Sensor Size and Number of Pixels

The physical size of the image sensor can play an important role in the performance of the camera, particularly when examined in the context of the effective number of pixels provided by the sensor (commonly referred to as the sensor's "resolution").

For example, the GRAS-14S5 model uses a Sony^® ICX285 CCD, which has fewer pixels (1.4 million, or 1.4MP) than the ICX274 CCD used on the GRAS-20S4 (2.0MP). However, the ICX285 is a larger format sensor (2/3") than the ICX274 (1/1.8"). As a result of being able to spread a smaller number of pixels across a larger physical area, the size of a single pixel is significantly bigger on the ICX285 (6.45 x 6.45um) than on the ICX274 (4.4 x 4.4um). Among other things, this typically results in increased sensitivity, a higher signal-to-noise (SNR) ratio, and an overall increase in image quality, at the expense of fewer effective pixels.

Both sensor size and resolution must also be taken into account when selecting optics. For example, the specification for a particular lens may state that it will fit a 2/3" sensor, but it is also important that it be able to accommodate the correct number of pixels. The GRAS-50S5, which uses a Sony ICX625 2/3" 5.0MP CCD, is an example of where some 2/3" lenses may not have the resolving power to accommodate a 5.0MP sensor. Contact the lens manufacturer if there is any uncertainty regarding compatibility.

Frame Rate

A number of factors can affect a camera's maximum possible full resolution frame rate, including:

1. Bus bandwidth - the total bus bandwidth provides a hard limit on the maximum frame rate (data rate) that any camera(s) attached to it can provide. However, in many cases the maximum frame rate is significantly less than that allowed by the 1394 bus, but it at least serves as a guide for what could be possible. See Knowledge Base Article 22 for more information on the bandwidth available with 1394a and 1394b buses.
2. Image sensor clock rate - most CCD manufacturers provide a maximum clock rate, often referred to as "horizontal drive frequency" or "pixel clock speed", in their datasheets that describes the rate at which pixels can be clocked off (or read out) of the sensor. The clock rate will vary between sensors. For example, Sony recommends a maximum horizontal drive frequency of 36MHz for the ICX274 CCD, and 28MHz for the ICX285. In some cases, overclocking the CCD is desirable, such as wanting to obtain higher than normal frame rates at high resolution. In other cases, limiting the clock rate to within the limit specified by the sensor manufacturer helps to ensure the maximum possible image quality.
3. A/D converter clock rate - different analog-to-digital converters can operate at different speeds. For example, the 12-bit A/D used on the Flea2 FL2-20S4 can operate at pixel rates up to 36MHz, while the 14-bit A/D used on the Grasshopper GRAS-20S4 can operate at pixel rates up to 65MHz. This, among other things, allows the Grasshopper to run the 2.0MP ICX274 at 30FPS, versus 15FPS on the Flea2.
4. Heat and power - after taking into account the previous three factors, it may seem that a particular camera should be able to run at a faster frame rate than is advertised. However, what is theoretically possible may not actually be desirable with the given electrical and/or mechanical design. For example, running a sensor at its maximum pixel clock rate generally requires more power, which in turn generates more heat. With some cameras, such as the ultra-compact Flea2, the case is simply not large enough to dissipate the additional heat.

Special Sensor Characteristics

Some Sony sensors, such as the ICX285, use Sony's EXView HAD CCD^® technology to, "drastically improve light efficiency by including near infrared light." EXView HAD CCD's are also known for their high sensitivity and low smear. Other special characteristics, such as special readout modes, that are supported by the sensor and implemented by the camera should also be taken into account when selecting a Grasshopper model.

Summary

Before making a buying decision, it is important to have a good idea of the minimum requirements of the camera system, then do some research into the options available.

• Contact Point Grey Research technical sales staff
• Browse image sensor datasheets
• Search the Point Grey knowledge base
• Download Point Grey product documents