The present invention provides a method and apparatus for estimating a
property of a fluid downhole by exposing the fluid to modulated light
downhole and sensing changes in intensity of infrared radiation from the
downhole fluid to estimate the property of the downhole fluid. The
present invention senses changes in intensity of light by converting the
changes to transient changes in temperature of a detector, such as a
pyroelectric detector. The present invention performs spectroscopic
analysis of fluids by optically filtering the light allowed to impinge on
a pyroelectric detector, converting the changes in temperature of the
pyroelectric detector to a signal and converting the signal to estimate
the property of the downhole fluid. The light source is modulated by
mechanically chopping the beam or by electrically pulsing the light
source or by steering the beam between different path lengths of sample
or between a reference cell (filled with a reference fluid or empty) and
a sample-filled cell.