Bright Lenses and Optical Depth

Abstract

In gravitational lensing, the concept of optical depth assumes that the lens is dark. Several microlensing detections have recently been made in which the lens may be bright. Relations are developed between apparent and absolute optical depth in the regime of the apparent and absolute brightness of the lens. An apparent optical depth through bright lenses is always less than the true, absolute optical depth. The greater the intrinsic brightness of the lens, the more likely that it will be found nearer the source.