Hitachi Ltd announced a finger vein authentication module as thin as 3mm Aug 26, 2009.
Compared with Hitachi's existing product, which is 23.5mm in thickness, the new module is about seven times thinner. The company aims to start volume production of the module within two years for use in mobile devices such as mobile phones, it said.
Finger vein authentication is a technology to check and authenticate finger vein patterns by transmitting near-infrared light through a finger. The probability that the new module recognizes the right person as a stranger is 0.01%. And the probability that the module recognizes a stranger as the right person is 0.0001%. The authentication accuracy is equivalent to that of the existing product, Hitachi said.
The time it takes for the module to recognize a person is about one second including the time to take a picture of a vein pattern and authenticate it. The dimensions of the module are 30 x 25 x 3mm.
Image sensor revamped
Hitachi realized the thickness of 3mm by newly developing the "contactless flat sensor," a sensor that uses a contact image sensor and is dedicated to taking pictures of finger vein patterns. Each pixel of the CMOS sensor has a micro-lens array.
The pixel count of the CMOS sensor is 150 x 100, and the pitch between micro-lens arrays is 0.1mm. There is a light-blocking layer between each two of the pixels.
Only perpendicular lights that pass through finger veins are collected by the micro-lens arrays and captured by the CMOS sensor. With this method, it is possible to take well-focused pictures of finger vein patterns even when the distance between a finger and the sensor is short, Hitachi said.
The image sensor in the existing product has a structure to take pictures of finger vein patterns by irradiating finger veins with near-infrared light and collecting the light with one lens. Therefore, to reduce distortion of images, a certain space is required between the lens and a finger, making it difficult to design a thin module.
Furthermore, Hitachi improved the accuracy in capturing finger vein patterns by reducing the influence of external light such as sunlight. The company diminished the area of the sensor part to 15 x 10mm to prevent external light from coming in and added a new function to control the exposure time of the CMOS sensor in accordance with external light.
When light comes in from a certain direction, the light intensity of LEDs arranged in the direction is adjusted. With those methods, it is possible to authenticate finger veins even under light of 20,000lx.