Focimeter Research Articles

Error model analysis and correlation correction of Hartmann's focimeter

In order to reduce the error and improve the measurement accuracy, a more detailed error model is established for the Hartmann method of focal power measurement in this paper. It focuses on the analysis of several problems that cause the error of refraction problems, including the dispersion error of the light source, the inaccurate of the photodetector’s central positing, the tilt of lens, misalignment between incidence axis and main axis of lens, and the incident light and the lens are not perpendicular. At last, it is concluded that the inaccuracy of the center extraction on the photodetector will cause a large error to the final result. For all these reasons, a method of dual bilinear interpolation combined with a fitting method to find the centroid is proposed, proving its effectiveness and accuracy.

ECONOMIC EFFICIENCY OF USING AN OPTICAL-ELECTRONIC INSTRUMENT FOR TESTING A SET OF TRIAL EYE GLASSES

The article discusses the economic efficiency of using an optoelectronic device for verifying a set of test eyeglass lenses in a metrological laboratory, and calculates the payback from introducing the device for MedtechStandard LLC. A characteristic is given to an optical-electronic device, namely, a lensmeter standard multi-functional LEM-1. The calculation of the number of jobs planned for verification of sets of trial eyeglass lenses is carried out. Also, for the company MedtechStandard LLC, the positive effects from the use of an optical-electronic device of the standard multifunctional lens meter LEM-1 were considered, which include reducing the cost of subcontracting, reducing the time it takes to test a set of trial spectacle lenses, additional profit for the enterprise, reducing the risk of loss contract and attracting new customers. The estimated annual program of attorney spectacle lenses is calculated.

중첩된 시험렌즈의 합성굴절력에 대한 신뢰도 평가

목적: 구면렌즈와 원주렌즈를 시험테에 중첩하였을 때 합성굴절력의 신뢰도를 알아보고자 하였다. 방법: 마이너스 구면 시험렌즈와 원주 시험렌즈의 굴절력, 중심두께 및 주변부두께를 측정하였으며 구면 시험렌즈와 원주 시험렌즈를 장입순서를 바꿔가며 시험테에 중첩되었을 때의 굴절력을 자동렌즈미터로 측정하여 합성굴절력 계산값과 비교 평가하였다. 결과: 시험렌즈의 중심부와 주변부의 두께증감이 도수의 증감과 상관성을 보이지 않았으며 79개 시험렌즈 중 3개의 시험렌즈에서 국제기준규격(ISO-9801)에서 벗어난 굴절력이 측정되었다. 정점간거리를 보정하는 굴스트랜드 공식 굴절력값이 얇은 렌즈 공식 굴절력값보다 실측값과의 오차가 적었으나, 여전히 실측값과는 통계적으로 유의한 차이가 있었다. 굴절력의 크기는 원주렌즈와 구면렌즈의 위치와 상관없이 평균적으로 얇은 렌즈 공식 굴절력값 > 실측 굴절력값 > 굴스트랜드 공식 굴절력값 순으로 나타났다. 구면렌즈가 안쪽, 원주렌즈가 바깥쪽에 장입되었을 경우에는 굴스트랜드 공식에 대입하였을 때 원주굴절력만 오차가 발생하지만 원주렌즈가 안쪽, 구면렌즈가 바깥쪽에 장입되었을 때는 구면굴절력과 원주굴절력 모두에서 오차가 나타났다. 실측값의 등가구면굴절력을 비교하였을 때 구면렌즈가 시험테의 안쪽, 원주렌즈가 시험테의 바깥쪽에 장입된 경우가 정확도가 더 높았으며 과교정의 우려가 작았다. 결론: 본 연구 결과 시험렌즈테에 렌즈를 중첩하였을 때의 합성굴절력은 정점간거리 외에도 중첩되는 렌즈의 두께와 광학중심 등의 영향을 받으며 구면렌즈와 원주렌즈의 중첩 순서에 따라 굴절력에 차이가 있으므로 이에 대한 기준의 정립이 필요함을 제안한다. Purpose: The current study aimed to evaluate the reliability for the combined refractive power when a spherical lens and a cylindrical lens were overlapped in a trial frame. Methods: The refractive powers, central thickness and peripheral thickness of spherical trial lenses and cylindrical lenses with negative power were measured. The combined refractive power of the spherical and cylindrical lenses was measured by auto lens meter. Measurement was repeated by changing the insertion order, and their results were further compared with the calculated combined refractive power. Results: There was no correlation between the variation of central and peripheral thickness in trial lenses and that of the lens power. Among 79 trial lenses, 3 trial lenses wasn't met the international standard. The refractive power calculated by Gullstrand's formula that could compensate vertex distance had smaller difference with the estimated power when compared with that calculated by thin lens formula however, it was significantly different from the estimated power. The refractive powers were generally apparent regardless of the insertion order of a spherical lens and a cylindrical lens: thin lens formula > actual measurements > Gullstrand's formula. The error was only found in cylindrical power calculated by Gullstrand's formula when inserted a spherical lens inside and a cylindrical lens outside however, the error was found in both of cylindrical and spherical powers calculated by Gullstrand's formula when inserted as a opposite order. By comparing actual measurements of equivalent spherical power, the accuracy was higher and the possibility of over-correction was lower when inserted a spherical lens inside and a cylindrical lens outside. Conclusions: From the results, those were revealed that the combined refractive power is influenced by the factors other than the vertex distance and the refractive power varies in accordance with the insertion order of a spherical lens and a cylindrical lens. Thus, it can be suggested that the establishment of standard for these is neccesaty.

How to measure slab–off and reverse slab prism in spectacle lenses

It is well known that new spectacle lenses for the correction of anisometropia can induce diplopia with reading. The difference in the powers of the lenses induces a net prismatic effect that can cause double vision through off-center areas of the lenses. This is particularly bothersome when patients try to read, often noting vertical double vision in attempted downgaze, especially through multifocal add segments. This induced prismatic effect can be compensated at one level of downgaze by the use of slab-off or reverse slab prism. Typically the slab-off correction is ground into the stronger minus, or weaker plus lens. Reverse slab is ground into the weaker minus, or stronger plus, lens. Unfortunately, determining the amount of slab-off prism already incorporated into spectacle lenses is nonintuitive and inconvenient. This usually requires the use of a lens clock, which is not widely accessible to many ophthalmology practices. A simple technique, described in the past but poorly known, is illustrated here for quickly measuring slab-off and reverse slab prism prescription lenses in the clinic with a common manual lens meter.

Refractive correction measurement in an ophthalmic lens

An ophthalmic lens is designed so that its refractive power is constant when the observer looks through the center of the lens, as well as when the observation is made through the edge of the lens. This state of correction of an ophthalmic lens can be confirmed by direct measurements of the dioptric refractive power by means of a typical lens meter, if some special precautions are taken. Here we describe a method to perform these measurements.

The eye in cystic fibrosis.

To investigate modifications of ocular surface and lens transparency in patients with cystic fibrosis in relation to the stage of digestive insufficiency. Forty consecutive patients with cystic fibrosis and 24 age- and sex-matched healthy volunteers were examined. The tear tests (Schirmer's basic test, tear film break-up time) and conjunctival exfoliative cytology (CC) were used to study the ocular surface. The lens transparency was measured with the Opacity Lens Meter 701 (OLM 701, Interzeag AG, Switzerland). Digestive insufficiency was evaluated by the steatocrit method. Significant changes in conjunctival cytology and lens opacity, and abnormal tear tests were detected in CF patients; the alterations were more pronounced in patients with severe digestive insufficiency. Cystic fibrosis patients present ocular surface abnormalities and lens transparency modifications and their severity is related to the digestive insufficiency. Simple, rapid and non-invasive tear tests and cytological procedures might be used as additional tests for assessing the severity of cystic fibrosis.

Cystic Fibrosis and Lens Opacity

Cystic fibrosis (CF) is a fairly common lethal genetic disease in whites, inherited via an autosomal recessive pattern. Several clinically evident anomalies of the eye such as xerophthalmia, papilledema, retinal alterations and optic neuropathy have been described. In the present study we investigated lens changes in 40 patients affected by CF, to relate the results to the stage of digestive insufficency. Each patient underwent an accurate ocular examination including visual acuity, slit-lamp examination, and measurement of lens transparency with the Opacity Lens Meter 701. The results were compared with those of an age- and sex-matched control group. The comparison between the lens opacity values obtained in the CF patients and those measured in the control group showed a statistically significant difference; moreover, lens transparency was more decreased in patients with severe digestive insufficiency.

Relationship cataract density and visual field damage.

In interpreting visual field results, two questions arise: a) what type of visual field damage is produced by cataract, and b) can the influence on the visual field somehow be predicted? To answer these questions, cataract density was quantified with the Opacity Lens Meter (OLM) 701, and visual field tests were done before and after IOL-implantation surgery with Octopus Program G1 in 58 eyes of 58 patients (mean age 71 +/- 8 years) with cataract but with no other detectable ocular diseases. The average improvement of mean damage (MD) after surgery was 5.4 dB, and that of D (20) (defect 20 on the Bebie Curve) was 5.7 dB. The improvement of the visual fields was, as expected, statistically highly significant (p < 0.0001). The corrected loss variance (CLV), however, increased on the average only by 2.5 dB2, which was not significant. The predictive value of the OLM reading for opacity-induced MD depends on the type of cataract. It is good for cortical and nuclear cataracts but poor for posterior subcapsular opacifications. The overall predictive value (R = 0.66) is, nevertheless, better than for preoperative visual acuity (R = -0.54). If OLM and visual acuity (VA) are considered together, the predictive value is slightly higher (R = 0.72). Thus, optical density influences on visual field performance can be subtracted from general visual field results.

Exfoliation syndrome as a risk factor for optic disc changes in nonglaucomatous eyes.

Exfoliation syndrome as a possible risk factor for morphologic changes of the optic nerve was examined in 66 patients with unilateral exfoliation and no glaucoma. K-readings (7.74 +/- 0.3 mm and 7.75 +/- 0.3 mm), axial lengths (23.1 +/- 1.1 mm and 23.1 +/- 1.1 mm), and refraction (+ 0.9 +/- 2.3 mm and + 1.1 +/- 2.3 mm) did not differ in exfoliative and contralateral nonexfoliative eyes. The mean intraocular pressure (IOP) difference, 17.2 +/- 3.3 mmHg and 15.6 +/- 3.2 mmHg, respectively, was statistically highly significant (P < 0.001). The mean visual acuity difference, 0.8 +/- 0.3 and 0.9 +/- 0.2, respectively, was significant (P < 0.05). The difference in visual acuity between the pairs of eyes was explained by the more frequent subcapsular cataract in exfoliative eyes. Lens opacity values (opacity lens meter), 27.9 +/- 8.3 and 28.0 +/- 8.4 opacity units, respectively, were similar. Disc area, neuroretinal rim area, rim/disc ratio, cup area, and cup volume values analyzed with the Imagenet (Topcon) nerve head analyzer did not differ significantly between the eyes. It was concluded that exfoliation as such does not induce optic nerve head changes but indicates a risk factor for elevated IOP and lens opacification.

Lens opacity: a population study.

Age-corrected normal values for the Opacity Lens Meter (OLM) 701 were established by measuring 1598 eyes of 799 healthy volunteers. This study revealed a remarkable age-relation for the normal values. The average readings of the OLM of the 30-year-old subjects were around 10 units whereas the average for the 65-year-olds was close to 20 units. The best fit of the opacity vs. age was a quadratic function. The interindividual variation was small in the young and quite large in the older population. The correlation between contralateral eyes was high (R = 0.93).

Opacity Lens Meter 701: Klinische Erfahrungen

The quantitative influence of cataracts on visual fields was studied in 11 patients before and after cataract extraction with intraocular lens implantation. Lens opacity, measured with the Opacity Lens Meter 701, correlated closely with the visual field changes. Measurement of stray light backscattered from the cataractous lens permits quantitative prediction of the influence of cataract on the visual field.

Equipment Review The Model 70 Vertometer by Bausch and Lomb

Equipment Review The Model 70 Vertometer by Bausch and Lomb

Investigation of an automatic lensometer

Investigation of an automatic lensometer