Spectral images, which contain more color information than is obtainable with a typical camera, reveal characteristics of tissue and other biological samples that can't be seen by the naked eye. A new smartphone-compatible device that is held like a pencil could make it practical to acquire spectral images of everyday objects and may eventually be used for point-of-care medical diagnosis in remote locations.

A new pencil-like wireless spectrometer can be used with a smartphone to collect 3D spectral images of the body and other objects. This design could make the device useful for point-of-care diagnostics. (Image Credit: Dan Wang, Beijing University of Chemical Technology)

Potential applications of the new device include detecting oxygen saturation in a person's blood, determining the freshness of meat in the grocery store, and identifying fruit that is the perfect ripeness. The spectrometer could also make it easier to acquire spectral data in the field for scientific studies.

The Hainan University research team have described how to make a new pencil-like spectrometer and demonstrated its ability to acquire spectral images of bananas, pork, and a person's hand. The device can detect wavelengths from 400 to 676 nanometers at 186 spots simultaneously.

The easiest way to use a spectrometer is to wave it over the part of the body or object being examined; however, many home-made portable spectrometers use a smartphone camera to acquire data and a phone cradle that contains other necessary optics. The cradle can be hard to align correctly and makes it awkward to wave the smartphone over the body.

Rather than using a smartphone camera to acquire images, the new spectrometer uses a commercially available CMOS camera that wirelessly transmits images to a smartphone. This approach allowed the researchers to assemble a cylindrical spectral imaging device weighing just 140 grams (about 5 ounces) that is about the length of a smartphone and just over 3 centimeters in diameter.

The new pencil-like spectrometer uses all commercially-available components that can be purchased for less than $300 (US). The light source is an array of white LEDs, which connects to an off -the-shelf optical lens tube with the CMOS detector and other optical components necessary for spectral imaging.

One can use the spectrometer simply by moving it across the target area by hand. This manual push-broom scanning process builds up a series of spectral images that are sent to a smartphone or computer where software stitches the spectral images together into a 3D spectral image data cube.

The researchers tested the spectrometer by using it to detect banana ripeness and levels of myoglobin — the iron-containing protein that gives meat its color—in a piece of pork. They also used it to scan a person's hand, obtaining a 16-second video containing 200 spectral images. From the 3D spectral images, the researchers could distinguish five fingers and the palm and saw differences in hemoglobin distribution in various parts of the hand.

The researchers are also interested in developing distributed spectral cameras that could be used for a wide range of ocean surveys, such as detecting dissolved organic matter in water or pigments that indicate early signs of harmful algal blooms. Since the spectrometer can connect to any type of camera, it could possibly be connected to the camera of an autonomous vehicle to create a remote ocean sensing system.

Using commercially-available components to make the prototype means that anyone can assemble the device, but it also places some limits on resolution and sensitivity. For example, their prototype can only resolve wavelengths that differ by at least 17 nanometers.

The next step is to improve the camera by using a long focal length lens to achieve significant improvement in the spectral resolution.

The researchers also plan to develop software to make the spectral imager even more useful by using machine learning algorithms to analyze the massive amounts of data that could be collected.

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