How Digital Images Are Displayed

The Human Eye’s Subjective View of Color

Elements of a good photo include composition, color, and brightness. One of your jobs
as a photographer is to capture the colors you see as intentionally as possible. Whether
you intend to show the color exactly as you see it or you want to enhance the color by
adjusting the color temperature, it is your job to understand your choices and
intentionally compose your picture.
Unfortunately, human eyes and brains can’t be trusted to see colors objectively. Unless
you can make side-by-side comparisons of your image on the screen, the photographic
print, and the actual subject, it may be hard to tell in what ways the color shifts from one
medium to another. Even when making side-by-side comparisons, it is nearly impossible
to objectively measure what the differences are when using your eyes alone.

The subjective nature of visual perception should not necessarily be viewed as a
handicap. If anything, it may be a blessing. Many challenges in photography come
from the fact that the technology is so unforgivingly objective. A common example of
this is the issue of white balance. Both film stocks and digital image sensors are
designed to interpret white under specific conditions. Outdoor light (daylight)
contains a lot more blue light than indoor (incandescent) light bulbs and candlelight.
White objects in these different lighting conditions objectively look more blue
(daylight), more red (incandescent), or more green (fluorescent), but the brain uses a
number of psychological clues to infer that white objects are white, even if they are
objectively different.
A white car during sunset objectively looks quite orange, but if someone asks you what
color the car is, you would reply with certainty that the car is white. That’s because you
know the car is white even if it doesn’t look white at the moment. In the morning, the
car has a bluish tint, and yet again, you would simply say it is white. Digital image
sensors and film, on the other hand, record only what they objectively receive, and
don’t interpret it. The auto white balance feature on many digital cameras measures the
scene in the viewfinder and tells the camera to interpret the brightest point as white.
This is important to know when switching between different lighting scenarios.
Light and color can be objectively measured and characterized. The scientific analysis
of light and color is necessary to build reliable, consistent photographic tools such as
film, digital image sensors, displays, and printers. The goal is not necessarily to make
all these devices capture or display colors the same way (although this would make
things a lot easier), but to develop terminology and processes to objectively measure
how these devices are different and adjust output accordingly, so that results match
visual perception.

Understanding How the Eye Sees Light and Color
Digital image sensors and the human eye perceive color in similar ways. One of the
remarkable things about human vision is the incredible range it has. A healthy eye can
see in very bright sunlight and in nearly total darkness. If you have spent much time
working with a camera, you know how amazing this range is. Film that works well
outdoors is nearly useless indoors, and vice versa. The range of human sight comes
from three different parts of the eye:
Pupil or iris: The pupil (also known as the iris) contracts and expands depending on
the amount of light entering the eye.
Rod cells in the retina: One of the two different types of cells that sense light. Rod
cells perceive levels of brightness (but not color) and work best in low light.
Cone cells in the retina: One of the two different types of cells that sense light. Cone
cells can perceive color in bright light.
Just as digital image sensors have light-sensitive elements that read red, green, and
blue light, the eye has three kinds of cone cells, each sensitive to a different part of the
visible electromagnetic spectrum:
 Cone R: Perceives colors with red hues with wavelengths in the visible spectrum
roughly between 600–700 nanometers (nm).
Cone G: Perceives colors with green hues with wavelengths in the visible spectrum
roughly between 500–600 nm.
 Cone B: Perceives colors with blue hues with wavelengths in the visible spectrum
roughly between 400–500 nm.
The human eye has roughly twice as many green cone cells as red and blue cone cells.
This color arrangement is similar to the arrangement of color elements on a digital
image sensor. (For more information about how digital image sensors capture images,
see “Digital Image Sensor”
The color the eye sees in a scene depends on which cells are stimulated. Blue light, for
example, stimulates the blue-sensitive cones, which the brain then interprets as blue.
The brain interprets combinations of responses from multiple cones at once and
secondary colors are seen as a result. For example, red light and blue light stimulate
both the red cones and blue cones, respectively, and the brain interprets this
combination as magenta (red + blue). If all three types of cone cells are stimulated by
an equal amount of light, the eye sees white or some neutral shade of gray.
Cones are more spread out in the eye than rods. Also, they are much less light-sensitive,
so they aren’t even active unless the brightness of a scene or object is beyond a certain
threshold. The result is that low-light situations tend to look monochromatic (like black
and white), whereas brighter scenes are detected by the cones and thus seen in full color.

Sources of Light
Prior to the invention of electric lights, electromagnetic energy originated from only a
few sources. Even today, the sun is the primary source of light. Fire and candlelight
provided evening light for thousands of years, though considerably weaker than
modern electric lights. Newer sources of light include incandescent light bulbs,
fluorescent light tubes, cathode-ray tubes (CRTs), liquid crystal displays (LCDs), lightemitting
diodes (LEDs), and some phosphorescent materials. These light sources
directly influence the images you create as a photographer.
The Color Temperature of Light
Color temperature is a term used to describe the color of light. Every light source has a
color temperature. However, color temperature refers to the color value of the light
rather than its heat value. Light’s color temperature is measured in units called kelvin
(K). This temperature scale measures the relative intensity of red to blue light. Warmer
light—light that tends to cast an orangish-red tint across the image—has a lower
temperature. Neutral or balanced light occupies the midranges, and has no effect on
the image’s color values because of its white qualities. Cooler light—light that is blue in
appearance—has a higher temperature.

With the invention of color film came a whole new set of considerations. In addition to
correctly exposing the image, photographers had to take into account the various color
tints different light sources cast across their film emulsion. Film manufacturers
improved the situation by developing film emulsions rated for daylight and tungsten
lamp color temperature ranges. Camera manufacturers also jumped in and developed
color filters, attached to the camera’s lens, to help photographers shoot outside the
temperature range of the film. However, these solutions didn’t completely eliminate
the problem because images shot in unforeseen and adverse lighting conditions
remained irreparable during the printing stage.
How White Balance Establishes Color Temperature
When you take a photograph with a digital camera, the color temperature of the scene
is not taken into account until the image is processed by the camera’s processor. The
camera refers to its white balance setting when it processes the image. When the
camera’s white balance is set to auto, the camera assumes the brightest value is white
and adjusts all other colors in the image accordingly. If the brightest value is white, the
colors in the image are rendered correctly. If the brightest color is yellow, the camera
still assumes that value is white, and shifts all the colors out of balance.
However, you can adjust the color temperature of a digital image. White balance is a
mathematical process that calculates an image’s color temperature and applies the
effects to the color values in the image after the RAW image is stored. That color
temperature data is stored as metadata in the image. The digital data that makes up
the original RAW file is unchanged. So, no matter what white balance or color
temperature setting was applied at the time the image was shot, the color temperature
of the image can always be corrected after the fact. Digital cameras’ RAW files solved the
problem of color temperature flexibility that the chemistry of film never could.
Measuring the Intensity of Light
In order to shoot an image with the correct exposure, you have to know the correct
value of the intensity of light. Photographers use light meters to measure the intensity
of the reflective light in a scene. Digital cameras have built-in light meters that are very
sophisticated and incredibly accurate. However, their accuracy is subjective. The
recommended aperture and shutter values are determined by how light falls in the
scene and by how the light meter is set. The camera’s light meter may recommend an
aperture and shutter combination that offers a decent exposure. However, it may not
give you the perfect exposure because it doesn’t know what you’re photographing.
Light meters can’t evaluate colors or contrast. They only see luminance, which is the
brightness of the reflected light in a scene.

Cameras with sophisticated light meters can be set to meter, or test, specific areas of
the scene. Most DSLRs allow you to choose the portion of the viewfinder to meter.
These meter settings include, but are not limited to:
- Evaluative: Evaluative metering operates by dividing the frame into several small
segments, taking a reading from each individual segment, and processing the average
of the total segments to recommend the best exposure value for the overall image.
- Spot: Spot metering operates by metering within a small target area that is usually in
the center of the frame. Spot metering is particularly useful when your subject is
placed in front of a relatively bright or dark background. Spot metering ensures that
you will correctly expose your subject. The drawback is that the background may be
incredibly under- or overexposed. This is why you should bracket (shoot multiple
exposures of the same image) when shooting in a situation that requires the use of
the spot meter. For more information on bracketing, see “Bracketing the Exposure of
an Image,” below.
- Center-weighted: When the camera’s light meter is set to center-weighted, the
camera measures the light in the entire viewfinder but gives extra emphasis to the
center of the frame. This setting is typically used by portrait photographers, because
the subject is usually centered and the background isn’t ignored. If the subject
moves out of the center of the frame, the meter assumes the background is the
correct exposure, leaving your subject incorrectly exposed.
It’s important to point out that light meters provide recommendations only. If the
details in the highlights of the scene are more valuable to you, you may choose to
expose the image shorter than the light meter recommends. Likewise, if the details in
the shadows of the scene are of more value, you may choose to expose the image
longer than the light meter recommends. It’s your prerogative as a photographer to
use the light meter to obtain the best exposure of the scene in your image.
 

Bracketing the Exposure of an Image
Even careful metering sometimes yields an under- or overexposed image. This is why
professional photographers bracket their images, whenever possible, to be absolutely
sure they have a correctly exposed image. Bracketing involves taking three shots of
the same image based on the aperture and shutter values recommended by the light
meter: one shot underexposed one stop, one shot at the recommended exposure,
and one shot overexposed one stop. Shooting the image with a range of three
exposure stops is the best way to ensure you’ll have a properly exposed image.
Note: Most DSLR models have a built-in, automatic exposure-bracketing feature.
Refer to your owner’s manual for directions about how to use it.

Understanding How a Digital Image Is Displayed

Photographers display their digital images in two basic ways: onscreen or in print. The
method by which an image is displayed onscreen and the way it is displayed as a print
hanging on a wall are completely different. Computers, televisions, and video and
digital still cameras create color images by combining red, green, and blue (RGB)
primary colors emitted from a light source. This approach is based on the additive color
theory. Printed images require an external light source from which to reflect light.
Printing technology uses subtractive color theory, typically with four primary
colors: cyan, magenta, yellow, and black.
Additive vs. Subtractive Color
Images with color elements derived from the light source itself are considered to have
additive color, while images that subtract or absorb certain wavelengths of light,
reflecting back specific colors to the viewer, are considered to have subtractive color.
Because of these differences, an image displayed with additive color (for example, on
an LCD display) will always look different from the same image displayed with
subtractive color (such as on a magazine cover). The reason for this is that digital
devices like LCD displays combine red, green, and blue light in different combinations
to produce the desired color. All colors combined at their maximum intensities create
white, and the absence of color creates black. On the other hand, a printed piece like a
magazine cover combines cyan (C), magenta (M), and yellow (Y) inks in different
combinations to create a color that reflects the proper color of light. Black ink (K) is
added to the image last to generate pure black on the page. The addition of ink creates
a darker color, and the absence of ink creates a lighter color. This color process is also
known as CMYK

Understanding Color Gamut
In 1931, a group of scientists and intellectuals who called themselves the Commission
Internationale de l’Eclairage (CIE) had the goal of defining standards for color. Using as
much objectivity as is possible with this highly subjective topic, they developed a
coordinate system for categorizing the world of colors. According to this system, every
hue the eye can see can be described in terms of x and y coordinates. Taking it one step
further, every device that reproduces colors can have its RGB color primaries described
by the CIE x and y values. This provides the basis for color-management systems such as
ColorSync. The total number of colors described by the two-dimensional plot of these x
and y coordinates is often referred to as the device’s color gamut. In other words, a
system’s color gamut refers to the total set of possible colors that system is capable of
displaying. In addition to this two-dimensional color description, color gamut has a third
dimension: its brightness. Unfortunately, the color gamut of displays does not
correspond exactly to the subtractive color of print. For example, certain colors that
appear onscreen cannot be exactly reproduced in print, and vice versa.

Displaying Images Onscreen
As mentioned earlier, when working with images on your computer screen, you are
working with additive light. The display converts electricity into light and the pixels on
the screen produce an image by using an RGB color space model. (Color space refers
to the limits, or parameters, of a given visible spectrum. Common color spaces are
sRGB and Apple RGB.) This process begins when the image file on the computer’s hard
disk is processed and then sent to the graphics card for further processing and
temporary storage in memory. The graphics card processes the image, preparing to
display it in the specific resolution and color profile of the display or displays
connected to the computer. (A color profile is a compilation of data on a specific
device’s color information, including its gamut, color space, and modes of operation.)
Processing the image may take some time, depending on the size and bit depth of the
image file, the size and number of displays in the system, and the resolution of the
displays. Whether an image was scanned or downloaded directly from a camera, the
image was recorded digitally in an RGB color space.
The essence of RGB is the combination of red, green, and blue colors emitted from a
light source to form a wide variety of additional colors. On color displays, three colored
elements (one red, one green, and one blue) combine to form a pixel. When red, green,
and blue are combined at their maximum intensities, the color white is created. When
there is an absence of light in all three colored elements, the color black is inferred.

The Importance of Color Calibrating Your Display
It’s incredibly important to color calibrate your display or displays to ensure that the
color on your screen matches the color you intend to output to print or to the web.
Your digital workflow depends on successful color calibration, from capturing to
displaying to printing. The adjustments you make to your digital image won’t
reproduce faithfully in print if your display isn’t calibrated. They’ll also look different
when viewed on other displays. Calibrating your display allows ColorSync to adjust
your image for consistent viewing results. Calibrating involves attaching an optical
device to your screen that evaluates your screen for luminance and color temperature.
There are several companies that manufacture color-calibration tools. The tools can be
expensive and can vary greatly in quality, so make sure you do an adequate amount of
research before you make your purchase. For a list of available color-calibration tools
and devices, see the Mac Products Guide at http://guide.apple.com

Apple Cinema Displays Are Proof Perfect
Apple Cinema Displays are so good at displaying color that you can use them in a
SWOP-certified soft-proofing workflow. Display-based proofing systems Remote
Director 2.0 from Integrated Color Solutions, Inc. and Matchprint Virtual Proofing
System-LCD from Kodak Polychrome Graphics both have Specifications for Web Offset
Publications (SWOP) certification. The prestigious SWOP certification means you can use
Remote Director 2.0 to approve jobs for press production onscreen without the need
for paper proofs, providing significant time and cost savings for print professionals.
Certified systems are capable of producing proofs visually identical to the
SWOP Certified Press Proof as defined in ANSI CGATS TR 001, Graphic Technology.
Integrated Color Solutions, Inc. and Kodak Polychrome Graphics chose Apple flatpanel
displays because they are capable of providing the luminance and color gamut
necessary to create an onscreen proof that has the same brightness and feel as paper.
Note: Your Apple Cinema Displays must be color-calibrated to achieve accurate
results when soft-proofing your images.

Displaying Images in Print
Displaying images in print requires converting the color from the RGB color space to
CMYK. The reason for this is that printed images need to reflect light from external light
sources to be viewed. Images are usually printed on white paper, so no white ink is
necessary. Darker colors are created by adding colors together, whereas lighter colors
are produced by reducing the color mix.
Printer Types
The following printer types are divided into two groups: personal printers and
professional printers.
Personal Printers
There are two basic types of printers that are affordable for most photographers.
- Inkjet: Inkjet printers create images by spraying little ink droplets onto the paper.
Inkjet printers are capable of placing the microscopic droplets on the paper with
great precision, resulting in high-resolution photographs. There are two methods of
applying the ink to the paper. One technique involves heating the ink to a
temperature warm enough to allow the ink to drip. The second method involves
vibrating a tiny valve filled with ink, forcing it to fling a droplet onto the page.
- Dye sublimation: Dye sublimation printers create images by heating colored ribbon
to a gaseous state, bonding the ink to the paper. The ribbon is a plastic material
that makes the print nearly waterproof and difficult to tear. The incredible
durability of dye sublimation prints gives them a longevity that cannot be
surpassed by any other medium.
The quality of inkjet printers has improved remarkably in the past few years, making
their resolution and color gamut superior to those of dye sublimation printers.
Professional Printers
There are two basic types of printers employed for professional use. Unlike personal
printers, these printers are relatively expensive.
- Offset press: Offset presses are used for high-volume printing for items such as
magazines and brochures. Offset printing presses deposit ink in lines of halftone dots
to produce images on the page. The printer uses a fixed drum to roll the image onto
the paper.
- RA-4: RA-4 printers are capable of printing digital files on traditional photographic
paper. They use a series of colored lights to expose the paper, which blends the
colors together to produce continuous-tone prints. Due to their expense and size,
most photo-direct printers are only available at professional photo labs.

Related Posts

• Taking a good picture…
• Sports photography
• Taking better holiday pictures
• Taking pictures in winter
• Taking artistic photographs
• Top 10 Photography tips
• Taking pictures in bright light
• The art of the closeup
• Taking photographs of animals
• Taking better landscape pictures