# US 8,138,463 B2 · Photosensor and ambient light sensor with constant bias voltage > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 8,138,463 B2 - Document ID: US8138463B2 - Status: Granted - Kind: B2 - Application number: 12/444,032 - Filed: 2007-10-04 - Published: 2010-02-04 - Granted: 2012-03-20 - Earliest claimed: 2006-10-04 - Assignees: Sharp Kabushiki Kaisha - Inventors: Benjamin James Hadwen; Christopher James Brown - Web page: https://patentgrep.com/patent/US8138463B2 ## CPC - G01J 1/44: Electric circuits {(for command of an exposure part G03B7/02)} - G01J 1/1626: {Arrangements with two photodetectors, the signals of which are compared} - G01J 1/4204: {with determination of ambient light (solar light G01J2001/4266)} - G09G 3/3406: {Control of illumination source (illumination devices structurally associated with liquid crystal cells G02F1/1336)} - H10F 30/223: the potential barrier being a PIN barrier - H10F 30/26: the devices having three or more potential barriers, e.g. photothyristors - G09G 2360/144: the light being ambient light - G01J: MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY (light sources F21, H01J, H01K, H05B; investigating properties of materials by optical means G01N) - G09G: ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION (arrangements for transferring data between digital computers and displays G06F3/14; static indicating arrangements comprising an association of a number of separate sources or light control cells G09F9/00; static indicating arrangements comprising integral associations of a number of light sources H01J, H01K, H10H, H05B33/12; scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission, details thereof H04N1/00) - H10F: INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION ## Abstract A method of operating a photosensor comprising: applying a bias voltage to a photosensor (12) comprising n (n>1) photo-sensitive elements (8) connected in series, and determining the photocurrent in the photosensor (12) at a time when the applied bias voltage across the photosensor maintains the photosensor at or close to the point at which it has the greatest signal-to-noise ratio. This may conveniently be done by determining the current in the photosensor at a time when the applied bias voltage across the photosensor is equal or approximately equal to n×Vbi, where Vbi is the bias voltage about which the current in a single one of the photo-sensitive elements (8), in the dark, changes sign. In an embodiment in which the photo-sensitive elements (8) are photodiodes, the bias voltage Vbi is the “built-in” voltage of the photodiodes. The photocurrent generated when n series-connected photodiodes are illuminated is approximately equal to the photocurrent generated when one photodiode is illuminated. However, the leakage current (i.e., the dark current) for the n series-connected photodiodes is significantly lower than the leakage current for one photodiode. The signal-to-noise-ratio is therefore significantly increased. ## Claims ### Claim 1 A method of operating a photosensor comprising: applying a bias voltage to a photosensor comprising n photo-sensitive elements connected in series, where n is an integer greater than one; determining the current in the photosensor at a time when the applied bias voltage across the photosensor maintains the photosensor at or close to the point at which it has the greatest signal-to-noise ratio; and maintaining a constant bias voltage across the photosensor irrespective of the light level on the photosensor. ### Claim 2 (depends on 1) A method as claimed in claim 1 and comprising determining the current in the photosensor at a time when the applied bias voltage across the photosensor is equal or approximately equal to n×Vbi, where Vbi is the bias voltage about which the current in a single one of the photo-sensitive elements, in the dark, changes sign. ### Claim 3 (depends on 2) A method as claimed in claim 2 wherein the step of applying the bias voltage comprises applying a constant bias voltage equal or approximately equal to n×Vbi. ### Claim 4 (depends on 2) A method as claimed in claim 2 and comprising determining the current in the photosensor at a time when the applied bias voltage across the photosensor is equal or approximately equal to zero. ### Claim 5 (depends on 4) A method as claimed in claim 4 wherein the step of applying the bias voltage comprises applying a constant bias voltage equal or approximately equal to zero. ### Claim 6 (depends on 1) A method as claimed in claim 1, wherein n≧10. ### Claim 7 (depends on 1) A method as claimed in claim 1, wherein n≧20. ### Claim 8 (depends on 1) A method as claimed in claim 1, wherein n≧50. ### Claim 9 (depends on 1) A method as claimed in claim 1, wherein n≧100. ### Claim 10 (depends on 1) A method as claimed in claim 1 wherein each photo-sensitive element is a photodiode. ### Claim 11 (depends on 10) A method as claimed in claim 10 wherein each photo-sensitive element is a lateral photodiode. ### Claim 12 (depends on 1) A method as claimed in claim 1 wherein each photo-sensitive element is a phototransistor. ### Claim 13 (depends on 1) A method as claimed in claim 1 wherein each photo-sensitive element is a thin film photo-sensitive element. ### Claim 14 (depends on 13) A method as claimed in claim 13 wherein each photo-sensitive element is a silicon thin film photo-sensitive element. ### Claim 15 (depends on 1) A method as claimed in claim 1 wherein the photo-sensitive elements are nominally identical to one another. ### Claim 16 (depends on 1) A method as claimed in claim 1 and comprising connecting the n photo-sensitive elements in series between a first input of an operational amplifier and the second input of the operational amplifier. ### Claim 17 (depends on 1) A method as claimed in claim 1 and comprising connecting the n photo-sensitive elements in series between a first input of an operational amplifier and an output of the operational amplifier. ### Claim 18 (depends on 1) A method of operating a display device comprising the steps of: sensing an ambient light intensity according to a method as defined in claim 1; and controlling the intensity of a backlight of the display device in accordance with an output from the photosensor. ### Claim 19 A light-sensing system comprising: a photosensor comprising n photo-sensitive elements connected in series, where n is an integer greater than one; bias means for applying a bias voltage to the photosensor; and determining means for determining the current in the photosensor at a time when the applied bias voltage across the photosensor maintains the photosensor at or close to the point at which it has the greatest signal-to-noise ratio, wherein the bias means are adapted to maintain a constant bias voltage across the photosensor irrespective of the light level on the photosensor. ### Claim 20 (depends on 19) A system as claimed in claim 19 wherein the determining means are adapted to determine the current in the photosensor at a time when the bias means applies a bias voltage across the photosensor that is equal or approximately equal to n×Vbi, where Vbi is the bias voltage about which the current in a single one of the photo-sensitive elements, in the dark, changes sign. ### Claim 21 (depends on 19) A system as claimed in claim 19 wherein the bias means applies, in use, a constant bias voltage equal or approximately equal to n×Vbi. ### Claim 22 (depends on 19) A system as claimed in claim 19, wherein the determining means are adapted to determine the current in the photosensor at a time when the bias means applies a bias voltage across the photosensor that is equal or approximately equal to zero. ### Claim 23 (depends on 22) A system as claimed in claim 22, and further comprising a second photosensor connected in parallel to the first photosensor, the second photosensor comprising n photo-sensitive elements connected in series, wherein the second photosensor is connected in an opposite sense to the first photosensor. ### Claim 24 (depends on 23) A system as claimed in claim 23 wherein at least one cross-connection is provided between an inter-element node of the first photosensor and a corresponding inter-element node of the second photosensor. ### Claim 25 (depends on 19) A system as claimed in claim 19 wherein the bias means are adapted to apply a constant bias voltage equal or approximately equal to zero. ### Claim 26 (depends on 19) A system as claimed in claim 19 wherein n≧10. ### Claim 27 (depends on 19) A system as claimed in claim 19 wherein n≧20. ### Claim 28 (depends on 19) A system as claimed in claim 19 wherein n≧50. ### Claim 29 (depends on 19) A system as claimed in claim 19 wherein n≧100. ### Claim 30 (depends on 19) A system as claimed in claim 19 wherein each photo-sensitive element is a photodiode. ### Claim 31 (depends on 30) A system as claimed in claim 30 wherein each photo-sensitive element is a lateral photodiode. ### Claim 32 (depends on 19) A system as claimed in claim 19 wherein each photo-sensitive element is a phototransistor. ### Claim 33 (depends on 19) A system as claimed in claim 19 wherein each photo-sensitive element is a thin film photo-sensitive element. ### Claim 34 (depends on 33) A system as claimed in claim 33 wherein each photo-sensitive element is a silicon thin film photo-sensitive element. ### Claim 35 (depends on 19) A system as claimed in claim 19 wherein the photo-sensitive elements are nominally identical to one another. ### Claim 36 (depends on 19) A system as claimed in claim 19 wherein the n photo-sensitive elements are connected in series between a first input of an operational amplifier and the second input of the operational amplifier. ### Claim 37 (depends on 19) A system as claimed in claim 19 wherein the n photo-sensitive elements are connected in series between a first input of an operational amplifier and an output of the operational amplifier. ### Claim 38 (depends on 19) A system as claimed in claim 19 wherein the bias means and the determining means are comprised in a single circuit. ### Claim 39 (depends on 19) A display system comprising: a display device having a backlight; a light-sensing system as defined in claim 19 for sensing an ambient light intensity; and a controller for controlling the intensity of the backlight of the display device in accordance with an output from the light-sensing system.