The Ability to Perceive a Minor Change in Luminosity - Lab Report Example

The ability to perceive a minor change in luminosity Introduction The ability to perceive a minor change in luminosity between regions that are not separated by definite borders is termed as contrast sensitivity (CS) (Khurana 2008). Like visual acuity, in certain pathological conditions CS can also be impaired, leading to sever visual complications, both in mobility and orientation. CS testing is a powerful technique for determining the visual system's ability to process spatial and temporal information about the daily seen objects. CS value can be determined through a variety of CS tests (CSTs) (Jindra and Zemon 1989). The objective of this lab is a comparative study of four CS tests, namely Pelli-Robson CS test, Cardiff CS test, Distich CS test and Cambridge plates test. Method To perform the CS test (CST), the basic requirements for all four tests were: 1) to have an adequate luminance level required for each test; 2) to have precise test distance; and 3) to perform each test approximately at the same eye sight level as that of test charts. The CST luminance requirement for Pelli-Robson charts was 60–120 cd/m2, while for Vistech6000 and Cambridge plates the adequate luminance levels were 103–240 cd/m2 and 100 cd/m2 respectively. The Lux value required for Cardiff cards CST was equal to that of the gray background cards. For each test the required Lux level was verified by using a Lux meter. The value of precise test distance varies from test to test. For Pelli-Robson CST the required separation was 1 m. For Vistech and Cambridge plates CSTs, the required separation was 3.048 m and 6 m respectively, and within the acuity limit for Cardiff cards CST (Wood and Wood 1995). After having all basic requirements for each test, the test charts/cards were hung vertically on a wall one after the other. To determine CS values, corresponding observations were made: first with the right eye (without any correction), starting horizontally on each line of charts/cards and moving from the left hand side to the right hand side, and then from top to bottom for each letter/symbol on the chart /card. The tests were repeated for a plastic in front of an eye pretended to be a cataract. The tests were concluded when two or three letters/symbols were named incorrectly. The corresponding observations for CS were recorded. Results The CS function (CSF) value obtained for monocular right eye (RE) testing without cataract through Pelli-Robson CST is 1.95 while with cataract it is 1.65 – a quite lower value (fig. 4 and 5) . From Cardiff CST the evaluated contrast (%) values for monocular RE without cataract and with cataract are 2% and 6%, hence the corresponding CSF values are 50 and 16.67 respectively. For Cambridge low contrast gratings test as the total scores obtained from contrast grating sheets were 35 and 23 for normal monocular RE without and with cataract respectively. Therefore, the equivalent CSF values are 370 and 140. The CST results obtained from Vistech test are represented graphically as shown (in fig. 1) in the appendices section both for monocular RE testing without and with cataract. From the Vistech CS value key chart configuration A, B and C, the corresponding graph entries for upper threshold values of contrast sensitivity are 1.5 and 3 respectively. Discussion Commercial available CS testing charts/cards, either with gratings or more complex patterns (Moseley, Hill, 1994), are powerful tools to measure the visual system's ability to process spatial and temporal information about the daily seen objects (Fiser, Bex and Makous 2003; Jindra and Zemon 1989). Each CS testing technique has its pros and cons. Letters/patterns CS, as determined in Pelli-Robson and Vistech CSTs, is generally measured at photopic light levels, whereas grating/cards CS, as evaluated by Cambridge low contrast gratings and Cardiff cards CSTs, is usually measured mesopic (i.e. a combination of photopic and scotopic ) light levels. Though with normal acuity, mesopic tests typically exhibit greater correlations with driving performance, yet such tests are by nature are more difficult and unpredictable than photopic tests (Drum, Calogero and Rorer, 2007). In the context of the above mentioned test results, both spatial and temporal contrast informations are evaluated. Pelli-Robson and Vistech CSTs measure the spatial contrast information while renaming two tests provide temporal contrast information (T. Grosvenor and G. Grosvenor 2007). All four test techniques are forced choice psychometric methods. Both Cardiff cards and Cambridge low contrast grating CSTs are 2 to 4 alternative forced-choice, preferential looking methods. Except the Pelli-Robson test that accesses CS performance for reading low contrast signs, the remaining tests either use patterns or shapes to determine CSF, hence these tests are suitable for all age groups, including children and physically impaired people. An important shortcoming of Pelli-Robson CST that it may not deliver an accurate valuation of performance perceiving and recognizing objects with sizes different than the letters test. Cambridge low contrast gratings cover the longest test distance (6 m) while Vistech requires maximum illumination level and more flexibility as it assesses CSF from the lower spatial frequency to higher spatial frequency with 8 contrast level to each spatial frequency. However, in contrast to Pelli-Robson test that requires maximum 8 minutes to complete the test, Cardiff cards, Cambridge low contrast gratings and Vistech tests are more time consuming and the test results have more variation than the standard acuity test results (Drum, Calogero and Rorer, 2007). Due to its short time duration and reliability, Pelli-Robson test has a substantial endorsement in clinical practices for visual performance description (Wood and Wood 1995). For monocular RE testing with cataracts, except Vistech test all three test results give a degradation in contrast sensitivity. Studies have shown that reduction in vision function including the CS with cataracts probably due to narrow angle light scattering from the white areas of CS testing charts (Hess and Woo, 1978; Elliott and Hurst, 1990). Reference List Drum, B., Calogero, D. and Rorer, E. (2007) ‘Assessment of visual performance in the evaluation of new medical products’ Drug Discovery Today: Technologies 4(2): 55 – 61. Elliott, D. B. and Hurst, M. A. (1990) ‘Simple Clinical Techniques to evaluate visual function in patients’ with early cataract. Optometry & Vision Science 67: 822 – 828. Fiser J, Bex B J and Makous W (2003) Contrast conservation in human vision. Vision Research 43: 2637–2648. Grosvenor T and Grosvenor G P (2007) Primary care optometry. 5th ed. St. Louis: Butterworth Heinmann Elsevier Inc. Hess, R., and Woo, G. ‘Vision through Cataracts.’ Investigative Ophthalmology & Visual Science. 17(5): 428 – 435. Jindra L F and Zemon V (1989) Contrast sensitivity testing: a more complete assessment of vision. Journal of Cataract and Refractive Surgery 15(2): 141–148. Khurana A K. (2008) Theory and practice of optics and refraction. 2nd ed. New Delhi: Reed Elsevier, India Private Limited. Moseley, M. J. and Hill, R. A. (1994) ‘Contrast sensitivity testing in clinical practice’ British Journal of Ophthalmology 78: 795-797. Wood R L and Wood J M (1995) The role of contrast sensitivity charts and contrast letter charts in clinical practices. Clinical and Experimental Optometry 78(2): 43–57. Appendices 1) Graphs Monocular right eye Vistech contrast sensitivity test curves 2) Figures a) Monocular right eye Vistech contrast sensitivity test observations with and without cataract. b) Monocular (RE) Pelli- Robson CS test result without cataract. c) Monocular (RE) Pelli- Robson CS test result with cataract. d) Cambridge low contrast grating CS test results with and without cataract. e) low contrast gratings test results (Left – red without cataract; Right – blue with cataract), Read More