Red GreenBlue Tri Color Imaging

This section describes classic red, green, and blue color filter imaging—RGB tricolor imaging—as the basis for understanding more sophisticated color capture, correction, and display methods covered later in this chapter. Color imaging consists of three steps:

• Capture filtered images. A basic set consists of red, green, and blue filters. Images taken through each of the color filters, and appropriate dark frames and flat frames are taken as well. After dark subtraction and flat-fielding calibration, we designate the set of filtered images as SR, Sa, and SB.

Wavelength (nanometers)

Figure 20.4 Color separation filters divide the white-light spectrum into well-defined wavelength regions. Solid curves show the transmittance of actual filters; the gray lines show "ideal" transmittance curves. Many red/green/blue filters sets require an extra infrared blocking filter to stop their infrared leak.

Wavelength (nanometers)

Red, Green, Blue, and IR-Blocking Filter Set

Figure 20.4 Color separation filters divide the white-light spectrum into well-defined wavelength regions. Solid curves show the transmittance of actual filters; the gray lines show "ideal" transmittance curves. Many red/green/blue filters sets require an extra infrared blocking filter to stop their infrared leak.

• Correct the filtered images. Filtered color images retain the effects of filter passbands and transmittances, CCD quantum efficiency, atmospheric attenuation, and background sky light.

We can measure and correct for these effects. After correction, we designate the images as S*R, S*G, and .

• Create a color image. The final stage in this process consists of scaling the corrected image values into the storage range of an image file or the display range of a computer monitor. After scaling, we designate the color display components (now usually called color channels) as DR, DG, and DB .

We now examine each of these steps in detail. 20.3.1 Step 1: Capture Filtered Images

To make color images, it is necessary to acquire images in well-defined portions of the spectrum. Although color filters do isolate portions of the spectrum, the light from the celestial object, the atmospheric transmittance, spectral sensitivity of the CCD, and background sky light also influence the signal produced in the CCD. The CCD output can be expressed by the following equation:

f= llOOnm

In this equation, SCCD is the output of the CCD, and:

• F(X) is the flux arriving from the celestial source as a function of wavelength;

• is the transmittance of the atmosphere as a function of wavelength;

• T(X) is the transmittance of the color filter(s) used as a function of wavelength;

• Q(X) represents the combined transmission of the optics and quantum efficiency of the detector as a function of wavelength;

• g is the gain of the amplifier, in electrons per ADU in the output image; and

• t is the integration time used to capture the image.

To explore color imaging, we need to simplify this equation. We can accomplish this by lumping together factors that remain the same and integrating over each spectral band to obtain a single figure that represents the integrated characteristics.

In this simplified picture, we treat light from celestial objects as consisting of only five passbands: infrared, red, green, blue, and ultraviolet. The actual passbands do not matter except insofar as they excite the color sensors in the human retina. Infrared and ultraviolet fall outside the range of human vision, but are detectable by CCDs. These passbands correspond to the following ranges:

• Infrared consists of wavelengths longer than 700 nm;

• Red with wavelengths from 700 nm to 600 nm;

Table 20.3 Atmospheric Transmittance*

Altitude

Red (%)

Green (%)

Blue (%)

90° (zenith)

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