An apparatus obtaining a pH level of a tissue comprises an array of at least three optical sensors configured to absorb light shined on the tissue and thereby obtain at least three electronic digital signals, wherein the first electronic signal A1(X,t) represents the light absorbed by the tissue in location X=(X,Y,Z) at time t and at wavelength L1, and similarly the second and third electronic signal A2(X,t) and A3(X,t), and a finite absorption range of the three wavelength L1, L2 and L3 is such that 0