Measurement of Needed Diopter for Near Low Vision Aids with Telescope
Welfare Centre for the Blind, Nippon Lighthouse
Graduate School of Medical Science, Kitasato University

Tadaaki Tanabe
Hiroshi Uozato

How do we select a near low vision aid?
Estimate the magnification or the diopter for near low vision aids.
Conventional methods
Sloan M system
Keeler A system
The N point system
Kestenbaum method
Logarithmic scale by Bailey(standard distance: 40cm)(standard distance: 25cm)(standard distance: 25cm)(reciprocal of Snellen distance chart)(any reading distance)[Chiristine M. Dickinson, 1991]
Other methods in terms of reading speed
MNREAD(standard distance: 30cm)
MNREAD-J (modified for Japanese people)

Are there any gadgets to assess the diopter for near low vision aids?
Adjustable focus telescope for near vision allows people with low vision toread the target letters. If a person with low vision can read the target lettersthrough the telescope, the equivalent viewing power (EVP) of the telescopeshould indicate the proper diopter for a near low vision aid.
Methods
1．Calibration
To measure the diopter or the EVP for a near low vision aidwith the telescope, calibrations were put on the tube of thetelescope and the optical bench.

Figure 1. Principle of the telescopic method
to determine the diopter
Dr. Reich LN(1991) found a linear relationshipbetween the front vertex focal distance and theEquivalent Viewing Distance (EVD).

2．How can we measure the diopter with the telescope?
1) The corrected eye is placed near the eye piece.
2) Set the target letters at the nearest working space calibrated on the optical bench.
3) Focus the telescope to the same value on the tube as the calibration of the optical bench.
4) Move the target letters to the next calibration until the critical size to be read is determined.
5) Lastly, the indicated EVP on the tube of the telescope and the optical bench is the appropriate
diopter for a near low vision aid.

Figure 2. Calibration of the EVP and
the working space on the tube of thetelescope
Figure 3. Target letters are set on
the optical bench

Figure 4. Corresponding calibrations are put on the tube of the telescope and the optical bench

3．Investigation
Six normal sighted subjects simulated low vision with the right eye. The diopter measured with the telescope wascompared with the diopter which was calculated using the MNREAD-J reading chart.
Results
The mean diopter needed for near low vision aids measured with the telescope was 7.0±0.8D. The mean critical size of
MNREAD-J was logMAR0.68±0.04, which could be converted to 6.4±0.5D for the mean diopter needed for near low vision
aids. There was no significant difference between the telescope and MNREAD-J to estimate the diopter needed for near lowvision aids.
Bland-Altman plot is as follows. The mean difference between the values of the telescope and MNREAD-J is -0.6D. The 95%
limits of agreement for the difference is ±2.3, meaning that on average there is 95% confidence that the difference is within
1.7 and -2.8.
Table 1. Needed diopter measured with the telescope and MNREAD-J

Subjects

TelescopeAge, sex

DiopterneededA（27、F）
B(30、F)
C(33、F)
D(29、M)E(23、F)
F(23、M)

MNREADJ

Criticalletter size
(logMAR)
6.0
0.7
8.0
0.7
7.0
0.7
8.0
0.6
7.0
0.7
6.0
0.7

Diopterneeded6.7
6.7
6.7
5.3
6.7
6.7

(D)

(D)

Figure 5. Bland-Altman plot

Discussion
・Measurement with the telescope was as useful as MNREAD-J.
・The diopter needed for near low vision aids was directly estimated with the calibration using the adjustable focus telescope fornear vision without requiring any calculations.
Vision 2014 in Melbourne, Australia. Melbourne Convention and Exhibition Centre. 31 March ～3 April, 2014.