GB1560943A - Device for determining colorimetric data from a sample - Google Patents

Description

(54) DEVICE FOR DETERMINING COLORIMETRIC DATA FROM A SAMPLE (71) We, KODAK LIMITED, a Company registered under the law of England, of Kodak House, Station Road, Hemel Hempstead, Hertfordshire, 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: This invention relates to light meters, and more particularly to devices for determining colorimetric data from a sample.

There are two main approaches to colour measurement by instrument. The first involves calculation from measured spectral reflectance or transmittance curves using known source characteristics and standard visual colour curves. An instrument for making these measurements is usually large and requires a sample, whose colour characteristics are being determined, to be positioned within or in contact with the instrument. The second approach involves use of a multiple colour filter device, which may be portable, to provide more quickly an abridged series of photometric results which are used as the basis of calculation. The accuracy of the results obtained thereby depends on the stability of the filters and light sensor used.

The present invention provides an improved device for determining colorimetric data.

In accordance with the present invention there is provided a device to determine colorimetric data from a sample, the device comprising a variable wavelength interference filter presenting a plurality of different passband portions, a light sensor arranged to receive light from the sample after it has passed through one of the pass-band portions, a stepping motor for moving the filter in a succession of steps to position different passband portions thereof in the light path, a presettable control circuit for the motor to determine the length of each of the steps, and means for producing a series of signals indicative of the response of the light sensor at the wavelengths of the said pass-band portions.

The interference filter may be annular with the wavelength of the pass-band portions being dependent on angular position.

Alternatively the interference filter may be a linear wedge interference filter element, and the said means for directing light may comprise means for advancing the element longitudinally.

The means for producing the series of signals may include amplifier means, and means for modifying the gain for one or more signals in accordance with the pass-band portion from which the signal has been derived.

One embodiment of spectroradiometer in accordance with the invention will now be described by way of example with reference to the drawings accompanying the Provisional Specification in which: Figure 1 shows a diagrammatic sectional view of important parts forming the measuring device; Figure 2 shows a plan of an interference filter used therein; and Figure 3 shows a block diagram of a control system for the measuring device.

Referring now to the drawings, the device has a telephoto lens 1 adapted to image a sample (not shown) onto a light sensor in the form of a photodiode 2. Viewing means in the form of an alignment and focus viewing screen 3 for use in setting up the device is provided together with a movable cooperating mirror 4 which can be retracted from the position shown after the sample has been viewed thereby through the lens and the lens has been focused. Light from the lens 1, which is preferably a telephoto lens and may be a zoom lens, then passes through a lightchopper 6, comprising an aluminium disc 7 formed around its periphery with an equallyspaced series of holes, and an electric motor 8 arranged to rotate the disc. The chopper modulates the light at equal mark-space ratio at, say, 1000 hertz.

Light passing the chopper 6 falls on an interference filter 9, and a filtered narrow band of radiant energy passes through the filter 9 and onto the photodiode 2. The interference filter 9, seen in plan view in Figure 2, comprises a pair of glass discs carrying between them a part-annular, dichroic interference filter zone 11 of progressively increasing passband wavelength. The pass-band wavelength of any portion thereof depends on the angular position of the portion on the filter, and a datum mark 12 indicative of the start of filter rotation is provided by a slot adjacent to the low frequency end of the filter zone and formed on an aluminium annulus 10. The discs are axially mounted for rotation by a stepping motor 13 to present selected portions of the filter zone to the light passing to the light sensor photodiode 2.

These selected portions are such that a desired series of narrow wavebands distributed across the light spectrum emanating from the sample is allowed to fall in turn on the light sensor. Where the characteristics of the filter zone progress uniformly along its length then a series of equi-angular rotations of the filter, with intervening stops for taking readings, will produce a series of uniformly spaced pass-bands in the light spectrum from the sample, and the stepping motor can rotate the filter in a series of equal steps of appropriate angle.

In a practical case however the characteristics of the filter zone may not be uniform, or it may be desired to take readings at nonuniformly spaced positions in the light spectrum. The filter can then be moved in angular steps which are not uniform. The control system may be arranged so that the stepping motor moves through a variable number of steps to advance the filter through the different angles required. The program necessary for stepping motor operation in this manner can be determined and set during initial calibration of the device.

Referring now to Figure 3 of the drawings, which shows the components of the control unit as well as the device, the output from the photocell 2 is fed to a pre-amplifier 20 and thence through a coarse gain control 21 to a variable gain amplifier 22. The gain of amplifier 22 is selected in accordance with a predetermined program by a decoder 23. One output of variable gain amplifier 22 is fed to peak/phase detector 24 and through an analogue-to-digital converter 25 to a digital display device 26 and a magnetic-tape recording point 27. Analogue recording could be effected from a tapping point, not shown, connected to the outlet of the detector 24.

The output (i.e. spectral radiance) displayed in digital form on device 26 is obtained when light from the sample has passed through a selected portion of the interference filter 11.

The wavelength allowed to pass through the interference filter is between 400 and 700 manometers, say, and the particular selected value is shown on a digital display device 30.

A manually pre-settable resolution logic circuit 31 determines the angular setting of the interference filter 11 for each successive measurement of the photocell output, and has a manual control 32 for setting the difference in wavelength (e.g. 5, 10, 20 or 50 nanometers) between successive measurements.

Resolution logic circuit 31 triggers motor step logic 33 which controls how many cycles from a master oscillator 34 are allowed to energise a drive circuit 35 for the stepping motor 13. Interlocking of the output from the photocell and the rate at which the light passing to the photocell is modulated by the chopper 6 is effected through a speed lock circuit 36 triggered from the chopper disc.

To conserve power from the power source 37, conveniently a rechargeable battery particularly where the device is easily portable, a stand-by logic circuit 38 changes the apparatus to a stand-by condition after a predetermined display time set by a manually set logic circuit 39. Reset circuit 40 is arranged to reset the interference disc to its pre-scanning condition ready to start a series of readings.

If logic circuit 41 is set to "automatic" then the readings are obtained automatically. On the alternative "manual" setting of circuit 41 the operator has to actuate a control to obtain each reading.

Calculation of colorimetric data, from the readings of spectral radiance obtained at predetermined wavelength intervals over the chosen portion of the visible spectrum, may be effected manually or automatically. The device may be provided with a built-in computing facility, or the measured data may be recorded on magnetic tape or on punch tape, or otherwise keyed into a separate computer.

In an alternative construction of telespectroradiometer a linear wedge interference filter element is used, and this is moved longitudinally to position the different filter zones of the element progressively into the light path from the sample. A stepping motor is used to produce this movement of the filter element. Any suitable mechanical drive for the filter element may be incorporated, for example the stepping motor may rotate a pinion which is in mesh with a rack attached to the element parallel to the longitudinal axis of the element. As before, the filter may be moved by the programmed stepping motor through a series of non-uniformly spaced positions in which the passband portions of the filter element are positioned for light from the sample to pass therethrough.

WHAT WE CLAIM IS: 1. A device to determine colorimetric data from a sample, the device comprising a

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

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. Light passing the chopper 6 falls on an interference filter 9, and a filtered narrow band of radiant energy passes through the filter 9 and onto the photodiode 2. The interference filter 9, seen in plan view in Figure 2, comprises a pair of glass discs carrying between them a part-annular, dichroic interference filter zone 11 of progressively increasing passband wavelength. The pass-band wavelength of any portion thereof depends on the angular position of the portion on the filter, and a datum mark 12 indicative of the start of filter rotation is provided by a slot adjacent to the low frequency end of the filter zone and formed on an aluminium annulus 10. The discs are axially mounted for rotation by a stepping motor 13 to present selected portions of the filter zone to the light passing to the light sensor photodiode 2. These selected portions are such that a desired series of narrow wavebands distributed across the light spectrum emanating from the sample is allowed to fall in turn on the light sensor. Where the characteristics of the filter zone progress uniformly along its length then a series of equi-angular rotations of the filter, with intervening stops for taking readings, will produce a series of uniformly spaced pass-bands in the light spectrum from the sample, and the stepping motor can rotate the filter in a series of equal steps of appropriate angle. In a practical case however the characteristics of the filter zone may not be uniform, or it may be desired to take readings at nonuniformly spaced positions in the light spectrum. The filter can then be moved in angular steps which are not uniform. The control system may be arranged so that the stepping motor moves through a variable number of steps to advance the filter through the different angles required. The program necessary for stepping motor operation in this manner can be determined and set during initial calibration of the device. Referring now to Figure 3 of the drawings, which shows the components of the control unit as well as the device, the output from the photocell 2 is fed to a pre-amplifier 20 and thence through a coarse gain control 21 to a variable gain amplifier 22. The gain of amplifier 22 is selected in accordance with a predetermined program by a decoder 23. One output of variable gain amplifier 22 is fed to peak/phase detector 24 and through an analogue-to-digital converter 25 to a digital display device 26 and a magnetic-tape recording point 27. Analogue recording could be effected from a tapping point, not shown, connected to the outlet of the detector 24. The output (i.e. spectral radiance) displayed in digital form on device 26 is obtained when light from the sample has passed through a selected portion of the interference filter 11. The wavelength allowed to pass through the interference filter is between 400 and 700 manometers, say, and the particular selected value is shown on a digital display device 30. A manually pre-settable resolution logic circuit 31 determines the angular setting of the interference filter 11 for each successive measurement of the photocell output, and has a manual control 32 for setting the difference in wavelength (e.g. 5, 10, 20 or 50 nanometers) between successive measurements. Resolution logic circuit 31 triggers motor step logic 33 which controls how many cycles from a master oscillator 34 are allowed to energise a drive circuit 35 for the stepping motor 13. Interlocking of the output from the photocell and the rate at which the light passing to the photocell is modulated by the chopper 6 is effected through a speed lock circuit 36 triggered from the chopper disc. To conserve power from the power source 37, conveniently a rechargeable battery particularly where the device is easily portable, a stand-by logic circuit 38 changes the apparatus to a stand-by condition after a predetermined display time set by a manually set logic circuit 39. Reset circuit 40 is arranged to reset the interference disc to its pre-scanning condition ready to start a series of readings. If logic circuit 41 is set to "automatic" then the readings are obtained automatically. On the alternative "manual" setting of circuit 41 the operator has to actuate a control to obtain each reading. Calculation of colorimetric data, from the readings of spectral radiance obtained at predetermined wavelength intervals over the chosen portion of the visible spectrum, may be effected manually or automatically. The device may be provided with a built-in computing facility, or the measured data may be recorded on magnetic tape or on punch tape, or otherwise keyed into a separate computer. In an alternative construction of telespectroradiometer a linear wedge interference filter element is used, and this is moved longitudinally to position the different filter zones of the element progressively into the light path from the sample. A stepping motor is used to produce this movement of the filter element. Any suitable mechanical drive for the filter element may be incorporated, for example the stepping motor may rotate a pinion which is in mesh with a rack attached to the element parallel to the longitudinal axis of the element. As before, the filter may be moved by the programmed stepping motor through a series of non-uniformly spaced positions in which the passband portions of the filter element are positioned for light from the sample to pass therethrough. WHAT WE CLAIM IS:

1. A device to determine colorimetric data from a sample, the device comprising a

variable wavelength interference filter presenting a plurality of different pass-band portions, a light sensor arranged to receive light from the sample after it has passed through one of the pass-band portions, a stepping motor for moving the filter in a succession of steps to position different passband portions thereof in the light path, a presettable control circuit for the motor to determine the length of each of the steps, and means for producing a series of signals indicative of the response of the light sensor.

2. A device as claimed in Claim 1, wherein the interference filter is annular with the wavelength of the pass-band portions being dependent on angular position.

3. A device as claimed in Claim 1, wherein the interference filter is a linear wedge interference element, and the said means for directing light comprises means for advancing the element longitudinally.

4. A device as claimed in any preceding claim, wherein the stepping motor is controlled to move the filter through a series of non-uniform steps.

5. A device as claimed in any preceding claim, comprising a telephoto lens arranged to image light from the sample onto the said light sensor, and means for viewing the sample through the lens and focusing the lens.

6. A device as claimed in claim 1, wherein the means for producing the said series of signals includes amplifier means, and means for modifying the gain for one or more signals in accordance with the filter pass-band portion from which the signal has been derived.

7. A device for determining colorimetric data as claimed in Claim 1 and substantially as hereinbefore described.

8. A device for determining colorimetric data substantially as hereinbefore described with reference to the drawings accompanying the Provisional Specification.