A portion of the light emitted by laser 850 passes through a lens , the semi reflective layer, or coating, 822 and is reflected by highly reflective layer, or coating, 834 and sensed by detector 852, which senses modulations in light intensity based on the presence or absence of a pit or {HEAD ~ land} in a particular spot on {TAIL ~ the highly reflective layer} or coating 822 
A portion of the light emitted by laser 850 passes through a lens , the semi reflective layer , or coating , 822 and is reflected by highly reflective layer , or coating , 834 and sensed by detector 852 , which senses modulations in light intensity based on the presence or absence of a pit or { HEAD ~ land } in a particular spot on { TAIL ~ the highly reflective layer } or coating 822
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