A new method for visualising the air exhaled while someone is speaking or singing could shed light on how diseases such as COVID-19 spread, and help evaluate the effectiveness of face masks, according to a study. The novel system, described in the journal Applied Optics, images temperature differences between exhaled breath and the surrounding air to estimate how far the breath travels before being dispersed into the surrounding air.

According to study author Thomas Moore from Rollins College in the US, the new technique can also be used to study the details of how breath flows from the mouth while speaking or singing, which could be useful for music instruction and speech therapy. Originally developed to study the flow of air through musical instruments such as organ pipes, Moore said he began imaging the breath of people speaking and singing.

"I realized that by scaling up my existing system, I could likely determine how far the breath extends and how effective masks may be in limiting the extent of the breath," he added. While most existing approaches used to image exhaled breath require expensive equipment and can image only a relatively small area, Moore said the new design uses common commercially available optical components to overcome these limitations. The new technique, Moore explained, is based on the fact that the speed of light changes depending on the temperature of air it passes through.

As breath is warmer than the surrounding air, the light transmitted through the exhaled air arrives at the camera slightly sooner than light that did not pass through it, which he said can be used to create images of the air. According to Moore, the technique can reveal new information that may affect how we approach distancing and masking requirements, especially when outdoors. "The pandemic has caused an economic catastrophe for many musicians, and any information we can give them that will help them get back to work is important," he added.