The invention is related to a retina implant, comprising a surface with a plurality of pixel elements disposed thereon for receiving and converting incoming light energy into electric energy, at least one amplifier and a plurality of stimulation electrodes being supplied via the at least one amplifier as a function of the signals received by the pixel elements. 
A retina implant of the afore-specified kind is disclosed in U.S. Pat.  No.  4,628,933. 
This prior art retina implant is a so-called epiretinal implant which is placed onto the surface of the retina.  Essentially it consists of a chip with a plurality of light-sensitive elements being provided on a surface thereof facing the lens of the eye, and converting the image projected through the lens into corresponding electric signals which shall stimulate the photoreceptors disposed in the retina.  The individual electric signals received by the light-sensitive elements may be amplified in order to supply associated stimulation electrodes disposed on the rear side of the chip which are provided for stimulating the retina cells. 
Due to inherent disadvantages of this system, the recent development has taught away from epiretinal implants and has suggested subretinal implants which are implanted into lower retina layers.  A subretinal implant of that kind is disclosed, for example, in WO 98/17343. 
This prior art subretinal retina implant comprises a plurality of light-sensitive pixel elements disposed on its surface facing the lens.  The pixel elements convert the image incoming through the lens into corresponding electric signals.  With the aid of infrared radiation being additionally irradiated into the eye for energy supply purposes via a photovoltaic layer positioned behind the pixel elements, the electric signals are converted into electric stimulating signals.  These stimulating signals are fed to the adjacent retina cells via stimulation electrodes which likewise are disposed on the surface of the implant facing the lens. 
For epiretinal as well as for subretinal implants it has turned out that it is necessary to couple external energy into the implants in order to convert the incoming light signals into corresponding electrical stimuli being of sufficient stimulation for the cells adjacent the implant.  Besides the coupling of invisible infrared light, also other options for coupling energy have been described, for example the option of coupling rf energy via a coil or by utilizing an external battery or an implanted battery. 
However, with prior art retina implants a problem has come up, namely the difficulty to convert the incoming light energy into corresponding electrical stimulating signals approximately linearly over a wide intensity range which under natural fluctuations of light intensity may span over various decades. 
It is, therefore, an object underlying the invention to provide an improved retina implant allowing the conversion of incoming light energy into appropriate electrical stimulating signals over an intensity range as wide as possible. 
According to the implant specified at the outset, this object is achieved by at least one light-sensitive reference element being coupled with the at least one amplifier for controlling the amplification thereof as a function of the light energy impinging on the at least one reference element. 
By doing so, the discharged electrical stimulating signals may be adapted to an average light intensity, as is done also in the course of natural adaptation of the eye relative to a change in ambient light conditions.  Hence, it is avoided on the one hand that under relatively bright ambient light the stimulating electrodes transmit too strong electric signals to the adjacent retina cells, which would result in an over-stimulation or even in a damage on these cells.  On the other hand, under very weak ambient light conditions, it becomes possible to transmit stimulating signals of sufficient intensity to the adjoining retina cells, so that a substantially improved viewing aid may be offered to a patient even under varying ambient light conditions. 
The at least one reference element should, as a matter of fact, be provided with at least a surface area being larger than the surface area of an individual pixel element, or a plurality of reference elements should be provided allowing an averaging over a selected portion of the surface.  The at least one reference element, therefore, is the standard for determining the operational range of the individual pixel elements or the amplification characteristic curve is shifted as a function of the mean luminescence intensity. 
The size of the at least one reference element depends on which surface portion the average light intensity shall be determined.  The size may, therefore, vary between a few xcexcm and a few mm2. 
In a preferred embodiment of the invention, a common reference element is provided extending over a selective portion of the surface with the pixel elements being disposed on the reference element as isolated areas, preferably along a grid pattern. 
This feature enables a relatively simple design and the possibility of adaptation to the entire mean light intensity. 
According to another variation of the invention, a plurality of reference elements is distributed over a selected portion of the surface along a predetermined pattern between the pixel elements. 
By doing so, a special characteristic during the determination of the mean light intensity may be achieved that can be also adapted to the personal physiological conditions of the patient. 
According to a modification of this embodiment, the reference elements are distributed over a selected portion of the surface as stripes and/or as a rectangular or an oblique grid. 
Such an arrangement allows different variations for the determination of the mean light intensity which, on the one hand, allow a simple design and on the other hand an optimum adaptation to predetermined parameters. 
According to the number, arrangement and circuitry of the reference elements, one reference element each may be associated to a plurality of pixel elements, or each pixel element may have one reference element associated thereto, or one pixel element may have a plurality of reference elements associated thereto. 
According to another preferred embodiment of the invention, each pixel element has an amplifier and a stimulation electrode associated thereto. 
This makes it possible to utilize the light signals received by the individual pixel elements with an utmost resolution for stimulating adjacent cells. 
According to another modification of the invention, the reference elements are distributed as stripes over a selected portion of the surface, further reference elements being disposed between the individual stripes and the pixel elements. 
This, too, allows an optimum adaptation to predetermined external or physiological parameters of the patient. 
According to still another embodiment of the invention, a plurality of reference elements is distributed over a selected portion of the surface in a chessboard pattern, wherein each pixel element is surrounded by a plurality of reference elements arranged at an angle with respect to each other. 
When doing so, the reference elements may have a larger length or width as compared to the associated pixel elements, depending on the particularly selected geometrical arrangement, or may be subdivided into individual segments associated to an individual pixel element. 
By doing so, it is possible to let selected portions of the surface overlap which are required for the determination of the reference signal for an individual pixel element.  Several reference elements or several segments of a reference element, respectively, may be associated to a pixel element in order to determine the mean light intensity over a larger portion of the surface.  On the other hand it is also possible that individual reference elements distributed over a larger selected portion of the surface are used as a reference for a plurality of pixel elements or for the entire retina implant. 
In such a manner, chip segments are generated utilizing the entire surface of the retina implant for determining the mean light intensity so that if a reference element should become inoperative, only a portion of the retina implant would lose its operability. 
According to still another embodiment of the invention, the at least one amplifier amplifies the difference between the output signals of the pixel elements and of the associated reference elements, and controls the associated stimulation electrodes as a function of the amplified signals. 
This results in a particularly simple and reliable design of the amplifier circuit.  No further averaging means or the like are required. 
According to still another embodiment of the invention, the pixel elements and the at least one reference element have the same wavelength sensitivity. 
By doing so, the at least one reference element may directly be used for determining the operational range of the prevailing amplifier. 
According to another embodiment of the invention, the pixel elements and the at least one reference element are configured as photodiodes. 
Appropriate methods for manufacturing same are well-known, allowing a regular arrangement of a high number of pixel elements in an array having a high packing density. 
The amplifier or amplifiers may have a linear or a logarithmic characteristic curve. 
The utilization of amplifiers having a logarithmic characteristic curve may be advantageous because even in a situation with a large variation of the input signals over a wide intensity range, the output signals only vary over a relatively small range. 
It goes without saying that the features mentioned before and those that will be mentioned hereinafter may not only be used in the particularly given combination, but also in other combinations or alone, without leaving the scope of the present invention.