Flat-Format Lens-Free Imaging Using an Organic Sensor With Guided Illumination and Application to Fingerprints

The cheap basis of organic sensors likely to be soon available for large areas (over 25 cm²) opens novel opportunities for flat capture photonic schemes. We investigate one such scheme whereby a thin coverslip-type glass above an organic sensor is used as an illumination medium. To preserve sufficient resolution of the order of 50 μm in spite of the nonimaging optics of considerable numerical aperture ~1.5, a design frame for directional filters is given. The response of various kinds of filters to the bandwidth and polarization of the illumination source, typically an LED, is discussed. Illustrations for fingerprint detections are provided. A possible layout for cheap, dense, parallel detection of scattering by nanosized systems is finally suggested.

I. Introduction

Flat formats are increasingly popular in photonics, a trend historically nurtured by progresses in displays. The prospect of available large area sensors (photodiode or photoconductive arrays) increasingly becomes a reality with the recent advent of organic photonics, itself propelled by organic light-emitting diode (OLED) applications [1]–[4]. Giants Samsung, Fuji and Panasonic are active in the field, press news tell. Non-organic approaches exist, notably since the 90s amorphous Si in X-ray sensors (see at companies Dpix or Trixell), or LTPS (low-temperature crystalline silicon) Thin-Film Transistors (TFT) [5] or oxides. But their quantum efficiency for visible light seems quite low. With the use of TFT cheap backplanes first intended for displays [6]–[8], it is possible to deposit at low cost a set of organic photosensitive layers that acts as a detector. The working principle exploits a photodiode similar to an OLED in much the same way as a traditional semiconductor photodiode resembles an inorganic light-emitting diode (LED) for common III-V semiconductors.