GB1589891A

GB1589891A - Optical filters

Info

Publication number: GB1589891A
Application number: GB2991377A
Authority: GB
Original assignee: C Reichert Optische Werke AG
Current assignee: C Reichert Optische Werke AG
Priority date: 1976-07-17
Filing date: 1977-07-15
Publication date: 1981-05-20
Also published as: HK97484A; FR2358667A1; FR2358667B1; DE2632341A1

Description

(54) IMPROVEMENTS IN AND RELATING TO OPTICAL FILTERS (71) We, C. REICHERT OPTISCHE WERKE AG., an Austrian Corporation of Hernalsa, Haupstrasse 219, Vienna, Austria, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: The invention relates generally to colour correcting interference filters for use with optical apparatus in which electromagnetic radiation from a light source passes through an optical system having light refracting and/or reflecting optical elements for impingement on to a radiation sensitive receiver, such as a photographic film or a television camera.

Colour films and also television cameras are adjusted by the manufacturer to respond in a certain manner to a very definite colour temperature of the light source. For artificial light, this colour temperature generally is 3,200"K, for day light, 6,000 K. If the colour temperature of the light source deviates from the colour temperature for which the receiver is intended, whether it be a photographic film or a television camera, one does not get true colour reproduction of the specimen being photographed or observed via television.

While there exists the possibility of excluding colour alterations by the choice of an appropriate light source, such a solution must take into account that optical instruments nowadays, practically without exception, contain reflection-reducing coatings on the optical elements whether they be prisms or lenses. The action of such coatings is not uniform across the whole visible spectrum, but results in relating greater amounts of reflection being present towards the short and long wavelengths of the visible spectrum than at the centre.

When several optical elements are present in succession in the optical path, a corresponding heightening of this colour-dependent differential reflection occurs, which means that the amount of red or blue radiation reaching the receiver is too small. Due to this, a sometimes very appreciable colour alteration occurs. It should be further considered that highly refractive glasses also absorb the near UV-range which increases the amount of yellowish tinge. Such colour alterations, arising from the properties of the optical elements of an instrument, will hereinafter be called instrument dependent colour alterations. Hence, if one wishes to get perfect colour photographs in conjunction with optical intruments having coatings in the optical elements, then it is necessary to be concerned with correction of such instrument dependent colour alterations. To this end, attempts have been made to use several coloured filters, for instance glass or gelatin filters. However, practically all coloured materials show a strong absorption in the UV and blue range which results in the blue portion of the spectrum, which has been already adversely affected by the optical system, being even furher weakened, and the yellowish tinge being further enhanced. The use of multiple coloured filters is especially disadvantageous in that intensive attenuation causes an appreciable prolonging of the exposure time required for the film or camera.

The spectral transmissivity of available glasses or laminates used for coloured filters is limited, and even by careful combination of several different kinds of filters, only an inadequate correction is possible. Even if one attempts to achieve an optimal correction using multiple coloured filters, the result in optical instruments having many optical elements (especially with microscopes) may be a correct specimen colour, but a background which is yellow. Although a white background is achievable, a certain distortion of the speciment colours is then unavoidable.

In addition to the above-discussed deficiencies, there are also difficulties in conjunction with the use of cemented laminates because they have limitations as to their location in the instrument due to optical and thermal rcasons. For instance, due to lack of planoparallelism and differing indices of refraction of cover glass laminate, and cement, they are unsuited for location in an imaging system. The use of cemented laminates in the illumination system of an apparatus causcs difficulties due to the high temperatures involved.

Thc use of coloured glasses is also undesirable because selection of thickness and colour shade is critical. Furthermore, with colour filters, the exposure times of photographic films are determined experimentally and then must be very accurately followed, even though the film would normally have a much greater tolerance to variations in exposure. The reason for this lics in the fact that the spectral sensitivity of the film is not consistent with unnattiral" colour transmission of the given optical instrument. Coloured glasses have the further disadvantage that, due to variations from filter to filter, one gets differing spectral distribution curves although the "same" coloured glass was used.

Filters for optical instruments have also been proposed which make it possible to change the colour temperature (spectral energy distribution) of-the light source into a colour temperature suitable for a specific radiation sensitive receiver. In conjunction with such conversion filters, the light source. generally a temperature radiator or incandescent lamp, possesses the correct energy distribution. The energy distribution of the light source is redistributed into light of another colour temperature. Even with such redistribution, the above-discussed instrument dependent colour alterations still occur because of the optical system of the instrument. Therefore colour-tinging of the photograph caused by the wrong colour temperature is avoided. but certain colour alterations still occur.

T hc present invention provides an optical instrument for use with a light source having a chosen spectral energy distribution, the instrument comprising:- an optical system including optical elements that change in an undesired manner the energy distribution of light passing therethrough so as to produce an instrument dependent colour alteration (as hereinbefore defined); detection means responsive to light and adapted to respond in a given manner to said chosen energy distribution of the source; and an interference filter whose spectral character compensites for said instrument dependent colour alteration, the interference filter comprising: a transparcnt substrate and a plurality of alternating layers of first and second dielectric coating materials: said first material having a given index of refraction; said second material having an index of refraction of about 1.5 times said given index of refraction; the total number of layers being an odd integer and including at least a first layer, second layer middle layer, penultimate layer, and last layer; said first, middle, last and other odd numbered livers being one of said first and second materials: said first and last layers being of equal optical thickness and being thinner than said middle layer; said second, penultimate, and other even numbered layers being of the other said first and second materials; and said second and penultimate layers having an optical thickness greater than the optical thickness of said first and last layers but less than the optical thickness of said middle laver.

Toe invention corrccts colour alterations originating from the instrument itself. Each instrument will. of coursc, have its own particular instrument dependent colour alteration.

For this purpose the coatings of the interference filter consist of materials with good tflinsmission characteristics. and sequential layers of different indices of refraction and thicknesses, the arrangement being designed to cancel the instrument dependent colour alterations caused hy the optical system of the instrument, whereby the energy distribution of the radiation reaching the receiver is consistent with the emission spectrum of the light source.

In the preferred embodinent of the invention. the colour errors caused by the optical elements of the instrument itself are cancelled by using the multiple-laycr interference filter of the invention. The previously described attempts to correct this source of error (by the optical system) by insertion of conversion filters for changing the colour temperature did not compensate for instrument dependent alteration of the energy distribution caused by the optical system. Even the well-known corrective photographic filters afforded, heretofore, only the possibility of selectively correcting a very select spectral range, where, in given ciretinistinces. a liege numher of corrective filters had to be inserted one after the other. In a microscope, the use of many filters can lead to such an intensive diminution of the energy of the light source. that there is insufficient residual light at the microscopic receiver for pictures. The invention nOs provides a device cancelling colour alterations individuilly for the specific instrument. whereby the same filter can be used with its respective instrument for all films of the same colour temperature, inasmuch as the energy distribution of the light source. which is representative of colour temperature, is Ic-cstnhlishctl in each and ever'. case in front of the receiver. A multiple interference filter constructed in accordance with the invention can be introduced in various locations along the light p;ih of tloc optical instrument. In ecueral. filters are introduced where the light rays are not concentrated and where the ray-divergence is small. In a microscope having a photographic attachment, it is moreover favourable to introduce the filter in the light-path in front of the eyepiece/photography axis separation, inasmuch as both the eye-piece and the photographic images are colour corrected.

In the inventive interference filter, there is the advantage that absorption in the blue and UV spectral range does not occur. The spectral transmission of the filter can readily be adjusted to a particular optical system, as the thickness of the layers and their composition or sequence can be varied within the constraints of the invention. By measurement of the spectral curve of an optical system, the required wavelength corrections can be found, and the multiple layer interference filter adjusted by selection of thickness, sequence, and refractive index of the dielectric layers. The filter is relatively transmission in view of the total number of layers being limited to an odd number less than fourteen, and the original spectral energy distribution of the light source is re-established at the receiver, providing tolerance to exposure variations as indicated by the manufacturer for films.

It is further proposed that the interference filter may additionally effect an adjustment of the energy distribution of the light source to the spectral sensitivity of the receiver. This can likewise be accomplished by selection of the filter coating, for which purpose and under given circumstances, the number of the dielectric layers may have to be increased over the number of layers provided in the preferred embodiment described hereafter. In so doing, the possibility of adjusting for difference in the colour temperature of the light source and of the receiver is accommodated. On the other hand, it is already known that some receivers possess a spectral sensitivity different from the spectral distribution corresponding to the actual energy distribution of light sources. Such differences can also be corrected by insertion of multiple interference filters depending on the receiver.

In the preferred form of instrument, the possibility exists therefore, for instance, of effecting a correction without change of the colour temperature, by only correcting the instrument dependent colour alterations (briefly stated, from 3,200 K "false", to 3,200 K "correct"). On the other hand, the possibility is likewise afforded of effecting correction of both the instrument dependent colour alterations and simultaneously to change the colour temperature (which briefly can be expressed as a change of, for instance, from 3,200 k "false" to 6,0000K "correct").

An especially simple way to manufacture such interference filters is by evaporating layers forming the coating of the interference filter on to a transparent substrate.

Experiments have shown that the coating should by constructed of an odd number of layers, preferably between 4 and 14, with alternating high and low refractive indices in which the ratio of the refractive indices (nH:nL) of the sequential high and low refractive layers is about 1.5:1; also the middle layer should preferably have an optical thickness of about 2 to 8 times the thickness of the layers closest to and furthest from the substrate, and the next closest and furthest layers should preferably have an optical thickness of about 1.5 to 2.5 the thickness of the closest and furthest layers, but should be less than the thickness of the middle layer.

In order to achieve an especially good colour correction, it is desirable that the coating consists of more than seven dielectric layers and the two layers closest the substrate and the two layers furthest from the substrate each possess an optical layer-thickness less than h,/4, (in which 480 nm S kO S 670 nm) and also it may be desirable to have the layers therebetween have an optical thickness of about fro/4.

Features and advantages of the invention will become apparent from the following description of an embodiment thereof, given by way of example with reference to the accompanying drawing, in which: Figure 1 is a graphic representation of the spectral energy distribution of a light source having a colour temperature of 3,200 K and the energy distribution curve after passing through an optical instrument; Figure 2 is a graphic representation of the transmissivity of an embodiment of an interference filter according to the present invention as a function of the wavelength; and Figure 3 shows a section through the interference filter of Figure 2 used in conjunction with the optical instrument.

As Figure 1 shows, there occurs, on transmission of light having a colour temperature of 3,200 K (dotted curve) through an instrument optical system, as a consequence of the above described circumstances, a shift of the spectral distribution, as represented by the solid line in Figure 1. By using a filter (details of which are given later), a colour correction cancelling the effects of the optical system is obtained so that on a radiation sensitive receiver such as a photographic film or television camera, the same spectral distribution is presented as corresponds to the ideal energy distribution of a light source with 3,2000K, that is, according to the dotted curve in Figure 1. To accomplish this, a filter is used having the transmission curve corresponding to Figure 2. The transmission curve according to Figure 2 can be obtained with a filter having the construction shown in Figure 3 and described in the following tahlc.

TABLE Layer Refractive Index Material Layer-Thickness (nm) 1 2.05 ZrO2 35 2 1.38 MgF2 70 3 2.05 ZrO2 140 4 1.38 MgF2 140 5 2.()5 ZrO2 140 6 1.38 MgF 140 7 2.05 ZrO2 140 8 i.38 MgF2 70 9 2.()5 ZrO2 35 Substrate 1.52 Glass As is evident from the foregoing tahle. layers 3 to 7 of the filter, according to the example, possess the same thickness (i.e. optical thickness), about k"/4, with kt, being in the range 48() Om to 67() nm and having an average wavelength of 560 nm. The layers 8 and 9 nearest the substrate and the layers 1 and 2 furthest from the substrate have a thickness less than f"/4. Preferably layers 1 and 9 have a thickness of about o/16, and layers 2 and 8 have a thickness of about fro/8. In so doing. the first and last layers to be deposited are only 35 nm thick in each case, while the second and penultimate layers have, in each case, a thickness of 7() nm.

The modification of the interference filter to correct for the colour alterations of different instruments cm be accomplished in a simple manner by changing the thicknesses of layers, by the use of materials having other refractive indices, by changing of the sequence of layers, etc. in conjunction with which there are practically no restrictions other than that such changes accord with the ranges set hy the invention. In so doing, the number of the layers is a function of the material used in each case, but also is a function of what modification is desired. With more intensive modification, e.g. of the colour temperature, more layers may have to be used than would be necessary for cancelling colour errors without change of the overall colour temperature, e.g. more layers than as described for the above embodiment. The complexity of the instrument optical system, the number of components, and the nature of coatings used in the optical system can also affect the amount of correction required, and thus the number and type of layers on the interference filter.

Referring to the example, it is further evident that the ratio of the indices of refraction of the high and low refracting layers. in sequence, amounts to about 1.5 : 1.

Coating materials having a low index of refraction. include magnesium fluoride, calcium fluoride, cryolite and lithium fluoride, while coating materials having a high refractive index include cciium oxide. niohium (V) oxide. yttrium oxide. and zirconium oxide.

In conclusion, reference should once more be made to the fact that the filter in accordance with the preferred embodiment of the invention naturally can be used not only with microscopes, (in conjunction with which, however, these instruments do represent the man field of utilisation). it would he entirely conceivable to have other optical instruments in which colour-correct reproduction is desirable. Further, it should be mentioned that the transmission characteristic of the interference filter is achieved by selective reflection, whereby and as IS zit consequence of the good transmission properties, a particularly small light loss results in arias which are not to be altered.

The optical insi rument is preferably a microscope.

WllAT WE ('LAIM lS:- l. An optical instrument for use with a light source having a chosen spectral energy distribution, the instrument comprising: - an optical system including optical elements that

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

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can be obtained with a filter having the construction shown in Figure 3 and described in the following tahlc.

TABLE Layer Refractive Index Material Layer-Thickness (nm) 1 2.05 ZrO2 35
2 1.38 MgF2 70
3 2.05 ZrO2 140
4 1.38 MgF2 140
5. An instrument according to claim 4 wherein the interference filter has the following parameters: Layer Refractive Index Material Optical Layer thickness (nm)

1 2.05 ZrO2 35

2 1.38 MgF2 70

3 2.05 ZrO2 140

4 1.38 MgF2 140

5 2.05 ZrO2 140

6 1.38 MgF2 140

7 2.05 ZrO2 140

8 1.38 MgF2 70

9 2.05 ZrO2 35 Substrate 1.52 Glass
6. An optical instrument substantially as hereinbefore described with reference to the accompanying drawings.

5 2.()5 ZrO2 140

6 1.38 MgF 140

7 2.05 ZrO2 140

8 i.38 MgF2 70

9 2.()5 ZrO2 35 Substrate 1.52 Glass As is evident from the foregoing tahle. layers 3 to 7 of the filter, according to the example, possess the same thickness (i.e. optical thickness), about k"/4, with kt, being in the range 48() Om to 67() nm and having an average wavelength of 560 nm. The layers 8 and 9 nearest the substrate and the layers 1 and 2 furthest from the substrate have a thickness less than f"/4. Preferably layers 1 and 9 have a thickness of about o/16, and layers 2 and 8 have a thickness of about fro/8. In so doing. the first and last layers to be deposited are only 35 nm thick in each case, while the second and penultimate layers have, in each case, a thickness of 7() nm.

The modification of the interference filter to correct for the colour alterations of different instruments cm be accomplished in a simple manner by changing the thicknesses of layers, by the use of materials having other refractive indices, by changing of the sequence of layers, etc. in conjunction with which there are practically no restrictions other than that such changes accord with the ranges set hy the invention. In so doing, the number of the layers is a function of the material used in each case, but also is a function of what modification is desired. With more intensive modification, e.g. of the colour temperature, more layers may have to be used than would be necessary for cancelling colour errors without change of the overall colour temperature, e.g. more layers than as described for the above embodiment. The complexity of the instrument optical system, the number of components, and the nature of coatings used in the optical system can also affect the amount of correction required, and thus the number and type of layers on the interference filter.

Referring to the example, it is further evident that the ratio of the indices of refraction of the high and low refracting layers. in sequence, amounts to about 1.5 : 1.

Coating materials having a low index of refraction. include magnesium fluoride, calcium fluoride, cryolite and lithium fluoride, while coating materials having a high refractive index include cciium oxide. niohium (V) oxide. yttrium oxide. and zirconium oxide.

In conclusion, reference should once more be made to the fact that the filter in accordance with the preferred embodiment of the invention naturally can be used not only with microscopes, (in conjunction with which, however, these instruments do represent the man field of utilisation). it would he entirely conceivable to have other optical instruments in which colour-correct reproduction is desirable. Further, it should be mentioned that the transmission characteristic of the interference filter is achieved by selective reflection, whereby and as IS zit consequence of the good transmission properties, a particularly small light loss results in arias which are not to be altered.

The optical insi rument is preferably a microscope.

WllAT WE ('LAIM lS:- l. An optical instrument for use with a light source having a chosen spectral energy distribution, the instrument comprising: - an optical system including optical elements that

change in an undesired manner the energy distribution of light passing therethrough so as to produce an instrument dependent colour alteration (as hereinbefore defined); detection means responsive to light and adapted to respond in a given manner to said chosen energy distribution of the source; and an interference filter whose spectral character compensates for said instrument dependent colour alteration, the interference filter comprising: a transparent substrate and a plurality of alternating layers of first and second dielectric coating materials; said first material having a given index of refraction; said second material having an index of refraction of about 1.5 times said given index of refraction; the total number of layers being an odd integer and including at least a first layer, second layer, middle layer, penultimate layer, and last layer; said first, middle, last and other odd numbered layers being one of said first and second materials; said first and last layers being of equal optical thickness and being thinner than said middle layer; said second, penultimate, and other even numbered layers being of the other said first and second materials; and said second and penultimate layers having an optical thickness greater than the optical thickness of said first and last layers but less than the optical thickness of said middle layer.

2. An instrument according to claim 1 wherein said transparent substrate is an optical element of said optical system.

3. An instrument according to claim 1 or 2 wherein there are more than seven layers.

4. An instrument according to claim 3 wherein there are nine layers; the first and last layers having an optical thickness of h,/16, the second and penultimate layers having an optical thickness of h,/8, and the remaining layers having an optical thickness of h,/4, wherein 480 nm S B" S 670 nm.