GB2249389A - Densitometers - Google Patents

Abstract

A densitometer for measuring optical density in which a sample 9 is illuminated with light and the emergent light is separated into its spectral components, for example by use of a ruled blazed diffraction grating 23, after which the separated components are measured, for example on a linear array of photo diodes 25. Various means for calibrating the instrument are described. For example a light guide 14 normally conducting light from a module 7 (housing the sample 9) to a module 15 (housing the grating 23) may be unplugged from the module 7 and plugged into a calibration module 16 (containing a cadmium lamp 17). A measurement light source module 1 contains a cold light source. Light is transmitted between modules 1 and 7 by a water-filled light guide 6. <IMAGE>

Description

DENSITOMETERS The present invention relates to densitometers.

A densitometer is used for measuring the optical reflection or transmission density of a sample such as photographic film or paper. The sample is illuminated and the incident light is compared with that reflected or transmitted in order to determine the optical density of the sample.

For spectrally selective materials, various high quality filters need to be used in conjunction with known instrument spectral characteristics in order to measure the response in different colours (e.g red, green, blue, visual) as defined for the particular measurement. (e.g. International Standard ISO 5/3 Photography - Density Measurements - Spectral Conditions).

The present invention seeks to overcome the problems which arise due to the use of colour filters for spectral measurements in a densitometer.

According to the present invention the densitometer comprises means for illuminating a sample with light, a spectograph for separating the emergent light from the sample into its spectral components and means for measuring the separated spectral components and calculating the optical density of said sample.

The emergent light can be separated by relative refraction by means of an optical prism, but preferably a diffraction grating is used particularly a ruled blazed grating.

The emergent separated light will preferably be arranged to strike a linear array of photo diodes.

The output of the photo diodes can then be fed to a computer and be analysed in order to measure the spectral transmittance of the sample and thence combining it with the required spectral products such as those defined in ISO 5/3 to determine the optical density. Other spectral products may be used e.g.

spectral sensitivity of printing paper or response function of the human eye.

With such an arrangement a principal wide band illuminating source, such as essentially white visible light from a quartz halogen lamp, and various calibration light sources can be provided.

A spectral calibration light source can be a cadmium lamp which has a line spectrum of known wavelengths and can be used as a substitute for the emergent sample light for infrequent spectral calibration of the instrument. A lamp such as a neon lamp can be provided for frequent rapid checking purposes.

An embodiment of the invention will now be described by way of example with reference to the accompanying single schematic drawing.

Referring to the drawing, a measurement light source module 1 houses a quartz halogen lamp with IR transmitting reflector 2 to give an essentially white cold light source. This is arranged to direct light through a heat absorbing filter 3 via a shutter system 4 to a light guide mount 5. A water filled light guide 6 then takes the light to the input of a sample illuminating module 7. The light then passes through an opal diffuser 8 against which is positioned the sample 9. The entire optical system complies with International Standard ISO 5/2 - Photography - Density Measurements - Geometric conditions for transmission density.

The sample is moved into position by a transport system which is not shown. The sample is gripped between two belts which are moved by a stepping motor.

The sample is pushed along a guide into an entry nip formed by the two belts. Sprung pads push the two belts together causing the belts to grip the sample.

The sample is then guided through the illumination zone while the belts grip its bottom edge. A reciprocating aperture ensures that the sample remains flat in the illumination area during measurements. The sample follows a pre-defined path under computer control, measurements being made at the appropriate points.

The central portion of the light passing through the sample, defined by reciprocating aperture 10, is collected by the fibre optic light guide 14 positioned so as to comply with the geometric conditions defined in ISO 5/2.

The fibre optic bundle 14 then passes the light to a spectograph module 15. A main spectral calibration module 16 comprises a cadmium lamp 17 and a fibre bundle coupling unit 17a. Thus, the fibre bundle 14 can be unplugged from the unit 12 and plugged into the unit 17a for spectral calibration.

The fibre bundle 14 terminates as a rectangular group of the fibres 18 transmitting light into the spectrograph and a secondary circular bundle of fibres which terminate at a gallium arsenide photo-diode 19 which is used to monitor the output in the case of wide dynamic range spectrally non-selective samples, i.e to monitor very low levels of output.

Attached to the fibre termination 18 is a neon lamp 20 and both are located to direct light in the direction of a concave mirror 21. The function of the neon lamp 20 is to serve as a comparative check of wavelength. The focal length of the convex mirror 21 enables a cone of illumination from the fibre termination 18 and neon lamp 20 to be reflected as parallel rays to a grating 22. The grating surface is in the form of a ruled blazed grating which diffracts the light so as to separate it into its spectral components. The diffracted light is then directed to a second concave mirror 24 whose focal length brings parallel light to a focus at a linear array of photo-diodes 25.The spacing of the photo-diodes and the degree of cut of the ruled blazed grating 23 ensures that the separated light spreads out across the array (as per the dotted ray diagram) so as to to give adequate sensitivity and wavelength resolution to the measurement.

The output of the array of photo-diodes then passes to an analogue signal module ASM where the low current signals from the array of photo-diodes, 46 in this case, are passed to seven amplifier cards (7AC) which each support seven amplifiers as well as de-coupling capacitors, programmable gain multiplexers, integrators and control circuits. The outputs are then fed to a host computer HC where they are digitised and where the density calculations take place.

The output of the whole system is calibrated from time to time by comparison with the output of the spectral calibration module 15, and spot checks on wavelength can be made by comparison with the output of the neon lamp 20.

The described example is, by virtue of the design of the light source module 1, arranged as a transmission densitometer. By using reflective optics in place of the opal glass the device can be arranged as a reflection densitometer.

Claims (4)

1. A densitometer comprising means for illuminating a sample with light, a spectograph for separating the emergent light from the sample into its spectral components, and means for measuring the separated spectral components and calculating the optical density of said sample.

2. A densitometer according to claim 1 in which the spectograph comprises a diffraction grating for separating the emergent light into its spectral components by diffraction.

3. A densitometer according to claim 1 or 2 comprising a linear array of photo-diodes located so that the emergent separated light will fall on said array of photo-diodes so as to be capable of measurement by measurement of the output of said diodes.

4. A transmission or reflective densitometer substantially as hereinbefore described with reference to the accompanying drawing.