49
To understand anti-blooming it is convenient to think of a CCD charge site as a mechanism for
converting light (photons) to charge (which is then converted to voltage) in a linear manner. The
more photons that are incident on a charge site during the integration time (1/60 sec) the more
the charge that gets collected in the CCD site; this translates to more voltage corresponding to a
particular charge site. The Voltage vs. Light curve is, therefore, linear, with a positive slope.
Unfortunately, there is a limit to the amount of charge that can accumulate in the charge site.
Once that limit is reached the charge site saturates and tends to overflow to the neighboring
charge sites.
Blooming occurs when a CCD charge site that is exposed to very bright light fills up with charge
and then overflows to its neighboring charge sites. The overflow of charge causes a bright spot
of light in the field of view of a video camera to appear larger in size. Since the charge sites are
like "buckets" full of charge, they can only hold so much charge. When that limit is reached, the
charge site is saturated and can hold no more charge.
Continuing with the "buckets" analogy, if the rate at which a "bucket" is filled with water is
decreased significantly after it is half-full, then it takes a lot more water to fill it !
If several holes are made in the side of a bucket half way up, the rate at which the bucket is filled
decreases significantly once it is half-full.
For a CCD charge site, this is achieved by decreasing the positive slope in the Voltage vs. Light
curve. This can be described as a deliberately introduced "knee" in the Voltage vs. Light curve
beyond which the slope of the curve is significantly reduced. Although the slope is still positive, it
takes a lot more light to reach the saturation point.
The slope of the curve is reduced by means of electron-hole recombination via the anti-blooming
gate which is built into each CCD charge site. The excess charge is dumped into the substrate of
the device.
Voltage vs. Light curve for the TC-245 CCD
0
50
100
150
200
250
300
350
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
2.8
3
Light (lux)
Voltage (mV)
Reaches Vsat at 100lux !
Vsat
Reaches Vsat at 1lux !
Vn = Noise Floor (not to scale)
Vuse = Camera White Clip
Vsat /2
Anti-blooming disabled
Anti-blooming Enabled
Figure 8-5: Anti-blooming curve
Summary of Contents for DigitEyes Series
Page 14: ...9 Figure 4 3 TC 245 Gate Level Drawing Texas Instruments 1994 ...
Page 61: ...56 12 Appendix D Camera Mechanical Drawings Figure 12 1 Camera Mechanical Drawings ...
Page 63: ...58 Figure 13 2 Camera Noise Spectrum Min Gain Bandwidth 10kHz to 4 2MHz ...
Page 64: ...59 Figure 13 3 Camera Noise Spectrum Max Gain Bandwidth 100kHz to 4 2Mhz ...
Page 65: ...60 Figure 13 4 Camera Noise Spectrum Max Gain Bandwidth 10kHz to full ...