Multi-focal type optical lenses having a plurality of focal points are known from the past, as one type of ophthalmic lenses used for a human eye optical system. For example, with contact lenses used as corrective optical elements for refractive error, alternative optical elements after lens extraction or the like with the optical system of the human eye, or with intraocular lenses used for insertion in the human eye, by applying multi-focal lenses, it is possible to compensate for the decrease or loss of accommodation function of eye in the human body.
Particularly in recent years, there is an increase in people continuing to use contact lenses even when they reach the age of having presbyopia. People with presbyopia have a decrease in focus accommodation function, so a symptom appears of having difficulty focusing on nearby items. Thus, multi-focal contact lenses which can also focus on nearby objects become necessary for presbyopia patients. Also, for patients who have undergone cataract surgery, the lens which is in charge of the adjustment function is removed, so even if an intraocular lens is inserted as a replacement, the symptom of difficulty seeing close up remains. A multi-focal function that offers a plurality of focal points is necessary for that intraocular lens as well. Thus, there is a great increase in the need for multi-focal lenses reflecting the aging society of recent years.
However, as a method for realizing this multi-focal lens, examples are known of a refraction type multi-focal lens for which a plurality of focal points are formed based on the principle of refraction, and of a diffractive type multi-focal lens for which a plurality of focal points are formed based on the principle of diffraction. With the latter diffractive type multi-focal lens (diffractive multi-focal lens), equipped are a plurality of diffractive structures formed in concentric circle formed on the optical part of the lens, and a plurality of focal points are given by the mutual interference effect of light waves that passed through the plurality of diffractive structures (zones). Thus, compared to the refraction type lens with which a focal point is given by the refraction effect of light waves at a refracting surface comprising boundary surfaces with different refractive indexes, with the diffractive type multi-focal lens, there are advantages such as being able to set a high lens power while inhibiting an increase in lens thickness.
Typically, the diffractive multi-focal lens has a diffractive structure by which the diffractive zone pitch gradually becomes smaller as it goes from the lens center toward the periphery according to a rule called the Fresnel zone, and this has multiple focal points by using different orders of diffracted light generated from that structure. In particular, when using a diffractive multi-focal lens as a contact lens or an intraocular lens, normally, 0th order diffracted light is the focal point for far vision, and +1 order diffracted light is the focal point for near vision. By distribution of this diffracted light, it is possible to make a bifocal lens having focal points for far and near vision. The general Fresnel zone constitution is basically the zone pitches having the zone radius determined by Equation 1 below. This Equation 1 is hereafter called a Fresnel zone setting equation. Besides, the zone radius and the zone diameter refer to the radius of the zone outer diameter.
                              r          n                =                              nK            P                                              [                  Equation          ⁢                                          ⁢          1                ]            
rn is the outer diameter radius of the nth zone obtained from Equation 1. K is a constant. P is addition power for setting the focus point of first order diffracted light with the focus point of 0th order diffracted light as a reference, and by varying this, it is possible to change the focal point position of the first order diffracted light.
For example when the focal point by 0th order diffracted light is a focal point for far vision, and first order diffracted light is set as the focal point for near vision, when P (the addition power noted above) is made larger, the focal point position for near vision moves closer to the lens. Specifically, when using that lens for the human eye, objects that are closer become visible. Conversely, when P is made smaller, the focal position for near vision recedes away from the lens. In this case, when the lens is used in the human eye, the near points that are visible recede away.
For patients with advanced presbyopia, or patients who have an intraocular lens inserted, power of accommodation of the crystalline lens decreases or is lost, so it is preferable to use a lens for which the focal point is matched in the nearer direction as with the former example. In other words, an item is needed for which the addition power is set to be large. On the other hand, for patients for which the power of accommodation has not decreased that much, even if the near focal point position is not made that near, it is possible to see near objects by joint use with one's own residual power of accommodation, so there are cases when large addition power does not need to be set. Taking into consideration the status of the eyes of these patients, it is possible to obtain bifocal lenses that can be suitably used at different required powers for each patient by setting P.
Furthermore, in recent years, with the goal of improving visual performance in the intermediate region between two focal points of a focal point for far vision and a focal point for near vision, diffractive multi-focal lenses with a focal point set for intermediate vision have been proposed. This diffractive multi-focal lens with three or more focal points set has a plurality of types of reliefs for which their respective diffractive primary lights give mutually different focal point distances formed overlapping, having a synchronous structure for which the grating pitches for the reliefs overlap with each other periodically. As specific examples, for example, disclosed previously by this patent applicant, there are Japanese Unexamined Patent Publication No. JP-A-2010-158315 (Patent Document 1) and PCT Japanese Translation Patent Publication No. JP-A-2013-517822 showing the subordinate concepts thereof (Patent Document 2).
However, with the diffractive multi-focal lens with three or more focal points set using the background art constitution, there was the problem that it was difficult to sufficiently ensure the degree of freedom for tuning the optical characteristics respectively requested for the plurality of focal points. In particular, with the inventions noted in Patent Documents 1 and 2, when setting the plurality of focal points, effective tuning technology was not yet established by which while adjusting the light intensity of each focal point, there is suppression of noise form peaks due to multi-order light or the like that is generated secondarily on the optical axis other than the target focal points.
Also, with the invention noted in Patent Document 1, when setting the intermediate focal point between the far focal point and the near focal point, there was the problem that it is difficult to set the intermediate focal point to an optional target position on the optical axis.