Selecting an external light source for fetal biophysical testing

OBJECTIVE: To select the most intense light source that penetrates tissues and is safe for fetal biophysical testing STUDY DESIGN: A 3-step series of experiments was undertaken using a digital light meter (Extech Light Probe Meter, Extech Instruments, Waltham, Massachusetts). First, the density of light through a light filter was compared between the sun and 4 commercially available light sources. Second, penetration of light through the abdominal subcutaneous tissue (3-4 cm thick) of 6 pigs was compared between these light sources. Last, the skin reaction to the preferred light source and the distance in penetrating the uterine cavity were assessed in 50 pregnant women. RESULTS: Light emitted from a halogen bulb was significantly more intense than from a photographic flash, krypton bulb or penlight. Maximal intensity was gained with the light source placed against the skin. The halogen bulb penetrated a pig's abdominal wall more than the photographic flash or krypton bulb. The thickness of a pig's abdominal wall is similar to the distance in pregnant humans near term from the skin to the uterine cavity. No thermal injury or discomfort to the skin was observed for exposures <10 seconds. CONCLUSION: The halogen bulb was safe, penetrated tissues most effectively and was the best light source for fetal biophysical testing.