OM t of c.iii - Vision Research Coordinating Center - Washington ...

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2/22/96 Chapter 6 Vision Assessment page 6-1 6.1 Measurement of Visual Acuity Chapter 6 Vision Assessment High and low contrast visual acuity are measured by a member of the CLEK Participating Clinic personnel Study-certified for visual acuity. The procedure for measuring visual acuity was originally developed for the Early Treatment of Diabetic Retinopathy Study (ETDRS) using Bailey-Lovie high contrast visual acuity charts. The Clinician and/or Technician is/are certified by the Chairman’s Office according to procedures outlined in Chapter 12 of this Operations Manual. 6.1.1 Introduction Bailey-Lovie distance visual acuities are utilized during the course of the CLEK Study. All Bailey-Lovie visual acuity charts are supplied by the Chairman’s Office from the School of Optometry, University of California, Berkeley, CA. The chart is located at 4 meters, and the white background of the chart has a luminance as specified in Section 6.1.2 below. At each Study Visit, visual acuity is measured in three ways in the following order: (1) High and low contrast Bailey-Lovie visual acuity with habitual correction, for each eye separately and both eyes together; (2) High and low contrast Bailey-Lovie visual acuity with best correction (for rigid contact lens wearers (including Softperm and piggyback wearers): contact lenses with optimal over-refraction, for non-rigid contact lens wearers: a CLEK Study Trial Lens Set contact lens with base curve equal to steep “K” plus optimal over-refraction), for each eye separately; and (3) High contrast Bailey-Lovie visual acuity with manifest refraction, for each eye separately. Table 6-1 shows the sequence of visual acuity measurements and the chart to be used for each.
2/22/96 Chapter 6 Vision Assessment page 6-2 Table 6-1. Mandatory Sequence of Visual Acuity Testing for all CLEK Study Visits. Correction Test Eye Chart Used Type of Patient Habitual High contrast OD HC-Chart 4 All Habitual High contrast OS HC-Chart 5 All Habitual High contrast OU HC-Chart 4 All Habitual Low contrast OD LC-Chart 7 All Habitual Low contrast OS LC-Chart 6 All Habitual Low contrast OU LC-Chart 7 All Over correction High contrast OD HC-Chart 5 All* Over correction High contrast OS HC-Chart 4 All* Over correction Low contrast OD LC-Chart 6 All* Over correction Low contrast OS LC-Chart 7 All* Manifest refraction High contrast OD HC-Chart 4 All Manifest refraction High contrast OS HC-Chart 2 All *Over correction for contact lens-wearing patients is their habitual contact lenses plus sphero-cylindrical over-refraction results. Over correction for spectacle wearers is over the rigid contact lens from the CLEK Study Trial Lens Set with base curve equal to the steep keratometric reading. 6.1.2 Calibration of Chart Lighting Light meters are provided by the CLEK Chairman’s Office. The light meter is removed from the pouch. The M button is pressed, and the LCD display illuminates. The only other buttons needed are the horizontal FUNCTION arrows on the right. These move the square cursor along the settings at the top of the screen. The Cursor must be set to the EV icon (as it should be when your meter arrives). To calibrate, the investigator should be positioned as close as possible to the letter charts without blocking the illumination falling on them. The M button should be pressed, and the two numbers on the bottom left should be inspected (“94” in Figure 6- 1). Figure 6-1. Light meter display. EV t f EV cal ISO 9 4 t' 125 100 1.4 2 2.8 4 5.6 8 11 16 22 32 45 64 90 S This EV scale is arbitrary (ie, not in candela/m 2 ), but it has been calibrated such that the target range is 96 to 102 EV. Too low and more light is needed. Too high and
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