THERMO Spoken Here! ~ J. Pohl  ©
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Laser Retina Surgery

Retina Image

A blow to the head can cause the retina, the sight-giving lining of one's eye, to tear or detach from the inner surface of the orb, to float in its aqueous. For a minor tear sight might be restored by "welding" the retina back to the wall of the eye with carefully applied blasts of a laser powered at 103W/mm2. Use the information given.

Calculate the correct time, of duration, of applied laser power?
♦  The power a laser delivers is focused and substantial; 103W/mm2. The target is one square millimeter of surface of one cubic millimeter of injured retinal tissue. The tool is to sear the tissue and cause its temperature to attain 98°C - in one firing. The initial temperature of the tissue is 38°C.

The energy equation is applied with the cubic millimeter of retinal tissue as the system. Kinetic and potential energy changes are zero.

Ret Img 1

Bring the term, "pdV", th the left. Pressure is constant, the term combines with "du" to become "dh." Next apply integrals:

Ret Img 3

The differential, dh, is called exact; it integrates immediately. We use an average specific heat to represent the enthalpy difference. The laser power is a function of time, W-dotlaser/A(t) and area. It might be that the device has a start-up transient. But our calculation is preliminary so we apply the mean value theorem to effect the integration. Also apply the numbers and properties of water.

Ret Img 4

The mechanism of the laser is work. The effect is increased temperature. The trauma of being seared causes the tissue to adopt a "heal and repair" physiological mode. The sear does not occur as a consequence of temperature difference, hence is not heat.