DENSITOMETRY METHOD

Abstract: A method of performing densitometry on photographic films and papers or the like, in which a processed film control strip (1) is digitally scanned (3) and analysed to produce a set of constant exposure information tables (5). The information contained in these tables is cross referenced (9) with appropriate transmission/reflection density values (7) and statistical techniques (11) are applied to the data to give a mapping of the transmission/reflection density values (7) to the constant exposure information (5).

Description: Densitometry method This invention relates to a method and apparatus for performing densitometry . The process of densitometry is the measurement of optical properties of a material, specifically the optical transmission properties and/or the optical reflection properties of the material. Densitometry is particularly but not exclusively applicable to measuring the optical density (light absorbance or reflectance) and/or colour content of photographic images which have been exposed and processed. In the context of processing photographic films and papers, quality control is commonly based on the use of pre-exposed control tools or reference materials which are processed and/or printed, and then measured for optical density (light absorbance or reflectance) and/or colour content by means of an instrument for performing densitometry. The control tools or reference materials may be known as control strips, step wedges, printer tests, or monitors. The reference materials or samples may be supplied (together with a processed and/or printed sample) by the film manufacturer in a pre-exposed state (under controlled exposure conditions) , or the reference films may be generated using a controlled exposure instrument known as a Sensitometer . The reference materials normally contain areas of constant exposure, referred to as 'steps' . A densitometer (or apparatus for performing densitometry) is a measuring instrument which provides controlled illumination of the film or paper sample, and one or more photo-cells disposed to be illuminated by light transmitted through or reflected by the sample such that the photo-cell or cells produce a light - dependent output. The densitometer measures the photo- cell output, and converts the outputs to a universal measuring scale denoted in "densitometric units" . Typically, a densitometer will use filtration to sample light intensities at different bands of the visible spectrum, in order to characterise the light intensity in terms of its constituent colours, for example red, green and blue, or cyan magenta and yellow. In respect of measurements performed on a given sample, a densitometer output (in densitometric units) may be manually recorded and used for process control computations, or the densitometer output may alternatively be coupled either directly or indirectly (eg via a modem) to a process control computer. The densitometer may additionally be arranged to perform some rudimentary computations and result storage internally. A densitometer therefore provides a highly accurate and direct measure of the light transmitted through or reflected from an object. As a consequence, such instruments require the use of very high quality optical components to ensure that accurate and reproducible results are obtained. 3 Furthermore, sophisticated scientific instruments, such as densitometers, are necessarily expensive, and do not have a market size which would allow extensive competition and economies of scale to reduce prices. According to a first aspect of the present invention there is provided a method of performing densitometry comprising the steps of providing a document or image scanner, inserting a processed pre-exposed control tool or reference material into the scanner in place of a conventional document or image, operating the scanner to produce a digital output which is a measure of the optical density and/or of the colour content of the inserted control tool or reference material, and passing the digital output of the scanner to a digital computing means programmed to analyze the scanner output for the purposes of quality control in the context of image reproduction, such as processing photographic films or papers, reproduction of graphic arts, X-ray films or the like. Document or image scanners for scanning and digitising the images on documents (eg illustrations and/or text on film or paper) are increasingly common in the context of personal computing ("PC"), and present the price and availability advantage concomitant with a large market, competition, and economies of scale. A document or image scanner provides controlled illumination of a light source, together with a computer-compatible output representative of colour and density (optical absorbance or reflectance) . Use of a document or image scanner in conjunction with a suitably programmed digital computing means (eg a personal computer) enables the combination to perform densitometry and so allows quality control in the context of processing photographic films or papers, particularly in commercial or professional processing of films or papers (whether of visible scenes or objects, or X-ray films, or other images), as well as in the context of other forms of image reproduction. According to a second aspect of the present invention there is provided apparatus for performing densitometry, the apparatus comprising the combination of a document or image scanner and a digital computing means, the document or image scanner being operable to receive a pre-exposed control tool or reference material in place of a conventional document or image and to scan the control tool or reference material to produce a digital output which is a measure of the optical density and/or the colour content of the received control tool or reference material, the digital computing means being coupled to the document scanner to receive the digital output of the scanner, the digital computing means being programmed to analyze the scanner output for the purposes of quality control in the context of image reproduction, such as processing photographic films or papers, reproduction of graphic arts, X-ray film or the like. Embodiments of the invention will now be described by way of example. A commercially available image, film or hardcopy (paper-printed documents) scanner is obtained, being a device capable of digitising images for input to a computer for viewing, manipulation, transmission or storage. The scanner comprises light source (s) and light sensor (s), together with the appropriate control circuitry necessary to accomplish the digitisation. The output of the scanner is coupled to a personal computer (a self-contained digital computer) . The computer is programmed to receive and to process the scanner output in a manner which approximates the optical densities of transmitted or reflected light either at specific wavelengths or through specific filters in respect of a control tool or reference film or sample film or other material being processed by the scanner. In the photographic processing industry, the filters may be one of a number of the industry-defined colour or density filters, for example "Status A" and "Status M" colour filters. A calibration pack of processed film or paper control strips, together with tables of their densitometric values, is also supplied. Ideally, but not essentially, the calibration pack will comprise at least one strip for each of the three photographic filter statuses, and each strip is exposed at a number of discrete levels (usually 21 or more) . A flow chart for calibration of the scanner is shown in figure 1. In order to calibrate the scanner, a processed control strip from the calibration pack is inserted into a scanner. The control strip may be made of negative film 1, positive film 1A or paper IB. The control strip is scanned and results stored 3 in a standard digital format such as bit-map. The stored image should contain at least 8 bits per colour, which gives a range of intensity values from 0 to 255 for each colour. The stored image is analyzed digitally to identify the areas of constant exposure ('steps') and three tables 5 are produced, one for each colour red green and blue. The information in these tables is then cross referenced 9 with the information contained in the appropriate/known transmission/reflection density value 6 table 7 for each colour. This is done to provide a comparison between colour intensity values obtained from the scanner and the known density values given in the transmission/reflection density value table. In cases where a given density value gives only a single colour intensity value then this density value is mapped to the measured colour intensity value. Where there are several samples used of the same density then the corresponding colour intensity would be an average of the measured colour intensities. Statistical techniques such as Legrangian or Cubic Spline Interpolation are used to construct a set of tables 11, one each for red, green and blue, based on the above mappings. The above process can be undertaken in a similar manner for positive control strips 1A or paper control strips IB. This technique gives a mapping or correspondence between density values and the scanned colour intensity values for the photographic filter status of the control strip. This technique should be performed with a sample and table of densitometric values for each of the three photographic filter statuses. The mapping or correspondence is used to calibrate the scanner in order that it may provide an accurate representation of the transmission/reflection density as would be obtainable from a densitometer. The use of the scanner along with a digital computer and known densitometric transmission/reflection values allows quality control of photographic printing and the like to be tested without having to use a densitometer for routine measurements. The present invention therefore allows the calibration of a scanner to give 7 accurate and inexpensive quality control. The present invention may be used as a routine means of quality control by obtaining photographic density values for a control strip and a reference strip as follows. The reference strip is scanned and the image stored. The image is analyzed to produce 3 tables giving the colour intensity value (0 to 255 or greater) for red, green and blue. The appropriate red, green or blue colour intensity to density look-up tables are selected depending on the photographic filter. Using the calibration tables established above, a statistical analysis is performed, as above, to determine the densitometric values of the reference strip from the scanned colour intensity values. In the same manner, a control strip is scanned and its densitometric values obtained. The density values of both the reference strip and control strip are now known which allows photographic process control to be performed as if the reference and control strip density values had been read on a densitometer. The present invention also allows the routine comparison of photographic materials. A processed sample of a material with known densitometric characteristics is scanned and the density values are obtained from the scanner, as above. The unknown sample, which has been processed in the same conditions as the known material is scanned and the photographic densities are determined. From the comparison of the densitometric values of the two samples, the difference in photographic parameters, such as speed or contrast or any such calculation as may be performed on the densitometric values can be determined. In a further example, the present invention may be used as a means of determining the print quality of a printer by scanning a test print, determining the photographic densities of the test print as above. The quality of subsequent prints can be determined by comparison with the density values obtained from the test print . In general, for a given known input to the scanner, eg a control tool or reference material, information regarding the status of control of the process can be ascertained by the computer. Given the status of control of the process, appropriate action can be taken, eg the correction of deviations from optimum parameters . Without knowledge of the input material, eg with a film of unknown properties in the scanner, but with knowledge of the state of the control of the process through which the input material has passed (which may be obtained as described in the preceding paragraph) , information about the input material can be ascertained. Improved performance can be obtained by various measures (alone or in any suitable combination) , such as use of a more accurate scanner, use of more robust algorithms to approximate optical densities, use of known standard inputs to characterise hardware and system responses, and use of other such improvements. It is also envisaged that the present invention could be used as a spectrophotometer , by being adapted to analyze the intensity of each colour or band of wavelength in an optical spectrum. In a modification, for application in a spectral unit, the steps of filtering the light source or photocells may be included. It is also envisaged that the digital scanner and computing means could be incorporated into a single dedicated unit for the purpose of the present invention. Modifications and variations of the above-described embodiments can be adopted without departing from the scope of the invention. While the invention has been particularly described in respect of process quality control in the context of photographic film processing, the invention can also be applied to other forms of image reproduction, eg to graphic arts, ie to the professional, commercial and industrial printing of illustrations, text, X-ray films or other images on paper or the like.

Claims: 10 Claims 1. A method of performing densitometry comprising the steps of providing a document or image scanner, inserting a pre-exposed control tool or reference material into the scanner in place of a conventional document, operating the scanner to produce a digital output which is a measure of the optical density and/or of the colour content of the inserted control tool or reference material, and passing the digital output of the scanner to a digital computing means programmed to analyze the scanner output for the purposes of quality control in the context of image reproduction, such as processing photographic films or papers, reproduction of graphic arts, X-ray films or the like. 2. Apparatus for performing densitometry, the apparatus comprising the combination of a document or image scanner and a digital computing means, the document or image scanner being operable to receive a pre-exposed control tool or reference material in place of a conventional document or image and to scan the control tool or reference material to produce a digital output which is a measure of the optical density and/or the colour content of the received control tool or reference material, the digital computing means being coupled to the document scanner to receive the digital output of the scanner, the digital computing means being programmed to analyze the scanner output for the purposes of quality control in the context of image reproduction, such as processing photographic films or papers, reproduction of graphic arts, X-ray film or the like.

Inventors: HOWELL MICHAEL JOHN, HOWELL, MICHAEL, JOHN
Applicants: HOWELL MICHAEL JOHN, HOWELL, MICHAEL, JOHN
Application year: 1999
Main classification: H04N1
IPC codes: G01N21, H04N1