Most optical tweezer measurements of polymers' elastic properties involve balancing the polymer's restoring force against the tweezers' trapping force. Not only are such measurements technically challenging, but even indirect laser illumination may affect the materials being measured. Another approach to optical tweezer force measurements avoids both of these shortcomings.
Optical tweezers can position colloidal particles reproducibly in interesting configurations. Turning off the tweezers releases the particles to move under the combined influences of random thermal forces and other interactions. The freed particles' trajectories can be measured with great precision using computerized digital video microscopy . Alternately trapping and releasing particles by blinking the tweezers on and off makes possible precise measurements of single-particle dynamics under tightly controlled conditions. Residual influences of the trapping beam such as local heating have been shown to relax in a few microseconds [26, 27].