To measure the true add power of a higher Rx flat top bifocal lens, you should place the _______ of the _________ against the lensometer aperture.

In measuring the true add power of a higher Rx flat top bifocal lens, placing the front of the segment against the lensometer aperture is essential because it allows for an accurate assessment of the additional power the bifocal provides for near vision. The flat top design of bifocal lenses incorporates a distinct segment at the bottom, which serves a different optical function compared to the distance portion of the lens.

When the front surface of the segment is aligned with the aperture, it ensures that the measuring instrument can properly evaluate the power of the segment. This is crucial for determining the additional dioptric strength required for reading or other near visual tasks.

Aligning the wrong part of the lens or measuring from the incorrect surface can lead to inaccuracies in determining the add power. The other options would not align with the measurement protocol necessary for determining the true add power, as they do not correctly consider the lens design and how bifocal optics function.