To further evaluate the effect of CCT on IOP measurement, the method and instruments used to measure IOP were compared. In a clinical setting IOP's are measured by four instruments, Goldmann applanation tonometer, ocular blood flow pneumatonometer, pneumatonometer or Tono-Pen. Singh et al (2001) concluded that the effect of central corneal thickness on Goldmann applanation tonometry accuracy appears not to be clinically relevant. However, Bhan et al (2002) showed that in eyes with normal corneas, the ocular blood flow pneumatonometer appears to be more affected by variation in CCT than the Goldmann tonometer, and the Tono-Pen is least affected by CCT. A limitation of this study is that corneal curvature (a measure of astigmatism) was not performed prior to measuring IOP. High amounts of astigmatism can impact IOP readings obtained from the Goldmann tonometer.