Urbach’s formula for evaluation of electron trapping parameter that happens to be the earliest has emerged as a popular method amongst researchers of persistent luminescence. In this study, we analysed the thermoluminescence curves of glow-in-the-dark phosphors. Those included some curves recorded by us and others that solely used the Urbach’s formula. Using the parameter obtained by rigorous analysis by fitting the whole TL curves, we presented a comprehensive picture of the potentialities and limitations of the simple Urbach’s relation E=T_m/500, where E is the trap-depth and T_m the peak temperature in absolute scale, first proposed by Urbach in 1930. Some simple thumb rules on the use of the formula were presented for beginners as well as non-specialists so that entry of physically unrealistic data into the literature was prevented. Urbach’s formula for evaluation of electron trapping parameter that happens to be the earliest has emerged as a popular method Among researchers of persistent luminescence. In this study, we have analyzed the thermoluminescence curves of glow-in-the-dark phosphors. by us and others that solely used the Urbach’s formula. Using the parameter obtained by rigorous analysis by fitting the whole TL curves, we presented a comprehensive picture of the potentialities and limitations of the simple Urbach’s relation E = T_m / 500, where E is the trap-depth and T_m the peak temperature in absolute scale, first proposed by Urbach in 1930. Some simple thumb rules on the use of the formula were presented for beginners as well as non-specialists so that entry of physically unrealistic data into the literature was prevented.