GB2243265A

GB2243265A - Thermal recording apparatus

Info

Publication number: GB2243265A
Application number: GB9106528A
Authority: GB
Inventors: John Edward Baxendale
Original assignee: Dowty Maritime Ltd
Current assignee: Dowty Maritime Ltd
Priority date: 1990-03-29
Filing date: 1991-03-27
Publication date: 1991-10-23
Also published as: WO1991014577A1; GB9106528D0; GB9007014D0; AU7489691A

Abstract

A method of compensating for uneven heating effects in the heating elements (1) of thermal recording apparatus energised by input gradation data (8) comprising measuring the optical density of spots recorded by the heating elements when energised, using said optical density measurements to derive gradation correction data for each heating element, storing said gradation correction data (7), and using said gradation correction data to produce corrected outputs (3) to energise the heating elements in response to input gradation data. The correction memory (7) may be a look-up table storing corrected values each corresponding to the identity of a particular heating element and a particular level of the input gradation data. The corrected values are derived by averaging the optical density of the spots recorded by the heating elements at one grey level, comparing the optical density of each spot with the average to produce a correction ratio, and applying a correction equation for all grey levels.

Description

THERMAL RECORDING APPARATUS Technical Field This invention relates to thermal recording apparatus for recording a grey scale image using a thermal recording head, and to a method of compensating for spurious heating effects in thermal recording apparatus.

Known thermal recording heads comprise a linear array of heating elements each of which can be selectively energised from an electric current source so as to heat a portion of a recording medium and produce a spot or printed pixel along a print line. Successive lines are printed by moving the recording head and recording medium relative to one another. The greyness or optical density of each spot is controlled by controlling the electrical energy applied to each heating element in accordance with gradation data corresponding to the image to be recorded.

It has been appreciated that it is difficult to ensure that the heating elements all have the same electrical resistance, and thus the same electrical voltage applied to the different elements for the same duration does not always produce the same grey level. The quality of the recorded image therefore suffers. It has been proposed to overcome this problem by energising the heating elements in accordance with their resistance so as to compensate for uneven heating effects. Resistance values are recorded in a memory and used in conjunction with input gradation data to produce corresponding corrected gradation data from data stored in a correction memory.For every grey level of the input gradation data, there is corresponding corrected gradation data available depending on the resistance of the heating element being energised and how much this deviates from a predetermined average or nominal value.

It has been found that this proposed solution to the non-uniformity of the heating elements is not fully effective, and an object of the present invention is to provide a more effective solution.

Disclosure of the Invention The present invention is based on an appreciation of the fact that the variation in the performance of the heating elements is not simply due to variations in resistance but is due to variations in their geometry. In particular, heating elements comprising thick or thin film elements on a ceramic substrate with an insulating wear resistant surface glaze, are found to vary in surface geometry and also in the thickness of the heating film and surface glaze. These variations effect the thermal properties of the heating elements independently of their resistance. Thus variations in the performance of each heating element is best compensated for by reference to correction data derived from measurement of the optical density of spots recorded by the heating elements.

Thus, thermal recording apparatus according to the invention comprises a plurality of heating elements each operative to record a spot on a recording medium corresponding to input gradation data, and a correction memory storing gradation correction data for producing a corresponding output to each heating element in accordance with the input gradation data and the identity of each heating element, said gradation correction data corresponding to measurements of the optical density of spots recorded by said heating elements in response to input gradation data uncorrected by said gradation correction data.

Preferably, the gradation correction data stored in the memory consists of individual corrected values of input gradation data for each combination of heating element and grey level of the input gradation data, these corrected values being stored in the memory in the form of a look-up table so that the address of each heating element serves to access the corresponding corrected value for the particular grey level of the input gradation data.

Preferably, the corrected values of the input gradation data are derived by averaging the optical density of the spots recorded by all the heating elements at one particular grey level of the input gradation data before any correction is applied, and comparing the optical density of each spot with the average optical density value so as to produce a correction ratio for each heating element. An equation is then used which relates the input gradation data to corrected values in terms of the correction ratio for each heating element so as to generate the corrected values for all the other grey levels of the input gradation data.This equation is established from an analysis of the optical density measurements of the spots recorded by the heating elements at different grey levels of the input gradation data, and once established it is found to apply to any recording head employing the same type of heating element.

Description of the Drawings The invention will now be described by way of example with reference to the accompanying drawings in which Figure 1 is a schematic drawing of the drive circuit of a thermal recording head according to the invention, Figure 2 is a graph showing the variation in optical density of the spots recorded by a prior art recording head at different grey levels, Figure 3 is a graph showing the average and lightest and darkest optical densities for each grey level in Figure 2.

Mode of Carrying Out the Invention The thermal recording head 1 is of a known type and comprises a plurality of heating elements fabricated on a ceramic substrate as a linear array so as to record a line of spots or pixels on a recording medium. The heating elements may be fabricated using thick film technology in which the heating elements are still screen printed, or thin film technology in which the heating elements are deposited by vacuum evaporation. In both cases, an insulating, wear-resistant surface glaze is formed over the heating elements. The recording medium may be thermally sensitive paper or plastic film on which the image is recorded directly, or thermally sensitive carrier film from which the image is transferred to paper.In all cases, the grey level or optical density of a spot formed on the recording medium is generally proportional to the heat emitted by the heating element.

A recording head drive unit 2 serves to control energisation of the heating elements in accordance with image data received via a a connection 3 from a buffer memory 4. The image data is quantised in terms of levels on a grey scale, and the drive unit 2 energises the heating elements with energies corresponding to these grey levels or gradation data so as to record the required image.

The buffer memory 4 typically stores sufficient data to record one line of the image at a time, this data being supplied via a data input 5 in conjunction with address data at an address input 6 so that the gradation data is related to the corresponding heating elements that are to be energised.

As described so far, the recording head is of known construction. The uniformity of the head in recording grey scales can be evaluated by energising all of the heating elements at each successive grey level and measuring the optical density of the resulting spots. The results of these tests at grey levels 1 to 15 are plotted in Figure 2, and illustrate how the optical density i varies across the width w of the recording head, and how this variation becomes more pronounced at higher grey levels. These same results are represented in a different manner in Figure 3 by plotting normalised values of the average optical density i and the lightest and the darkest optical densities for each grey level.This graph snows, for example, that one of the heating elements operating at a grey level 10.5 produces a lightest spot of optical density i = 0.9 and that in order to produce a spot of average optical density I = 1, this heating element needs to be energised at a level corresponding to a grey level 11.5.

The illustrated recording head is adapted to counter the described non-uniformity of grey level recording by providing a correction memory 7 which stores correction data derived from optical density measurements. The memory 7 takes the form of an EPROM and is configured as a look-up table so as to respond to input image data at data input 8 and heating element address data at address input 9 and produce corresponding corrected gradation data at output 10 which is applied to the data input 5 of the buffer memory 4.

The correction data is derived from the optical density measurements of the spots recorded by the heating elements at a predetermined grey level without the correction memory 7 being effective, for example, the optical density measurements i shown in Figure 2 for grey level 10. The optical density of each spot is compared with the average optical density I of all the spots at grey level 10 so as to produce a correction ratio R = i/I for each heating element.

In order to accommodate grey levels other than level 10, the relationship between the optical density characteristics shown in Figure 2 is established in the form of an algebraic equation which relates the corrected gradation data C to the input image data D in terms of the correction ratio R for each particular heating element. This equation is based on the curves shown in Figure 3, and for a recording head model NH-B28-42, manufactured by Ricoh has been determined as follows: C = D + D 1 - R A where 1 s A 4 1.5.

This equation is used to calculate the corrected output data C for each heating element at each grey level of the input gradation data D.

The resulting corrected output data C is then rounded up or rounded down to the nearest grey level and is stored in the look-up table of the memory 7.

In other embodiments of the invention, the equation quoted above may be used with a different value for the factor A; for example, with 1 < A 5 2 although preferably 1.1 < A 4 1.2.

As described above, the recording head has a 16 level grey scale, but the invention is especially applicable to apparatus for high quality, high resolution recording which would preferably use a larger number of grey levels, for example, 121 levels. Additional grey levels may be obtained using the known technique of "dithering" between adjacent grey levels in an array of successive lines. For example, an array of eight spots, four each on successive lines adjacent one another, may be recorded at levels selected from either of two adjacent levels so as to produce up to seven intermediate levels.

For high resolution recording, the recording head will preferably have a large number of heating elements. Typically, there can be 8, 12, 16 or 24 heating elements per millimetre with a typical overall total of 3967 heating elements.

The optical density measurements made on the spots produced by the heating elements may be made using an optical reflection densitometer that is scanned along the line of spots.

Claims

1. A method of compensating for uneven heating effects in the heating elements of thermal recording apparatus when energised by input gradation data comprising deriving gradation correction data for each heating element at different levels of the input gradation data, and using said gradation correction data to produce corresponding outputs to energise the heating elements in response to input gradation data, characterised in that said gradation correction data is derived from measurements of the optical density of spots recorded by the heating elements in response to input gradation data uncorrected by said gradation correction data.

2. A method as claimed in claim 1 in which the gradation correction data comprises individual corrected values of input gradation data stored in the correction memory in the form of a look-up table, each corrected value producing a corrected output corresponding to the identity of a particular heating element and a particular level of the input gradation data.

3. A method as claimed in claim 2 in which the gradation correction data corresponding to each heating element is derived by averaging the optical density of the spots recorded by the heating elements at one particular level of the input gradation data, and comparing the optical density of each spot with the average optical density value so as to produce a correction ratio.

4. A method as claimed in claim 3 in which the gradation correction data for each level of the input gradation data to which each particular heating element can be energised is derived from the correction ratio for that heating element at said one particular level.

5. A method as claimed in claim 4 in which the gradation correction data (C) is derived from the input gradation data (D) and the corresponding correction ratio (R) using the equation C=D+D[1-R] A where 1 ç A 4 1.5.

6. A method as claimed in claim 5 in which 1.1 4 A 4 1.2.

7. A method as claimed in any one of the preceding claims in which the optical density measurements of the spots is made using an optical reflection densitometer which is scanned along the spots.

8. Thermal recording apparatus comprising a plurality of heating elements each operative to record a spot on a recording medium corresponding to input gradation data, and a correction memory storing gradation correction data for producing a corresponding corrected output to each heating element in accordance with the input gradation data and the identity of each heating element, characterised in that said gradation correction data corresponds to measurements of the optical density of spots recorded by said heating elements in response to input gradation data uncorrected by said gradation correction data.

9. Apparatus as claimed in claim 8 in which the gradation correction data stored in the correction memory comprises individual corrected values of input gradation data in the form of a look-up table, each corrected value producing a corrected output corresponding to the identity of a particular heating element and a particular level of the input gradation data.

10. A method of compensating for uneven heating effects in the heating elements of thermal recording apparatus substantially as herein described with reference to the accompanying drawings.