Vertical bleeding or smearing from a saturated portion of an image

In the sample image below, a faint bright line can be seen that extends from the bottom to the top of the image, passing through the light saturated spot.



This bleeding effect is called "smearing" or "blooming", which is an undesired signal that appears as a brighter vertical (from top to bottom) stripe emanating from a bright part of the image. Smearing is a common problem in high light intensity settings.

Smear Explained

With the interline transfer CCDs used by PGR, each pixel consists of a light sensitive area called the photosensor and a light-shielded area called the interline transfer cell (also known as the vertical shift register). The interline transfer is used to shift the charge out of the photosensor and off of the sensor, and is approximately 100,000 times less sensitive than the light sensitive region.

Basically, smear is produced by scattered photons tunneling into the darkened interline transfer cell rather than being collected in the photosensors of the image sensing area. Every pixel in an image goes by the brightest spot in the image; blank pixels are shifted in from the top of the image pixel array so that even a pixel that is "at the bottom" of the array has been by the bright pixel. Therefore, the vertical stripe above a bright spot is generated by shifting the previous image, while the smear stripe below is produced during the shift of the actual image.

Effect of Integration Time and Lens Aperture on Smear

The final value of a pixel is the sum of the signal from the time spent in the photosensor (light-sensitive portion) and the time spent in the interline transfer (light-shielded portion). The sensitivity for light collection in the interline transfer is approximately 1/100,000 of that in the photosensor. Therefore, smear is often more noticeable when using very short (microsecond) shutter times. The reason for this is that the ratio of the time integrating in the photosensor to the time integrating in the interline transfer is small, which decreases the relative ratio of contributions from the two sources; hence more smear.

The other factor that affects smear is the angle that the light hits the sensor. Many CCDs use on-chip
microlenses that cover each pixel in order to increase sensitivity. When light hitting the sensor is not highly
collimated (this can be exacerbated by using a lens with a very wide aperture) light can be refracted by the outer edges of the lens in such a way that it hits the light-shielded portions of the CCD. This effectively increases the amount of light hitting the interline transfer cell.

Reducing Smear

With respect to the factors described above, in one case you can reduce smear by reducing the amount of light landing on the interline transfer cells relative to the amount hitting the photosensors. In the other case you can decrease the aperture size and collimate the light to prevent it from reaching the interline transfer cells. Some specific means of doing this include:

• Increase the shutter (integration) time. This will increase the amount of time light is being collected in the photosensors relative to the amount of time in the interline transfer cells.
• Turn the
light source off before and after an exposure by using a mechanical or LCD shutter.
• Use a pulsed or flashed light source. A pulsed light of 1/10,000 duration is sufficient in most cases to allow an extremely short 100ns exposure without smear effect.
• Increase light collimation, either by using a lens with variable aperture and stopping it down (close the aperture) or use a lens with a smaller aperture. However, a byproduct of closing the iris will be darker images.