The 3 basic value types for visual field representations

Perimetry knows a wide range of "image types": grayscale, defect map, indices, cumulativ curve... They are always based on 1 of 3 potential basic values. The basic value is decisive for the meaning of a visual field representation.

Inside the PeriData program, the basic value is given in the lower right corner . Representations on the basis of defect depth / total deviation are always in color, while representations with other basic values are shown black on white

Threshold values

Threshold values are the untreated measured values of the examination (raw data). The importance is more traditional: In the 1970 decade, threshold values were the only representation option for the young computer perimetry

Threshold values depend on many factors:

Position of the measured point (normal value is lower in the periphery than in the center, nasal lower than temporal)
Age of patient
Instrument type
Stimulus size
Background luminance and other parameters
Strategy (measurement algorithm)
Damage
individual variation
Measurement error

CONCLUSION: Visual field representations on the basis of threshold values are subject to various influencing factors and are not suitable for the safe evaluation of visual field damage.

Defect depth / total deviation

Defect depth values are the deviation of the measured values from the age-corrected normal value for the instrument type at the specific measurement point. In case of double measurement, the average measured value is taken. The formula is:

measured value – normal value

Therefore, defect depth values are already corrected for measurement position, age, instrument, parameter and strategy. The values only depend on:

Damage
individual variation (usually less than 1 db shift)
Measurement error

CONCLUSION: Visual field representations on the basis of defect depth (total deviation) are well suitable for the safe evaluation of visual field damage and the visual field change over time.

Pattern deviation

Pattern deviation is derived from total deviation (defect depth) by shifting all values with a shift value. The formula is:

total deviation (defect depth) - shift value

The shift value is calculated in a way to get the better values in the field being around zero damage pattern is better visible by the shifting, because the diffuse damage is removed from total damage.

NOTE: Pattern deviation values are mostly lower than total deviation values. It is possible to underestimate the defect depth and the damage
NOTE in a follow-up analysis: the shift value is different for each examination. Pattern deviation values may differ even when the actual defect depth is equal - and vice versa
NOTE: Different instrument types use different calculation methods

CONCLUSION: Visual field representations on the basis of PATTERN deviation values show only a part of the damage. These representations are only suitable for the recognition of the damage pattern