Table 2.

CH, CRF, and custom ORA variable descriptions

Group	Variable	Operational Definition	Related to:
1: Applanation Signal Intensity	A1	Peak intensity of 1st applanation event	Maximum surface area achieving planarity during inward deformation
	A2	Peak intensity of 2nd applanation event	Maximum surface area achieving planarity during recovery
	ApplanationPeakDiff	A2 – A1	Difference in maximum planarity between inward and recovery phases
	ConcavityMin	Minimum applanation intensity between A1 and A2	Depth and irregularity (non-planarity) of deformation
	ConcavityMean	Mean applanation intensity between A1 and A2	Depth and irregularity of deformation, averaged
2: Pressure	Corneal Resistance Factor (CRF), mmHg	P1 - 0.7P2	Difference in applanation pressures, weighted toward pressure required to produce the first applanation, maximizes correlation to central corneal thickness
	Corneal Hysteresis (CH), mmHg	P2 - P1	Difference in pressures between the two applanation events (a single cross-section of the pressure-deformation relationship)
	P1P2Avg	(P1+P2)/2	Average of the pressures at the two applanation events
	Pmax	Peak value of pressure signal	Force and time required to reach first applanation event
3: Response Time (msec)	ConcavityDuration	Time lapse between A1 and A2	Temporal delay of deformation recovery between applanation events
	ConcavityTime	Time from onset of applied pressure to ConcavityMin	Time required to achieve maximum deformation from onset of impulse
	LagTime	Time between Pmax and ConcavityMin	Delay between peak applied pressure and maximal deformation
	ApplanationOnsetTime (AOT)	Time from onset of applied pressure to A1	Time required to achieve first applanation from onset of impulse
4: Applanation Intensity and Response Time (msec−1)	SlopeUp	Positive slope of the first applanation peak, from inflection point to peak	Rate of achieving peak planarity
	SlopeDown	Negative slope of the first applanation peak, from peak to inflection point	Rate of loss of peak planarity
5: Pressure and Applanation Intensity 6: Pressure and Time	Hysteresis Loop Area (HLA)	Area enclosed by pressure vs. applanation function	Hysteresis aggregated over entire deformation cycle except concavity
	Impulse	Area under pressure vs. time curve	Air pressure intensity

Adapted from Hallahan et al, Ophthalmol 2014.