The Photon Transfer Curve

Purchasing a camera for high performance imaging applications is frequently a confusing and irritating process. Two cameras with seemingly identical printed specifications can perform completely different during a side by side “Shoot-off”. One of the sources for this confusion is that a camera specified as 12 bits from one vendor may mean simply that a 12 bit A/D converter is used somewhere in the camera – while to another (more rigorous) camera vendor, 12 bits means a dynamic range of 72dB.

The inability to perform a quantitative comparison based upon technical specifications alone can be solved in one of two ways.   The first (and most common) approach would be to do side by side comparisons of all cameras which “look” like they might work. This brute force approach will work, but it is very expensive and time consuming for the camera system integrator. In addition to coordinating the delivery of the camera, frame grabber, cables and power supply, the system integrator must familiarize thereself with new software and determine a suitable method for measuring camera performance. While this approach might be intriguing as a doctoral thesis topic, it is very inefficient for the camera consumer. Moreover, it places all of the burden of proving technical capability on the consumer rather than the camera manufacturer.

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Photon Transfer Curve

An alternative approach is to place the burden of technical proof on the camera manufacturer. With this plan, a standardized test procedure is used during camera manufacturing to provide consistent, quantitative and verifiable performance data such as read noise, dark current, full well capacity, sensitivity, dynamic range, gain, and linearity. Fortunately, a test method of this type has existed for well over a decade and is known as the Photon Transfer Curve (PTC). The PTC characterization method is used by NASA’s Goddard Space Flight Center, NASA Jet Propulsion Labs and leading camera manufacturers around the world to allow for an apples-to-apples comparison of key performance parameters. The fundamentals of PTC calibration are derived from simple system theory where knowledge of a system’s input and output signals are used to derive the characteristics of the system itself.