WO2001057583A2

WO2001057583A2 - Method for correcting colour deficiency, the filter used in the method and method for providing the filter

Info

Publication number: WO2001057583A2
Application number: PCT/HU2001/000015
Authority: WO
Inventors: György ÁBRAHÁM; Gábor KOVÁCS; Itala Kucsera; Gottfriedné Wenzel
Original assignee: Coloryte Hungary Rt.
Priority date: 2000-02-07
Filing date: 2001-02-07
Publication date: 2001-08-09
Also published as: WO2001057583A3; HU0000531D0; HUP0000531A2; AU3398801A

Abstract

There is disclosed a method for providing a filter (3) for correcting colour deficiency. The method includes the following steps: a, the spectral response curves of the colour deficient person are determined; b, the spectrum to be observed is determined; c, the colour perception response signals generated by the given spectrum and associated to the spectral response curves of a person with normal colour vision are determined; d, the spectral transmission function of the filter is defined with parameters; e, the spectral response signals generated by the given spectrum and using the filter, associated to the colour deficient spectral response curves are determined; f, the filtered response signals are compared to the response signals of the person with normal colour vision; g, the parameters providing the best approximation are determined. There is also provided a method for correcting colour deficiency with a filter fitted to the specific colour deficiency of that person, so that the visual colour perception resulting from the observation of a given spectrum resembles the colour perception of people with normal colour vision.

Description

METHOD FOR CORRECTING COLOUR DEFICIENCY, THE FILTER USED IN THE METHOD AND METHOD FOR PROVIDING THE FILTER

Technical Field The invention relates to a method for correcting colour deficiency, and to a method for providing the filter correcting the colour deficiency. The invention also concerns the filter manufactured with the method. In the inventive method for correcting colour deficiency the colour vision of a person having colour deficiency is improved with a filter fitted to the specific colour deficiency of that person, so that the visual colour perception resulting from the observation of a given spectrum resembles the colour perception of people with normal colour vision.

Background Art

Such a method is disclosed in the international publication No. WO95/0562 I . This known method is substantially based on improving the colour vision of a colour deficient person by using a filter fitted to the specific colour deficiency of that person. It is sought to make the spectral sensitivity curves (cone response curves) of the colour deficient person similar to the spectral sensitivity curves of a person with normal colour vision, in any situation, more precisely, when observing a colour having arbitrary spectral composition. For this reason, a specially designed filter is placed before the eyes of the colour deficient person. The parameters of the filter are determined based only on the spectral sensitivity (spectral response) of the eye of the colour deficient person.

In practice, the filters manufactured with this method did not always turn out to be satisfactory, and the colour vision could not be restored to the desired degree.

Therefore, it is an object of the present invention to improve the resulting colour perception (colour response), instead of changing the effective shape of the spectral response curves. The invention is based on the recognition that if in the design of the filters not all possible spectra should be improved, but only the colour perception should be corrected when one or more well-defined spectra is being observed, than in this more limited spectrum the correction of the colour vision may be better achieved.

Summary of the Invention

Accordingly, the present invention concerns a method for correcting colour deficiency, where the colour vision of a person having colour deficiency is improved with a filter fitted to the specific colour deficiency of that person, so that the visual colour perception resulting from the observation of a given spectrum resembles the colour perception of people with normal colour vision. The method comprises the following steps: a, the spectral response curves of the colour deficient person are determined, b, the spectrum to be observed is determined, c, the colour perception response signals (P,D,T or L,M,S) generated by the given spectrum and associated to the spectral response curves of a person with normal colour vision are determined, d, a filter is used with a spectral transmission where observing the given spectrum through the filter, the colour perception response signals associated to the spectral response curves of the colour deficient person and to the observed spectrum are substantially equal to the adapted colour perception response signals associated to the normal spectral response curves and to the observed spectrum.

The term „spectral response curve" are also known as „spectral sensitivity curve" in the art. In practice, the method is performed by determining the spectral components constituting the spectrum to be observed, or by dividing the spectrum to be observed is into spectral components. A filter is used, where observing the given spectrum through the filter, the colour perception response signals associated to the spectral rcsponse curves of the colour deficient person and to each spectral component are substantially equal to the adapted colour perception response signals associated to the normal spectral response curves and to the respective spectral component.

An essential aspect of the invention is the method for providing the filter for correcting the colour deficiency. This method comprises the following steps: a, the spectral response curves of the colour deficient person are determined, b, the spectrum to be observed is determined, c, the colour perception response signals generated by the given spectrum and associated to the spectral response curves of a person with normal colour vision are determined, d, the spectral transmission function of the filter is defined with parameters, e, the spectral response signals generated by the given spectrum and using the filter, associated to the colour deficient spectral response curves arc determined, f, the filtered response signals are compared to the response signals of the person with normal colour vision, g, the parameters providing the best approximation arc determined, practically with a mathematical method.

Preferably, the best parameters are found by determining the difference function of the filtered response signal and the normal response signals, and the difference function is minimised. Usually, the minimum value of the difference function is calculated by iteration or by determining its vanishing differential.

The generation of the difference function requires further mathematical instruments. Therefore, the following intermediate steps are foreseen: a, the spectral components constituting the spectrum to be observed are determined, or the spectrum to be observed is divided into spectral components, b, the normal and colour deficient colour perception response signals generated in each cone by each spectral component arc calculated, c, the normal and colour deficient colour matrices associated to the spectrum to be observed arc calculated, d, a filter is determined which makes the colour matrix of the colour deficient person similar to the colour matrix of a person with normal colour vision.

Preferably, the filter is described by its spectral transmission function, and the spectral transmission function is approximated by linear sections or spline functions, or other suitable curves. However, it is also feasible to describe the function by the function values defined in a large number of points.

The methods according to the invention is particularly successful if the spectrum to be observed is the spectrum of a colour CRT (Cathode Ray Tube), plasma or LCD (Liquid Crystal Display) monitor. These spectra may be decomposed into relatively few and simple spectral components, which partly makes the calculations easier, and partly makes the separation of the blending colours easier.

The invention is generally useful in other fields as well. The use of the invention is particularly suggested where the spectrum to be observed is the spectrum of traffic lights and/or traffic signs and/or equipment characteristic for a given profession. In this manner the unhindered participation in the traffic or the exercise of a certain profession will be achievable for a large number of people, which would otherwise presume a basic colour vision (e. g. for the correct recognition of cables, signal lights having a certain colour, etc.).

Finally, the invention also concerns the filter based on the methods. Generally, the filter is realised by an optical thin film system provided on an ophthalmic eyeglass (spectacle). Brief Description of Drawings

The invention is explained in more detail with the help of the attached drawings, where Fig. 1 is a schematic figure of an eyeglass comprising a filter made according to the inventive method, and a schematic figure of a CRT computer monitor having a specific spectrum, Fig. 2 is the emission spectrum of the phosphor used in the monitor shown in

Fig. I , Fig. 3a illustrate the spectral response curves of the cones in the eye of a colour deficient person, Fig. 3b illustrate the spectral response curves of the cones in the eye of a person with normal colour vision, Fig. 4 is the theoretical spectral transmission curve of the filter made according to the inventive method, and

Fig. 5 illustrates the practical implementation of the theoretical transmission curve with a concrete filter.

Best Mode for Carrying out the Invention

During the method according to the invention, the correction of the colour deficiency is made with an appropriate filter, which has been fitted to the specific, individual colour deficiency. Referring to Fig. 1 , the filter 3 is normally applied on the glasses 2 of an eyeglass 1 , usually in the form of optical thin films or optically absorbent dyes, deposited on an external surface of the glass 2 (this embodiment is also illustrated in Fig. 1.) The dye may be also included in the material of the glass 2. The filter may be produced by mixing the available technologies. Thc filter 3 is manufactured so that the visual colour perception generated by a given spectrum in the colour deficient person is made similar, preferably substantially equal to the colour perception in a person with normal colour vision. The invention is demonstrated with an example where the given spectrum is provided by the imaging tube of a monitor 4. When the eyeglass 1 is supplemented with the filter 3 made according to the invention, the colour perception of the colour deficient person wearing the eyeglass 1 , i. e. his/her colour distinction ability improves significantly for colours generated by the monitor 4.

The first step of the method is determining the spectral sensitivity curves (spectral response curves) of the colour deficient person. Fig. 3a shows the spectral sensitivity curves in a concrete case. For comparison, in Fig. 3b the spectral sensitivity curves of a person with normal colour vision are also presented.

The measurement method of these spectral sensitivity functions is known per so for the skilled person. Here we refer to the method disclosed in the international publication No. WO 95/28125, and further to the publications: Wyszecki - Styles: Color Science; Concepts and Methods, Quantitative Data and Formulas (John Wiley and Sons, London, 1966 2^nd ed. pp 614.) and Backhaus - licgl - Werner: Color Vision; Perspectives from Different Disciplines (Walter dc Gruyter, Berlin - New York 1998. pp 309). It must be noted that it is not necessary for performing the method to measure every time the spectral sensitivity curves. Available data from earlier measurements made with the affected person may be used as well.

For the method it is also necessary to know the spectrum to be observed. It is an essential element of the method that always a specific spectrum is considered, and the colour vision of the colour deficient person is improved for this specific spectrum. It must be noted that when observing another spectrum through the filter obtained with the method, the resulting colour vision may not necessarily improve, and in certain cases the colour vision may even differ more from the normal colour vision, if compared with the case when the colour deficient person would not use a filter at all.

The method is particularly well suited for using with devices which have a known spectrum, composed of well-defined spectral components. Such devices are the known CRT, plasma- or LCD-type television tubes or computer monitors. Normally, the spectrum of these is composed of the discrete spectral components E(λ)_h, E(λ)_g, E(λ)_r of the phosphors associated to the three basic colours (red, green, blue), as it is shown in Fig. 2. The spectrum in Fig. 2 belongs to a CRT-type display, the spectrum of LCD or plasma monitors is slightly different.

The method is also applicable to other known spectra, e. g. for the spectra of traffic lights or other signal lights, or other important spectra occurring in certain professions.

In a further step of the method, the colour perception response signals generated by the given spectrum and associated to the spectral response curves (spectral sensitivity curves) of a person with normal colour vision are determined. These response signals are calculated in the related field based on the spectral sensitivity function, i.e. the sensitivity function or response curve of the colour sensitive cones in the eye. This is discussed among others in detail in the publication: F. Vienot, A. Ben M'BArek, L. Ott: „Screening colour vision with an LMS calibrated display", Colour Vision Deficiencies XIII, Proceedings of the thirteenth Symposium of the International Research Group on Colour Vision Deficiencies, Pau France 1995. In the present case, the colour perception response signals are calculated by determining the colour perception response signals generated by the spectral components associated to the individual phosphors, and therefrom the response signals for the complete spectrum are calculated. With other words, the magnitude of the colour perception response signals, i. e. magnitude of the effective biological signals physically generated by the cones in the eye are calculated from the response curves of the respective cones. In effect, the magnitude of the signals is calculated by integrating the product of the spectral response curve and the energy (intensity) distribution of the observed spectral components over the relevant wavelength interval.

The discrete spectral emission bands of a CRT display are particularly well suited for obtaining an effective correction with a filter having a relatively simple spectral transmission function, i. e. a filter that may be easily realised in practice. Accordingly, the filter is obtained through determining its spectral transmission function. Determining the spectral transmission function of the filter is one of the main objects of the invention. For this purpose the following method is suggested:

Let us consider the response signals generated in each L,M,S type of cones (receptors) by each spectral component. The L,M,S cones arc the receptors sensitive to light with long, medium and short wavelength, usually termed as protos, deutcros and tritos. The r,g,b indices refer to the spectral components associated to the red, green and blue CRT colours. The function E(λ) describes the wavelength dependent energy distribution in the observed spectrum, with other words, the actual shape of the spectrum (see also Fig. 2), while the functions L(λ), M(λ), S(λ) describe the amplitude of the response generated in the cones by the incident light having different wavelength and equal energy. With other words, L(λ), M(λ), S(λ) describe the spectral sensitivity curves (spectral response curves). These latter functions are also shown in Figs. 3a and 3b.

The response signals L_r,M_r, S_r, L_g, M_g, S_g and L_b, M_b, S_b generated by the each spectral components in the each type of receptors are given by the equations I-IX below: _-- = / ,J^*E-(λ)/_(λ)c.λ I

M_r = k_mfE_r(λ)M(.λ)dλ IV

M_κ - k_mjE_κ(.λ)M(λ)dλ V

M_h - k_mjE_h(λ)M(λ)dλ VI

S, - k,fE,(.λ)S(λ)dλ VII

S_g - k,fE_g(λ)S(λ)dλ VIII

Usually, it may be assumed that the eye is observing the spectrum to be observed in a state adapted to white light. This is a state where the total response of an individual receptor is largely equal to the response generated by a uniform white light having an average intensity. It is stressed that in this case we consider the white light with an average intensity which is produced by the monitor, i. e. the light which is seen as white by the user, and not the theoretical white light having a spectrum with equal energy (homogenous spectrum). The magnitude of these responses generated by this ..monitor-white" may be arbitrarily considered as unity. This means that the k constants in the above equations may be selected so that the value of the response generated by the combined effect of the different spectral components in the individual receptors also will be unity. In this case the so-called monitor colour matrix will have the following form:

M = J S = 1

where J /. = _-. + /-,. + L_A = I , etc..

This latter condition is the mathematical expression of the state adapted to the „monitor-whitc". The above monitor colour matrix describes the effect on the receptors of the eye by a given spectrum, here the spectrum of a CRT display. In a specific example, the values of the colour matrix for persons having a normal colour vision and for a CRT monitor using a P 22 type phosphor are as follows:

0,43 0,06 0,02 0,52 0,81 0,06 0,05 0,13 0,92

For a colour deficient person the magnitude of the generated responses will be different, due to the different shape of the spectral sensitivity curves L (λ), M (λ) es S^*(λ), where the shape of the spectral sensitivity curves L (λ), M^*(λ) es S^'(λ) differ from the shape of the normal curves (compare Figs. 3a and 3b). The magnitude of the response signals is given by the equations X-XVIII below:

_. - k^',jE_κ(λ)L^'(λ)dλ XI

M = /.^'-,/ E_r(λ)/V ^"(λ)-/λ XIII

M = kX}E_h(λ)M'(λ)dλ XV

XVIII

The method is based on the assumption that the colour adaptation mechanisms function largely in the same way in colour deficient persons as with persons having normal colour vision. Accordingly, the k* adaptation constants must be calculated anew. Thereafter, similar to the case of normal colour vision, the colour matrix is generated as follows:

where 2 = 2 =

= ι

For a CRT monitor using a P 22 type phosphor, as in the previous case, the values of the colour matrix in the state adapted for white light, for a specific colour deficient person, will be as follows:

0,67 0,06 0,02 0,30 0,81 0,06 0,03 0,13 0,92

It is apparent that there is an appreciable difference in the values for the L receptor (the protos). This difference may be reduced with the method according to the invention, with the help of an appropriate filter. Let us define the spectral transmission function F(λ) of the filter. In this case the cone responses (colour perception response signals) generated by the filtered spectrum are given by the equations XIX-XXVII below, which arc completely analogous to the previously discussed response equations I-XVIII.

L^'lt\ = k^{'υ )},fE_r(λ)L * (λ)F(λ)dλ (XIX)

L^'if = k^{'{ l} $E_χ{λ) {λ)F{λ)dλ (XX)

L^'{F = k^'(V),$E_h(k)L{k)F{k)dk (XXI)

M^{' f} = k^'{F) _mJE_r(λ)M^'(λ)F(λ)dλ (XXII)

Λ ^'(F)„ = £^{,< )}„J^'E,,(λ) Λ/^"(λ)E(λ)c.λ (XXIII)

M'^,F) _Λ = k^{' F)} _mfE_h(λ)M'(λ)F(γ )dλ (XXIV)

s^w„ =

(xxvii)

The goal is to make the response values L^"(|,) ... S^{*(( -)} equal or similar to the response values L..S of persons with normal colour vision, in the maximum possible extent. With other words, a filter must be used, and the given spectrum must be observed through the filter. This filter should be designed to fulfil the following condition: if the spectrum is observed through the filter, the adapted colour perception response signals associated to the spectral response curves of the colour deficient person and to the observed spectrum are substantially equal to the adapted colour perception response signals associated to the normal spectral response curves and to the observed spectrum. More precisely, in the discussed case, the adapted colour perception response signals associated to the spectral response curves of the colour deficient person and associated to the spectral components of the CRT base colours will be equal to the adapted colour perception response signals associated to the normal spectral response curves and assocatiated to the respective spectral components.

Of course, the colour adaptation constants k^{*(l )} must be calculated again, and here they will be also dependent on the spectral transmission function F(λ). For this puφose, it is still assumed that the colour matrix fulfils the conditions below:

It may be realised that with this boundary condition the colour adaptation constants k^{*(, )} may be unequivocally defined for a given spectral transmission function F(λ). Therefore, the next task is to find a suitable spectral transmission function F(λ). In a suggested realisation of the method according to the invention, this is found by determining the difference function of the filtered response signals and the normal rcsponsc signals, and by minimising the difference function. In practice, it is not necessary, and generally it is not even possible to minimise the difference functions separately, because of the common spectral transmission function F(λ). Therefore, it is straightforward to treat the difference of the respective response signals together. These may be combined in a single equation, and the following difference function Φ is defined:

Φ = (C - L_rY + (L^'<' \ - Z,J ₊ ( ⁽' - L_h)² ₊ ( Λ/'"Λ - M_rf ₊ ( ^'"Λ- - /Vf ₊ ₊ ( M^'" - M_h)² ₊ (s' X - s_r)² ₊ (s™ - S ₊ (S'^<' - s_hY

It is apparent that for the specific spectrum the colour matrix of the colour deficient person will be identical to the colour matrix of a person with normal colour vision, if the value of the above defined difference function Φ is zero.

The minimum value of the difference function is calculated by iteration or by determining its vanishing differential, i. c. finding such values for F(λ) which satisfy the equation dΦ(F(λ))/dλ = 0

Considering the fact that the difference function Φ may have a very complicated form, due to the adaptation boundary condition and due to the form of the spectral transmission function F(λ), which all may involve non-linear relationships, it is possible that the minimum value of the difference function Φ can not be found by analytical calculations, e. g. by differential calculus. Therefore, it is also foreseen to apply iteration or heuristic methods.

In a specific case, the spectral transmission function F(λ) was approximated with spline functions. The approximation may be performed also with linear sections. The effect of this modelled filter on the response values generated in the receptors, i. e. its effect on the colour matrix was calculated with an interactive computer program, which displayed in real time the normal colour matrix and the filtered colour matrix of the colour deficient person. The computer program allowed the „manual" adjustment of certain sections of the filter. The filter, being described with linear sections, and shaped with heuristic approximations, was finally optimised with an automatic method. The resulting colour matrix was the following:

0,43 0,06 0,02 0,54 0, 1 0,06 0,03 0,13 0,92

It is apparent that the normal colour matrix could be restored with a very good approximation. As a result, the colour perception of the colour deficient person will be practically identical to the colour perception of a person with normal colour vision, when observing the device with the specific spectrum, in the present case when observing the CRT display. It must be stressed repeatedly that the filter thus obtained will not necessarily improve the colour vision when observing devices having a spectrum different from the target spectrum used for defining the filter.

With another approach, it is also feasible to describe the spectral transmission function of the filter by the function values defined in a large number of points.

Clearly, in this case the function values in the individual points will be the variables of the iteration calculation.

Fig. 4 shows such a spectral transmission function F_S7(λ), which has been calculated according to the inventive method. Once such a spectral transmission function F _/(λ) has been obtained, known methods exist to manufacture the filter. With the presently available thin film technologies theoretically any filter may be produced with suitable accuracy, and having the spectral transmission function as calculated with the inventive method. There are commercially available application software for calculating filter parameters (typically, standard thin film thicknesses) resulting in a given spectral transmission function. Fig. 5 illustrates the measured spectral transmission function τ(λ) of a filter which embodies in practice the calculated spectral transmission function F_s/(λ) of Fig. 4. It is apparent that the sought function behaves practically completely as desired in the critical interval, namely between 450-750 nm. The filter consists of sixteen thin film layers, the parameters of which arc listed in Table I. The material of the high refractive index layer is Ti₂θ₃, while the material of the low refractive index layer is Si0₂. The refractive index of the substrate is 1.52.

Table I. (the structure of the filter with the spectral transmission function τ(λ) of Fig. 5) Layer Number Optical path length refractive index Physical thickness (d) d*n (n) (100 nm)

(1000 nm) H =high L=low

0 Substrate (glass) 1,52 Substrate

1 ,27283 H 3,34 0,818

2 ,28925 L 2,09 1,381

3 ,23925 H 3,35 0,715

4 ,27345 L 2,09 1,306

5 ,22392 H 3,34 0,671

6 ,26557 L 2,09 1 ,268

7 ,22111 H 3,33 0,663

8 ,26588 L 2,10 1,268

9 ,23366 H 3,33 0,701

10 ,29287 L 2,09 1,398

11 ,29363 H 3,34 0,88

12 ,31931 L 2,10 1,524

13 ,27192 H 3,34 0,815

14 ,2841 L 2,10 1,356

15 ,25771 H 3,33 0,773

16 ,14998 L 2,09 0,716 Air Air

Claims

Claims:

1. Method for correcting colour deficiency, where the colour vision of a person having colour deficiency is improved with a filter fitted to the specific colour deficiency of that person, so that the visual colour perception resulting from the observation of a given spectrum resembles the colour perception of people with normal colour vision, the method comprising the following steps: a, the spectral response curves of the colour deficient person arc determined, b, the spectrum to be observed is determined, c, the colour perception response signals generated by the given spectrum and associated to the spectral response curves of a person with normal colour vision are determined, d, a filter is used with a spectral transmission where observing the given spectrum through the filter, the adapted colour perception response signals associated to the spectral response curves of the colour deficient person and to the observed spectrum arc substantially equal to the adapted colour perception response signals associated to the normal spectral response curves and to the observed spectrum.

2. The method of claim 1 , characterised in that the spectral components constituting the spectrum to be observed are determined, or the spectrum to be observed is divided into spectral components, and a filter is used where observing the given spectrum through the filter, the colour perception response signals associated to the spectral response curves of the colour deficient person and to each spectral component are substantially equal to the adapted colour perception response signals associated to the normal spectral response curves and to the respective spectral component.

3. A method for providing a filter for correcting colour deficiency, characterised by the following steps: a, the spectral response curves of the colour deficient person arc determined, b, the spectrum to be observed is determined, c, the colour perception response signals generated by the given spectrum and associated to the spectral response curves of a person with normal colour vision are determined, d, the spectral transmission function of the filter is defined with parameters, e, the spectral response signals generated by the given spectrum and using the filter, associated to the colour deficient spectral response curves arc determined, f, the filtered response signals are compared to the response signals of the person with normal colour vision, g, the parameters providing the best approximation are determined.

4. The method of claim 3, characterised in that the difference function of the filtered response signal and the normal response signals is determined, and the difference function is minimised.

5. The method of claim 4, characterised in that the minimum value of the difference function is calculated by iteration or by determining its vanishing differential.

6. The method of any one of claims 3 to 5, characterised in that a, the spectral components constituting the spectrum to be observed are determined, or the spectrum to be observed is divided into spectral components, b, the normal and colour deficient colour perception response signals generated in each type of cone by each spectral component are calculated, c, the normal and colour deficient colour matrices associated to the spectrum to be observed are calculated, d, a filter is determined which makes the colour matrix of the colour deficient person similar to the colour matrix of a person with normal colour vision.

7. The method of any one of claims 3 to 6, characterised in that the filter is described by its spectral transmission function, and the spectral transmission function is approximated by linear sections.

8. The method of any one of claims 3 to 7, characterised in that the filter is described by its spectral transmission function, and the function is described by the function values defined in a large number of points.

9. The method of any one of claims 3 to 8, characterised in that the spectrum to be observed is the spectrum of a colour CRT, plasma or LCD monitor.

10. The method of any one of claims 3 to 8, characterised in that the spectrum to be observed is the spectrum of traffic lights and/or traffic signs and/or equipment characteristic for a given profession.

1 l .The method of any one of claims 3 to 10, characterised in that the spectral response curves of a normal person and/or the spectral response curves of the colour deficient person and/or the spectrum to be observed is determined by measuring the necessary values or using readily available data.

12. A filter for the correction of colour deficiency, manufactured according to any of the claims 2 to 10.

13. The filter of claim 12, characterised in comprising optical thin film and/or optically absorbing dye.

14. An eyeglass comprising a filter according to claim 12 or 13.