Black and White in Photoshop I - Black and White in Photoshop CS4 and Photoshop Lightroom - page 130

in Photoshop are stored and edited within a working document. The example
below (Fig. 1) illustrates an Alpha channel storing a selection of the horses.

Note:
Generally, under default
program settings,
Photoshop is set up
to reveal channels in
grayscale. This can be
changed in Photoshop
CS4 Preferences
Interface Show
Channels in Color

Digital RGB capture is actually grayscale i rst!
This may be surprising, but digital cameras do not actually capture in
color at all. Camera imaging sensors have the capacity to capture luminance
values or brightness alone, not color. All digital images, therefore, are
fundamentally grayscale at the time of capture and then converted to
color. How this works is a little complex but, basically, colored i lters cover
each photosite on a digital camera's imaging sensor. These i lters capture the
brightness of the light that passes through them. With the i lters in place, each
pixel can record only the brightness of the light that matches its i lter and
passes through it, while other colors are blocked.

The grayscale in digital terms is a series of 256 increasingly darker tones
ranging from pure white to pure black. 0 represents pure white,
255 represents pure black with the other 254 brightness values being varying
shades of gray that increasingly darken from pure white to pure black.

How the digital camera creates a color image from this brightness scale
recorded on the sensor is much like Maxwell's 1860 experiment as well. Since
daylight is made up of red, green and blue light, placing red, green and blue

FIG 1

0 255

i lters over individual pixels on the image sensor can create color images
just as they did for Maxwell in 1860. For example, a pixel with a green i lter
knows only the brightness of the green light that strikes it. A process called
interpolation then determines the actual color of a pixel, which uses the
colors of neighboring pixels to calculate the two colors that the pixel did
not record directly. By combining these two interpolated colors with the
directly measured colors, the full color of the pixel can be calculated and then
recorded. Interestingly, the human eye is more sensitive to green than it is to
red or blue, so most digital camera sensors have twice as many green i lters as
red or blue i lters to accommodate this preference.