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