Frequently Asked Questions

Measurement Parameters

What is Hysteresis?

What is the difference between viscous and elastic resistance?

How is Corneal Hysteresis measured?

What is Corneal Hysteresis

What is IOPg?

Is IOPg different from the IOP value presented by a traditional Non-Contact Tonometer (NCT)?

Is there a massage effect caused by repeated measurements with the Ocular Response Analyzer?

What is IOPcc?

How is IOPcc measured?

What is CRF?

CRF is a measurement of the cumulative effects of both the viscous and elastic resistance encountered by the air jet while deforming the
corneal surface. CRF exhibits the expected property of increasing at significantly elevated pressures. Though CH and CRF are, on average, the same for a normal population, they differ from person to person, providing us with distinct corneal information.

Corneal Hysteresis is simply the raw difference between the inward and outward applanation pressure values.  CRF uses these same two data points but employs a specific algorithm to arrive at the CRF.  This equation was developed by maximizing the correlation of P1-(k*P2) with CCT.

What is the range of Corneal Hysteresis in normal Subjects?

What is the range of Corneal Resistance Factor (CRF) in normal subjects

What is the average value of Corneal Hysteresis in normal subjects?

What is the average value of Corneal Resistance factor (CRF) in normal subjects?

Are CH and CRF Related to the “Modulus of Elasticity” of the cornea?

What is the acceptable repeat measurement variability for the corneal hysteresis measurement?

What is the acceptable repeat measurement variability for the corneal resistance factor (CRF) measurement?

Why is IOPg higher/lower than my Goldmann IOP value?

How do Corneal Hysteresis (CH) and Corneal Reistance Factor (CRF) differ?

Is Corneal Hysteresis (CH) related to Central Corneal Thickness (CCT)?

Interpreting results

What is the average value of Corneal Hysteresis in normal subjects?

What is the acceptable repeat measurement variability for the corneal hysteresis measurement?

What is the acceptable repeat measurement variability for the corneal resistance factor (CRF) measurement?

Why is IOPg higher/lower than my Goldmann IOP value?

What are the characteristics of a "good" measurement signal for a normal subject?

How can I tell a "bad signal" from a "good signal"?

Using the instrument

What is the acceptable repeat measurement variability for the corneal hysteresis measurement?

What is the acceptable repeat measurement variability for the corneal resistance factor (CRF) measurement?

Why is IOPg higher/lower than my Goldmann IOP value?

Why are the average values in the software different from the true mathematical average?

Where are the results saved when I use the pachymeter?

Should I use anesthesia before taking an Ocular Response Analyzer measurement?