Fig. 8. Corneal abnormalities and tonometry. Tonometry gives the most accurate estimate of IOP when the cornea is normal. Goldmann's instrument was designed assuming a corneal thickness of 520 μm. (1) If the cornea is thinner than this, the readings are artificially low in proportion to the degree of thinning. (2) When the cornea is thicker than normal, the readings are low if the thickening is due to epithelial edema. (3a) If the cornea is thickened because of an increase in normal stroma, or (3b) if the stroma is scarred—with or without thickening—(3c), the readings are artificially high.