IOLMaster — White-to-White Measurement (WTW).

Determination of "White-to-White" (optional)

• Activate the WTW mode by:

  • clicking on the WTW icon
  • pressing the <W> key
  • pressing the <SPACE BAR> in ACD mode (ACD)
• Ask the patient to relax and look at the at the yellow fixation light .
• Align the IOLMaster so that the six peripheral light spots are symmetrical to the cross hairs and the iris structures or the edge of the pupil appears optimally focused. The fixation point in the center of the 6 light dots is usually not in the center of the pupil or iris, because only in the rarest cases does the visual axis correspond to the optical axis of the eye.
• Focus on the iris, not on the light spots. Adequate room lighting will facilitate the detection of iris structures. Avoid direct exposure of the eye and device front panel to extraneous light.
• In particular, ensure that the visible right and left edge of the iris is not disturbed by reflections from lamps and windows.
• If the iris structure is not discernible, focus on the edge of either iris or pupil. Serious defocusing will result in incorrect data.

WARNING: Ask the patient if he or she sees the fixation point. If the patient fails to fixate properly, the visual axis will not be correctly recognized, which may result in measuring errors.

• Take the measurement by pressing the button on the joystick.

Fig. 34, White to White determination

Each time the joystick knob is pressed, an image of the eye is displayed, in which the detected iris edge is marked. After checking that the iris and fixation point have been correctly recognized, confirm with OK. Only then will the data be valid and available for further processing.

• After the image has been taken, the operator should check if the software has correctly detected the edge of the iris. If the circle segments drawn in the image do not define the iris correctly, the result must be discarded.

WARNING: The validity of the WTW determination depends on this check of correct recognition of the iris edge.

The WTW value is the horizontal diameter of the iris. In addition to the WTW value, the deviation of the visual axis from the center of the iris (x, y) will also be displayed (as seen above).

The values are stated in millimeters with reference to a Cartesian coordinate system, the zero point of which is assumed to be in the established center of the iris or pupil. If the visual axis is above the iris or pupil center, the Y value will be positive; if it is below, the value will be negative. X values to the left of the center are negative; those to the right are positive.

Note If the software has difficulty detecting the iris or fixation point, this may be due to inadequate room lighting. It is recommended that the front panel and examined eye be shielded from direct or lateral light. The best results will be obtained when the examination room is slightly darkened.

WTW measurement may be repeated as often as desired.