determined as 2.3% and 4.4% respectively. The energy saving potential of skylight rooms for selected climatic locations in India is also presented in this paper. (C) 2010 Elsevier Ltd. All rights reserved.