How 3D Sensors Achieve Increased Speed: Active Window and Resolution

The camera technology used in 3D sensors consists of one of two types––either CCD, or CMOS. Both technologies convert light into electrons. CCDs need an external analog to digital converter whereas CMOS incorporates internal conversion to produce digital directly from the camera chip. CCD has traditionally been used in earlier sensor design, however, more recent sensors leverage CMOS.

Benefits of CMOS Over CCD

Although a CCD can deliver greater sensitivity, it is also more costly to implement, has limited frame rates that slow sensor speed below acceptable production rates, and produces “bloom” when pixels are saturated.

CMOS, on the other hand, offers simpler design, is immune to blooming, offers logarithmic compression to achieve greater dynamic range and produces very high frame rates suitable for factory speeds. CMOS allows users to trade resolution for increased speed by utilizing subsampling, binning or setting smaller regions-of-interest that read out at higher frame rates.

Active Window Sizing

In CMOS, the frame rate is largely determined by the active window––a programmable region of pixels made up of rows and columns that is read out from the camera with each exposure event.

Active Area

Active Area: Z=Rows, X=Columns

In 3D sensors, the rows are often mapped to height (Z) and the columns represent the lateral field of view (X). When the active window is reduced in rows (commonly referred to as “windowing down”), the result is a corresponding increase in frame rate. In some cases, reducing columns can also increase the sensor’s scan speed.

Gocator Sensor Active Area ViewerSetting Active Area

Gocator Setup: Active Area and Spacing (Resolution)

Subsampling and Binning

A sensor’s frame rate can also be sped up either by subsampling (skipping pixels) or binning (summing pixels). Both of these techniques affect the resolution in Z (skipping or summing rows) or X (skipping or summing columns). So, by subsampling or binning by 2 or 4 rows, the user can speed up the sensor’s frame rate by a factor of 2 or 4.

Controlling Active Area and Resolution to Optimize Speed

In Gocator’s smart sensor web interface, the user can set the active window. In many applications, the full measurement range (Z) for a Gocator model isn’t needed, which means the user is able to reduce this range to cover their part height variation and achieve a significant speed increase as a result. For example, a sensor calibrated with a 50mm measurement range can be windowed down to 10mm and achieve a 5x frame rate increase. Similarly, the Z resolution may not be required, and can therefore be reduced by ½ or ¼ to increase speed by x2 or x4.

Speed vs. Measurement Range