Presentation on theme: "1 ColourColour David Molyneaux. 2 Light is electromagnetic energy Light is a wave Colour is the way the eye perceives light at different wavelengths inside."— Presentation transcript:
2 Light is electromagnetic energy Light is a wave Colour is the way the eye perceives light at different wavelengths inside the visible spectrum What is Light?
3 The Visible Spectrum When energy at these wavelengths reaches our eye, it causes us to perceive a colour. The rainbow - ROYGBIV
4 The Eye + How we see colour The retina of the eye (D) contains millions of photoreceptors Rods Number about 120 million, and are very sensitive to light Cones Number 6 – 7 million, and are sensitive to colour
5 The Colour Wheel Understanding how colours interact makes colour adjustments much easier
6 Describing Colour Hue –Hue refers to the attribute of colors that permits them to be classed as red, yellow, green, blue, or an intermediate between any contiguous pair of these colors. Hue differences depend primarily on variations in the wavelength of light. Brightness –Brightness refers to the relative lightness or darkness of the color. It is determined by the degree of reflectivity of the physical surface receiving the light. The higher the brightness, the lighter the color. Saturation –Saturation refers to how vivid a color appears. It is measured in terms of the difference of a color from a colorless (neutral) gray with the same degree of brightness. The lower the saturation, the grayer the color. When saturation is zero, the color is gray.
7 Additive and Subtractive Colour Objects in the real world absorb and reflect particular wavelengths of light – this is what gives them a colour. Black (K) is usually added to CMY in printers Subtractive – Colour printers, prints made in the darkroom Additive – Monitors, TV, projectors, etc..
8 Adding, Subtracting The RGB colour wheel demonstrates the relationship between the colours. Two adjacent colours can be mixed to form intermediates –E.g. Magenta can be made from red and blue –Yellow and cyan, when mixed together, produce green. Colors opposite each other on the color wheel are complementary. –For example, if the photograph you've taken has too much green in it, you can suppress this effect by adding in its complementary color, magenta (or red and blue in the RGB model). –Conversely, you can make red appear brighter by reducing cyan (or reducing green and blue, in the RGB model).
10 How Colour Film Works Colour film is composed of several layers of light sensitive chemicals Each layer is sensitive to a different wavelength of light –(A) is Blue sensitive –(C) is Green sesitive –(D) is Red sensitive Mask layers (B) filter out certain wavelengths, so that lower layers are not exposed Everything sits on the film base (E)
11 Developing Colour Film C41 Process C41 for colour negatives is relatively straightforward Two bath processes are available - a developer and a bleach-fix (BLIX) bath Both require constant temperature around 35°C +/- 0.5 E6 Process E6 colour reversal chemistry for slides is more complicated than C41 You had to develop the negative image, bleach it away, then develop the positive image… Again all steps require constant temperature around 35°C +/- 0.5
12 Colour Paper Both similar process to black and white, but with an additional bleach stage to form the colours. This may be combined with the fixer to create a bleach- fix (BLIX) RA4 Process: –RA4 process for printing from Colour Negatives –Chromogenic printing works by colour developer acting on "colour couplers" in the paper. Color dyes that are in the chemistry interact with the developer to form a color image on the paper. –Typically 3 layers: Red sensitive (forms a cyan colour on developed) Green sensitive (forms a magenta layer on development) Blue sensitve (forms a yellow layer on development) Ilfochrome / Cibachrome Process: –Ilfochrome (formerly Cibachrome) / P3 / P30 process for printing from Colour Positives (slides) –The necessary colors (azo dyes) are built into three emulsion layers of silver salts sensitized to one of three colors: red, blue, or green. The image is formed by selectively bleaching dyes already existing within the paper
14 Enlarger Operation Set an initial filtration (e.g. 75M 45Y 0C) Set enlarging lens to f11 or f16 Focus your negative using the focus-scope Switch off all lights except the minilux torch Expose a test strip to determine the correct exposure (ignore colours) Expose a test strip for colours – change filters in large increments to start with Important: Generally you should only use one or two of the filters, as using all three gives you a neutral density (only useful if you need to lengthen the exposure) Important: Exposure also changes as the colour filters change (increase in filters = longer exposure required) Viewing Filters can help you decide what filtration changes are required
15 Filtering for Colour Correction For colour negatives: Cast Correction GreenSubtract magenta BlueSubtract yellow MagentaAdd magenta RedAdd yellow and magenta YellowAdd yellow CyanSubtract yellow and magenta 10 point cast 20 point cast Do the opposite for slides as this is positive-positive.
16 Colour Analysers A colour analyser can reduce time spent colour printing However, it must be calibrated first! –Take a photo of the test card using the film type you want to print from (e.g. Kodak Gold Ultra) outside on an overcast day –Have this developed and put it in the enlarger –Manually print a correctly exposed and colour corrected print –Switch on the enlarger with the correct settings as determined by the manual print and switch on the analyser –Zero the dials / centre the LEDs on the analyser –The analyser is now calibrated to print from that one type of film. See analyser instructions for actual usage with other images on this film type.
17 Durst Printo and Jobo CPE2 The Durst Printo system is a roller transport process –The paper travels through the heated chemical baths on rollers For RA4 the system consists of : –Intro Module – where you introduce the paper –Two Heated Chemical Baths for Dev and Blix –Plus an external tray rinse Paper requires 45 seconds per bath @ 35°C Uses 2.5L of chemicals per bath, but replenishable Good for high volume work The Jobo CPE2 is a heated water bath with drums for paper and film development Paper requires 45 seconds per chemical @ 35°C –May also need a stop bath (same chemicals as B+W) Difficult to get good results due to very short dev times, so can process for longer at lower temperature if required Only requires 150-200ml of chemical per paper, but is one-shot
22 Colour Managed Workflows in the Digital Darkroom
23 Digital Darkroom Equipment Computer, Monitors, Flabed Scanner, Slide Scanner, Digital Camera, Printer
24 RGB Many digital imaging systems use the RGB system to describe colour RGB are three values, specifying the intensity of the Red, Green and Blue channels, which when mixed will create the correct colour Most digital cameras and computers use 24 bit colour, where each channel uses 8 bits to describe its intensity (a range of 0 to 255 in decimal) 0 is low intensity, 255 is high intensity (bright), so: –Red = [ 255, 0, 0] –Green = [ 0, 255, 0 ] –Blue = [ 0, 0, 255 ] –Mid Grey = [ 128, 128, 128 ] –Light pink = [ 236, 152, 234 ] Higher bits values (e.g. 16 bits per channel) allow a greater range of colours to be described and more natural looking images, as the possible number of colours is larger Unfortunately higher bit images take up a lot more room on your disk…
25 Colourspaces and Colour Gamut This is the CIE XYZ device independent model for describing colour This describes a colour mathematically without referring to any particular device Difficult to work with directly as it is an abstract model Device gamut's can be described inside this model
26 Device Gamuts Each device will have a slightly different gamut associated with it, depending on its capabilities Monitors and Televisions (additive colour) have a different gamut to printers (subtractive colour) This is why it is difficult to print exactly what you see on a monitor in an uncalibrated system
27 Profiles To solve this gamut difference problem, we can create a device profile A profile mathematically describes the size and shape of gamut for the device we are using With known device profiles, we can convert images between each device colour space and what we see on each device should be roughly the same To generate these profiles we can use spectrophotometers (or colorimeters) to compare the colours our devices produce against a calibrated standard Profiles have a.icc or.icm extension
28 Rendering Intent Perceptural or Relative Colourmetric mappings are between profiles are favored Perceptural attempts to maintain the relationship between colours, so will compress out of gamut colours (saturation decreases) Relative Colourmetric keeps colours inside destination gamut unchanged, but maps colours outside to the nearest hue. (saturation is clipped) White is mapped to white.
29 Where is all this used in Windows? Where are the profiles kept? –C:\WINDOWS\system32\spool\drivers\color How do I add/install a profile? –Right click on the icc or icm file and choose Install Where do I set my monitor profile? Right click on the desktop, select Properties. Go to the Settings tab, Click Advanced Go to the Colour Management Tab You can now select your monitor profile (only useful if you have calibrated your monitor) What about all this perceptual mapping stuff? –See Colour Management Menus in Adobe Photoshop Paint Shop Pro
30 Colour Correction Photoshop, Paint Shop Pro, The GIMP, or other image editing software can be used to correct any remaining colour casts. Tip: Use the Levels or Colour Balance function to introduce a large colour shift in the preview, then slowly return to what you think is correct…
31 The Magenta Problem Magenta is most likely the colour problem you will have. It is a common result of atmospheric scattering of light from haze or other particulate matter. However, our eyes have difficulty seeing it, so most people may not notice a slight magenta cast. Tip: Increasing the saturation of an image to maximum will clearly show you what colours are present in an image – a good way to identify colours that shouldnt be there.
32 Metamerism Colours can look different in different lighting conditions Can be beneficial, as it allows us to produce the same colour with different inks and pigments TIP: Always evaluate a print under the lighting conditions in which it will be displayed 5000 Kelvin Proofing LightTungsten Light
33 The End One Recommended Book for Colour Management on Computers: Color Confidence: The Digital Photographers Guide to Color Management By Tim Grey £21.00 from Amazon.co.uk
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