2.1 Comparative Pathways for Image Capture.The project examined the effects on digitised image quality of capturing images using the capture routes shown in Figure 2.1. A representative subject for each type of source material was captured. Not all images can be captured using all the prescribed capture devices, e.g. the Canon Ion still video camera has an unsophisticated zoom lens unsuitable for connection to a microscope. Other mutually exclusive permutations of image and capture device are identified in later sections.
Figure 2.1 Capture options for various source materials and test charts
2.2 Image quality
2.21 IntroductionThe quality of a digitised image is dependent on both the quality of the original source material and the adoption of a digitisation pathway which minimises quality loss. This study has worked with source material of as high a quality as practicable to effectively test the various capture routes. Quality judgements on the attributes of a particular capture path or piece of equipment were made by employing both quantitative and qualitative techniques. To isolate the effect of capture path on overall quality as defined in section 1.1, Spatial Resolution and Grey Scale test charts (standard throughout television broadcasting) were used for quantitative assessment of spatial resolution and contrast ratio respectively, while a qualitative assessment was made of the colour information in full colour images.
It would have been preferable to undertake a quantitative analysis of degradation in colour information but the project did not have access to a Spectraradiometer or similar device to enable such a study. It is intended that quantitative colour assessment should be included as part of the on-going research programme.
2.22 Quantitative measurement of quality - test chartsThe standard Spatial Resolution test chart (figure 2.2), used in broadcasting to assess vertical and horizontal spatial resolution and system frequency response, carries over satisfactorily for use with computer display monitors. This chart enables assessment of spatial resolution through testing the systemÆs ability to differentiate between distinct black straight lines against a light background. When the lines merge together indistinctly then resolution is said to have reached its limit. As can be seen from the diagram, spatial resolution between 200 and 800 TV Lines can be assessed at various points across the screen and in this case horizontal resolution was measured in units of 25 lines."TV lines" are a unit of measurement to establish the number of discreet picture elements (in this case lines) that can be resolved horizontally and vertically over the full screen. Although originally designed for use with video scanning systems the chart does provide a convenient means of comparing the spatial resolving power of the different systems used for capture.
Where the lines converge moir patterning appears before resolution is measurably impaired. Moiré is a rainbow-coloured patterning introduced when lines in television picture content are closer together than the monitor is able to display, and it is commonly seen in television presenters' striped shirts before they learn not to wear that style. Moiré is not as apparent on a computer monitor as it is on a video monitor because on the computer monitor colour information comes from digital red, green and blue (RGB) signals rather than being a part of the analogue coded Phase Alteration Line (PAL) video signal. For this reason moir is not always a relevant factor in comparisons and on this project was eliminated on the video monitor by turning down the colour to give a truer assessment of spatial resolution.
The Grey Scale chart (figure 2.3) displays 9 half tone shades of grey between white and black - 11 shades in all. It is designed to assess whether the system can differentiate between steps all the way from black to white, but particularly at the extremes of the chart where contrast limitations are most apparent. This chart is also designed to test the ability of a colour capture device to show a grey image with no colour casts across the half tone range provided. A colour capture device utilises equal parts of the additive primary colours red, green and blue when reproducing a black, white and half tone image, and is effectively lined up to deliver good colour output when it displays an accurate and complete grey scale. For this reason broadcast television cameras are lined up initially on the Grey Scale chart and colour casts in particular half tones are revealed and eliminated. Depending on the number of half tones presented for grey scale resolution, it is reasonable to assume that those in between, and not specifically covered by the steps in the chart, would appear in any case.
2.23 Qualitative measurement of quality - full colour imagesSubjective assessments were made of the remaining 9 full colour images based on the criteria of sharpness (a less precise version of spatial resolution), colour (faithfulness to earliest available version) and contrast ratio (ability to resolve perceived range of light and dark shades). These assessments were qualitative and were scored on a scale of 1-5 as follows:
5 Excellent 4 Good 3 Satisfactory 2 Poor 1 Unacceptable
Assessments were made on the above technical criteria by two members of the project team working together. All the high budget and PhotoCD versions of an image were displayed on screen together when scoring the individual images, and all the low budget versions were also viewed simultaneously. Assessment was made without reference to educational effectiveness in any specific context. The assessment of the images in a specific educational context using students was not possible due to the time constraints of the project but this is part of on-going research.
2.3 Source materialThe selection of types of original source material for image capture was made on the basis of the likely requirements of different university faculties. The 9 image types were:
Low magnification microscope specimen
The Microscope Slide was a biological thin section, mounted on glass, of the lactating mammary gland of a sheep, designed to be viewed through a high magnification microscope and in this case seen at 50x magnification. The magnification was the product of a 10x objective and 5x objective in the microscope and did not reflect displayed magnifications on 35mm film or video monitor. It required the resolution of a range of half tones stained magenta and some seemingly monochrome grey material. This slide demonstrated the effect of using a transmitted light source to illuminate an image; in this case at just over half power on its thyristor control, providing 'warm' illumination from the red end of the spectrum. For microscope work the same video rostrum camera head (a JVC TK-1280E, hereinafter designated Rostrum 2A), set to white balance automatically was used because of the need for a custom built adapter to interface the camera with the microscope. Usually a separate adapter is needed for each microscope used because the focal distance between the cameraÆs charge coupled device (CCD) and the eyepiece in the microscope is crucial. Each adapter can cost up to £500. The magenta staining in the slide tested the video system because historically tube television cameras have had difficulty resolving magenta, which is in the area between red and blue, although it is now recognised that this is less of a problem with CCD than with Tube cameras.
The Low Magnification Microscope specimen was an apical dissection of a developing ear of barley, designed to be viewed at low magnification and in this instance seen at about 12.5x magnification through the microscope. This image was side lit with a fibre optic source used at just over half power, with a piece of aluminium foil presenting its matte side providing a fill light.
The Plant was a flowering example of Charlock, captured with the sun high on a clear autumn day. Lighting conditions were ideal because a range of half tones and shadows had to be resolved. However it was difficult to isolate a single plant from its background because it was in a densely colonised field - a familiar problem for photographers in a biological context. It is recognised that in some circumstances it may be possible to remove the specimen for photography under studio conditions. On this occasion it made it difficult to separate the flower from its background in low resolution versions of the image.
The Soil Profile was part of the side of a granite quarry below Bennachie - a hill in Aberdeenshire - regularly used for teaching purposes by soil scientists at Aberdeen University. The section exhibited shades of light pink; not what a photographic printer would expect to reproduce accurately without some reference. The image was purposely captured out of direct sunlight because sunlight and shadow can interfere with perception of subtle colour shadings associated with soil profiles. For this reason it was not considered appropriate to consider contrast ratio with this subject, and accordingly only colour and spatial resolution assessments appear in the results.
The Landscape was a view looking north-west towards the hills from a point about two miles west of Tarland in Aberdeenshire. It showed a range of land uses from gorse and woodland alongside a burn, upwards through arable farmland to hill grazing, coniferous plantation and eventually heather moorland to the skyline. The sun was out and quite high, providing a range of half tones and possible shadow detail to test contrast.
The head and shoulders Portrait required capture of the mixture of primary colours associated with flesh tones, which although not identical between faces is nevertheless familiar as a concept to most of us. The subject was captured against a textured monochrome grey wall to reveal small colour shifts not immediately apparent on the face. No fill light was used on the unlit side of the face to test the amount of shadow detail resolvable by different methods of capture.
The Skull was chosen to exemplify capture of a three dimensional subject displayed under controlled studio lighting conditions. The subject was that of a male capercaillie which, although mainly monochrome, exhibited many shades of the colour associated with bone structure.
The Map detail was a section of the 1:250,000 Soil Survey of Scotland Soil Map, Sheet 5: Eastern Scotland. The physical dimensions of the example were chosen to provide legible names at initial capture when viewed full screen on the computer monitor. This particular map was chosen to test the ability of the capture systems to resolve subtle variations of colour required to differentiate between the mapping units.
The Line Drawing was a detail of a coloured Victorian engraving of the view through a microscope of a cryptogram, Volvox globator (The Rolling Sphere) which included subtle colour variations and fine line engraving.
2.4 Capture devices
2.41 IntroductionEquipment for image capture was chosen to fulfil two budgetary scenarios as defined in section 1.1:
Some of the capture devices used were common to both Central and Departmental models, and some specific to one or the other. Table 2.1 lists the equipment reviewed, and fuller descriptions of each item are given in following sections and in the Appendix.
Table 2.1 Capture Devices
The slide was designated the benchmark for quality. There is a need for such a benchmark because seasonal and lighting changes plus remoteness make it impractical to go back to check original source material in most cases. The colour slide, which is the actual film from the camera and in development is not prone to errors of interpretation as are prints from a negative. The other possibility would have been to use the negative but it is not practical to compare the other images against the negative.
2.42 Capture devices common to both resource modelsThe 35mm Slides were shot on Kodachrome 64 film stock which has a resolving power of between 63 and 100 lines per millimetre (LPM) and an exposure latitude of less than + or - half a stop (Kodak trade literature).
35mm Negative. The film used was Kodak Ektar 100. It is half a stop faster than the Kodachrome 64 used for slide origination but the closest comparable stock, providing resolving power between 63 and 160 LPM despite the extra speed and with a much greater exposure latitude between -1 and +3 complete stops (Kodak trade literature). Designed for the provision of copies, colour negative film provides greater contrast range, colour temperature flexibility and exposure latitude than 35mm slide film, although it does introduce the possibility of errors in the copying process.
Photographic Prints were processed by Boots the Chemist, whose output conforms to British Standard 5750. The print size used as a standard in this project was 7 x 5ins. This minimises any loss of sharpness inherent in the texture of the emulsion on the paper, which might become apparent at smaller sizes.
All photographic capture was undertaken on Nikon F3 cameras with appropriate Nikkor lenses and filters.
The Still Video camera used was a Canon Ion RC-560 camera with an 8-24mm (3-1) integral zoom lens and a stated maximum resolution of 736 x 544 pixels and 450 TV lines. This camera is capable of recording 50 field (alternate television lines scanned) or 25 full frame images onto a small floppy disc and replaying them individually at a standard 1 volt of video peak to peak (75 ohms), either through its own board or through a video capture board for digitisation.
Single Frames of Videotape were selected from continuous recordings on Hi8 metal-E tape from a Sony Video Hi8 Pro Camcorder with 8-80mm zoom lens c/w macro facility.
All 35mm slides and negative images were also transferred onto PhotoCD at Photo-Technical Services, a local processing firm.
£ Panrix 486DX 66MHz 16Mb RAM computer 2474.00 'Wizard' 9000VL 24 bit (true colour) video card 295.00 SVGA monitor 900.00 CD-ROM drive (multisession Photo CD compatible) 200.00 1Gb Read/Write Optical Disc Drive 1800.00 Video Monitor 600.00 Kaiser RS1 rostrum copy stand 200.00 Kaiser Tungsten copy lights (2 x 150w) 201.60 Master light box 131.00
2.43 Capture devices specific to the central resource model
£ Rostrum camera 1 JVC KY-F30B 3x Charge Coupled Device (CCD) Rostrum Camera + 7-98mm zoom lens c/w macro and 6 diopter close up attachment 6240.00 Video capture board 1 Screen Machine II Video Capture Board 725.00 Scanner 1 Hewlett Packard Scanjet Iic Scanner 1675.00 Still Video Canon Ion 560 1000.00
2.44 Capture devices specific to the departmental resource model.
£ Rostrum camera 2 JVC TK-1280E 1xCCD Rostrum Camera + 18-108mm zoom lens c/w macro and 3 diopter close up attachment 1100.00 Rostrum camera 2A JVC TK-1280E 1xCCD Rostrum Camera 900.00 no lens, adapted to the microscope Video capture board 2 Videologic Captivator Video Capture Board 300.00 Scanner 2 Logitech Scanman Colour Scanner 306.00