LBCAST

Last updated April 16, 2019

LBCAST (lateral buried charge accumulator and sensing transistor array) is a type of photo sensor which the manufacturer claims is simpler and thus smaller and faster than CMOS sensors. It was developed over ten years by Nikon, in parallel with other manufacturer's development of CMOS, and resulted in shipping product in 2003.

A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material usually with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals controls the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal. Today, some transistors are packaged individually, but many more are found embedded in integrated circuits.

Nikon Corporation, also known just as Nikon, is a Japanese multinational corporation headquartered in Tokyo, Japan, specializing in optics and imaging products.

A charge-coupled device (CCD) is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time. CCDs move charge between capacitive bins in the device, with the shift allowing for the transfer of charge between bins.

From the Nikon Website:

"In July 2003, Nikon introduced LBCAST- a completely new type of image sensor, different from CCD and CMOS, that is a high-speed, power-efficient, low-noise device to be installed in Nikon's flagship camera, the D2Hs."

"... Compared with conventional sensors, it saves more power and achieves less dark noise. (Dark noise is a phenomenon in which randomly spaced bright pixels appear in images due to the heat from the image device during shooting). Also, LBCAST increases image processing speed and improves sensitivity, contrast and color reproduction."^[1]

Comparison between LBCAST and CMOS photo sensors

The main differences between LBCAST and CMOS-based sensors appear to be those given below:^[1]

LBCAST divides the photosites to be read out into two channels by colour, red and blue photosites—making up 50% of the total number of photosites between them, as in CMOS and CCD colour sensors—accumulate using one read-channel, while green photosites—which make up the other 50% of total pixels, as in CMOS and CCD—accumulate in a dedicated channel.

Red is the color at the end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–740 nanometres. It is a primary color in the RGB color model and the CMYK color model, and is the complementary color of cyan. Reds range from the brilliant yellow-tinged scarlet and vermillion to bluish-red crimson, and vary in shade from the pale red pink to the dark red burgundy. The red sky at sunset results from Rayleigh scattering, while the red color of the Grand Canyon and other geological features is caused by hematite or red ochre, both forms of iron oxide. Iron oxide also gives the red color to the planet Mars. The red colour of blood comes from protein hemoglobin, while ripe strawberries, red apples and reddish autumn leaves are colored by anthocyanins.

Blue is one of the three primary colours of pigments in painting and traditional colour theory, as well as in the RGB colour model. It lies between violet and green on the spectrum of visible light. The eye perceives blue when observing light with a dominant wavelength between approximately 450 and 495 nanometres. Most blues contain a slight mixture of other colours; azure contains some green, while ultramarine contains some violet. The clear daytime sky and the deep sea appear blue because of an optical effect known as Rayleigh scattering. An optical effect called Tyndall scattering explains blue eyes. Distant objects appear more blue because of another optical effect called aerial perspective.

This division is to speed reading, and the separation of green pixels, to which the human eye is most sensitive, reduces noise artifacts which might otherwise be introduced by residual electrical charge in the accumulation circuitry, acquired previously from reading a pixel of a different colour.

Red and blue are significantly less important in human sight, and so presumably a decision was made not to keep separate read channels for the two colours, in order to simplify the circuit design for practical use -- at least in the described version of LBCAST technology.

Each photosite in LBCAST uses a single JFET transistor in place of two MOSFET transistors used for the separate tasks of photosite read-out selection and signal amplification, in CMOS technology. In total, CMOS uses four transistors per photosite, where LBCAST manages with three.
Charge to be (re-)distributed between components in the sensor during use are channeled via the LBCAST lower layers, where CMOS channels this charge over the surface layer. This difference is also claimed to reduce the presence of noise artifacts.
The simpler circuitry required translates into more space for light to be received, since wiring and opaque masking takes less space.
The simpler circuitry required translates into fewer layers of material in the silicon chip meaning that tangential light requires less correction in order to traverse to the depth at which the photons result in signal, simplifying the need for lensing at each pixel or photosite, and therefore theoretically, attaining greater image uniformity.

The junction gate field-effect transistor is one of the simple type of field-effect transistor. JFETs are three-terminal semiconductor devices that can be used as electronically-controlled switches, amplifiers, or voltage-controlled resistors.

The metal-oxide-semiconductor field-effect transistor is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, whose voltage determines the conductivity of the device. This ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals. A metal-insulator-semiconductor field-effect transistor or MISFET is a term almost synonymous with MOSFET. Another synonym is IGFET for insulated-gate field-effect transistor.

Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word usually refers to visible light, which is the visible spectrum that is visible to the human eye and is responsible for the sense of sight. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), or 4.00 × 10⁻⁷ to 7.00 × 10⁻⁷ m, between the infrared and the ultraviolet. This wavelength means a frequency range of roughly 430–750 terahertz (THz).

Uses of LBCAST sensors

As at end 2006, the Nikon D2H and the D2Hs were the only publicly available cameras known to carry the sensor. Nikon has opted to use CCDs sourced from Sony in most of their low and mid range cameras and used a CMOS sensor in the flagship D2Xs/D2x. The LBCAST sensor in the D2Hs has remained at 4.1MP.

The Nikon D2H is a professional-grade digital single-lens reflex camera introduced by Nikon Corporation on July 22, 2003. It uses Nikon's own JFET-LBCAST sensor with a 4.1-megapixel resolution, and is optimised for sports and action shooting that require a high frame rate. In 2005, the D2H was replaced by the D2Hs, which added new features derived from the 12-megapixel D2X digital SLR. The D2Hs was discontinued after the introduction of the D300 and D3 models.

With the advent of the D3, D700 and D300 cameras in 2007 and 2008, all featuring CMOS sensor technology, it is unknown whether LBCAST plays a part in the design of the CMOS sensor of either, since Nikon's implementation of LBCAST is an adaptation of CMOS, and it is therefore technically correct to refer to the known instances of LBCAST as CMOS, Nikon has not been forthcoming to requests for specific information on the D3 sensor^[2] and Nikon have claimed in the past that LBCAST would be further developed.^[1]

Criticisms of LBCAST sensors

The following weaknesses have been cited as affecting the Nikon D2H, although whether these issues have origins specific to LBCAST, and whether LBCAST necessitates these problems, is not known.

Colour cast—generally (and by definition) colour correction is a fundamental requirement of imaging in any medium where incident light quality is variable, including unaided human vision, and so this possibly a software issue.^[3]
Infra-red pollution—incident infra-red light is usually filtered on image sensors, using a specific thin film dedicated to this task (but which may serve an additional purpose in protecting the relatively much more expensive sensor component from contamination and damage), and so this issue, if truly the result of ir pollution, may be a problem which is independent of the LBCAST technology of the sensor.^[3]
Low resolution—it is not known whether the relatively low resolution of the D2H and D2Hs in comparison with other professional cameras of the day is a technological limitation of LBCAST, a business management or marketing decision, or whether it has other causes such as gradual ramping-up of manufacturing capability of a new technology by Nikon, who did not appear previously to have the capacity to have novel sensors manufactured to their specification. It is known that there is a market for high-speed, power efficient cameras where higher resolution is not required, for example newspaper journalism.

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References

1 2 3 4 "LBCAST". Nikon Corporation. Retrieved December 15, 2009.
↑ Hogan, Thom (April 9, 2007). "Answers to Nikon D3 Questions" . Retrieved December 15, 2009.
1 2 Chambers, Lloyd (November 11, 2006). "Infrared Contamination: Good Color Gone Bad" . Retrieved December 15, 2009.

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[nik_lbcast-1] 1 2 3 4 "LBCAST". Nikon Corporation. Retrieved December 15, 2009.

[2] Hogan, Thom (April 9, 2007). "Answers to Nikon D3 Questions" . Retrieved December 15, 2009.

[l_cham-3] 1 2 Chambers, Lloyd (November 11, 2006). "Infrared Contamination: Good Color Gone Bad" . Retrieved December 15, 2009.