Fig. 1.

This series of neutral density filters was scanned with each tissue section and used in Scion ImagePC to calibrate optical density (OD) measurements of CO activity. The percent transmittance (T) was derived from the relationship OD = log 1/T. Using the flatbed scanner to digitize images from CO sections allowed easy, direct measurement of optical density. The calibration bar guaranteed that optical density measurements remained accurate from one CO section to the next. All CO images processed in this manner display a calibration bar.