Thank you for the question. Basically in Transmission mode normally you fix 100%T at 0Abs. To fix the dark correction we can measure 0%T (infinity in Abs). To measure 0%T, block the sample beam completely with a not transparent material, make sure there is no scatter that can reach the detector and execute the dark correction. If you are working with low absorbing materials it is not necessary, but as you approach the limits of linearity (3 to 4Abs for the V-770), if you are working in transmission mode and only performed baseline correction, you may see the transmission for the sample becoming a negative value. The dark correction prevents this from happening.
