GB2178167A - Apparatus for photographic image area measurement - Google Patents

Abstract

The apparatus comprises a light- proof enclosure having therein an evenly illuminated support surface 2; means 4 to define an area of specimen supported on the illuminated support surface; and a semiconductor photodiode 5 arranged at a distance from the support surface to receive such light from the illuminated support surface as is transmitted through the specimen 3 on the support surface and provide an electrical signal linearly related to the intensity of transmitted light so permitting measurement of the area of image. <IMAGE>

Description

SPECIFICATION Apparatus for photographic image area measurement This invention relates to apparatus for measuring the area of a contrasting image in a transparent or translucent field.

The invention is especially, though not exclusively, applicable to the measurement of areas of high-contrast photographic images in negative or positive form for calculating the ink quantity required for printing on such articles as packaging, greetings cards, textiles, wall paper and ceramics. Great expense is incurred and wasted if excess ink is prepared for printing runs.

The invention can also, with advantage, be applied to setting ink duct control equipment; the analysis of high-contrast photomicrographs; the analysis of botanical specimens (such as plant, leaf or root systems); and in various metallurgical investigations and processes.

British Patent No. 646520 describes apparatus for measuring the area of a non-transparent sheet-like article, the apparatus comprising an array of several lamps which illuminate a luminous surface on which the article whose area is to be measured is supported. Light from the luminous surface is collected at a light-sensitive cell, preferably of the emissive type. The signal from the light-sensitive cell is preferably amplified and rectified before being measured by a microammeter. This apparatus requires control of an array of lamps and amplification of the signal before measurement is made. The apparatus is accordingly complicated and susceptible to instability of signal output.

British Patent Application No. 1,019,540 describes a method of determining the difference in projected area of an article from a known area between a source of light and an intensity-measuring means and measuring the intensity of light received and then placing an article of unknown area between the source of light and the intensity-measuring means and measuring the light then received. Comparison of the light received permits comparison of areas. This apparatus comprises a lamp; a first (convex) lens to convert light from the lamp to a parallel beam; a second (concave) lens to receive light, arriving past the article, and focus the beam onto a photoelectric cell which converts the light to an electrical voltage signal proportional to the light received.

The voltage signal is passed to a calibrated visual indicator so that for example damaged turbine blades may be discovered. This apparatus requires use of a lens to illuminate the article and a further lens to focus the signal light onto the photoelectric cell and accuracy is subject to any variation in the voltage produced by the cell.

The present invention provides apparatus for measuring the area of a contrasting image in a transparent or translucent field of a specimen, said apparatus comprising a light-proof enclosure having therein an evenly illuminated support surface; means to define an area of specimen supported on the illuminated support surface and a semiconductor photodiode arranged at a distance from the illuminated support surface to receive such light from the illuminated support surface as is transmitted through the specimen and provide an electrical signal linearly related to the intensity of the transmitted light so permitting measurement of the area of the image.

The benefits of using a semi-conducting photodiode are: 1. a direct linear relationship of the intensity of light received to the current passing through or from the photodiode; 2. a semiconducting photodiode gives improved sensitivity over a wide spectral range, low noise, fast response, low capacitance, and long-term stability; 3. a semiconducting photodiode does not suffer from the hysteresis and aging effects which can give rise to inconsistent response in photoelectric cells; 4. the area measurement made permits accurate estimation of the ink requirement so that expensive waste of ink at the end of print runs is avoided.

The semiconducting photodiode may be used in a current-producing mode and connected directly to a microammeter. A benefit arising from this mode of use is that no battery or power pack is needed.

Alternatively, the semiconducting diode may be used in a photoconductive mode in which light striking the photodiode reduces its electrical resistance so that when a voltage is applied across the photodiode a current, related to the amount of light striking the photodiode, will flow for measurement at a microammeter.

If a small-area photodiode (sometimes called a pin photodiode) is used it may be desirable to amplifiy the current produced or, alternatively, use the pin photodiode in the photoconducting mode. However, in a preferred embodiment a large area photodiode is used which (a) permits use in the current producing mode without amplification; and (b) provides a large optical capture angle for received light, so the optical length is a convenient length.

The semiconductor photodiode is preferably a silicon photodiode having rapid response characteristics.

The means to define the area of specimen to be studied may be in the form of a masking frame defining the study area which may, if desired, be a small chosen area within a larger specimen. Use of suitable masking frames ensures that the light-receiving area of the photodiode is not exceeded.

The current signal from the photodiode may be passed to a microammeter for visual dis play which may be digital. However, it is desirable to provide means for calibrating the galvanometer readings against relevant standard specimens of known opacity and clarity so that the image area of interest can be read off as a percentage of the total area of study.

From this figure the quantity of ink required may be read off published data.

Figure 1 shows the optical layout of one embodiment; Figure 2 is a diagrammatic front view of the apparatus; Figure 3 is a diagram showing a large-area photodiode connected directly in current-producing mode to a galvanometer; Figure 4 is graphs of signal current v area of specimen occupied by an image for two specimen "format" areas; Figure 5 is a graph of measured area v known areas; Figure 6 is a diagram showing a small area photodiode and current signal amplifier; and Figure 7 is a diagram showing a photodiode in photoconductive mode.

In Figure 1 the length of the specimen is denoted 8 and a photodetector 5 is mounted centrally above the specimen on the optical axis 6. The angle of acceptance 7 is defined by lines drawn from each edge of the specimen to each respective edge of the sensitive area of the detector 5. The distance from specimen to detector measured along the optical axis 6 is governed by the angle of acceptance 7 of the detector 5.

For example, a detector 5 having a sensitive area of 100 mm2 has a wide angle of acceptance 7, so the distance from detector to specimen would be about 0.5 m or 19.7" which permits easy access for positioning of specimens or access to the detector. Such an arrangement can study specimens, such as photographic images of printing forms, from 24 mm X 26 mm up to 300 mm X 400 mm.

If a smaller sensitive area of detector 5 is used, the apparatus has to be taller and therefore, in order to capture the same study area, more expensive.

Referring to Figure 2 the apparatus comprises a source of diffused light in the form of a large-area light source 1, such as a fluorescent tube or array of such tubes, and a diffuser screen 2, which may be a frosted glass plate or the like, arranged to provide uniform illumination of a specimen 3 resting on it.

A masking frame 4 provides means for defining the area of the specimen 3 to be studied. A large-area photodiode 5 having a receptive area of 100 mm2 is mounted centrally over the diffuser screen 2 to receive all light transmitted from the source 1 through the specimen 3. The photodiode 5 is, for this purpose, optimally positioned along the optical axis 6 in such a way that its angle of acceptance 7 provides total coverage of the full area which is illuminated by the source 1. The photodiode 5 is preferably a silicon photodiode, having a fast response time. A suitable photodiode is No. 303-674, available from R.

S. Components Ltd., Corby, Northamptonshire, which has a sensitive area of 100 mm2. However, other photodiodes may be used. The amount of light arriving on the sensitive surface of the photodiode 5 induces a current through or from the photodiode which linearly relates to the amount of incident light. The photo-current is measured by an electronic galvanometer 9.

The electronic galvanometer 9 incorporates a light-emitting diode (LED) display 10 for providing a visual digital display representative of said current. The galvanometer 9 comprises means for calibrating the apparatus in the form of a zero control 11, a coarse control 12 and a fine control 13. The galvanometer 9 is also provided with an on/off switch 14.

A light-proof cabinet 15 contains the light source 1, diffuser screen 2, specimen 3 and a masking frame 4 in correct optical relationship with the photodiode 5. The galvanometer 9 and display 10 are conveniently mounted within the roof area of the cabinet 15. A lightproof door (not shown) closes the area 16 between the specimen 3 and photodiode 5 to complete a light-proof enclosure.

In Figure 3 a large-area silicon-type photodiode 5 is connected directly to a galvanometer 9 so that when light is received on the receptive surface of the photodiode 5, a current is induced to flow for measurement at the galvanometer 9. In this specification a photodiode so working is said to be in the current-producing mode.

A photodiode suitable for inclusion in the circuit of Figure 3 is No. 303-674 available from R. S. Components Ltd., Corby, Northamptonshire. This silicon-type semiconducting photodiode has a light-sensitive area of 100 mm2 so that when positioned at a distance of 0.5 m from a specimen it captures a specimen area up to 300 mm X 400 mm.

A moving-coil microammeter may be used as galvanometer 9 in which case an operator would first set the zero reading on the microammeter. It is necessary to set the zero because these semiconducting photodiodes are sensitive to radiation beyond the visible spectrum. He would then place within the masked area a clear specimen, of the same area and thickness as those of the eventual study specimen, on the illuminated diffuser screen 2 and take a first reading of current i,. After removal of the clear specimen used for calibration, he would place the study specimen on the diffuser screen and measure the current i2. The ratio of i2 to ii multiplied by 100 gives a percentage of the study area occupied by an image. Whilst this "manual" technique is useful the study may be made less laborious by use of an electronic galvanometer preferably provided with a digital readout such as denoted 9 and 10, respectively, in Figure 2.

A suitable digital galvanometer 9 is the "DI GITAL UNIGALVO DS29" available from Diffusion Systems Ltd., London. This particular instrument includes potentiometers so the zero setting, low intensity field reading and maximum light readings can be calibrated against dark and light fields to read off on the visual display as a figure between 0 and 100%.

When the circuit of Figure 3 including such a digital galvanometer is incorporated in the apparatus of Figure 2, the procedure to measure an area of image in a specimen area is as follows: 1. Select a suitable masking frame 4 to define the desired area of study.

2. Select a "clear" film of the material to be studied and lay it on the illuminated diffuser screen 2 with the mask 4 defining the area of study. Close the door 16 to keep out stray light.

3. Adjust the galvanometer reading to 100% (steps 1 to 3 compensate for any light losses arising within the "film" thickness of a specimen, be it glass or photographic film material or the like).

4. Remove "clear" film and replace it with an opaque or black layer, close door 16 and adjust the galvanometer reading to zero. (The galvanometer is then ready to read off a percentage of dark image in a light field of specimen.) 5. Remove the opaque or black calibration layer and replace it with a test specimen, close the door 16 and read off percentage image area shown by the galvanometer.

Once the apparatus is calibrated the actual measurement of each test specimen of the material is rapid and convenient.

The circuit of Figure 3 incorporating the 303-674 photodiode and UNIGALVO DS29 was used to collect the data shown graphically in Figure 4. Several known areas of image in a specimen format of area 194 mm X 158 mm (suitably masked) were studied and the current produced was noted for\preparation of Graph A. Graph A shows that the relationship between image area and current produced is linear.

A second test graph, denoted B, was prepared from data collected using a specimen format of area 85 mm X 163 mm and again the relationship between the current and image area is linear but at a lower slope because this smaller format lets less light through to reach the photodiode.

On making sure that the relationship of current to image area is linear, the apparatus of Figure 2 including circuit of Figure 3 may be used to measure the percentage area of images of known area in a format.

The graph shown in Figure 5 shows that the correlation between actual percentage image area and measured percentage is good.

The circuit of Figure 3 is particularly suitable for use with a large-area photodiode such as the 303-674 giving the benefits of good response across the specimen area and accurate results without recourse to a power supply such as batteries or a rectifier.

However, a small-area photodiode, such as is sometimes called a "pin" photodiode, may be used but the current produced by it will be smaller for an area of specimen such as is discussed above, because it will have to be held further away from the specimen and it has a smaller receptive area. Thus it may be necessary to amplify the current.

Figure 6 shows a circuit in which a pin photodiode 25 is connected to an amplifier 29 the amplified signal from which is read off as a voltage on a millivoltmeter 9A. It will be noticed that the amplifier requires a source of power such as the batteries 26, 27.

In Figure 6 the pin photodiode 25 is connected to an operational amplifier 29 so that the pin photodiode works in current-producing mode. Two separate batteries 26, 27 are connected in series about ground level. The output signal level, proportional to the light intensity is dependent on the resistance r and is measured as a voltage V between the output side of the amplifier and ground.

The embodiments of Figures 3 and 6 use the photodiode in current-producing mode. Alternatively photodiodes may, if desired, be used in a photoconductive mode because the resistance across the photodiode decreases proportionally with the amount of light received by the photodiode.

Figure 7 shows a circuit in which a pin photodiode is connected to work in the photoconductive mode. In Figure 7 a pin photodiode, such as No. 305-462 availabe from R. S.

Components Ltd., is used in a linear photometer circuit with reverse bias. The leakage current increases proportionally to the incident light on the photodiode. The output signal from the FET operational amplifier, such as for example No. 305-456 available from R. S.

Components Ltd., is measured as a voltage the magnitude of which is dependent on the chosen value of resistance R.

A 10 kQ adjustable resistance on the amplifier permits setting of the zero reading.

Having measured the % area of field or format occupied by an image an operator uses this figure to calculate the print run, and the desired ink film thickness. In effect a volume of ink is calculated but in practice this volume is multiplied by the specific gravity of the ink to be used so that the correct weight of ink is prepared.

Claims (10)

1. Apparatus for measuring the area of a contrasting image in a transparent or translucent field of a specimen, said apparatus comprising a light-proof enclosure having therein an evenly illuminated support surface; means to define a area of specimen supported on the illuminated support surface; and a semiconductor photodiode arranged at a distance from the support surface to receive such light from the illuminated support surface as is transmitted through the specimen and provide an electrical signal linearly related to the intensity of transmitted light so permitting measurement of the area of image.

2. Apparatus according to Claim 1 wherein the photodiode is used in a current-producing mode and connected directly to a microammeter.

3. Apparatus according to Claim 1 wherein the photodiode-is used in a photoconducting mode and the light received on the photodiode controls the current flowing in a circuit comprising a source of power and a microammeter.

4. Apparatus according to any preceding claim wherein the photodiode is a large-area photodiode as hereinbefore defined.

5. Apparatus according to any preceding claim wherein the photodiode is a silicon photodiode with fast response characteristics.

6. Apparatus according to any preceding claim wherein the means for defining the area of specimen to be studied is a masking frame defining the area of study.

7. Apparatus according to any preceding claim wherein the photodiode is connected to a galvanometer.

8. Apparatus according to Claim 7 wherein the galvanometer is a microammeter having a visual digital display.

9. Apparatus according to Claim 7 or Claim 8 having means to permit calibration of the galvanometer readings so that the image area is read off as a percentage of total area of study.

10. Apparatus substantially as hereinbefore described with reference to Figures 1, 2 and 3 or 1, 2 and 4 of the accompanying drawings.