Hear us out: Smartphone lidar can test blood, milk

Using tiny samples, too, a claim we're sure you haven't heard before

Could the light detection and ranging (lidar) sensors in your future smartphone take the place of laboratory equipment in health and food safety applications? It's looking like a possibility.

Researchers at the University of Washington reckon handset lidar can determine fluid properties, sparing you from having to use expensive lab gear. The amount of liquid needed for a smartphone lidar test is significantly less than that for a medical lab, too, they said.

One application of lidar would be testing blood for coagulation. The researchers also found their method was able to determine the fat properties of milk and whether the liquid had been adulterated, and to identify a particular liquid from among ten samples.

Typically, measuring liquid for things like blood viscosity is done using radio signals, which display phase and amplitude differences based on the clotting factor. That particular test requires a vessel filled with blood to a depth of nearly an inch. And patients who need blood coagulation measurements have to take them regularly. 

Understanding how the researchers repurposed smartphone lidar sensors for their experiments means understanding the basics of how lidar works. Your phone's lidar fires pulses of light to measure the distance to various objects in the camera's view to help the device take better pictures.

Point a laser, including a near-infrared lidar pulse, at a diffuse surface and you'll see what's known as the laser speckle phenomenon. This speckle can be picked up by a phone's camera. Ergo, you can fire a lidar pulse from a handset at a surface, and have the same device observe the resulting speckling.

"When the variation in the surface height is larger than the laser wavelength, light from different points on the surface within the camera's pixel resolution traverses different paths and hence superpose with each other to create constructive and destructive interference patterns," as the university team put it in a paper about the tech. 

Laser speckles in liquids are affected by any particle in the liquid larger than the wavelength of the laser, usually about 800nm. When those particles move, the speckle pattern changes, and the phone camera can capture that for analysis by software.

Here is where you'll find the science at the heart of the research. Based on Brownian motion, particles in a liquid move differently based on the liquid's viscosity. Armed with that knowledge, the academics were able to get a proxy measurement of viscosity by looking at the time variance of the laser speckle. 

That discovery, we're told, let the researchers distinguish between coagulated and uncoagulated blood with only a single finger-lancet-sized drop, differentiate between different types of milk and whether the milk had been adulterated, and distinguish between 10 classes of liquid with 91.5 percent average accuracy. 

Lidar transmission power is a problem. For some liquids, particularly clear or translucent ones, the output power of sensor's laser pulses is too weak, preventing readings from being taken. This is an academic study, after all, it's not a shipping product.

Beyond milk and blood, the researchers point to other uses of laser speckle imaging that could be tested with a smartphone, such as burn wound assessment, birthmark monitoring, blood flow tracking in retinas, and non-medical uses like analyzing microplastics. Again, this is only at the experimental stage.

Apple's iPhone 12 Pro and iPhone 13 Pro are handhelds with lidar that spring to mind. The university team said their paper was accepted into the March 2022 edition of Proceedings of the ACM on Interactive, Mobile, Wearable and Ubiquitous Technologies. ®