Flexible liquid crystal-containing lenses

Abstract

The present invention provides an electrically switchable flexible contact lens conforming to the eye of a user. The lens includes: a liquid crystal cell for changing the power of the contact lens, the liquid crystal cell having a cell gap thickness between a first inner surface and a second inner surface, the liquid crystal cell including a diffraction optical element for correcting the user's vision, wherein the diffraction optical element is configured to maintain the cell gap thickness by providing support at one or more locations within the cell.

Description

Flexible liquid crystal-containing lens The present invention relates to flexible contact lenses. The present invention relates to flexible contact lenses. More specifically but not exclusively, the present invention relates to an electro-switchable contact lens. Flexible contact lenses including diffractive optical elements are known. The lens may comprise a liquid crystal that is electro-switchable between two states. In a first state, the refractive index of the liquid crystal does not match the refractive index of the diffractive optical element, and the diffractive optical element interacts with light and diffracts the light. In a second state, the refractive index of the liquid crystal matches the refractive index of the diffractive optical element and there is no diffraction or very little diffraction of the incident light. The present invention seeks to provide an improved flexible contact lens including a diffractive optical element. According to a first aspect of the present invention, there is provided an electro-switchable flexible contact lens having the features set out in Technical Solution 1 below. According to a second aspect of the present invention, there is provided an electro-switchable flexible contact lens having the features set out in Technical Solution 10 below. According to a third aspect of the present invention, there is provided an electro-switchable flexible contact lens having the features set out in Technical Solution 15 below. According to a fourth aspect of the present invention, there is provided an electro-switchable flexible contact lens having the features set out in Technical Solution 19 below. According to a fifth aspect of the present invention, there is provided an electro-switchable flexible contact lens having the features set out in Technical Solution 20 below. According to a sixth aspect of the present invention, there is provided an electro-switchable flexible contact lens having the features set out in Technical Solution 21 below. According to a seventh aspect of the present invention, there is provided an electro-switchable flexible contact lens having the features set out in Technical Solution 25 below. The preferred but optional features of the present invention are set out below and in the dependent technical solutions. Of course, it should be understood that features described in relation to one aspect of the present invention may be incorporated into other aspects. According to a third aspect of the present invention, there is provided an electro-switchable flexible contact lens for conforming to a user's eye, the lens comprising: a first liquid crystal cell for changing the diopter of the contact lens and having a cell gap thickness between a first inner surface and a second inner surface, the first liquid crystal cell including a diffractive optical element for correcting a user's vision, wherein the diffractive optical element is configured to maintain the cell gap thickness by providing support at one or more positions within the cell. The applicant has found that it is feasible to use a diffractive optical element as a spacer to maintain a cell gap thickness in a flexible ophthalmic lens. Maintaining the cell gap thickness is important for maintaining the optical properties of the ophthalmic lens. Specifically, maintaining the cell gap thickness maintains a sufficiently small cell gap such that the liquid crystal can be aligned without haze. In this regard, maintaining the cell gap thickness can include an average reduction in the cell gap thickness across the first liquid crystal cell of no greater than 15%, optionally no greater than 12%, optionally no greater than 10%, optionally no greater than 8%, and optionally no greater than 5%. Maintaining the cell gap thickness can only allow a relatively small reduction and / or increase in the cell gap thickness. In this regard, when the ophthalmic lens is deformed due to being placed on a user's eye, the average change in the cell gap thickness across the first liquid crystal cell can optionally be no greater than 15%, optionally no greater than 12%, optionally no greater than 10%, optionally no greater than 8%, and optionally no greater than 5%. The average cell gap thickness in an undeformed lens is optionally at least 2.0 microns, optionally at least 3.0 microns, optionally at least 3.5 microns, optionally at least 4.0 microns, optionally at least 4.5 microns, and optionally at least 5.0 microns. The average cell gap thickness in an undeformed lens is optionally no greater than 7.0 microns, optionally no greater than 6.5 microns, optionally no greater than 6.0 microns, optionally no greater than 5.5 microns, optionally no greater than 5.0 microns, optionally no greater than 4.5 microns, and optionally no greater than 4.0 microns. As used herein and as understood by those skilled in the art, micron is the same as micrometer. The average cell gap thickness in an undeformed lens is optionally from 2.0 microns to 7.0 microns, optionally from 2.5 microns to 5.5 microns, optionally from 3.5 microns to 5.0 microns, and optionally from 3.5 microns to 4.5 microns. The average height of the diffractive optical element can correspond to the average cell gap thickness. Similarly, the height of the diffractive element at a point in the first liquid crystal cell can correspond to the cell gap thickness at that point in the first liquid crystal cell. The first liquid crystal cell can be substantially free of other elements (such as spacers) that are used to maintain the cell gap thickness by providing support at positions within the cell. However, optionally, a peripheral spacer can also be provided at the periphery of the first liquid crystal cell in addition to the diffractive optical element. For the avoidance of doubt, a diffractive optical element is one whose size and scale are small enough relative to the wavelength of the light incident thereon so as to cause a controlled and desired optical effect that is solely due to the diffraction of the light. For the avoidance of doubt, a diffractive optical element does not include a Fresnel lens. The first liquid crystal cell can operate between a first non-switched state and a second switched state. In the first state, the difference between the effective refractive index of the liquid crystal and the refractive index of the diffractive optical element can be greater than or less than that in the second state. In the first state or the second state, the difference between the effective refractive index of the liquid crystal and the refractive index of the diffractive optical element can be small or zero, i.e., the effective refractive indices of the liquid crystal and the diffractive optical element can be approximately the same. In this context, the difference between the effective refractive index of the liquid crystal and the refractive index of the diffractive optical element can be, depending on the situation, not greater than 0.03, not greater than 0.02 and not greater than 0.01. This difference in refractive index is to be calculated for a wavelength in the visible part of the electromagnetic spectrum (radiation having a wavelength from 450 to 700 nm). This difference in refractive index can be calculated at a wavelength of 450 nm and, depending on the situation, at a wavelength of 700 nm. Optionally, this difference in refractive index can be calculated at a plurality of wavelengths from 450 nm to 700 nm. In the other of the first and second states, the effective refractive index of the liquid crystal and the refractive index of the diffractive optical element do not match, and thus, the diffractive optical element diffracts the incident light. The first state can be a non-switched state, i.e., where no or a low voltage is applied to the first liquid crystal cell. The second state can be a switched state, i.e., where a voltage is applied that switches the liquid crystal in the cell into the second state. For the avoidance of doubt, the effective refractive index is the refractive index of the liquid crystal for light normally incident on the contact lens and on the first liquid crystal cell. In the first non-switched state, the effective refractive index can be n ave = 0.5(n o + n e )), where n o is the ordinary refractive index of the liquid crystal and n e is the extraordinary refractive index of the liquid crystal. In the second switched state, the effective refractive index can be n o . For the avoidance of doubt, "for correcting a user's vision" includes being suitable for correcting a user's myopia, hyperopia and / or intermediate vision. The maximum height of the diffractive optical element need not be the same across the diffractive optical element. For example, the diffractive optical element may include an inner portion and an outer portion. The maximum height of the diffractive optical element at the outer portion may be greater than the maximum height of the diffractive optical element at the inner portion. The maximum height of the diffractive optical element at the outer portion may be up to 20% greater than the maximum height of the diffractive optical element at the inner portion, optionally up to 15%, optionally up to 10%, optionally up to 7.5%, optionally up to 5%, optionally up to 2.5% and optionally up to 1%. The height of the diffractive optical element is typically the height along the optical axis. The maximum height of the diffractive optical element at the outer portion may be at least 1% greater than the maximum height of the diffractive optical element at the inner portion, optionally at least 2% greater than the maximum height of the diffractive optical element at the inner portion, optionally at least 3%, optionally at least 4%, optionally at least 5%, optionally at least 6%, optionally at least 7%, optionally at least 8%, optionally at least 10%, optionally at least 12.5%, optionally at least 15% and optionally at least 20%. The inner portion may be at or near the center of the diffractive optical element. The height of the diffractive optical element may increase with the distance from the center of the diffractive optical element. The height of the diffractive optical element may increase linearly with the distance from the center of the diffractive optical element. The height of the diffractive optical element may increase sub-linearly with the distance from the center of the diffractive optical element. The height of the diffractive optical element may increase super-linearly with the distance from the center of the diffractive optical element. At least a portion of the diffractive optical element may be attached to the first surface and at least a portion of the diffractive optical element may be attached to the second surface. As mentioned above, the diffractive optical element may include a plurality of peaks and valleys, optionally annular peaks and valleys. The outer portion may include an outer peak, for example, optionally one of the ten outermost peaks, optionally one of the eight outermost peaks, optionally one of the five outermost peaks and optionally one of the three outermost peaks. The outer portion may include the outermost portion. The height of the peak may increase with the distance from the center of the diffractive optical element. The height of the peak may increase linearly with the distance from the center of the diffractive optical element. The height of the peak may increase sub-linearly with the distance from the center of the diffractive optical element. The height of the peak may increase super-linearly with the distance from the center of the diffractive optical element. At least a portion of a peak may be attached to the second surface. The liquid crystal may include a cholesteric liquid crystal. In an unswitched state, a director of the liquid crystal adjacent to the first and / or second inner surface may be at an angle of not greater than 20 degrees, optionally not greater than 15 degrees, optionally not greater than 10 degrees, optionally not greater than 8 degrees, optionally not greater than 5 degrees, and optionally not greater than 3 degrees with respect to the first and second inner surfaces. In an unswitched state, a director of the liquid crystal remote from the first and second inner surfaces (and optionally the middle between the first and second inner surfaces) may be at an angle of not greater than 20 degrees, optionally not greater than 15 degrees, optionally not greater than 10 degrees, optionally not greater than 8 degrees, optionally not greater than 5 degrees, and optionally not greater than 3 degrees with respect to the first and second inner surfaces. In a switched state, a director of the liquid crystal remote from the first and second inner surfaces (and optionally the middle between the first and second inner surfaces) may be at an angle of at least 60 degrees, optionally at least 70 degrees, optionally at least 80 degrees, and optionally at least 85 degrees with respect to the first and second inner surfaces. The refractive index of the diffractive optical element may be optionally at least 1.40, optionally at least 1.42, optionally at least 1.44, optionally at least 1.46, and optionally at least 1.48. The refractive index of the diffractive optical element may be optionally not greater than 1.72, optionally not greater than 1.70, optionally not greater than 1.68, optionally not greater than 1.66, and optionally not greater than 1.64. For example, the refractive index of the diffractive optical element may be from 1.40 to 1.72, preferably from 1.42 to 1.70, and more preferably from 1.44 to 1.68. The contact lens may include a further optical element for correcting a user's vision. In this regard, the contact lens may include a lens body for correcting a user's vision. The lens body may provide a positive optical diopter, such as +0.5, +1.0, or +1.5 D. The optical diopter of the lens body may be fixed. The addition of such a lens body may be particularly useful when the first liquid crystal unit is in the second switched state if the effective refractive index of the liquid crystal matches the refractive index of the diffractive optical element. At both 450 nm and 700 nm, the refractive index of the liquid crystal is optionally 0.80 to 1.20 times the refractive index of the diffractive optical element. At both 450 nm and 700 nm, the refractive index of the liquid crystal may be 0.90 to 1.10 times the refractive index of the diffractive optical element. At both 450 nm and 700 nm, the refractive index of the liquid crystal may be 0.95 to 1.05 times the refractive index of the diffractive optical element. At both 450 nm and 700 nm, the refractive index of the liquid crystal may be 0.97 to 1.03 times the refractive index of the diffractive optical element. At both 450 nm and 700 nm, the refractive index of the liquid crystal may be 0.98 to 1.02 times the refractive index of the diffractive optical element. At 500 nm, the refractive index of the liquid crystal can be 0.80 to 1.20 times the refractive index of the diffractive optical element, optionally 0.90 to 1.10 times, optionally 0.95 to 1.05 times, optionally 0.97 to 1.03 times, and optionally 0.98 to 1.02 times. The refractive index of the liquid crystal is the average refractive index n ave , and thus calculated as: n ave = 0.5(n e + n o ), where n e is the extraordinary refractive index and n o is the ordinary refractive index. The first liquid crystal cell may include a cholesteric liquid crystal, and the refractive index of the diffractive optical element is optionally not greater than 1.57. The first liquid crystal cell may include a cholesteric liquid crystal, and the refractive index of the diffractive optical element is optionally at least 1.58. The diffractive optical element may include a plurality of peaks and valleys, optionally circular peaks and valleys. The peaks and valleys may be concentric. At least one of the peaks, optionally more than one, and optionally each of the peaks maintains the cell gap thickness by providing support at one or more positions within the cell. The diffractive optical element may include at least 5 peaks, optionally at least 7 peaks, and optionally at least 10 peaks. The diffractive optical element may optionally include no more than 20 peaks, optionally no more than 15 peaks, and optionally no more than 10 peaks. The contact lens may include more than one liquid crystal cell. For example, the contact lens may include a second liquid crystal cell. The first and second liquid crystal cells may be configured such that light travels through two liquid crystal cells before traveling into the wearer's eye. The second liquid crystal cell may include the features described above in relation to the first aspect of the invention. For example, the second liquid crystal cell may include a diffractive optical element that serves as a spacer within the second liquid crystal cell. According to a second aspect of the present invention, there is also provided an electro-switchable flexible contact lens for conforming to a user's eye, the lens comprising: a first liquid crystal cell for changing the diopter of the contact lens and having a cell gap thickness between a first inner surface and a second inner surface; the first liquid crystal cell includes a diffractive optical element for correcting a user's vision, the diffractive optical element including an inner portion and an outer portion, and the maximum height of the diffractive optical element at the outer portion is 1% to 20% greater than the maximum height of the diffractive optical element at the inner portion. The applicant has found it advantageous for a diffractive optical element to have a greater maximum height at an outer portion of a first liquid crystal cell than at an inner portion of the first liquid crystal cell, but the difference in the maximum height should be limited. In particular, the applicant has found that when a contact lens is placed on the cornea, this configuration of the diffractive optical element can compensate for the curvature of the cornea. In addition, especially at the edge of a lens including a diffractive optical element, this configuration of the diffractive optical element can help to improve "direct vision". This configuration of the diffractive optical element helps to ensure that the optical path of light at the edge of the diffractive optical element is approximately the same as the optical path of light at the center of the diffractive optical element. The height of the diffractive optical element is typically the height along the optical axis. The maximum height is typically measured locally (i.e., relative to a substrate or bottom of the diffractive optical element). The maximum height of the diffractive optical element at the outer portion can be up to 15% greater than the maximum height of the diffractive optical element at the inner portion, optionally up to 10%, optionally up to 7.5%, optionally up to 5% and optionally up to 2.5%. The maximum height of the diffractive optical element at the outer portion can be at least 2% greater than the maximum height of the diffractive optical element at the inner portion, optionally at least 3%, optionally at least 4%, optionally at least 5%, optionally at least 6%, optionally at least 7%, optionally at least 8%, optionally at least 10%, optionally at least 12.5% and optionally at least 15%. The height of the diffractive optical element can increase with the distance from the center of the diffractive optical element. The height of the diffractive optical element can increase linearly with the distance from the center of the diffractive optical element. The height of the diffractive optical element can increase sub-linearly with the distance from the center of the diffractive optical element. The height of the diffractive optical element can increase super-linearly with the distance from the center of the diffractive optical element. The inner portion can be at or near the center of the diffractive optical element. The diffractive optical element can extend within a chord length r. The inner portion can include the portion of the diffractive optical element within a chord length of r / 8 from the center of the diffractive optical element. The outer portion can include the portion of the diffractive optical element having a chord length from r / 4 (and optionally from 3r / 8) to r / 2. The diffractive optical element may include a plurality of peaks and valleys, optionally annular peaks and valleys. The outer portion may include an outer peak, e.g., optionally one of the ten outermost peaks, optionally one of the eight outermost peaks, optionally one of the five outermost peaks, and optionally one of the three outermost peaks. The outer portion may include the outermost portion. The height of the peaks may increase with the distance from the center of the diffractive optical element. The height of the peaks may increase linearly with the distance from the center of the diffractive optical element. The height of the peaks may increase sub-linearly with the distance from the center of the diffractive optical element. The height of the peaks may increase super-linearly with the distance from the center of the diffractive optical element. The liquid crystal may include a cholesteric liquid crystal. In an unswitched state, the director of one of the liquid crystals far from the first inner surface and the second inner surface (and optionally the middle between the first and second inner surfaces) may optionally form an angle of not more than 20 degrees, optionally not more than 15 degrees, optionally not more than 10 degrees, optionally not more than 8 degrees, optionally not more than 5 degrees, and optionally not more than 3 degrees with the first and second inner surfaces. In a switched state, the director of one of the liquid crystals far from the first and second inner surfaces (and optionally the middle between the first and second inner surfaces) may form an angle of at least 60 degrees, optionally at least 70 degrees, optionally at least 80 degrees, and optionally at least 85 degrees with the first and second inner surfaces. The average unit gap thickness in an undeformed lens is optionally at least 2.0 microns, optionally at least 3.0 microns, optionally at least 3.5 microns, optionally at least 4.0 microns, optionally at least 4.5 microns, and optionally at least 5.0 microns. The average unit gap thickness in an undeformed lens is optionally not more than 7.0 microns, optionally not more than 6.5 microns, optionally not more than 6.0 microns, optionally not more than 5.5 microns, optionally not more than 5.0 microns, optionally not more than 4.5 microns, and optionally not more than 4.0 microns. The average unit gap thickness in an undeformed lens is optionally from 2.0 microns to 7.0 microns, optionally from 2.5 microns to 5.5 microns, optionally from 3.5 microns to 5.0 microns, and optionally from 3.5 microns to 4.5 microns. The contact lens of the second aspect of the present invention may include any features of the contact lens of the first aspect of the present invention. According to a third aspect of the present invention, there is provided an electro-switchable flexible contact lens for conforming to a user's eye, the lens comprising: a first liquid crystal cell for changing the diopter of the contact lens and having a cell gap thickness between a first inner surface and a second inner surface, the first liquid crystal cell comprising a diffractive optical element for correcting a user's vision and a cholesteric liquid crystal, the first liquid crystal cell being operable between a first unswitched state and a second switched state, wherein in the first state, the difference between the effective refractive index of the liquid crystal and the refractive index of the diffractive optical element is greater than or less than that in the second state, wherein at both 450 nm and 700 nm, the average refractive index of the liquid crystal is 0.80 to 1.20 times the refractive index of the diffractive optical element, and the average refractive index of the liquid crystal is n ave , and thus calculated as: n ave = 0.5(n e + n o ), where n e is the extraordinary refractive index and n o is the ordinary refractive index. The applicant has found that it is advantageous for the average refractive index of the liquid crystal to be similar to the average refractive index of the diffractive optical element in the visible part of the electromagnetic spectrum. The average refractive index of the liquid crystal and the refractive index of the diffractive optical element can be determined at ambient temperature (e.g., 20˚C or 25˚C), or at a temperature corresponding to the temperature of the human body (about 37˚C), or at a temperature corresponding to the temperature of the cornea of an eye (about 34˚C). At both 450 nm and 700 nm, the average refractive index of the liquid crystal can be 0.90 to 1.10 times the refractive index of the diffractive optical element. At both 450 nm and 700 nm, the average refractive index of the liquid crystal can be 0.95 to 1.05 times the refractive index of the diffractive optical element. At both 450 nm and 700 nm, the average refractive index of the liquid crystal can be 0.97 to 1.03 times the refractive index of the diffractive optical element. At both 450 nm and 700 nm, the average refractive index of the liquid crystal can be 0.98 to 1.02 times the refractive index of the diffractive optical element. At 500 nm, the average refractive index of the liquid crystal can be 0.80 to 1.20 times the refractive index of the diffractive optical element, optionally 0.90 to 1.10 times the refractive index of the diffractive optical element, optionally 0.95 to 1.05 times, optionally 0.97 to 1.03 times and optionally 0.98 to 1.02 times. The average refractive index of the liquid crystal is the average refractive index n ave , and thus is calculated as: n ave = 0.5(n e + n o ), where n e is the extraordinary refractive index and n o is the ordinary refractive index. At 460 nm, 480 nm, 500 nm, 520 nm, 540 nm, 560 nm, 580 nm, 600 nm, 620 nm, 640 nm, 660 nm, and 680 nm, one refractive index of the liquid crystal is 0.80 to 1.20 times, 0.90 to 1.10 times, 0.95 to 1.05 times, 0.97 to 1.03 times, and 0.98 to 1.02 times, as the case may be, of the refractive index of the diffractive optical element. At substantially all wavelengths from 450 nm to 700 nm, one refractive index of the liquid crystal is 0.80 to 1.20 times, as the case may be, of the refractive index of the diffractive optical element. At substantially all wavelengths from 450 nm to 700 nm, the refractive index of the liquid crystal can be 0.90 to 1.10 times, 0.95 to 1.05 times, 0.97 to 1.03 times, and 0.98 to 1.02 times, as the case may be, of the refractive index of the diffractive optical element. Those skilled in the art will realize that it is not necessary to measure the refractive index at all wavelengths; the refractive index varies with wavelength in a similar manner for most materials. Generally speaking, the refractive index decreases continuously from 450 nm to 700 nm without a minimum value, maximum value, or inflection point. The first liquid crystal cell can operate between a first and a second state. In the first state (usually an unswitched state), the director of one liquid crystal far from the first and second inner surfaces (and the middle between the first and second inner surfaces, as the case may be) can form an angle of no more than 20 degrees, no more than 15 degrees, no more than 10 degrees, no more than 8 degrees, no more than 5 degrees, and no more than 3 degrees, as the case may be, with the first and second inner surfaces. In a switched state (e.g., the second state), the director of one liquid crystal far from the first and second inner surfaces (and the middle between the first and second inner surfaces, as the case may be) can form an angle of at least 60 degrees, at least 70 degrees, at least 80 degrees, and at least 85 degrees, as the case may be, with the first and second inner surfaces. The lens of the third aspect of the present invention may include any features of the lenses of the first and / or second aspects of the present invention. According to a fourth aspect of the present invention, there is provided an electro-switchable flexible contact lens for conforming to a user's eye, the lens comprising: a first liquid crystal cell for changing the diopter of the contact lens and having a cell gap thickness between a first inner surface and a second inner surface, the first liquid crystal cell including a diffractive optical element for correcting a user's vision and a cholesteric liquid crystal; the first liquid crystal cell being operable between a first unswitched state and a second switched state, wherein in the first state, the difference between the effective refractive index of the liquid crystal and the refractive index of the diffractive optical element is greater than or less than that in the second state, wherein the refractive index of the diffractive optical element is not greater than 1.57. The applicant has found that if the refractive index of the diffractive optical element is relatively low, the optical performance of a contact lens exhibits good refractive index matching with good near-field performance in the second state within a wide wavelength range. In the first state or the second state but preferably the second state, the difference between the effective refractive index of the liquid crystal and the refractive index of the diffractive optical element may be small or zero, i.e., the refractive indices of the liquid crystal and the diffractive optical element are approximately the same. In the second state, due to the refractive index matching, the diffractive element does not significantly diffract the incident light. In the first state or the second state but preferably the first state, the difference between the effective refractive index of the liquid crystal and the refractive index of the diffractive optical element may be greater than in other states, such that the diffractive optical element diffracts the incident light. In the first unswitched state, the effective refractive index of the liquid crystal may be the average refractive index of the liquid crystal, n ave =0.5(n e +n o ) where n e is the extraordinary refractive index and n o is the ordinary refractive index. In the second switched state, the effective refractive index of the liquid crystal may be n o , the ordinary refractive index. The contact lens may include a lens body for correcting a user's vision. The lens body may provide a positive optical diopter, such as +0.5, +1.0 or +1.5 D. The optical diopter of the lens body may be fixed. The lens body may be located at or near the uppermost part of the lens, the part that is away from the part of the lens that contacts the wearer's eye. The addition of such a lens body may be particularly useful when the first liquid crystal cell is in the second switched state and the effective refractive index of the liquid crystal matches the refractive index of the diffractive optical element. The refractive index of the diffractive optical element is optionally not greater than 1.55 and optionally not greater than 1.53. The refractive index of the diffractive optical element is optionally at least 1.43, optionally at least 1.45, optionally at least 1.47 and optionally at least 1.49. The refractive index of the diffractive optical element is optionally from 1.43 to 1.57, optionally from 1.45 to 1.55, and optionally from 1.47 to 1.55. In an unswitched state, the director of a liquid crystal far from the first and second inner surfaces (optionally the middle between the first and second inner surfaces) can optionally form an angle of not greater than 20 degrees, optionally not greater than 15 degrees, optionally not greater than 10 degrees, optionally not greater than 8 degrees, optionally not greater than 5 degrees and optionally not greater than 3 degrees with the first and second inner surfaces. In a switched state, the director of a liquid crystal far from the first and second inner surfaces (and optionally the middle between the first and second inner surfaces) can form an angle of at least 60 degrees, optionally at least 70 degrees, optionally at least 80 degrees and optionally at least 85 degrees with the first and second inner surfaces. The lens of the fourth aspect of the present invention may include any features of the lenses of the first, second and / or third aspects of the present invention. According to a fifth aspect of the present invention, there is provided an electro-switchable flexible contact lens for conforming to a user's eye, the lens comprising: a first liquid crystal unit for changing the diopter of the contact lens and having an inter-unit gap thickness between a first inner surface and a second inner surface, the first liquid crystal unit including a diffractive optical element for correcting a user's vision and a cholesteric liquid crystal; the first liquid crystal unit is operable between a first unswitched state and a second switched state, wherein in the first state, the difference between the effective refractive index of the liquid crystal and the refractive index of the diffractive optical element is greater than or less than that in the second state, wherein the refractive index of the diffractive optical element is at least 1.58. The applicant has found that if the refractive index of the diffractive optical element is relatively high, then in the first state, the optical performance of the lens exhibits less sensitivity to different wavelengths. In the first or second state (but preferably the first state), the difference between the effective refractive index of the liquid crystal and the refractive index of the diffractive optical element can be small or zero, that is, the effective refractive indices of the liquid crystal and the diffractive optical element are approximately the same. In the first state, due to the refractive index matching, the diffractive element does not significantly diffract the incident light. In the first state or the second state but preferably the second state, the difference between the effective refractive index of the liquid crystal and the refractive index of the diffractive optical element can be greater than in other states, so that the diffractive optical element diffracts the incident light and contributes to the optical diopter of the contact lens. In the first unswitched state, the effective refractive index of the liquid crystal can be the average refractive index of the liquid crystal, n ave = 0.5(n e + n o ), where n e is the extraordinary refractive index and n o is the ordinary refractive index. In the second switching state, the effective refractive index of the liquid crystal can be n o , the ordinary refractive index. The refractive index of the diffractive optical element is optionally at least 1.58, optionally at least 1.60, optionally at least 1.62, optionally at least 1.64 and optionally at least 1.66. The refractive index of the diffractive optical element is optionally not greater than 1.70, optionally not greater than 1.68 and optionally not greater than 1.66. The refractive index of the diffractive optical element is optionally from 1.58 to 1.70, optionally from 1.60 to 1.68, and optionally from 1.62 to 1.66. In an unswitched state, a director of a liquid crystal far from the first and second inner surfaces and optionally an intermediate between the first and second inner surfaces can form an angle not greater than 20 degrees, optionally not greater than 15 degrees, optionally not greater than 10 degrees, optionally not greater than 8 degrees, optionally not greater than 5 degrees and optionally not greater than 3 degrees with the first and second inner surfaces. In a switched state, a director of a liquid crystal far from the first and second inner surfaces and optionally an intermediate between the first and second inner surfaces can form an angle of at least 60 degrees, optionally at least 70 degrees, optionally at least 80 degrees and optionally at least 85 degrees with the first and second inner surfaces. According to a sixth aspect, there is provided an electro-switchable flexible contact lens for conforming to a user's eye, the lens comprising: a first liquid crystal cell for changing a diopter of the contact lens and having a cell gap thickness between a first inner surface and a second inner surface, the first liquid crystal cell including a diffractive optical element for correcting a user's vision, the diffractive optical element including a plurality of peaks and valleys, the peaks extending in a direction from the first inner surface to the second inner surface, at least a portion of at least one peak being attached to the second inner surface. Attachment of at least one peak of the diffractive optical element to the second inner surface can limit movement of the liquid crystal within the cell, which movement can be undesirable. The diffractive optical element can be configured to maintain the cell gap thickness by providing support at one or more locations within the cell. The diffractive optical element can include a central peak and a plurality of outer peaks, at least a portion of at least one of the outer peaks being attached to the second inner surface. At least one of the external peaks, more than one and optionally each of which is annular. More than one and optionally all of the external peaks may be concentric. At least most and substantially all of at least one peak may be attached to the second surface. In the event of deformation of the first liquid crystal cell, this configuration inhibits the liquid crystal from traveling through the peak. Substantially all of each of more than one peak may be attached to the second inner surface. Each peak may be attached to the second inner surface. Substantially all of each peak may be attached to the second inner surface. For example, an adhesive may be used to achieve attachment of a peak to the second inner surface. Alternatively, a fusible polymer that attaches the peak to the second inner surface when heated and cooled may be provided to at least one of the peak and the second inner surface. According to a seventh aspect of the present invention, there is provided an electro-switchable flexible contact lens for conforming to a user's eye, the lens comprising: a first liquid crystal cell for changing the dioptric power of the contact lens and having a cell gap thickness between a first inner surface and a second inner surface, the first liquid crystal cell including a diffractive optical element for correcting a user's vision, the diffractive optical element having an operating characteristic wavelength that ranges from 450 nm to 510 nm. The first liquid crystal cell includes a cholesteric liquid crystal, and the first liquid crystal cell is operable between a first unswitched state and a second switched state, wherein in the first state, the difference between the effective refractive index of the liquid crystal and the refractive index of the diffractive optical element is greater than or less than that in the second state. The applicant has found that it is advantageous to use a diffractive optical element having an operating characteristic wavelength that is lower than the operating characteristic wavelength that can typically be used for other lenses, such as lenses of glasses. For example, the operating characteristic wavelength may be no greater than 560 nm, optionally no greater than 550 nm, optionally no greater than 540 nm, optionally no greater than 530 nm, optionally no greater than 520 nm, optionally no greater than 510 nm, optionally no greater than 500 nm, optionally no greater than 490 nm and optionally no greater than 480 nm. The applicant has found that it is feasible to obtain a practical contact lens having an operating characteristic wavelength of up to about 550 nm, because blue diffractive artifacts that can cause problems in glasses at wavelengths up to 550 nm can also cause problems for contact lenses. For example, the operating characteristic wavelength may be at least 460 nm, optionally at least 470 nm and optionally at least 480 nm. For example, the operating characteristic wavelength can be from 460 to 560 nm, optionally from 460 to 550 nm, optionally from 470 to 550 nm, optionally from 460 to 490 nm, optionally from 470 to 490 nm, and optionally 480 nm. Exemplary embodiments will now be described by way of example only with reference to FIGS. 1 to 9. An example of an electro-switchable flexible contact lens according to an embodiment of a first and second aspect of the present invention is shown in FIGS. 1 and 2. FIG. 1 is a schematic cross-sectional view of a contact lens generally designated by the reference numeral 1 and includes a flexible lens body 2 incorporating therein a liquid crystal cell generally designated by the reference numeral 3. The flexible lens body 2 comprises any suitable material such as a polysiloxane hydrogel material, a hydrogel material without polysiloxane compounds, and a polysiloxane elastomer material. FIG. 2 is a cross-sectional view of a portion of the contact lens 1. For clarity and ease of illustration purposes, the liquid crystal lens 3 is depicted as flat / plane in FIG. 2. Those skilled in the art will appreciate that this is not the case and the shape of the liquid crystal cell is more appropriately shown in FIG. 1. The liquid crystal cell 3 includes a diffractive optical element 4 and a liquid crystal 5 for correcting a user's vision. Briefly, the liquid crystal can be switched between a first non-switched state and a second switched state. In one of the switched and non-switched states, the refractive index of the liquid crystal matches the refractive index of the diffractive optical element, and the diffractive optical element has a first focal power. In the other of the switched and non-switched states, the refractive index of the liquid crystal does not match the refractive index of the diffractive optical element, and the diffractive optical element has a second focal power different from the first focal power. Diffractive optical element 4 includes a plurality of peaks and valleys, with a central peak 10A and eight annular peaks 10B, 10C, 10D, 10E, 10F, 10G, 10H, 10I concentrically arranged around the central peak, thereby providing nine diffractive regions (one region is provided between each pair of adjacent peaks). For simplicity, the associated valleys are not labeled. Such diffractive elements are known to those skilled in the art of optical devices. In known lenses, spacer beads or the like would be used to maintain the inter-element spacing between the first inner surface 6 and the second inner surface 7. In this example, diffractive optical element 4 serves as a spacer to maintain the inter-element spacing between the first inner surface 6 and the second inner surface 7. Although FIG. 2 shows that the central peak 10A is not in contact with the second inner surface, this is only for illustration that the height of peak 10A is less than the height of 10I, which is described in more detail below. A spacer 11 is disposed around the periphery of the liquid crystal cell 3 outside the diffractive optical element 4. This spacer 11 is formed by beads having a diameter of about 4 microns dispersed in a glue. The spacing between the first inner surface 6 and the second inner surface 7 is approximately 4 microns, but the spacing is greater at an outer region 22 of the liquid crystal cell 3 than at an inner region 21, with a difference of about 5%, and the difference is due to the difference in the maximum height of the diffractive optical element 4 at the inner portion 21 and the outer portion 22. The maximum height h2 of the diffractive optical element 4 at the outer portion 22 is 4.0 microns, while the maximum height h1 of the diffractive optical element 4 at the inner portion 21 is 3.85 microns. This difference in maximum height is small, but it improves the optical performance. Specifically, the applicant has found that when the contact lens is placed on the cornea, this configuration of the diffractive optical element can compensate for the curvature of the cornea. In addition, especially at the edge of a lens including a diffractive optical element, this configuration of the diffractive optical element can help improve "direct vision". The increased height of the diffractive optical element at the edge of the diffractive optical element ensures that the optical path at the edge is approximately the same as that at the center of the diffractive optical element. This configuration of the diffractive optical element ensures that the wavelength for optimal optical performance is approximately the same across the width of the diffractive optical element. Without the increased height, the optical path at the edge of the diffractive optical element would be slightly shorter than that at the center of the diffractive optical element, which would be undesirable. The heights of peaks 10A to 10I increase with the distance from the center of the diffractive optical element 4. The heights of peaks 10A to 10I can increase sub-linearly, linearly, or super-linearly with the distance from the center of the diffractive optical element. Diffractive optical element 4 is a spherical surface that is "chopped up" such that the wavefront traveling through the diffractive optical element has a 2π phase shift across the region boundaries (it should be recognized that, strictly speaking, the 2π shift occurs only at a single wavelength). It is contemplated that one could modify the shape of the diffractive optical element to have different optical refractive powers across the region of the diffractive optical element rather than a simple spherical surface (giving a simple uniform spherical optical power across the region of the diffractive optical element). For example, an elliptical diffractive optical element could be used to handle astigmatism. The diffractive optical element 4 maintains the spacing between the first inner surface 6 and the second inner surface 7, and it inhibits a large reduction in the spacing that can occur when the lens 1 (and thus, the liquid crystal cell 3) is deformed. The cornea is typically aspherical, and the placement of a contact lens on a wearer's cornea causes the lens to deform. In the absence of a diffractive optical element 4 acting as a spacer, if the contact lens 1 is placed on a wearer's eye, a large reduction in the spacing between the first inner surface 6 and the second inner surface 7 can be observed for some portions of the liquid crystal cell 3. Additionally, maintaining the cell gap thickness also maintains a cell gap thickness small enough such that the liquid crystal can be aligned without haze. The applicant has observed that in the event that the peaks are not attached to the inner surface 7 and the contact lens would deform as it is placed on a user's cornea, the liquid crystal can move outward from the space between the peaks 10A and 10B towards the outer portion of the contact lens, thereby creating a large amount of excess liquid crystal between the diffractive optical element and the inner surface 7, increasing the spacing between the first inner surface 6 and the second inner surface 7 by up to 1 micron in the regions of the peaks 10G, 10H, and 10I. To help reduce the displacement of the liquid crystal, an adhesive can be used to attach at least one, and optionally more than one and optionally each, of the peaks 10A to 10I to the inner surface 7. As an alternative to an adhesive, the peaks 10A to 10I can be attached to the inner surface by melting, pressing, and cooling the polymer 9. Still further, the peaks can be attached by physical adhesion (such as by using a plasma treatment process and the like). However, attaching one (or some) of the peaks to the inner surface 7 is not critical because the optical properties of the optical device will not be significantly negatively affected by the liquid crystal displacement due to the diffractive element. In the liquid crystal device 3, the liquid crystal comprises a cholesteric liquid crystal which is a nematic liquid crystal doped with a pair of chiral dopants. These nematic liquid crystals are well known to those skilled in liquid crystal science and technology, and for example, may include E7, BL037 and / or BL038. These chiral dopants are well known to those skilled in liquid crystal science and technology, such as Merck ZLI-3786, CB15 and S811. The local alignment of the molecules in the liquid crystal device 3 is schematically shown in FIG. 3. Referring to FIG. 2, the inner surfaces 6 and 7 are formed by a pair of alignment polymers that impart alignment to the liquid crystal molecules adjacent to the adjacent rubber polymers. Depending on the flexoelectric properties of the liquid crystal, this alignment of the liquid crystal molecules adjacent to the alignment polymers imparts a specific orientation to the liquid crystal far from the alignment polymers. Referring to FIG. 2, a layer of the polymer 9 is adjacent to the liquid crystal 5. In this case, the alignment polymer 9 is a UV curable polymer (Rolic ROP-103 / 2CP). The polymer is cured using suitable polarized UV radiation such that it imparts a desired alignment to the liquid crystal adjacent to the polymer. A similar layer of UV curable polymer is disposed on top of the diffractive optical element 4 but is not shown. Rubbing the alignment polymer layer enables the director (the average direction of the liquid crystal molecules) adjacent to the rubber polymer layer to be approximately parallel to the first inner surface 6 and the second inner surface 7, but usually tilted by a few degrees (a phenomenon known to those skilled in the art as "pretilt"). (As shown by the small circles adjacent to the first inner surface 6 and the second inner surface 7). The tilt of the director adjacent to the first inner surface 6 and the second inner surface 7 can be greater than a few degrees. For example, the tilt of the director adjacent to the first inner surface 6 and the second inner surface 7 can be 10 to 30 degrees. Between the first inner surface 6 and the second inner surface 7, the director of the liquid crystal 5 forms a helical structure. This is shown in FIG. 3. The rod-shaped indicates that the director of the liquid crystal is approximately parallel to the first inner surface 6 and the second inner surface 7 and in the plane of the figure. In this non-switched state, the effective refractive index of the liquid crystal is independent of polarization and is given by n ave =0.5(n e +n o ) (Equation 1), where n e is the extraordinary refractive index and n o is the ordinary refractive index. Those skilled in the art will realize that it is desirable for the pitch of the liquid crystal to be no greater than 500 nm (i.e., approximately equal to or less than the wavelength of the incident light) in order for the liquid crystal to behave as a single refractive index material to a reasonable approximation. The applicant has found that if the liquid crystal has a higher pitch (i.e., 600 to 700 nm) without significant optical artifacts, it is feasible for the contact lens to operate satisfactorily. This can be beneficial because using a higher pitch can reduce the switching voltage. The alignment of the director of the liquid crystal adjacent to the alignment polymer 9 is shown in FIG. 10, where the arrows indicate the direction of the director of the liquid crystal adjacent to the alignment polymer. This alignment is used to provide polarization-independent operation of the contact lens. The alignment of the director of the liquid crystal adjacent to the polymer layer formed on the diffractive optical element 4 is substantially the same as the alignment shown in FIG. 10. The liquid crystal 5 and the material from which the diffractive optical element 4 is produced can be selected to achieve a desired optical result. For example, the liquid crystal and the material used to manufacture the diffractive optical element can be selected such that the effective refractive index of the liquid crystal matches the effective refractive index of the material used to manufacture the diffractive optical element, and in this case, the diffractive optical element does not contribute to the lens power. Alternatively, if the effective refractive index of the liquid crystal does not match the effective refractive index of the material used to manufacture the diffractive optical element, then the diffractive optical element contributes to the lens power. When a suitable voltage is applied to the electrodes (8A, 8B) of the liquid crystal cell, the liquid crystal molecules switch to an aligned state schematically shown in FIG. 4, where the director of the liquid crystal in the central portion 30 of the liquid crystal cell 3 is normal to the first inner surface 6 and the second inner surface 7. This realignment of the liquid crystal provides a different effective refractive index. In some cases, when the liquid crystal is in a switched state, the effective refractive index of the liquid crystal can match the refractive index of the material used to manufacture the diffractive optical element 4, in which case the diffractive optical element does not contribute to the lens power. Conversely, when the liquid crystal is in a switched state, the effective refractive index of the liquid crystal may not match the refractive index of the material used to manufacture the diffractive optical element 4, in which case the diffractive optical element contributes to the lens power. An example of an embodiment of a contact lens according to a third aspect of the present invention will now be described with reference to FIGS. 1 and 2. The electro-switchable flexible contact lens is denoted by the reference numeral 1. The lens 1 includes a liquid crystal cell 3 for changing the power of a contact lens. The liquid crystal cell 3 includes a diffractive optical element 4 for correcting a user's vision and a cholesteric liquid crystal 5. The liquid crystal cell 3 can operate between a switched state and an unswitched state. The diffractive optical element is made of MR10 and the diffractive optical element has a refractive index of 1.63. In the unswitched state, the effective refractive index of the liquid crystal matches that of the diffractive optical element, and thus, the diffractive optical element does not contribute to the lens power. The average refractive index of the liquid crystal is calculated according to Equation 1 above. The average refractive index of the liquid crystal effectively matches the refractive index of the diffractive optical element at all visible wavelengths (i.e., from 450 to 700 nm). This matching of the refractive index at all visible wavelengths provides improved optical performance because the amount of diffraction observed from the diffractive optical element is minimized due to the matching of the refractive index across the visible spectrum. Those skilled in the art will appreciate that an exact match of the refractive index across the entire visible spectrum is not required. An example of an embodiment of a contact lens according to a fourth aspect of the present invention will now be described with reference to FIGS. 1 and 2. The lens is generally denoted by the reference numeral 1 and includes a liquid crystal cell 3 for changing the diopter of a contact lens. The liquid crystal cell 3 includes a diffractive optical element 4 for correcting the vision of a user and a cholesteric liquid crystal 5. The lens further has a +1D bulge (not shown) on its upper surface. The liquid crystal cell 3 can operate between a first non-switched state and a second switched state. In the first state, the liquid crystal molecules are oriented as described above with reference to FIG. 3. The average refractive index of the liquid crystal is 1.63. The diffractive optical element 4 is made of Trivex and has a refractive index of 1.51. In the first non-switched state, there is a mismatch between the refractive index of the liquid crystal and that of the diffractive optical element. This causes the diffractive optical element to contribute to the diopter of the lens. The diffractive optical element contributes -1D to the lens, and thus, considering the contribution of the diffractive optical element 4 and the +1D bulge, the total diopter of the lens is 0D. This can be regarded as the state of far vision. In the second switched state, the effective refractive index of the liquid crystal 5 is 1.51, and there is a match between the refractive index of the liquid crystal and that of the diffractive optical element. This causes the diffractive optical element not to contribute to the diopter of the lens. Therefore, the total diopter of the lens is +1D. This is the state of near vision. In the event of a power failure, the contact lens is preset to far vision, i.e., an optical power of 0D. The Applicant has found that when the liquid crystal cell has been switched to the second state, it is feasible to obtain an excellent match between the effective refractive index of the liquid crystal and the material constituting the diffractive optical element for all visible wavelengths of light. FIG. 5 shows that when the liquid crystal cell is switched to the second state, the effective refractive indices (n o ) of the liquid crystal and the material constituting the diffractive optical element are substantially the same for all visible wavelengths of light. In this regard, the solid line shows the effective refractive indices of the liquid crystal and the diffractive optical element. The effective refractive indices of the liquid crystal and the diffractive optical element are effectively the same from 400 nm to 700 nm. This excellent match of the refractive indices causes the diffractive optical element not to contribute to the optical power of the lens. In addition, FIG. 5 shows the intensities of various order diffraction profiles observed from the diffractive optical element. In this regard, the dashed line shows the 0th order diffraction profile, the chain line (long dashed line separated by dots) shows the 1st order diffraction profile and the dotted line shows the 2nd order diffraction profile. Note that the 1st and 2nd order diffraction profiles shown in FIG. 5 are shown multiplied by 100. It can be seen that substantially all of the light incident on the contact lens is transmitted, with very little light being diffracted at very small 1st and 2nd order diffraction profiles at all wavelengths. Figure 6 shows the intensities of various diffraction peaks from the diffractive optical element as a function of wavelength when the liquid crystal cell 3 is in the first unswitched state. In this case, there is a mismatch between the effective refractive index of the liquid crystal and the diffractive optical element, and thus the diffractive optical element contributes to the lens power. In this regard, the dashed line shows the 0th order diffraction profile, the chain line (long dashed line separated by dots) shows the 1st order diffraction profile and the dotted line shows the 2nd order diffraction profile. Figure 6 shows that there is some wavelength dependence, but the optical performance of the lens is still good. Surprisingly, the applicant has found that although there is some wavelength dependence of the optical response of the contact lens, no lateral rainbow effect, which would be observed in glasses, is observed in the contact lens. Thus, the applicant has demonstrated that it is feasible to obtain a lens with good optical properties using a diffractive optical element having a relatively low refractive index (i.e., not greater than 1.57). An example of an embodiment of a contact lens according to a fifth aspect of the present invention will now be described with reference to Figures 1 and 2. The lens is generally denoted by the reference numeral 1 and includes a liquid crystal cell 3 for changing the power of a contact lens. The liquid crystal cell 3 includes a diffractive optical element 4 and a cholesteric liquid crystal 5 for correcting a user's vision. The liquid crystal cell 3 is operable between a first unswitched state and a second switched state, wherein in the first state the difference in refractive index between the liquid crystal and the diffractive optical element is less than in the second state. The refractive index of the diffractive optical element is 1.63. In the first unswitched state, the effective refractive index of the liquid crystal 5 is 1.63, and there is a match between the refractive index of the liquid crystal and the diffractive optical element. This results in the diffractive optical element not contributing to the lens power. Thus, the total power of the lens is 0D. This can be regarded as the state of distant vision. In the second switched state, the effective refractive index of the liquid crystal 5 is 1.51, and there is a mismatch between the refractive index of the liquid crystal and the diffractive optical element. This results in the diffractive optical element contributing to the lens power. Thus, the total power of the lens is +1D. This can be regarded as the state of myopia or near vision. In the event of a power failure, the contact lens is preset to distant vision, i.e., an optical diopter of 0D. The Applicant has found that when the liquid crystal cell is in the first non-switched state, it is possible to obtain a good match between the effective refractive indices of the liquid crystal and the material forming the diffractive optical element for all visible wavelengths of light, with only a small difference between the refractive indices at lower wavelengths (below 500 nm). In this regard, the refractive index of the liquid crystal is shown as a solid line, and the refractive index of the diffractive optical element is shown as a non-bold dashed line adjacent to the solid line. This excellent match of the refractive indices results in the diffractive optical element not significantly contributing to the optical diopter of the lens. Figure 7 shows the intensity of the various order diffraction profiles observed from the diffractive optical element. In this regard, the bold dashed line shows the 0th order diffraction profile, the chain line (long dashed line separated by dots) shows the 1st order diffraction profile and the dotted line shows the 2nd order diffraction profile. The 1st and 2nd order diffraction profiles shown in Figure 7 are shown multiplied by 10. It can be seen that substantially all of the light incident on the contact lens is transmitted, with very little light being diffracted at very small 1st and 2nd order diffraction profiles at all wavelengths. Figure 8 shows the intensity of the various diffraction peaks from the diffractive optical element as a function of wavelength when the liquid crystal cell 3 is in the second switched state. In this case, there is a mismatch between the effective refractive indices of the liquid crystal and the diffractive optical element, and thus, the diffractive optical element contributes to the power of the lens. In this regard, the bold dashed line shows the 0th order diffraction profile, the chain line (long dashed line separated by dots) shows the 1st order diffraction profile and the dotted line shows the 2nd order diffraction profile. Figure 8 shows that there is very little wavelength dependence and the optical performance of the lens remains good. Thus, the Applicant has demonstrated that it is possible to obtain a lens with good optical properties using a diffractive optical element having a relatively high refractive index (i.e., at least 1.58). Surprisingly, the Applicant has found that although there is some wavelength dependence of the optical response of the contact lens, no lateral rainbow effect such as would be observed in glasses is observed in the contact lens. The exemplary contact lens described above provides unexpectedly good peripheral imaging performance. In this regard, one would expect light to be incident on the diffractive optical element from a wide angular range, and this wide variation in the angle of incidence and the curvature of the cornea results in a large variation in the optical path length through the diffractive optical element, thereby resulting in poor optical performance. However, the Applicant has found that the peripheral light that enters the contact lens and travels through the pupil travels through a portion of the diffractive optical element that is approximately normal to the incoming light, i.e., the light is approximately from the same direction and there is less variation in the angle of incidence. This results in good peripheral imaging performance. In addition, as the angle of incidence of light increases from the normal, the amount of light traveling through the diffractive optical element decreases. At a certain angle, substantially no light will travel through the diffractive optical element and no optical ghosting will be seen. For example, for light incident at about 53° and a 5 mm pupil size, approximately half of a detected beam is incident on the diffractive optical element and the other half travels through the non-diffractive region of the contact lens. For higher angles of incidence of light, the effective area of the diffractive optical element exposed to light is even further reduced such that when the beam has completely exited the diffractive region, the optical ghosting becomes negligible. Thus, for the user, there is no sudden start / stop of the diffractive region, only a smoothly varying contribution magnitude. For an eyeglass lens implementation, there appears to be a sharp jump between looking through the diffractive optical element and not looking through it. The Applicant has also found that the peripheral light astigmatism caused by incident light traveling through the diffractive optical element at a non-normal angle is acceptable (on average about 0.3 D). An example of an embodiment of a flexible contact lens according to a seventh aspect of the invention will now be described with reference to FIG. 1. The lens is substantially the lens described above in connection with the fifth aspect of the invention. The lens is generally denoted by reference numeral 1 and it includes a liquid crystal cell 3 for changing the dioptric power of the contact lens and having a cell gap thickness between a first inner surface and a second inner surface. The liquid crystal cell 3 includes a diffractive optical element 4 and a cholesteric liquid crystal 5 for correcting a user's vision. The liquid crystal cell 3 can be operated between a first state and a second state, wherein in the first state, the difference in the effective refractive index of the liquid crystal and the diffractive optical element is greater than in the second state. The liquid crystal cell and the diffractive optical element have an operating characteristic wavelength which ranges from 450 nm to 510 nm. The operating characteristic wavelength is a wavelength at which one or more properties of the lens are optimized. For example, one or more properties can include the transmission of the lens when the liquid crystal cell is in a state in which the effective refractive index of the liquid crystal matches the effective refractive index of the diffractive optical element or in a state in which the effective refractive index of the liquid crystal does not match the effective refractive index of the diffractive optical element. FIG. 8 demonstrates how a property of a contact lens can be optimized at 480 nm. Those skilled in the art will appreciate that other optical properties can be optimized at or near 480 nm. Those skilled in the art will appreciate that optimization can be determined by changing the liquid crystal, for example, in order to change the refractive index of the liquid crystal or the cell spacing between the first and second inner surfaces. The effective refractive index of the liquid crystal depends on the orientation of the molecules of the liquid crystal. For example, in the non-switched state, the effective refractive index of the liquid crystal is the average refractive index calculated by Equation 1 as mentioned above. In the switched state, the effective refractive index of the liquid crystal is n o . Contact lenses in all aspects of the present invention may include more than one liquid crystal cell, as will now be described with reference to FIG. 9. An example of a contact lens according to a first aspect of the present invention is generally designated by reference numeral 101. The lens 101 includes a first liquid crystal cell 3 and a second liquid crystal cell 3' disposed in a lens body 2. The first liquid crystal cell 3 is substantially as described above with respect to the contact lens of the first aspect of the present invention. The second liquid crystal cell 3' is substantially the same as the first liquid crystal cell 3. Those skilled in the art will appreciate that this need not be the case. Those skilled in the art will appreciate that the lenses of the present invention may include diffractive optical elements different from those described above. In this regard, the number of diffractive regions is proportional to the optical power (in diopters) of the diffractive lens and is also proportional to the square of the diameter of the diffractive optical element. Thus, the smaller the diffractive optical element, the fewer diffractive regions are required to produce a given optical power. Table 1 below shows the number of diffractive regions required to produce a given focal power for a given diameter of a diffractive optical element. Table 1 - Number of diffractive regions required to achieve a specific focal power for a given diameter of a diffractive optical element Of course, it should be understood that features described in connection with one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the methods of the present invention may incorporate any features described in connection with the apparatus of the present invention and vice versa. The following clauses will now be used to describe further aspects of the present invention: Clause A1 - An electro-switchable flexible contact lens for conforming to a user's eye, the lens comprising: a first liquid crystal cell for changing the dioptric power of an ophthalmic lens and having a cell gap thickness between a first inner surface and a second inner surface, the first liquid crystal cell including a diffractive optical element for correcting a user's vision, wherein the diffractive optical element is configured to maintain the cell gap thickness by providing support at one or more locations within the first liquid crystal cell. Clause A2 - The lens of Clause A1, which includes a peripheral spacer disposed at the periphery of the first liquid crystal cell for maintaining the cell gap thickness in addition to the diffractive optical element. Clause A3 - The lens of Clause A1 or A2, wherein the first liquid crystal cell includes a cholesteric liquid crystal and the first liquid crystal cell is operable between a first non-switched state and a second switched state, wherein in one of the first and second states, the difference between the effective refractive index of the liquid crystal and the refractive index of the diffractive optical element is greater than in the other of the first and second states. Clause A4 - The lens of Clause A3, wherein in the other of the first and second states, the difference between the effective refractive index of the liquid crystal and the refractive index of the diffractive optical element is approximately zero. Clause A5 - The lens of any one of Clauses A1 to A4, wherein the diffractive optical element includes an inner portion and an outer portion, and the height of the diffractive optical element at the outer portion is greater than the height of the diffractive optical element at the inner portion. Clause A6 - The lens of any one of Clauses A1 to A5, wherein at least a portion of the diffractive optical element is attached to the first inner surface and at least a portion of the diffractive optical element may be attached to the second inner surface. Clause A7 - The lens of any one of Clauses A1 to A6, wherein at both 450 nm and 700 nm, the average refractive index of the liquid crystal is 0.80 to 1.20 times the refractive index of the diffractive optical element, the average refractive index being n ave = 0.5(n e + n o ), where n e is the extraordinary refractive index and n ois the ordinary refractive index. Clause A8 - A lens as in Clause A7, wherein at both 450 nm and 700 nm, the average refractive index of the liquid crystal is 0.95 to 1.05 times the refractive index of the diffractive optical element. Clause B1 - An electro-switchable flexible contact lens for conforming to a user's eye, the lens comprising: a first liquid crystal cell for changing the diopter of the contact lens and having a cell gap thickness between a first inner surface and a second inner surface; the first liquid crystal cell includes a diffractive optical element for correcting a user's vision, the diffractive optical element including an inner portion and an outer portion, the height of the diffractive optical element at the outer portion being 1% to 20% greater than the height of the diffractive optical element at the inner portion. Clause B2 - A lens as in Clause B1, wherein the height of the diffractive optical element at the outer portion is at least 3% and at most 15% greater than the height of the diffractive optical element at the inner portion. Clause B3 - A lens as in Clause B1 or B2, wherein the diffractive optical element includes a plurality of peaks and valleys, and the outer portion includes an outer peak, optionally the outermost peak. Clause B4 - A lens as in any one of Clauses B1 to B3, wherein the diffractive optical element extends within a chord length r, the inner portion includes the portion of the diffractive optical element within a chord length of r / 8 from the center of the diffractive optical element, and the outer portion includes the portion of the diffractive optical element having a chord length from 3r / 8 to r / 2. Clause B5 - A lens as in any one of Clauses B1 to B4, wherein the liquid crystal includes a cholesteric liquid crystal, and optionally, in an unswitched state, the director of the liquid crystal can form an angle of no more than 5 degrees with the first and second inner surfaces, and in a switched state, the director of the liquid crystal can form an angle of at least 60 degrees, optionally at least 70 degrees, optionally at least 80 degrees and optionally at least 85 degrees with the first and second inner surfaces. Clause C1 - An electro-switchable flexible contact lens for conforming to a user's eye, the lens comprising: a first liquid crystal cell for changing the diopter of the contact lens and having a cell gap thickness between a first inner surface and a second inner surface, the first liquid crystal cell includes a diffractive optical element for correcting a user's vision and a cholesteric liquid crystal, the first liquid crystal cell is operable between a first unswitched state and a second switched state, wherein in the first state, the difference between the effective refractive index of the liquid crystal and the refractive index of the diffractive optical element is greater than or less than that in the second state, wherein at both 450 nm and 700 nm, the average refractive index of the liquid crystal is 0.80 to 1.20 times the refractive index of the diffractive optical element, the average refractive index of the liquid crystal is n ave , thus calculated as n ave = 0.5(n e +n o ): where n e is the extraordinary refractive index and n ois the ordinary refractive index. Clause C2 - A lens as in Clause C1, wherein at both 450 nm and 700 nm, the refractive index of the liquid crystal is from 0.90 to 1.10 times the refractive index of the diffractive optical element. Clause C3 - A lens as in Clause C1 or C2, wherein at both 450 nm and 700 nm, the refractive index of the liquid crystal is from 0.95 to 1.05 times the refractive index of the diffractive optical element. Clause C4 - A lens as in any one of Clauses C1 to C3, wherein at 500 nm, the refractive index of the liquid crystal is from 0.95 to 1.05 times the refractive index of the diffractive optical element. Clause D1 - An electro-switchable flexible contact lens for conforming to a user's eye, the lens comprising: a first liquid crystal unit for changing a diopter of the contact lens and having a unit gap thickness between a first inner surface and a second inner surface, the first liquid crystal unit comprising a diffractive optical element for correcting a user's vision and a cholesteric liquid crystal, the first liquid crystal unit being operable between a first non-switched state and a second switched state, wherein in the first state, the difference between the effective refractive index of the liquid crystal and the refractive index of the diffractive optical element is greater than or less than that in the second state, wherein the refractive index of the diffractive optical element is not greater than 1.57. Clause E1 - An electro-switchable flexible contact lens for conforming to a user's eye, the lens comprising: a first liquid crystal unit for changing a diopter of the contact lens and having a unit gap thickness between a first inner surface and a second inner surface, the first liquid crystal unit comprising a diffractive optical element for correcting a user's vision and a cholesteric liquid crystal, the first liquid crystal unit being operable between a first non-switched state and a second switched state, wherein in the first state, the difference between the effective refractive index of the liquid crystal and the refractive index of the diffractive optical element is greater than or less than that in the second state, wherein the refractive index of the diffractive optical element is at least 1.58. Clause F1 - An electro-switchable flexible contact lens for conforming to a user's eye, the lens comprising: a first liquid crystal unit for changing a diopter of an ophthalmic lens and having a unit gap thickness between a first inner surface and a second inner surface, the first liquid crystal unit comprising a diffractive optical element for correcting a user's vision, the diffractive optical element comprising a plurality of peaks and valleys, the peaks extending in a direction from the first inner surface to the second inner surface, at least a part of at least one peak being attached to the second inner surface. Clause F2 - A lens as in Clause F1, wherein the diffractive optical element comprises a central peak and a plurality of outer peaks, at least a part of at least one of the outer peaks being attached to the second inner surface. Clause F3 - A lens as in Clause F2, wherein at least one of the outer peaks, optionally more than one and optionally each, is annular.Clause F4 - A lens as in any one of Clauses F1 to F3, at least most of at least one peak and optionally substantially the entire periphery is attached to the second inner surface. Clause G1 - An electro-switchable flexible contact lens for conforming to a user's eye, the lens comprising: a first liquid crystal cell for changing at least one optical property of the contact lens and having a cell gap thickness between a first inner surface and a second inner surface, the first liquid crystal cell comprising a diffractive optical element for correcting a user's vision, the diffractive optical element having an operating characteristic wavelength which ranges from 450 nm to 510 nm; the first liquid crystal cell comprising a cholesteric liquid crystal, the first liquid crystal cell being operable between a first non-switched state and a second switched state, wherein in the first state, the difference between the effective refractive index of the liquid crystal and the refractive index of the diffractive optical element is greater than or less than that in the second state. Clause G2 - A lens as in Clause G1, wherein the operating characteristic wavelength ranges from 460 to 490 nm, optionally from 470 to 490 nm and optionally 480 nm. Although the present invention has been described and illustrated with reference to specific embodiments, those of ordinary skill in the art will understand that the present invention is itself suitable for many different variations not specifically illustrated herein. By way of example only, some possible variations will now be described. The use of a diffractive optical element is not limited to a liquid crystal cell comprising a cholesteric liquid crystal. For example, other types of liquid crystals including un-doped nematic and smectic liquid crystals can be used. In cases where integers or elements are mentioned in the foregoing description which have known, obvious or foreseeable equivalents, then such equivalents are incorporated herein as if individually stated. Reference should be made to the claims for determining the true scope of the present invention, which should be construed to cover any such equivalents. The reader is also aware that the integers or features of the present invention described as being preferred, advantageous, convenient or the like are optional and do not limit the scope of the appended claims. Furthermore, it should be understood that although such optional integers or features may have possible benefits in some embodiments of the present invention, they may be undesirable in other embodiments and may therefore be absent. 1: Contact lens 2: Flexible lens body 3: Liquid crystal cell / Liquid crystal lens / First liquid crystal cell 3’: Second liquid crystal cell 4: Diffractive optical element 5: Liquid crystal / Cholesteric liquid crystal 6: First inner surface 7: Second inner surface 8A: Electrode 8B: Electrode 9: Polymer 10A: Central peak 10B: Annular peak 10C: Annular peak 10D: Annular peak 10E: Annular peak 10F: Annular peak 10G: Annular peak 10H: Annular peak 10I: Annular peak 11: Spacer 21: Inner part 22: Outer part 30: Central part 101: Contact lens h1: Maximum height h2: Maximum height Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings, in which: Figure 1 shows a schematic side cross-sectional view of an example of a contact lens according to a first aspect of the present invention, the contact lens including a liquid crystal cell; Figure 2 shows a schematic side view of the liquid crystal cell used in Figure 1; Figure 3 shows a schematic representation of the local alignment of liquid crystal molecules in a liquid crystal cell in an unswitched state; Figure 4 shows a schematic representation of the local alignment of liquid crystal molecules in a liquid crystal cell in a switched state; Figure 5 shows the intensity of the diffraction profile generated by a lens including a liquid crystal cell, where the diffractive optical element has a low refractive index and the liquid crystal cell is in a switched state; Figure 6 shows the intensity of the diffraction profile generated by a lens including a liquid crystal cell, where the diffractive optical element has a low refractive index and the liquid crystal cell is in an unswitched state; Figure 7 shows the intensity of the diffraction profile generated by a lens including a liquid crystal cell, where the diffractive optical element has a high refractive index and the liquid crystal cell is in an unswitched state; Figure 8 shows the intensity of the diffraction profile generated by a lens including a liquid crystal cell, where the diffractive optical element has a high refractive index and the liquid crystal cell is in a switched state; and Figure 9 shows a schematic cross-sectional view of an example of a contact lens according to various aspects of the present invention, the contact lens including two liquid crystal cells; and Figure 10 shows a schematic plan view of the alignment of the director vectors of the liquid crystals adjacent to an alignment polymer. 1: Contact lens 2: Flexible lens body 3: Liquid crystal cell / liquid crystal lens / first liquid crystal cell 4: Diffractive optical element 5: Liquid crystal / cholesteric liquid crystal 6: First inner surface 7: Second inner surface

Claims

1. An electrically switchable flexible contact lens for fitting into the eye of one of the users, the lens comprising: A first liquid crystal unit for changing the focal power of the contact lens and having a unit gap thickness between a first inner surface and a second inner surface. The first liquid crystal unit includes a diffractive optical element for correcting a user's vision and a cholesterol liquid crystal. The first liquid crystal unit can operate between a first unswitched state and a second switched state. In the first unswitched state, the difference between the effective refractive index of the liquid crystal and the refractive index of the diffractive optical element is greater than or less than in the second switched state. At both 450 nm and 700 nm, the average refractive index of the liquid crystal is 0.80 to 1.20 times that of the refractive index of the diffractive optical element. The average refractive index of the liquid crystal is nave, and is therefore calculated as nave = 0.5(ne + no), where ne is the unusual refractive index and no is the ordinary refractive index.

2. The electrically switchable flexible contact lens of claim 1, wherein at both 450 nm and 700 nm, the refractive index of the liquid crystal is 0.90 to 1.10 times that of the refractive index of the diffractive optical element.

3. The electrically switchable flexible contact lens of claim 1, wherein at both 450 nm and 700 nm, the refractive index of the liquid crystal is 0.95 to 1.05 times that of the refractive index of the diffractive optical element.

4. The electrically switchable flexible contact lens as claimed in claim 1, wherein at 500 nm, the refractive index of the liquid crystal is 0.95 to 1.05 times that of the refractive index of the diffractive optical element.

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