The present invention relates to thermodiffusion dyeing methods and op- tical elements prepared by applying such methods. More particularly, the present invention relates to an improved method of thermodiffusion dyeing for providing a transparent substrate with a predetermined spectral transmission, i. e. pre- scribed eg. by an ophthalmologist.

In the field of science, technology, industry, biology, medical sciences etc. one of the basic tasks is to provide transmission spectra with given spectral dis- tribution of arbitrary light-sources, eg. of a common electric bulb, of a laserdiode, or of the Sun, at the site of an observer by means of an optical system compris- ing one or more optical elements arranged between the source and the ob- server. Depending on the achievable object certain transmission spectra can easily be obtained by an optical element prepared using traditional and well- known industrial methods, i. e. by the process of lamination, simple colouring, coating with a gel-like material, or adding compounds having the required light absorption characteristics to the base materials. These transmission spectra are often limited to narrow ranges of wavelength, do not show abrupt running-ups or only slightly vary as a function of the wavelength. It is of extreme difficulty, how- ever, to realize more complex transmission spectra, and this can be achieved relatively simply only in some special cases.

The phenomenon of thermodiffusion is known per se. It is based on the fact that in presence of heating, molecules of small size diffuse and incorporate into a material comprising molecules of greater size, i. e. into a substrate. Ther- modiffusion can be used eg. for colouring of substrates made of plastics having macromolecules. In this special case tiny molecules of a dyestuff incorporate among macromolecules of the plastic substrate. The thus obtained coloured or

tinted plastic substrate shows different light transmission characteristics com- pared to the original untinted substrate, i. e. a significant difference in light- absorptions of tinted and untinted substrates exists due to the fact, that the tinted substrate absorbs incident light to a different extent and/or at a various wavelength or in a different range of wavelengths. It is also well-known that de- gree of thermodiffusion is determined by the concentration of interfering dyestuff, the value of temperature and/or pressure applied during the dyeing process and (via diffusion rates and incorporation of dye molecules) the time taken by the substrate within the dye solution.

HU Patent No. 206,780 describing a dyeing method of an optical means capable of filtering out radiation of a laser light-source damaging to human eye contains information on how the degree of filtering (i. e. the transmittivity) varies upon change in concentration of dye solutions which show selective light ab- sorption characteristics at well-defined wavelengths, i. e. using laser light of a fixed wavelength. Besides this, the specification at issue also exemplifies that simultaneous usage of two dyestuffs leads to selective light absorption appear- ing at more than one wavelengths at a time.

HU Patent No. 214,922 introduces a method for producing a layer of se- lective light-absorbance at predetermined wavelength wherein a dyestuff is dif- fused, however not by thermodiffusion, into a layer of gelatine applied onto a transparent substrate to ensure the selective light absorption characteristics thereof. Degree of light-absorbance is set by the length of the time during which the gelatine layer is actually within the dye solution.

And finally, HU Patent No. 205,997 teaches such a thermodiffusion method, wherein a blank of plastic spectacle lens is placed for about 0.4 minutes to about 5 minutes into a heated dye solution to produce spectacle lens showing a modified light-absorption throughout the transmission spectrum.

In the light of the above-mentioned things, optical elements aimed to be reached and having predetermined light absorption characteristics at discrete wavelengths are prepared in such a way by the known colouring methods, that a blank lens made of eg. a plastics of known composition is placed into a base solution made of dyestuffs being present in fixed amounts, compositions and

concentrations and other additives, and then the lens is left in this solution until the light-absorbance of the lens takes the value required. A common disadvan- tage of the above dyeing methods on the one hand is that in order to avoid ex- cess colouring and the undesirable light absorption appearing as a result thereof, a continuous monitoring of dyeing process is required. On the other hand, after the required duration of colouring has been over, the dyeing process should immediately be interrupted, as dyeing takes place in an irreversible man- ner. This means, that by nowadays'dyeing methods there is no way to adjust the degree of colouring if the dyeing process took place too long. As a consequence, any kind of mistake or unobservance results in spoilage, i. e. an optical element which cannot be used for its purpose planned.

Due to irreversibility of colouring, dyeing processes of the art can be used mostly in such cases, where values of transmittivity to be reached are lower than a certain threshold. This holds eg. for eye-guards against lasers, wherein the values of transmittivity aimed of the eye-guard at one or more predetermined characteristic wavelengths are lower than a value chosen to be reasonably small. The known processes are not suitable for producing optical elements characterized by complex transmission spectra. Furthermore, as the production of such optical elements follows a definite recipe, the dyeing used to prepare the optical elements allows no modifications in case there would be a need for al- tering the transmission spectrum obtained. This means, that there is no possibil- ity for the effects of the dyestuff applied to be enhanced or reduced. Further- more, it is also impossible to modify parts of the transmission spectrum having not been affected by the dyestuff applied earlier by means of subsequent appli- cations of other dyestuffs.

Therefore the object of the present invention is to improve dyeing proc- esses based on the phenomenon of thermodiffusion. More specifically, our ob- ject is to develop a new technology which allows feasible production of optical elements having arbitrary and complex transmission spectra, meanwhile the spoilage produced is kept at the lowest possible level.

The experiments carried out proved, that to achieve an optical element having the required transmission spectrum (which is called the target spectrum

hereafter) is impossible or possible but only with an inadmissibly high percent- age of defect if one follows the known techniques, i. e. when such dye composi- tions are applied, which are obtained as mixture of dye solutions with bearing the specific light transmitting behaviour of the dye solutions in mind and which aim to reach a"resulting"transmission spectrum in good agreement with the target spectrum. The deviation from the target spectrum is especially great for optical elements with dioptry and for optical lenses, wherein the course of diffusion and the dyestuff-content and the dyestuff-distribution of the surface layer after col- ouring are pretty much affected by the shape and the structure of the surface and the continuously changing material thickness.

After a careful study we have come to the conclusion, that there is a need for the application of at least one, but rather more than one dyestuffs of different types subsequently and, if required, more than once to reach the above- identified object of the present invention.

It was also realized that the object of the present invention cannot be reached by dyeing processes carried out under predetermined conditions, i. e. a recipe of colouring that defines the concentration of a certain dyestuff to be used or the duration of the colouring, etc. applicable generally to all cases cannot be given. During the dyeing process, various dyestuffs applied subsequently alter the initial specific thermodiffusion charateristics of the dyestuffs separately con- sidered, and there is no possibility for the correction of these characteristics.

According to our studies an eligible colouring of the optical elements can be achieved by applying various dyestuffs in subsequent steps, one at a time, and, if required, in a repeated manner. Furthermore, after each dyeing step the current transmission spectrum is recorded and compared to the target spectrum.

In this way the target spectrum is approached through a sequence of iterative dyeing steps carried out until an acceptable error limit is reached. In this proc- ess, besides the increase of the concentration of the dyestuff diffused into the substrate, dissolution of the dyestuff from the substrate is also considered and applied, if necessary.

Furthermore, it was also realized that from the point of view of dyeing the order of the application of different dyestuffs is not indifferent.

The above objects are fulfilled with a thermodiffusion dyeing method comprising of the following steps: (a) based on separate ground dyeings of identical substrates with dyestuffs ca- pable of thermodiffusion dyeing such a dyestuff is chosen which results in run- ning-down loci and running-up loci of the transmission spectrum of the substrate coloured with the dyestuff chosen in the ground dyeing being within a predeter- mined distance relative to the corresponding running-down loci and running-up loci of the target spectrum, and further has dyeing parameters resulting in a most effective change; (b) the substrate is placed into a heated dye solution made of a solvent contain- ing no dyestuff and the dyestuff chosen in step (a) and the substrate is coloured ; (c) transmission spectrum of the substrate coloured is recorded, and then a cur- rent deviation thereof relative to the target spectrum is determined; (d) depending on the current deviation thus obtained, the substrate coloured is either placed back into the heated dye solution for increasing its dyestuff content or placed into the heated solvent containing no dyestuff for decreasing its dye- stuff content; and (e) by a continuous and iterative repetition of steps (c) to (d) the transmission spectrum of the substrate recorded in each step is gradually approached to the target spectrum until the current deviation becomes smaller than a predeter- mined threshold.

Alternatively, separate dye solutions are prepared from more than one dyestuffs chosen, and dyeing of the substrate is performed with the dye solu- tions, one at a time, by starting the dyeing method with the dyestuff having the least diffusion rate.

The dyeing parameter resulting in the most effective change preferably is the concentration of the dye solution and/or the duration of the thermodiffusion dyeing. Preferably the distance between running-down loci of the target spec- trum and corresponding running-down loci of the transmission spectrum re- corded after ground dyeing of the substrate is at most 20 nm, and the same holds also for the running-up loci.

The thermodiffusion dyeing process of the present invention is performed using dye solution (s) having concentration (s) between 0.1 weight% and 1.5 weight%, preferably between 0.2 weight% and 1.0 weight%, in the temperature range of 90°C to 100°C. Furthermore, the threshold is chosen to fall between 1% and 5%, preferably between 1% and 2% of the transmission value of the target spectrum at any wavelength.

The solvent containing no dyestuff is preferably a polar or an apolar sol- vent, while the substrate is a spectacle lens made of diallyl-glycole-bis-allyl- carbonate.

In the present specification more than one dyestuffs of different types and hence of different colours and transmission functions of substrates coloured by these dyestuffs are disclosed. The base material of the transparent substrate is diallyl-glycole-bis-allyl-carbonate (slightly doped with an ultra-violet absorber material) vastly used in industry and referred to as CR-39 in literature. A detailed description of a spectacle lens made of CR-39 can be found eg. in the work of Harry Reach Jr. entitled to Allyl Resins and Monomers (1965, Reinhold Publish- ing Corp., New York, USA). The base material of the substrate, nevertheless, can be of course any kind of material, preferably a plastic material, resulting in a product, i. e. an optical element suitable for the applications aimed.

The dyeing process of the present invention and the optical elements col- oured thereby will be explained in detail through the following examples and with reference to the accompanying drawings, wherein Figure 1 shows a transmission spectrum prescribed by an eye-specialist (i. e. the target spectrum) that should be reached by the thermodiffusion dye- ing method of the present invention; Figure 2 illustrates current specific transmission spectra [curves (a), (b), (d) and (e)] taken after thermodiffusion dyeings of predetermined dura- tions of a substrate carried out iteratively using two different dyestuffs and a transmission spectrum [curve (c)] of the substrate which ap- proximates the target spectrum plotted in Fig. 1;

Figure 3 shows a further target spectrum measured and prescribed by the eye- specialist against the wavelength ; Figure 4 illustrates the specific transmission spectra [curves (a) to (d)] recorded after various durations of dyeings in the thermodiffusion dyeing proc- ess of the present invention and a transmission spectrum [curve (e)] approximating the target spectrum of Fig. 3 in case of using two dif- ferent dyestuffs in a repeated manner; Figure 5 shows a couple of current transmission spectra that were recorded after different durations of dyeings of a substrate coloured with only a single dyestuff, wherein the transmission spectrum approaching the target spectrum within tolerance is plotted by a broken line ; and finally Figure 6 illustrates a couple of transmission spectra of a further substrate col- oured by the iterative thermodiffusion dyeing method of the present invention using two different dyestuffs again, wherein the final trans- mission spectrum having recorded after the completion of the dyeing process and thus approximating the target spectrum defined by an eye-specialist within tolerance is plotted by a broken line.

Before starting with dyeing process of the present invention a suitable substrate is chosen, which can be coloured by a thermodiffusion dyeing method and is transparent within the range (s) of wavelengths required. In view of our ex- perimental results and of the publications presented up to this time, a plastic material of diallyl-glycole-bis-allyl-carbonate, which is denoted by CR-39 in lit- erature, possesses and unambiguously shows these features. Therefore, sub- strates preferably made of this material are used with the dyeing method of the invention. The substrate used, however, might be made of any kind of materials besides or instead of CR-39 that can be coloured in a thermodiffusive manner.

As a next step, several dyestuffs are selected that are thought of resulting in such specific transmission spectra which have shapes approximating a pre- determined target spectrum, if used for colouring. (Here and from now on the term"specific transmission spectrum"refers to a transmission spectrum obtained as a result of a dyeing process carried out applying only one dyestuff.) The dye- stuffs selected have to meet certain requirements, namely they should be suit-

able for a thermodiffusion dyeing process, i. e. as a consequence of heating to a temperature of about 100°C a decomposition thereof cannot take place, and further, their diffusion coefficients should have fairly reasonable values. After this step, separate solutions, i. e. dye solutions are prepared from the dyestuffs se- lected earlier and then ground dyeing of plurality of identical substrates, which are prepared from the same material and have identical surface structures, is performed with each of the dye solutions separately, bearing the approximate absorption and transmission characteristics of the dyestuff used in mind. Having the substrates thus coloured, dyestuffs mostly adaptable to our purposes and desirable values of further parameters during the dyeing process, such as the possible methods for dissolution of the dyestuff, the type of solvent, the value of the temperature applied, the diffusion rate of dye molecules into the substrate, etc. are determined based on predetermined criteria. As a next step, the specific transmission spectra of substrates coloured by the different dyestuffs are re- corded and evaluated. In order that further steps of the process could be done relatively fast and perfectly with no errors, in the present phase of the dyeing process according to the invention, which is actually a preparation phase, the thermodiffusion dyeing of substrates with the dyestuffs selected earlier should be done systematically and in an extremely precise manner.

Evaluation of the measurements means a comparison of a transmission spectra recorded to the target spectrum. The comparison can be either a simple visual comparison or a computer assisted one. Those dyestuffs that result in transmission spectra differing substantially in shape from the target spectrum, are not used in subsequent steps of the method at all. Here the term"differing substantially"refers to such spectra that show absorption just at those points (i. e. at wavelengths) where the coloured substrate should be highly transparent to light and/or locations of running-up edges and/or running-down edges of the transmission spectra are far away from locations of the corresponding edges of the target spectrum. Having this selection step fulfilled, those dyestuffs are cho- sen from the still remaining dyestuffs that outside the ranges of their characteris- tic running-up and/or running-down edges assuring just the required transmis- sion effects do not mutually modify, studying from region to region, the effects of

other dyestuffs, i. e. the running-up and/or running-down edges characteristic to other dyestuffs and locations thereof, and furthermore, the values of transmis- sion measured at points other than the special locations. This means, that the light incident upon a substrate coloured with a certain dyestuff is transmitted to a great extent, preferably at least about in 80%, at any locations of the spectrum outside the regions defined by the running-up and/or running-down edges char- acteristic of the dyestuff used.

After this step, the specific transmission spectra of substrates coloured with the dyestuffs chosen in view of criteria described previously in connection with the ground dyeing are compared point by point with the target spectrum. As a result of the comparison only those dyestuffs are kept and will be considered for thermodiffusion dyeing of substrates, in case of which the distance between the running-up and/or running-down loci of the specific transmission spectrum and the corresponding loci of the target spectrum is at most about 20 nm. The term"running-down locus"refers to the midpoint of that region of the transmis- sion spectrum, in which the value of (light-) transmittivity decreases from a great value (preferably from above about 80%) to a small one (preferably to about a value being in the range of 5% and 40%). Analogously, the term"running-up lo- cus"refers to the midpoint of that region of the transmission spectrum, in which the value of transmittivity increases from a small value (preferably from about a value being in the range of 5% and 40%) to a great one (preferably to above about 80%).

Having thus obtained the set of advantageous dyestuffs, all the parame- ters are run through the change of which can induce modifications in the specific transmission spectrum. These parameters, which vary from dyestuff to dyestuff, are the sort of the solvent, the dyeing temperature, the pressure applied during the dyeing, the concentration of the dye solution, the duration of thermodiffusion dyeing and the rate of dissolution of the dyestuff from the substrate in a clean solvent (i. e. in a solvent which contains no dyestuff at all). All the parameters listed here but one are fixed. This non-fixed parameter is varied in a predeter- mined manner and then the process is repeated for all the remaining parame- ters, one after another, that were not selected in the step concerned and are

kept fixed, in order to determine the parameter resulting in the most advanta- geous modification and the extent of the modification.

In knowledge of the dyestuffs selected as described above, the effects of the dyeing parameters and the target spectrum (the latter is unambiguously de- termined by the wavelengths corresponding to the running-down and/or running- up loci and by the amount of the running-down and/or running-up), the transmit- tivity values of the substrate at the required wavelengths are set within tolerance by thermodiffusion dyeing carried out at a temperature being in the range of about 90 °C to about 100 °C, preferably at about 100 °C. The threshold is cho- sen to be in the range of +1% and 5%, preferably in the range of +1% and 2% of transmittivity at wavelengths concerned in the target spectrum. Setting of transmittivity values itself is comprised of the following steps: the raw substrate is coloured with the dyestuffs thought to be adapted for our purposes; a current transmission spectrum of the substrate coloured is compared to the target spec- trum by means of which the extent of its deviation relative to the target spectrum is determined; in case a deviation is detected, according to the nature thereof, the value of the parameter (possibly parameters) resulting in the most effective change of the transmission spectrum is varied and thereby a proportional diffu- sion of dyestuff (s) into the substrate or a proportional dissolution of dyestuff (s) from the substrate is effected for each dyestuff applied. If on the one hand the transmittivity value at a given locus of the transmission spectrum influenced by the dyestuff at issue is smaller than the transmittivity value at the same locus but of the target spectrum, some of the dyestuff diffused into the substrate is dis- solved in the clean solvent. If on the other hand the transmittivity value at a given locus of the transmission spectrum set by the dyestuff at issue is greater than the transmittivity value at the same locus but of the target spectrum, a further amount of the dyestuff is introduced into the substrate via thermodiffusion. When more than one dyestuffs are used simultaneously to reach the target spectrum this last step is performed for each dyestuff used after each other, using one at a time. However, in these cases dyeing is always started with the dyestuff having the least diffusion rate among dyestuffs to be applied. As a next step, the trans-

mission spectrum of the substrate is compared again to the target spectrum, the extent of deviation of the two spectra is determined once more and depending on the nature of the deviation the step of diffusion or dissolution of dyestuff is carried out. The target spectrum is hence approached by coming after an itera- tive process comprising of generally more than one steps, wherein the steps are gradually being refined until the current deviation becomes within tolerance.

The investigations carried out in relation to the ground dyeings of sub- strates have proved the concentration of the dye solution and the duration of the thermodiffusion dyeing to be the parameters that result in changes of the trans- mission spectra in the most effective way. In the thermodiffusion dyeing method of the present invention the concentration (s) of dye solution (s) are throughout kept in the range of about 0.1 weight% to about 1.5 weight%, more preferably between about 0.2 weight% and about 1.0 weight%. Depending on chemical properties of the dyestuff (s) selected, the dye solution (s) can be prepared by means of either a polar or an apolar solvent. Furthermore, in accordance with the nature of the solvent (s) used to prepare the dye solution (s), the clean sol- vent (s) can be either a polar or an apolar solvent.

The dyeing method of the present invention will be exemplified further by special embodiments of spectacle lens aiming the correction of colour vision.

The dyeing method concerned, nevertheless, can be used in other fields of technology too.

Example 1 In the human eye there are three colour sensitive receptors (pigments) assuring the colour vision: the protos that is sensitive to red, the deuteros that is sensitive to green, and the tritos that is sensitive to violet. All of the other colours are produced by simultaneous stimuli of the three receptors with different inten- sities.

Functions of spectral sensitivity P (X), D (X) and T (X) of the protos, deu- teros and tritos, respectively, for people having normal (average) colour vision are well known. It is common knowledge that people do not have exactly the same colour vision, there are so-called colour blind people (they see only two

basic colours instead of three) and people with anomalous colour vision or para- chromatism (they have all the three receptors in their eyes, but they see the col- ours in a different way than normal people).

In practice, improvement and modification of the colour vision and the parachromatism take place with determination of the patient's spectral sensitivity diagrams, i. e. the functions of spectral sensitivity P (2.), D (/') and T (X), by an eye- specialist. Any of the diagrams or any combination thereof can deviate from the same diagrams of peoples having normal colour vision. Knowing the precise de- viation, the eye-specialist in the visible range of the electomagnetic spectrum determines a so-called transmission function or target spectrum that is suit- able for the correction and/or modification of the anomalous colour vision. Then the purpose is to produce eg. such a spectacle lens (or more generally a trans- parent substrate) that has a light transmission spectrum being in the visible range identical with the target spectrum or approaching that within a predeter- mined accuracy.

Figure 1 shows a target spectrum determined and prescribed by an eye- specialist for the correction of the anomalous colour vision of a patient. The tar- get spectrum determined should be reproduced within the accuracy of 1. 5% by thermodiffusion dyeing of a spectacle lens. As it can be seen from Fig. 1, one has to find such a dyestuff or dyestuffs to fulfill this requirement by the applica- tion of which (i) a continuously increasing transmittivity with a local maximum of 27. 3% 1. 5% at , =430 nm, (ii) a transmittivity of 23.5% 1.5% at A=550 nm and (iii) a transmittivity evenly running up to about at least 80% to 90% up to A=700 nm could be reached.

Due to excellent properties of the plastic material referred to as CR-39, the base material of the raw spectacle lens was chosen to be CR-39. Based on literature and on the ground dyeings carried out as a first step, it is known that in order to reach the target spectrum shown in Fig. 1 two dyestuffs, both sold by Anilin, are suitable, namely the dyestuff called"Allion Navy Blue EXSF" (referred to hereafter as the NB dyestuff) and the dyestuff called"Hisperse Orange CGS 340%" (referred to hereafter as the OR dyestuff). A dyeing with the NB dyestuff

modifies the transmission spectrum basically in the range of wavelengths greater than about 550 nm, while a dyeing performed with the OR dyestuff dominantly effects the transmission spectrum for wavelengths less than about 550 nm.

As a first step, the spectacle lens made of CR-39 is placed for a duration of 300 seconds into a dye solution prepared from the NB dyestuff of 5 grams and from a solvent, preferably water, having the volume of 1 litre and the tem- perature of about 100°C. The thus obtained dye solution has a concentration of about 0.5 weight%. After removing the spectacle lens from the dye solution, its transmission curve is recorded, see curve (a) of Fig. 2. As it can be seen from this curve, the increase in transmittivity (from about 23.5% up to about 80% to 90%) observed in the range of 550 nm to 750 nm is definitely assured by the NB dyestuff.

After the dyeing that took 300 seconds, the transmittivity value measured at R=550 nm is, however, proved to be much greater than the corresponding value of the target spectrum. Therefore the spectacle lens is placed back into the dye solution to be coloured further. Transmission spectrum of the spectacle lens taken after a total dyeing duration of 360 seconds is shown in curve (d) of Fig. 2. The total dyeing duration is the sum of time intervals spent by the specta- cle lens in the dyeing solution. Due to the fact, that dyeing of the present inven- tion is performed iteratively in gradually refining steps, the time required for reaching the adequate colouring of the lens is of course much longer than the total dyeing duration, since if a current transmission spectrum shows an excess amount of dyeing with the NB dyestuff, some of the NB dyestuff has to be dis- solved from the lens in a clean solvent, preferably in water, having the tempera- ture of about 100°C. Placement of the lens either into the dye solution (i. e. for having the dyestuff diffused into the lens) or into the solvent containing no dye- stuff (i. e. for dissolution of the dyestuff from the lens) is repeated after each other until in gradually refining steps the transmittivity value of 23.5% 1. 5% re- quired at A=550 nm is reached.

Having now the transmission spectrum of the spectacle lens recorded and evaluated again, one finds that transmittivity values for the range of X < 550 nm are higher than that of the target spectrum.

A substrate made of the same base material as the spectacle lens is placed for a duration of 30 seconds into a dye solution prepared from the OR dyestuff of 5 grams and from a solvent, preferably water, having the volume of 1 litre and the temperature of about 100°C. The thus obtained dye solution has a concentration of about 0.5 weight%. After removing the OR-coloured lens from the dye solution, its specific transmission curve is recorded. As it can be seen from curves (b) and (e) of Fig. 2 taken after different dyeing durations with the OR dyestuff, a dyeing with the OR dyestuff really effects the transmission spec- trum merely in the range of k < 550 nm. This means, that if the spectacle lens coloured previously with the NB dyestuff is now coloured further with the OR dyestuff, then it is also possible to reach the target spectrum by the thermodiffu- sion dyeing method of the present invention performed in an iterative manner in the range of X < 550 nm.

Thus in the following step the spectacle lens coloured previously with the NB dyestuff is placed for a duration of 5 seconds into the dye solution of the OR dyestuff prepared according to the above. After the dyeing has been completed the transmission spectrum of the lens is recorded and evaluated again. Consid- ering and simultaneously monitoring the transmittivity values for all the wave- lengths characteristic to the target spectrum, i. e. in the present case at X equals to 550 nm and to 430 nm, and taking into consideration the relation of these val- ues to the corresponding values of the target spectrum at first the transmittivity value required at 2, =550 nm is set via the modification of the NB dyestuff content of the lens, and then the OR dyestuff content of the lens required to reach ade- quate transmittivity value also at =430 nm is set. The reason for this order of the application of dyestuffs is, that the NB dyestuff has the lower diffusion rate.

In gradually refining steps this last procedure is continued having the two dye- stuffs not necessarily alternately diffused into or dissolved from the lens until the transmission spectrum of the spectacle lens compared to the target spectrum is

within tolerance. Curve (c) of Fig. 2 illustrates the transmission spectrum of the lens after a dyeing with the NB dyestuff for the duration of 300 seconds followed by a dyeing with the OR dyestuff for the duration of 30 seconds.

Example 2 Figure 3 shows another target spectrum T ( ?,) determined by an eye- specialist to improve the colour vision of a patient. It can be seen from Fig. 3, that the realization of the target spectrum at issue requires the usage of such dyestuff or dyestuffs by means of which a transmittivity of 50.0% 1.5% at A=500 nm, a transmittivity of 30.3% 1.5% at), =560 nm and transmittivities of at least 90% for Au560 nm can be obtained and further, the transmission spectrum is continuous and shows no notable jumps in transmittivity throughout the visible range.

Taking into consideration the results obtained by the evaluation of specific transmission spectra in ground dyeing of substrates (spectacle lenses) made of CR-39 one comes to the conclusion that the most adequate dyestuffs for the present purposes are a dyestuff called"Hisperse Violet C-3R 120%" (referred to hereafter as the HV dyestuff) produced by Anilin and a dyestuff called"Cibacet Blau GDF" (referred to hereafter as the CBG dyestuff) manufactured by Ciba Vi- sion. The running up in the range of 560 nm to 700 nm and a transmittivity value of at least 50% at B=500 nm can be both provided by a dyeing with the HV dye- stuff. It is noted that the absorption minimum of the HV dyestuff concerned is at ?, =570 nm. This means, that in a dyeing process performed with the CBG dye- stuff in order that an adequate colouring of the lens (i. e. providing the transmit- tivity value of 50.0% 1.5% at =500 nm thereof) be produced within tolerance in the range of B<560 nm, the location of the absorption minimum should also be shifted to A=560 nm.

Separate dye solutions are prepared from the above dyestuffs by solving 8 grams of each dyestuff in a solvent, preferably in water, of the volume of 1 litre each. In this way dye solutions with the concentration of about 0.8 weight% are obtained. The dye solutions are heated to a temperature of about 100°C. As clean solvents, i. e. solvents with no dyestuffs preferably water, heated to about

100°C are used. In the present example at first dyeing with the HV dyestuff hav- ing the lower diffusion rate is effected. As a consequence, such a spectacle lens is obtained the current transmission spectrum of which approaches the target spectrum quite well throughout that range of wavelengths which is affected by the HV dyestuff. Then the gradually refining steps of the thermodiffusion dyeing method of the present invention are performed in an iterative manner having the two dyestuffs not necessarily alternately diffused into or dissolved from the lens until the final transmission spectrum of the lens compared to the target spectrum is within tolerance. In Fig. 4 specific transmission spectra of thermodiffusion dyeings performed for different durations are shown; curves (a) and (c) are re- corded in a dyeing process done with the HV dyestuff for the total dyeing dura- tion of 200 seconds and 300 seconds, respectively; curves (b) and (d) are taken in a dyeing process performed using the CBG dyestuff for the total dyeing dura- tion of 10 seconds and 30 seconds, respectively; and curve (e) illustrates a transmission spectrum of the spectacle lens coloured with both dyestuffs ap- proaching the target spectrum.

Example 3 The substrate (or spectacle lens) of the present example should be col- oured in order to provide a transmission function having the following character- istics: the transmittivity value is at least 65.0% at ? W=500 nm and 35.0% 1.5% at =550 nm and this latter value survives up to about B=645 nm, where the trans- mission curve has an abrupt running-up.

Based on experience gained during ground dyeings, it can be seen that the target spectrumr (k) described above can be reached within tolerance using only one dyestuff. The dyestuff that suits best to the purposes is a dyestuff called"Cibanet Blau F-3R" (referred to as the dyestuff CBF hereafter) sold by Ciba Vision.

A dye solution is prepared from the CBF dyestuff by solving 5 grams thereof in a solvent, preferably in water, of the volume of 1 litre. The concentra- tion of the dye solution obtained in this way is about 0.5 weight%. The dye solu- tion is then heated to a temperature of about 100°C. As a solvent with no dye-

stuff preferably water heated to about 100°C is used. The thermodiffusion dyeing is performed as previously but in this case using only one dyestuff.

Figure 5 shows the transmission spectra of the spectacle lens made of plastic material CR-39 and coloured iteratively by the thermodiffusion dyeing method of the invention. The curves representing the transmission spectra are recorded after definite total dyeing durations, i. e. curve (a) and (c) are taken after a dyeing of 120 seconds and 330 seconds, respectively. Furthermore, curve (b) plotted by a broken line in Fig. 5 has been taken just on completion of the itera- tive thermodiffusion dyeing, which means, it approaches the target spectrum within tolerance.

Example 4 This example is aimed to illustrate, that it is possible to produce sub- strates (eg. spectacle lens) characterized by relatively complex target spectra using the iterative thermodiffusion dyeing method of the present invention. For illustration purposes let us consider a target spectrum X) having the following characteristics: the value of transmission runs up to 52.0% 2.0% for B=400 nm, the thus reached value survives for B=430 nm and then continuously decreases to 32.0% 1.5% for B=550 nm, then increases again up to at least 60.0% or higher for B=600 nm, and the thus reached value survives throughout the re- maining part of the spectrum, i. e. up to , =750 nm.

Relying upon the experience gained in ground dyeing we came to the conclusion, that the target spectrum having the above features cannot be reached with only one dyestuff. However, with two dyestuffs it can be realized within tolerance. The dyestuffs that suit best to our present purposes are a dye- stuff called"Cibacet Violett 2R" (referred to as the dyestuff CV hereafter) and a dyestuff called"Cibacet Braun JNH-02 150%" (referred to as the dyestuff CB hereafter) sold both by Ciba Vision.

Separate dye solutions are prepared by solving 5 grams of each of these dyestuffs in solvents, preferably in water, of the volume of 1 litre each. In this way dye solutions having the concentration of about 0.5 weight% are obtained.

The dye solutions are then heated to a temperature of about 100°C. As solvents

with no dyestuffs preferably water heated to about 100°C are used. Thermodif- fusion dyeing is done according to Examples 1 and 2 starting with the CV dye- stuff having larger diffusion rate. Transmission spectra of the substrate coloured with different dyestuffs for predetermined durations are illustrated in Fig. 6, wherein curves (a) and (c) are taken after total dyeing durations of 90 seconds and 300 seconds, respectively, with the CB dyestuff ; curves (b) and (e) are re- corded after total dyeing durations of 120 seconds and 240 seconds, respec- tively, with the CV dyestuff; and finally curve (d) shows the final transmission spectrum of the substrate dyed with both dyestuffs according to the iterative method of the present invention. The transmission spectrum shown by curve (d) approaches the target spectrum-c () within tolerance.

It should be obvious from the above Examples that the thermodiffusion dyeing method of the present invention performed in an iterative manner, wherein the dyestuffs diffused into a substrate can be dissolved from the sub- strate, if necessary, and then can be diffused into it again but now in a different amount, are suitable to prepare such substrates that are characterized by more complex transmission spectra showing essentially arbitrary behaviour compared to substrates prepared by the known dyeing methods.

As a summary, optical elements having complex target spectra eg. of band-pass filter characteristics, such as optical elements or spectacle lenses providing the correction of colour vision, or transparent layers that provide the filtering-off of a light component having a certain wavelength, can be obtained in an extremely simple way by the production of relatively low amount of spoilage with the thermodiffusion dyeing method of the present invention. For the produc- tion of these types of optical elements, the known dyeing methods are insuffi- cient.