PHOTODETECTOR

Abstract: A photodetector and light source assembly useful in a bar­ code reading system comprises a semiconductor substrate with holes therein. A ligh souce in the form of LED's mounted on one side of the substrate corresponding with the respective holes illuminates an object plane, via a lens system, on the other side of the substrate. Preferably, the light received by the photodetector is slightly out of focus. Advantages include a saving in cost and size together with an improved depth of field and efficiency of illumination.

Description: The present invention relates to photodetectors and more particularly to photodetectors made from semi-conductors, such as a silicon photodetector. A principal, but not exclusive use of the invention, is in the field of bar-code reading wherein a reflective sensor system (employing, for example, a silicon photodetector, a light source and a lens system) is used to read bar-codes normally printed on labels. In bar-code reading systems, it is necessary to illuminate a small spot in a bar-code scanning region so that sufficient light is reflected (from the bar-code) onto a photodetector which thereby generates a corresponding signal. Ideally, the photodetector receives all of the light reflected from the illuminated spot and there is no waste of illuminating power. Moreover, due to the nature of bar-code readers, the optical system should be small and relatively simple in construction. One known method of producing a reflective sensor system for bar-code reading is to utilise a fibre optic lead with one end split randomly into two ferrules. However, with such a fibre optic system, most of the light generated is wasted or lost before it enters the fibre optics and some light is lost in transmission. A high level of illumination is thereby required which reduces the efficiency of the system. Moreover, a fibre optic system has further disdadvantages with regard to cost and size. An alternative approach is to mount a light emitting diode (LED) and a photodetector side by side. A lens system is used to focus the incident light onto the bar-code scanning region and to focus the reflected light onto the photodetector. However, such an arrangement has the disadvantage of providing a very limited depth of focus due to the separation between the light source and photodetector. For example, the depth of focus of the lens is inversely proportional to its diameter and directly proportional to its focal length. Attempts to improve the depth of focus by stopping down the lens system clearly reduces the level of illumination of the object and hence severely decreases the efficiency of illumination. With more general regard to systems employing a photodetector spaced from a light source, there is only one arrangement, i.e. with regard to the relative positions of the light source, photodetector and object plane, in which all of the incident light falling on the object plane is reflected onto the photodetector (the object plane is normally arranged at the point of intersection of the incident and reflected light beams). Whilst other optical arrangements employing, for example, half-silvered mirrors, may be used to reduce or to overcome these problems, such arrangements are too cumbersome to be of any practical value in a bar-code reading system. IBM Technical Disclosure Bulletin, volume 17, No. 11, April 1975, Page 3217 discloses a large-area photosensor or solar cell in which a central hole is drilled to admit light from an unspecified source and a lens tip is mounted in the hole. In this optical arrangement, light would be scattered over a wide area and not effectively received by an object plane in order to provide an efficient return. Patent Abstracts of Japan, volume 6, No. 59 (P-110) (937), 16th April, 1982 and JP-A-571930 together with IBM Technical Disclosure Bulletin, volume 9, No. 7, December 1966, Page 934 both disclose photodetector and light source assemblies but neither of these assemblies employs a semiconductor body or substrate with a through hole. FR-A-2487551 describes a photo-detector assembly in which a lens is spaced from a light source and a photosensitive area and wherein incident and reflected light pass through the lens system. The present invention seeks to overcome the above-mentioned problems and to provide an improved bar-code reading system and reflective sensor system. The invention provides a bar-code reading system in accordance with Claim 1. Preferred embodiments of the invention are defined by the features of the dependent claims. The arrangement can be such that the light received by the photosensitive side of the body or substrate is blurred or slightly out of focus. The semiconductor body or substrate has one or more holes therethrough. For example, three holes may be arranged at the apices of a triangle within the peripheral boundary of, e.g. a circular, disc-shaped substrate and centred on said disc. The holes may be as shown in Fig. 2 or in the form of notches, slots or sector-shaped recesses around the periphery of a substrate. Such recesses may be provided separately from, or in addition to through holes. Holes which pass through the semiconductor substrate are used to avoid edge effects, such as optical diffusion. Since it is not normally practical to drill holes, for example, in crystalline silicon, the holes are preferably formed during the process of manufacturing the semiconductor device. For example, holes may be formed by etching an amorphous silicon substrate. The light sources can be provided by LED's bonded to the body or substrate. Suitably, the light sources, such as LED's, are selected with regard to the optical response of the photodetector to maximise the response of the system. For example, the photodetector and the LED's can be types which respectively respond and produce light which is biased into the visible portion of the spectrum. The size of the holes may be adjusted in order to vary the amount of light passing through the semiconductor body of substrate. The number of holes (and LED's) may also be varied to suit particular requirements. Besides saving costs in manufacture and reducing the size of the system due to mounting LED's on the same substrate as a photodetector, the invention enables an optical system to be constructed, for example, for use in a bar-code reading system, which has a better depth of focus and a better efficiency of illumination than the known systems described above. In arrangements according to the invention, the light source is closer to the optical axis of the photodetector hence effectively reducing the diameter of the lens system required to focus light onto an object plane. In a preferred arrangement wherein a bar-code reader employs a photodetector and a light source assembly according to the invention, together with a lens system for focusing light onto an object plane, the arrangement is such that the light received by the sensitive side of the photodetector is slightly blurred or out of focus. This is due to factors such as (a) the light source (LED's) not being in the same plane as the sensitive side of the photodetector and the relative locations of the lens system and the photodetector, (b) variations, which are experienced in practice, in the distance between the reflective system (bar-code) and the lens system, and (c) the nature of the lens system, for example, its optical aberrations. Such a "slightly out of focus" system is advantageous in a bar-code reader in that the reflected light is spread over the whole are of the sensitive side of the photodetector and a good depth of field is available for accommodating variations in the distance between the bar-code and the lens system. An example of the invention is described with reference to the accompanying schematic drawings, in which: Fig.1 illustrates an optical arrangement, and Fig.2 is an elevation of a part used in the arrangement of Fig. 1. Referring to the drawings, a reflective sensor system comprises a silicon photodetector in the form of a circular substrate having three holes 2 therethrough. The holes are located at the apices of an equilateral triangle which is centered on the circular disc or substrate 1. As shown in Fig.2, three LED's 3 are mounted on one side of the circular substrate 1 in correspondence with the respective holes 2. The LED's may be mounted on the photodetector by any suitable means, e.g. they may be bonded to the substrate 1. The photodetector is located at or adjacent the focus of a lens system 4 which is arranged to focus the light passing through the holes 2 onto a focal plane 5 corresponding with, for example, a bar-code scanning region. The light (from LED's 3) which passes through the holes 2 is focused onto plane 5 and is reflected therefrom back through the lens system 4 onto the photodetector (i.e. onto a light responsive face of the substrate on the opposite side to the face on which the LED's 3 are mounted ). The incident and reflected light follows closely adjacent paths, close to the optical axis, whereby the illumination of the light sensitive face of the substrate 1 is improved (e.g. compared with the prior art arrangement wherein LED's are arranged side-by-side with a photodetector). The relative positions of the lens 4 and the photodetector are adjusted for the best or required level of illumination.

Claims: A bar-code reading system comprising means including a lens system for illuminating a spot in a bar-code scanning region and a photodetector for receiving light reflected from a bar-code in said region in order to generate a corresponding signal, said photodetector comprising a body or substrate (1) made of semiconductor material, said system being characterised in that said body or substrate (1) has one or more holes therethrough whereby a light source mounted on one side of said body or substrate illuminates the bar-code scanning region through said one or more holes, the arrangement being such that the other photosensitive side of said body or substrate receives light from the illuminated spot through said lens system. A bar-code reading system according to Claim 1 and further including a lens system, the arrangement being such that the light received by the photosensitive side of said body or substrate is blurred or slightly out of focus. A system according to any of the preceding Claims, wherein body or substrate is a circular disc and said holes (2) are located at the apices of a triangle centered on said axis. A system according to any of the preceding Claims, wherein each light source (3) is in the form of a light emitting diode bonded to said body or substrate (1). A system according to any of the preceding Claims, wherein each hole is formed by a process of etching an amorphous silicon substrate.

Inventors: SIMS, GEORGE EDWARD
Applicants: S.B. ELECTRONIC SYSTEMS LIMITED
Application year: 1984
Main classification: G06K7
IPC codes: G06K7