GB2485991A

GB2485991A - Camera using a Single Photon Avalanche Diode (SPAD) array

Info

Publication number: GB2485991A
Application number: GB1020273.7A
Authority: GB
Inventors: Barry Somerville
Original assignee: STMicroelectronics Research and Development Ltd
Current assignee: STMicroelectronics Research and Development Ltd
Priority date: 2010-11-30
Filing date: 2010-11-30
Publication date: 2012-06-06
Also published as: GB201020273D0

Abstract

A camera 400 has a proximity detector for detecting the distance of a subject being photographed from the camera. The proximity detector 404 is an array of Single Photon Avalanche Diodes (SPADs) with an illumination source. The SPADs may be arranged in rows and columns and connected to a multiplex and a counter to enable measurement of the reflected illumination e.g. to assist in controlling the focussing of the camera. The output of the proximity detector may be passed to an operating system of the camera to switch it from a normal mode to a macro mode. The proximity detector may employ a phase shift extraction method for range determination. The camera may be part of a phone or computer.

Description

Application using a Single Photon Avalanche Diode (SPAD)

Description

Field of the invention

The present invention relates to an application using a single photon avalanche diode (SPAD).

Backciround of the invention A SPAD is based on a p-n junction device biased beyond its breakdown region. The high reverse bias voltage generates a sufficient magnitude of electric field such that a single charge carrier introduced into the depletion layer of the device can cause a self-sustaining avalanche via impact ion isation. The avalanche is quenched, either actively or passively to allow the device to be "reset" to detect further photons. The initiating charge carrier can be photo-electrically generated by means of a single incident photon striking the high field region. It is this feature which gives rise to the name Single Photon Avalanche Diode'. This single photon detection mode of operation is often referred to as Geiger Mode'.

US 7,262,402 discloses an imaging device using an array of SPADs for capturing a depth and intensity map of a scene, when the scene is illuminated by an optical pulse.

US 2007/0182949 discloses an arrangement for measuring the distance to an object. The arrangement uses a modulated photonic wave to illuminate the object and an array of SPADs to detect the reflected wave. Various methods of analysis are disclosed to reduce the effects of interference in the reflected wave.

An objective of the present invention is to use a SPAD as a solid state photo-detector for ranging, proximity detection, accelerometry etc. This requires the use of new techniques and the development of new applications.

One such application where SPAD range detection or proximity detection can be used is in autofocus (AF) cameras.

A digital camera with an autofocus (AF) lens can operate in a normal mode for most situations, but for very close objects the camera may operate in a macro mode. Typically the macro mode must be hand set by a user when required. This can cause delays and means that the user can miss the shot.

Obiects of the invention It is an object of the present invention to overcome at least some of the

problems associated with the prior art.

It is a further object of the present invention to make use of SPADs in new applications and circumstances.

One object of the present invention is to provide a camera with a proximity detector to assist with autofocus and mode selection functions.

Summary of the invention

The present invention provides a method and system as set out in the accompanying claims.

According to one aspect of the present invention there is provided a camera comprising a proximity detector for detecting the distance of a subject being photographed from the camera.

Optionally, the proximity detector comprises an array of single photon avalanche diodes (SPAD); and an illumination source, wherein the illumination from the illumination source is reflected by the subject to the array of single photon avalanche diodes.

Optionally, the array of single photon avalanche diodes comprises a plurality of single photon avalanche diodes arranged in rows and columns.

Optionally, the array of single photon avalanche diodes is connected to a multiplex and a counter to enable measurement of the reflected illumination.

Optionally, the output from the proximity detector is passed to operating system of the camera to switch the camera from a normal mode to a macro mode.

Optionally, the output from the proximity detector is passed to operating system of the camera to assist in controlling the focussing of the camera.

The present invention offers a number of advantages, in particular the ability to automatically identify the distance of an object and adjust the mode or focussing accordingly is a benefit. This means that there is no delay in adjusting the camera and the user is less likely to miss the shot.

Brief description of the drawincis

Reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a diagram for illustrating the determination of phase shift in a SPAD, in accordance with an embodiment of the invention, Figure 2 is a diagram of a SPAD and associated timing diagram, in accordance with an embodiment of the invention, Figure 3 is a block diagram of a proximity detector, in accordance with an embodiment of the invention, Figure 4 is a simplified diagram of a camera including a proximity detector, in accordance with an embodiment of the present invention.

Detailed description of the preferred embodiments

The idea that a SPAD can be used as in a ranging application is borne out by the application of a Phase Shift Extraction Method for range determination, although alternative methods exist for range determination using SPADs based on direct time of flight measurement. The term ranging in this application is intended to cover all ranging devices and methods including by not limited to ranging devices, proximity devices accelerometers etc. Ranging can occur in a number of applications, including proximity detection which is relative easy to implement and inexpensive; Laser ranging which is more complex and costly than a proximity detector; and three-dimensional imaging which is a high-end application that could be used to recognize gestures and facial expressions.

A proximity sensor is the most basic of the ranging applications. At its simplest the sensor is capable of indicating the presence or absence of a user or object. Additional computation and illuminator complexity can provide enhanced data such as the range to an object. A typical range is of the order 0.01 m to 0.5m. In a simple proximity sensor the illumination source could be a modulated LED, at a wavelength of about 850nm.

The next application group is that of laser ranging, where the illumination source is a modulated diode laser. Performance can range from <1cm to 20m range (and higher for top end systems) with millimetric accuracy.

Requirements on optics are enhanced, with hemispherical lenses and narrow bandpass filters being required. A near-field return may results in the introduction of parallax error, i.e. movement of the returned laser spot over the sensor pixel array dependent on distance to object. To overcome these problems the ranger includes calibration functions to enable the subtraction of the electronic and optical delay through the host system.

The illumination source wavelength should be visible so that the user can see what is being targeted and is typically around 635nm.

The third application group is that of 3D cameras. In this application a pixel array is used in order to avoid mechanical scanning of the array.

Systems can be based on a number of different architectures. Both time of flight (TOF) and modulated illuminator based architectures are used, however, the latter is more robust to ambient light and thus fits best with established photodiode construction. Additional features such as face and gesture recognition are applications of this type of ranging device.

Most optical ranging implementations use either stereoscopic, structured light, direct time of flight or phase extraction methods in order to ascertain the range to a target. Stereoscopic solutions use two conventional cameras, and can have a heavy computation overhead in order to extract range. The structured light scheme uses diffractive optics and the range is computed using a conventional camera based on how a known projected shape or matrix of spots is deformed as it strikes the target. The direct time of flight (TOF) method uses a narrow pulsed laser, with a time-digital converter (TDC) measuring the difference in time between transmission and first photon reception. Commonly, a reverse mode' is employed, where the TDC measures the back-portion of time, i.e. the time from first photon reception to next pulse transmission. This scheme minimizes system activity to only the occasions where a photon is detected, and is therefore well matched to tightly controlled, low photon flux levels and medical applications such as fluorescent lifetime microscopy (FLIM).

The phase extraction method is probably the most commonly used method as it is well suited to systems which implement computation of the generalized range equation using existing photodiode technology. It is also robust to background ambient light conditions, and may be adapted to allow for varying illuminator modulation wave-shapes (i.e. sinusoidal or square). This scheme is favored for S PADs in proximity detection applications.

The present invention takes advantage of the fact that the phase extraction method system incorporates an inherent ambient light level detection function which can be used in conjunction with a SPAD for many applications, including a camera having a range or proximity detector for determining the range of a subject being photographed.

It is important to understand the range equation derivation as it indicates the ease of applicability of SPADs to phase extraction proximity detection and ranging solutions. It also aids in the understanding of inherent features such as ambient light metering and measuring a depth of interest for a specific purpose.

Distance is determined from the speed of light and TOF, as follows: s=ct Where s is distance, c the speed of light and t is time. For a ranging system however, the distance is doubled due to the fact there are send and receive paths. As such the distance measured in a ranging system s is given by: s=?4ct The time shift component (= t') due to the photon TOF, is dependent on the modulation frequency and phase shift magnitude of the waveform.

t = % shift of the returned waveform x tmod period and if tmod period l/fmod t = 0-2irf t= 0 2zr of The units are in radians. Then by substituting the above equation back into the starting equation: the range equation' is expressed as. co�

S 4zrof

The critical component in this equation is 0, which is the unknown component of the % shift of the returned waveform. The following section discusses how this can be determined.

Since the values of c, f and ii are all constants; the range result simply scales with 0, (the % shift of the received light waveform in relation to that which was transmitted). Figure 2 demonstrates how 0 may be determined for a system employing a square wave modulated illuminator. The transmitted and received waveforms are shifted from one another by q5.

By measuring the photons that arrive in "a" and "b" in bins 1 and 2 respectively the value of q5 can be determined as follows: çô = 2w (a + In this type of system there is a range limit set by the illuminator modulation frequency, which is known as the unambiguous range.

Photons received from targets that are further away than this range can introduce an aliasing error by erroneously appearing in a legitimate bin for a subsequent measurement. Since determination of range is enabled by the modulation process, it is desirable to maximize the number of edges of the modulation waveform in order to accumulate data for averaging purposes as fast as possible. However, a high modulation frequency may lower the unambiguous range and introduces more technical complexity in the illuminator driver circuitry. Therefore, two or more different modulation frequencies may be interleaved or used intermittently, so as to reduce or negate the impact of aliased photons via appropriate data processing.

Figure 2 illustrates a possible implementation of a SPAD based proximity sensor with an associated waveform diagram. Figure 2 shows a SPAD 200 connected to a multiplexer 202. The output from the multiplexer passes through counters 1 and 2 (204). The SPAD device shown generally at 200 is of a standard type, including a photo diode 210, a p-type MOSFET 212 and a NOT gate 214.

The timing waveforms are shown in such a way so as to represent the relative photon arrival magnitudes. It can be seen that an extra phase has been added to enable computation of the background ambient light level offset c', although this can be significantly reduced by the use of a narrow optical band-pass filter matched to the illuminator wavelength if necessary.

The element c' is then accommodated in the computation of received light phase shift 0. The computed results for a, b, c are determined and written into either a temporary memory store or an 12C register. The computation of the phase shift, is calculated as follows: count (a + -2c The predetermined selection of modulation frequency is performed by dedicated logic or host system which selects a suitable frequency or frequencies for the application of the range sensor. The range sensor of figure 2 is dependent on the amount of light that can be transmitted on to the scene, system power consumption and the target reflectivity.

Since the system shown in Figure 2 needs to compute the background light condition in order to ascertain the offset of the returned light pulse from the target, ambient light metering is included. A simplified timing scheme is employed if only the ambient light level data is required, since the target illumination cycle is not necessary. If a narrow band IR filter is employed in the optical path the value of c will represent only the content of the filter passband. This can then be extrapolated to an approximation of the general ambient light conditions.

Referring to figure 3 a block diagram of a proximity sensor is shown. The proximity sensor 300 includes SPAD function and the quenching thereof in block 302. The quenching can be passive as shown or of any other suitable type. The bias voltage for the SPAD may be provided by a charge pump or any other suitable device 304. The sensor module also includes an LED or other illumination source and an associated driver 306 to ensure that the required modulation is applied to the illumination source.

The sensor may include a distance computation logic module to determine range. Alternatively this can be located in a host device in which the range sensor is used. The sensor also includes multiplexers and counters 308 and a storage means 310, such as a 120 module. The sensor may also include a Phase Locked Loop (PLL) for clocking and subsequent timed signal generation purposes.

The power consumption of SPADs and their readout circuits is dependent on the incident photon arrival rate. The average power consumption of a ranging system could be reduced by using power saving modes such as pulsed on/off operation, at a rate of -10Hz for example, at the expense of target motion distortion.

The sensor may be implemented on a 1mm die size and the 120 module could also be implemented on an appropriate die. The sensor may include an optical package, an integral IR band pass Filter (either coating or inherent in the optical elements) and an optimal field of view of about 30°.

As the sensor is not intended to "create an image" but is instead used to ensure that as many photons as possible are detected the optics could be made from injection molded hemispherical elements.

The illuminator source should ideally be of a non-visible wavelength, for example in the Near Infra Red (NIR) band, such as 850nm.

It should be noted that the terms "optical", "illumination" and "light" are intended to cover other wavelength ranges in the spectrum and are not limited to the visual spectrum.

The proximity sensor has been described with reference to simple low cost system, although it will be appreciated for certain applications the laser ranging and 3D camera technologies discussed above, could be used.

As previously indicated the proximity sensor of the present invention is very versatile and can be used in a vast array of different applications.

One such application is now described.

Referring to figure 4 a digital camera is shown at 400. The camera includes a lens arrangement 402 on the front face of the camera. The camera also includes all other standard elements although these are not shown per se. A SPAD proximity detector 404 is located on the front face of the camera in proximity with the lens and in the same plane. The SPAD proximity detector 404 includes an illumination source (not shown). The illumination source is capable of illuminating a subject being photographed so that at least some of the illumination is reflected back to the proximity detector 404.

The illumination source is located in any appropriate location that will enable the subject to be illuminated and reflection to be returned to the proximity detector. The illumination sources may include modulated light emitting diodes (LED5), modulated lasers or any other appropriate illumination source. Similarly the proximity detector can be located on any suitable surface or location as long as it functions as described above.

The output from the proximity detector will give a distance of the subject from the camera. This distance can be used by the operating system of the camera to set the camera to either a normal or macro mode based on manufacturer's limits; or otherwise to assist in the focussing of the camera.

The camera may be a stand alone camera or be incorporated into another device, such as a computer, telephone or any other suitable device.

It will be appreciated that many variations of the invention could apply and are intended to be encompassed within the scope of the claims.

Claims

Claims 1. A camera comprising a proximity detector for detecting the distance of a subject being photographed from the camera.
2. The camera of claim 1, wherein the proximity detector comprises an array of single photon avalanche diodes (SPAD); and an illumination source, wherein the illumination from the illumination source is reflected by the subject to the array of single photon avalanche diodes.
3. The camera of claim 2, wherein the array of single photon avalanche diodes comprises a plurality of single photon avalanche diodes arranged in rows and columns.
4. The camera of claim 2 or claim 3, wherein the array of single photon avalanche diodes is connected to a multiplex and a counter to enable measurement of the reflected illumination.
5. The camera of any preceding claim, wherein the output from the proximity detector is passed to operating system of the camera to switch the camera from a normal mode to a macro mode.
6. The camera of any preceding claim, wherein the output from the proximity detector is passed to operating system of the camera to assist in controlling the focussing of the camera.
7. A device including the camera of any preceding claim.
8. The device of claim 7, wherein the device is a phone.
9. The device of claim 7, wherein the device is a computer.