Snellen and LogMAR acuity testing
The Snellen Chart (fig 1), which traditionally has been used to measure visual acuity is easily recognised as one of the hallmarks of the ophthalmologists consulting room. This has been in use sine 1862 and more recently a newer LogMAR (fig 2) chart has been introduced into clinical practice. Initially used as a research tool because it is more accurate than other acuity charts, this accuracy is also valuable in the clinical setting and many eye departments now use it routinely. This is particularly the case for children; testing children’s acuity accurately is always a challenge and the best charts available now are those utilising LogMAR. In addition, studies performed on age related macular degeneration (ARMD) utilised LogMAR acuities and therefore, as this research has been translated into practice, most ARMD clinics record vision using this notation.
There is no direct correlation between LogMAR and the better recognised Snellen and this can cause difficulty during translaition or when some clinics use Snellen and others use LogMAR. Also it is not intuitive as poorer vision is recorded as a higher number eg 6/60 Snellen is 1.00 LogMAR and 6/6 Snellen is 0.00 LogMAR (see fig 3).
Tables are available to assist and this short article and use of these tables should be helpful, both in primary and secondary care, to ease the understanding of visual acuity measurements.
Figure 1 – Snellen Chart Figure 2 – LogMAR Chart
Figure 3 – conversion table