1. A zonal diffractive lens with phase reversal and with an alternation of optically active or \u201cfull\u201d annular zones and of optically inactive or \u201cempty\u201d annular zones, all of these annular zones being concentric or coaxial, wherein the \u201cempty\u201d annular zones are occupied by an optically inactive \u201ccement\u201d that interconnects the \u201cfull\u201d annular zones in order to ensure stability of these \u201cfull\u201d annular zones.
2. The diffractive lens as claimed in claim 1, designed as an intracorneal lens, wherein the \u201ccement\u201d of the inactive or \u201cempty\u201d annular zones has a permeability to nutrients and to oxygen that is comparable to a permeability of corneal tissue and has an optical index close to that of a cornea.
3. The intracorneal diffractive lens as claimed in claim 2, wherein the \u201cfull\u201d annular zones have a different optical index compared to the empty annular zones, such that the optical index of the full annular zones is greater than that of the empty annular zones.
4. The intracorneal diffractive lens as claimed in claim 2, wherein the \u201cfull\u201d annular zones have a different optical index compared to the empty annular zones, such that the optical index of the full annular zones is less than that of the empty annular zones.
5. The intracorneal diffractive lens as claimed in claim 3, wherein the \u201cempty\u201d annular zones are filled by a hydrogel which has a high water content and which is permeable but optically inactive and constitutes the \u201ccement\u201d interconnecting the \u201cfull\u201d annular zones.
6. The intracorneal diffractive lens as claimed in claim 5, wherein the hydrogel that serves as a \u201ccement\u201d connecting the \u201cfull\u201d annular zones is a hydrogel having a percentage of water equal to or greater than 78%.
7. The intracorneal diffractive lens as claimed in claim 6, wherein the \u201cfull\u201d annular zones are also made of a hydrogel whose percentage of water is less than 78%.
8. The intracorneal diffractive lens as claimed in claim 6, wherein the \u201cfull\u201d annular zones are also made of a hydrogel whose percentage of water is greater than 78%.
9. The intracorneal diffractive lens as claimed in claim 2, wherein the \u201cfull\u201d annular zones are not only connected by the parts made of permeable hydrogel but are also coated by parts made of the same hydrogel.
10. The intracorneal diffractive lens as claimed in claim 1, wherein it comprises, at a center, a profiled disk made of the same active material as the \u201cfull\u201d annular zones and surrounded concentrically or coaxially by these \u201cfull\u201d annular zones, the central disk constituting an optically active zone comprising a first ring with an inner radius of zero.
11. The intracorneal diffractive lens as claimed in claim 1, wherein it comprises, at a center, an \u201cempty\u201d and therefore optically inactive circular zone, which is surrounded concentrically by the first \u201cfull\u201d annular zone.
12. The intracorneal diffractive lens as claimed in claim 10, wherein the \u201cfull\u201d annular zones, and if appropriate the central disk, are connected by a fine membrane made of the same optically active material, said membrane remaining permeable to nutrients because of its very slight thickness.
13. The intracorneal diffractive lens as claimed in claim 10, wherein the \u201cfull\u201d annular zones, and if appropriate the central disk, are connected by material bridges whose general orientation is radial and which are made of the same optically active material, said material bridges extending across the \u201cempty\u201d annular zones.
14. The intracorneal diffractive lens as claimed in claim 1, wherein it is produced as a bifocal lens designed to correct presbyopia.
The claims below are in addition to those above.
All refrences to claim(s) which appear below refer to the numbering after this setence.
1. In a BAYER circuit including a preliminary agglomeration phase, a crystal growth phase and a classification phase, a process for controlling precipitation of alumina hydrate from a slurry resulting from introduction of recycled alumina trihydrate seed into an aluminate liquor, in which particle size quality of alumina hydrate produced in the circuit and circulating in feed tanks is monitored, comprising the steps of:
a) a calibration step including:
a1) measuring, versus time, of:
percent of alumina hydrate particles circulating in the feed tanks in the circuit that are finer than X2 \u03bcm; and
percent of alumina hydrate particles circulating in the feed tanks in the circuit that are finer than X1 \u03bcm;
where X1 and X2 are predetermined particle sizes in microns and X1 is smaller than X2; and
a2) determining a relationship R between percent finer than X1 and later changes in percent finer than X2 \u03bcm, and defining upper and lower trigger thresholds of percent finer than X1 \u03bcm which correspond to maximum permissible variations in percent finer than X2 \u03bcm; and
b) controlling the circuit, comprising measuring percent finer than X2 \u03bcm and forming a correlation between percent finer than X2 \u03bcm and the particle size of hydrate produced by the circuit, measuring percent finer than X1 \u03bcm and updating the relationship R, and causing corrective action to the slurry at the beginning of precipitation when the measured value of percent finer than X1 \u03bcm reaches an updated trigger threshold, to bring the percent finer than X2 \u03bcm within the maximum permissible variation.
2. Process according to claim 1, wherein said corrective action includes modifying amount of solid in the slurry at the beginning of the precipitation.
3. Process according to claim 2, wherein the modifying comprises varying amounts of pregnant aluminate liquor fed to a first agglomeration tank and a first feed tank.
4. Process according to claim 1, wherein X2 is greater than 40 \u03bcm and X1 is less than 20 \u03bcm.
5. Process according to claim 1, wherein the measurements of percent finer than X1 \u03bcm and percent finer than X2 \u03bcm are made on a slurry at the end of crystal growth phase.
6. Process according to claim 1, wherein pregnant aluminate liquor feeding a first agglomeration tank in the circuit has a caustic content less than or equal to 160 g of Na2Oliter.
7. Process according to claim 1, wherein said calibration step comprises:
1) daily measuring percent finer than X1 \u03bcm in the slurry at a predetermined point in the circuit, which is used to produce a first particle size vs. time diagram represented by a curve Y=%<X1(t);
2) daily measuring percent finer than X2 \u03bcm in the slurry at a predetermined point in the circuit, which is used to produce a second particle size vs. time diagram represented by a curve Y=%<X2(t) and in which X2>X1 is a value already known to be well correlated with the particle size of the hydrate produced;
3) creating of an empirical relation between the particle size vs. time diagrams, which characterizes the relation R as:
R(%<X2(t), %<X1(t\u2212\u03c4))=0
where t is the time at which percent finer than X2 \u03bcm is measured and \u03c4 is a characteristic time interval estimated by observing an occurrence of a same accidental phenomenon on each curve %<X2(t) and %<X1(t\u2212\u03c4); and
4) defining a maximum threshold and minimum threshold of percent finer than X1 \u03bcm obtained from the relationship R and a maximum interval of the permissible variation of values of percent finer than X2 \u03bcm.
8. Process according to claim 7, wherein said controlling comprises:
1) daily measuring percent finer than X1 \u03bcm in the slurry at a predetermined point in the circuit, in order to complete the first particle size time diagram represented by the curve Y=%<X1(t);
2) daily measuring percent finer than X2 \u03bcm in the slurry at a predetermined point in the circuit, in order to complete the first particle size time diagram represented by the curve Y=%<X2(t);
3) updating a relationship between curve Y=%<X1(t) and curve Y=%<X2(t) and the definition of trigger thresholds of percent finer than X1 \u03bcm; and
4) triggering of a corrective action to modify amount of solid in the slurry at the beginning of the precipitation when the measured value of percent finer than X1 \u03bcm reaches one of the thresholds defined in 3).