This invention relates to a method for evoking a colour of a surface other than that possessed by the surface. The visible region of the spectrum of electromagnetic radiation is situated between approximately 400 - 700 nanometres. The eye discriminates three primary colours in this region, namely blue, green and red. However, more than three colours are perceived because the incident light is analyzed in the brain and the intensities of the three primary colours in relation to one another determines the perceived colour.

If the primary colours are present in the same amount, the colour white is perceived, but if on the other hand none of the primary colours are present, black is perceived. The rich range of perceivable colours has its physical cause in the greater or lesser degree of presence or -absence of one or more primary colours.

For example, if the colour green is absent in the observed light the colour purple is perceived (the result of the detection of red and blue) . Colour can come into being as the result of absorbtion, reflection and/or emission.

These points w ll be clarified with the aid of so- called colour matrices. In these colour matrices, a "0" means that the dye does nothing to the light of a given colour, and a "1" means that the dye does do something to that coloured light.

1. Absorbtion

The colour matrix for absorbtion looks as follows:

incident light

dye

It is apparent from this colour matrix that the colour blue, for example, is perceived if the colours green and red are absorbed.

2. Reflection by a non-luminophore

For this form of reflection, the colour matrix is as foilows: incident light

aye

This colour matrix is the inverse of the matrix obtained by absorbtion. This is necessarily the case, since after all a blue dye can not reflect any green or red since otherwise the dye would not be experienced as blue.

3. Emission

A blue (daylight) luminophore emits only blue light on illumination with blue light; on excitation by red or green light, no emission takes place, on account of the lower energy of this radiation. This means that the blue luminophore absorbs incident blue light, but reflects incident red and green light. The red luminophore, on the other hand, emits only red light, but does this on excitation by blue, red and green light. This implies that the red luminophore absorbs all three primary colours, and displays no or at least limited reflection of red, blue and green light.

The associated colour matrix is as follows incident light

luminophore

The emission matrix of a luminophore which is irradiated with UV-light and visible light is as follows: incident light emission blue green red UV blue 1 0 0 1 luminophore green 1 1 0 1 red 1 1 1 1

This last emission matrix is the inverse matrix of the directly preceding matrix.

The method according to the invention for the evocation of a colour of a surface other than the colour which the surface possesses creates a situation in which the luminous object apparently loses its own colour. The information-processing carried out by the eye on an emission colour of a luminous object is determined by the relationship of the absorbtion/reflection characteristics of the luminous object, on the one hand, and the colour of the immediate surroundings, on the other. An ostensible change of colour of the emitted light takes place, whereby the wavelength of the irradiating light disappears from the emission perceived through the eye. This effect seems to be a consequence of a diffuse scattering of the irradiating light of the same wavelength as the emission wavelength.

These matters w ll be clarified by reference to a number of embodiments.

In the first embodiments, the luminous object is formed by a surface and, applied thereto, a luminophore which is made caused to luminesce. The absorbtion/reflection characteristic of the surface can be influenced by use of paint dyes, for example pigments. The colour of the surroundings can be influenced by use of a set of four lamps, namely a blue, a green, a yellow and a red lamp. Example 1 In the most extreme case, use is made of a white- emitting surface having the absorbtion/reflection properties of a black surface.

To this end a white-emitting luminophore which is bound in a dispersion to a black pigment is applied on a substrate. On irradiation with white light, the surface w ll be black - in other words, the incident light w ll be entirely absorbed by the black pigment. On irradiation with UV _* (wavelength 366 nm.) the surface will emit white light - in other words, the activated luminophore will emit white light. If such a surface which is provided with a white- emitting luminophore is disposed in a space, a colour for this surface different from that colour which the surface possesses can be experienced in the following manner.

- On irradiation with UV«-light the surface becomes white, - on irradiation with UV _Λ -light and blue light, the surface becomes yellow (colour number 200, to be defined below),

- on irradiation with UVA-light and red light, the surface becomes cyan (colour number 501) ",

- on irradiation with UV^-light and green light, the surface becomes magenta (colour number 362) , and

- on irradiation with UVΛ-light and yellow light, the surface becomes azure blue (colour number 526) .

It should be noted that the evoked colour is perceived immediately the coloured light is switched on. There is no period of adaptation involved.

_ _

Exam le 2

For comparison, and to give a better comprehension of the invention, another substrate is provided only with a white-light emitting luminophore. On irradiation with UV _A the surface will emit white light.

On simultaneous irradiation with UVA and coloured light, the surface will keep emitting white in all circum¬ stances. This is in contrast to the previous example, 1, in which a different colour is clearly evoked on irradiation with UVΛ and coloured light.

Example 3

Instead of the use of a black pigment bound to a white luminophore use can be made of, on the one hand, a white- light emitting mixture of blue, green, yellow and red luminophores, and, on the other hand, a black-coloured mixture of blue, green, yellow and red pigments. For example, use can be made of commercially available fluorescent pigments. These fluorescent pigments consist of a pigment with a luminophore emitting the same or a neighbouring colour bound to it. For example, use can be made of the fluorescent pigments Fluor Blue, Fluor Green, Fluor Primrose Yellow and Fluor Rocket Red (from Silkscreen Promotion, Alkmaar, The Netherlands), bound with the binding medium Mowilith DM4 (from De Witt, Groningen, The Netherlands) .

After mixing of the pigments, black results from subtractive mixture and white from additive mixture (on irradiation with UV _Λ ) .

The table shows the evoked colour of a surface which is provided with a fluorescent pigment (Primrose Yellow) and with a mixture thereof with another pigment.

It can be clearly seen, at least from the colour numbers, that by use of the method according to the invention striking and large shifts in the evoked colour occur on irradiation with UV _Λ and/or coloured light.

Table: Evdked colour of a surface which is provided with a fluorescent pigment and a mixture with a pigment on irradiation with UV _* . and optionally coloured light

Primrose Primrose Primrose Primrose Primrose Primrose Yellow Yellow Yellow Yeilow Yeilow Yellow

Colour of + + + + + irradiating Cadmium Red Chromium Emerald Cerulean pirthalo light light* Yellow Green Blue blue

Lemon Raw Deep Moss Greenish Olive

Blue + UV _A Yellow Sienna Yellow Green Yellow Green no. 205 no. 234 no. 202 no. 653 no. 243 no. 620

Mixture of Mixture of

Lemon Light Greenish Neutral Grey Neutral Grey Neutral

Green + UV _A Yellow Brown Yellow 3 no .723 + 1 no. 721 + Grey 3 no. 205 no. 401 no. 243 Emerald Green t Emerald Green 1 no. 723 no. 615 no. 615

Mixture of

Lemon Orange Greenish Green Light Green Brilliant

Yellow + UV _A Yellow no. 235 Yellow Intenso no. 601 + Green D no. 205 no. 243 no. 641 Turquoise no. 638 Green no. 661

Mixture of

Lemon Yellow Yellow Green Green Intenso Viridian

Red + UV _A Yellow Ochre no. 200 intenso no.641 + Cad¬ no. 616 no. 205 no. 227 no. 641 mium Yellow Light no. 213

Mixture of Mixture of Mixture of

Lemon Yellow no.200 Greenish Lemon Yellow Lemon Yellow Emerald

UV _A light Yellow + Yellow no. 205 + no. 205 + Green no. 205 Neutral Grey no. 243 Emerald Green i Emerald Green no. 615 1 no. 721 no. 615 no. 615

Note: The colour numbers are taken from the Extra Fine Gouache sample card of Royal Talens B.V., Apeldoorn, The Netherlands.

Examp l e 4

A number of experiments were carried out (by C.R. Ronda and C. Haas, Department of Inorganic Chemistry, University of Groningen, The Netherlands) to investigate what process underlies the method according to the invention for the evocation of a different colour. To that end, a number of spectra were recorded.

Figure 1 shows five emission spectra, the four separate fluorescent pigments Fluor Blue, Green, Primrose Yellow and Rocket Red, and a mixture thereof - spectra 1-5 respectively - being irradiated with UV _A (wavelength 366 nm.)

Figure 2 shows so-called activation spectra which were recorded by investigating, at the emission maxima of the various fluorescent pigments (spectra 6-9), which excitation wavelengths are capable of feeding the emission. (It should be noted that the texture which can be seen in these spectra is determined by the characteristic of the light source) .

Figure 3 shows activation spectra of the above-named fluorescent pigments during concurrent irradiation with UV _A light. Comparison of spectra 6-9 of figure 2 with spectra 10- 13 of figure 3 shows that the activation spectra of the luminophores are not dependent on concurrent irradiation with UV _Λ light. It can clearly be seen that the emiS'Sion is merely strengthened! Figure 4 shows emission spectra 14-17 in which irradiation takes place with UV _Λ light and a light of the same wavelength as the emission maximum of each respective fluorescent pigment. In each spectrum an arrow indicates the position of the irradiating visible light. From these spectra it is clearly apparent that no physical quenching occurs; after all, around the area of the wavelength of the irradiating light the intensity has neither disappeared nor is it strongly diminished.

Observable to the eye apparent quenching (physio- logical quenching) could be observed if, for example, the red luminophore is applied on black sandpaper which is irradiated

with UVΛ light and red light, the sandpaper and not the luminophore being covered with a sheet of white paper. Taking away the sheet of white paper causes the effect according to the invention to disappear. Nor can the apparent colour shift of the surface be a consequence of physical processes taking place in the luminophore, but may rather come about through the presence of much diffuse light of the same wavelength as the emission wavelength in the immediate space around the irradiated surface, as a consequence of the irradiation with light of that wavelength.

If the surroundings reflect more light than the luminophore, the luminophore then seems darker than its surroundings. This is the cause of the difference in perception in the experiment in which sandpaper is covered or not covered with a sheet of white paper. The apparent colour-change of the mixture can be explained in the same way; irradiation with UVA light and one of the emission colours results in the perception of the emissions.

Example 5

Another luminous surface can be constructed by covering a light source with a piece of white acrylic sheet (for example Perspex™) . The light passing through the acrylic sheet forms a diffuse white-radiant surface. In order to avoid edge-scattering, it is preferable to provide the piece of acrylic sheet with a black border. If a fine black grid is laid over the diffuse white radiating piece of acrylic sheet, there results a white—radiant surface with absorbent properties. The placing of a white border around the black grid avoids diffuse scattering at the longitudinal edges.

If the colour of the surroundings is controlled with the aid of blue, green, yellow and/or red light emitting

lamps, the same colour changes occur, corresponding to those of a black pigment bound to a white luminophore.

The colour of the surroundings can be controlled with a lighting unit, for example, in which there are mounted two blue flood lamps, two green flood lamps, a yellow flood lamp and a red flood lamp (Splendor , 220-230 V, 100 W) and optionally a 40 W "black light" fluorescent tube as UV _Λ source.

The lighting unit is further provided with a driving device to control the lamps in at varying speeds.

With the aid of simple electronic means it is possible to construct a lighting unit which can follow a program, in which:

1) the lamps go on an off independently or in combination in a selectable tempo;

2) the lamps are steplessly adjustable up and down either independently or in combination; and

3) options 1 and 2 are executed in combination.

Thus it is possible, with a painting which includes ^' a large number luminous areas of different colours, to evoke a different controlled colour for isolated, numerous or all areas, according to choice. It is, for example, possible to evoke movement.

The method and the apparatus according to the invention can be applied in a great number of different areas, in which areas use is made of colour and/or picture surfaces - for example, advertising, interior design, discotheques, shops, window displays and so on.

Since the luminescence of the luminophore can be caused by, for example, electroluminescence, chemi- luminescence, bioluminescence, radioluminescence or tribo- luminescence, the colour changes of the surface can be controlled over the entire breadth of the spectrum for visible light, and can be used for the construction of colour image reproduction systems, for example colour picture-tubes.

Since a pigment or mixture of pigments, possibly a black pigment, with a luminophore which emits a different colour bound to it, only finds application in a method or apparatus according to the invention, such pigments or mixtures fall within the scope of the idea of the invention which is described hereabove.