# Why the light contrast of a set $\mathcal{E}$ of VCRG produced by an encryption scheme for a binary image B is defined as follows

In the paper “Image encryption by multiple random grids”, Shyong Jian Shyu, 42(7):1582-1596,· July 2009, the light contrast of a set $\mathcal{E}$ of VCRG produced by an encryption scheme for a binary image B is defined as
$$c(\mathcal{E})=\frac{\mathcal{T}(S[B(0)])-\mathcal{T}(S[B(1)])}{1+\mathcal{T}(S[B(1)])}$$

My question is : denominator taken as $1+\mathcal{T}(S[B(1)])$ why?