Binocular vision provides for fine stereopsis at short distances of a few meters and gradually less so at longer distances. Beyond about 20 m binocular vision is relatively unimportant with regard to judgment of distances.
Normal eye muscle balance provides for adequate binocular vision and avoids the risk of asthenopia (eye fatigue) or diplopia. With normal eye muscle balance there may be a small heterophoria (misalignment between the eyes) that can be observed when one eye is covered and the need for exact alignment is thus removed. When uncovering the eye the eye muscle control system re-established fused binocular vision easily and quickly.
With poorer eye muscle balance (likely with large heterophoria) there may be eye strain while fusing images with a risk of breakdown of binocular fusion. This is more likely to occur when the visual system is fatigued after a long day's flying, and when there is diminished visual stimulus such as by night or flying in white or grey-out conditions.
Measurement of the phoria provides an indication as to how much strain is placed on the eyes during binocular vision. If the phoria measurements fall within the following parameters it is unlikely that asthenopia of diplopia will occur.
| At distance | Exophoria | < 12 ∆ |
| Esophoria | < 6 ∆ | |
| Hyperphoria | < 2 ∆ | |
| At near | Exophoria | < 12 ∆ |
| Esophoria | < 6 ∆ | |
| Hyperphoria | < 2 ∆ |
Some degree of heterophoria is common. However if phoria determination results fall outside these limits (but not to the extent of break-down of the binocular vision) a determination of fusional reserves (vergence reserves) should be obtained.
The result of this test will show the prism strength (measured in 'prism dioptres', symbol ∆) that can be applied to a prism placed in front of the eye before a breakdown of binocular fusion occurs. Three values are generally provided in the form: blur / break / recovery:
The prism can be placed with its base laterally; this is called Base Out (BO), or medially, this is called Base In (BI). Values for both positions are recorded.
The prism can also be placed with its base vertically in front of the eye; this is called Base Up Right eye (BUR), or Base Down Right eye (BDR) providing two readings. Values for both base up and base down in front of one eye are recorded.
Example – applicant with an heterophoria at distance of 7 ∆ eso, at near 9 ∆ eso [Tendency of one eye turning in, acceptable limit is < 6 ∆ in both cases]
Fusional reserves determination – blur, break, recovery:
| At distance | BI -/13/8 | BO 24/40/24 |
| At near | BI 18/22/16 | BO 24/40/33 |
The interpretation of the significance of these results is beyond the competencies expected from Medical Examiners. It is best left to the CAA Medical Officers who sometime need to seek advice from expert optometrists.
Two criteria are useful to evaluate whether compensating vergence reserves are likely to be sufficient to prevent asthenopic symptoms and possible diplopia. Here is how these results may be interpreted in that case, using the following criteria:
The compensating vergence reserves should be 2x the magnitude of the heterophoria.
In this case where the esophoria is recorded as 7 prism dioptre at distance (the eye tends to turn in, the compensating vergence reserves are measured with a Base In (to determine how well the eye can turn out to regain fused binocular images). The findings are recorded as: - / 13 BI / 8 BI (blur, double vision, recovery). In this example the eyes go straight to a break without blur.
The compensating vergence reserve of 13 prism dioptres before break down of fusion occurs is not quite twice the magnitude of the heterophoria, but it is close. It should be 14 to satisfy this criterion.
The required eye position (the demand line) for fused single binocular vision should lie in the middle third of the range that allows single binocular vision.
In the example above, the region of single binocular vision at distance is from 13 BI to 40 BO. This is a range of 53 ∆ dioptres. According to this criterion the zero prism value must lie within the middle third of this range. The middle third range is 53/3 = ~ 18 ∆ dioptres from each edge of the binocular vision determination. Thus this range is from 5 ∆ BO to 22 ∆ BO dioptres. The phoria at distance is 7 Eso and thus lies just inside that range.
By one criterion the applicant is likely to experience problems with asthenopia or double vision. By the other criterion, the applicant is borderline. Research has shown that Percival's criterion is more applicable to assessment of esophorias, so we would put more weight on the Percival's criterion result in this example of esophoria.
