Iris image acquisition system
An iris image acquisition system comprises an image sensor comprising an array of pixels including pixels sensitive to NIR wavelengths; at least one NIR light source capable of selectively emitting light with different discrete NIR wavelengths; and a processor, operably connected to the image sensor and the at least one NIR light source, to acquire image information from the sensor under illumination at one of the different discrete NIR wavelengths. A lens assembly comprises a plurality of lens elements with a total track length no more than 4.7 mm, each lens element comprising a material with a refractive index inversely proportional to wavelength. The different discrete NIR wavelengths are matched with the refractive index of the material for the lens elements to balance axial image shift induced by a change in object distance with axial image shift due to change in illumination wavelength.
1. An iris image acquisition system comprising: an image sensor comprising an array of pixels including pixels sensitive to near infra-red (NIR) wavelengths; at least one NIR light source capable of selectively emitting light with different discrete NIR wavelengths; a processor, operably connected to said image sensor and said at least one NIR light source, to acquire image information from said image sensor under illumination at one of said different discrete NIR wavelengths; a lens assembly comprising a plurality of lens elements with a total track length no more than 4.7 mm, each lens element comprising a material with a refractive index inversely proportional to wavelength, said different discrete NIR wavelengths being matched with the refractive index of the material for the lens elements to balance an axial image shift induced by a change in object distance with the axial image shift due to change in illumination wavelength, and to focus NIR light at a first NIR wavelength reflected from an iris imaged at a first object distance between about 200 mm and 270 mm on said image sensor and to focus NIR light at a second NIR wavelength, longer than said first NIR wavelength, reflected from the iris imaged at a second object distance, between about 270 mm to 350 mm, on said image sensor.
2. An iris image acquisition system according to claim 1 wherein said lens assembly comprises a front lens and a rear lens axially disposed between said front lens and said image sensor, a rear surface of said front lens and a front surface of said rear lens comprising even aspheric surfaces to correct for field aberrations across an approximately 40 mm field in object space.
3. An iris image acquisition system according to claim 1 wherein said lens assembly has an aperture stop of approximately 1.8 mm and a focal length at NIR wavelengths to provide an optical system for said NIR wavelengths of approximately f/2.
4. An iris image acquisition system according to claim 1 wherein said focal length of said lens assembly at NIR wavelengths is approximately 3.57 mm.
5. An iris image acquisition system according to claim 1 wherein said at least one NIR light source comprises at least one light emitting diode (LED).
6. An iris image acquisition system according to claim 1 wherein said first wavelength is approximately 780 nm and said longer second wavelength is either: 870 nm or 940 nm.
7. An iris image acquisition system according to claim 1 wherein said material of each lens element is glass.
8. An iris image acquisition system according to claim 1 comprising either a single tunable NIR light source; or a plurality of discrete NIR light sources.
9. An iris image acquisition system according to claim 1 comprising two light sources, each emitting light having a FWHM (full width at half maximum) of no more than 50 nm.
10. A smartphone including the iris image acquisition system according to claim 1 and further comprising a processor arranged to analyse images acquired from said image sensor to perform biometric recognition of a user of said smartphone.