<title>High-accuracy photometry using CCD technology</title>

Abstract

One of the most significant advances in light detection methods in recent years is the development of CCD technology. There is a great difference, however, in merely detecting light versus providing accurate photometric measurements of a field of view. Many factors must be taken into account in controlling CCD and camera characteristics, data acquisition and subsequent data processing. Calibration techniques are particularly significant if we are to ensure proper photometric analysis.This presentation will cover the important variables and controls needed, and provide illustrations of the value of videobased measurements in practical applications.The measurement of luminance distributions became a topic of research in the 1960's, when film cameras were used to perform absolute photometry.1 The procedures were complex, time-consuming and fraught with problems.Nevertheless, groundwork was laid which suggested that photographic photometry would be a viable and powerful technique if the chief source of the problem, the film itself, could be replaced by a more stable and reliable means of collecting the image. With the advent of the Charge Coupled Device, (CCD), most of the problems of the film cameras are removed. The CCD array, however, brings with it a new series of problems. To produce an accurate system of photometry, it is essential to understand the problems inherent to CCD camera systems. Techniques must be developed to overcome their idiosyncrasies, calibration methods must be developed, and controls must be exercised to ensure accuracy. Achieving these goals is not a trivial pursuit. This paper will address the many factors involved in the development of a high accuracy CCD measurement system to provide NIST-traceable reliable photometry. FORMS OF CCD SYSTEMMany types of CCD cameras exist. These range from inexpensive mass-produced systems for camcorders and low cost television cameras, to the highest quality systems used in scientific applications such as astronomy.Low cost cameras typically are used in non-broadcast television for surveillance and other non-critical applications. These normally use the NTS signal convention, and their purpose is to collect and transmit images rapidly. Photonic quality suffers due to high noise and low sensitivity, and such cameras can introduce insurmountable problems if it is attempted to use them to perform high accuracy photometry. At the other end of the range are cameras of very high sophistication, incredible sensitivity and stability. These are designed for critical applications in astronomical observations, but their cost is prohibitive for normal photometric applications.Between the two extremes is the "Slow Scan" camera, which offers technology quite different from the television systems. These offer a wide range of user controls, as is essential for high accuracy photometric applications. Such 28 / SPIE Vol. 2550 0-8194-1909-5/95/$6.00 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/23/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOf...

Radiometric calibration of charge-coupled-device video cameras

“…Taking into account the camera structure, the physical processes involved in the detection of light and the information required from the camera measurements, the radiometric characteristics to be determined are the spectral responsivity and its variation from pixel to pixel, the linearity of the response (to obtain reading ratios), the dark response and an estimate of the noise [11]. Since the response of individual pixels can be assumed to be independent, the CCD calibration may be thought of as a multiple calibration of individual detectors placed next to each other in the same plane.…”

“…CCD detectors are now produced with good isolation between pixels, the charge is stored without leakage in each detector element and the charge transfer efficiency is very high, as mentioned above. Over more than 95 % of the electronic dynamic range of most cameras the output is linear with respect to the absorbed optical power [11]. When the voltage response is converted to a digital value, automatic cameras may extend their dynamic range by producing a non-linear video signal, for example to compensate the non-linear response of the TV monitors on which the image will be presented.…”

“…FPN is small in those systems where electronic exposure control is not used. For high-precision radiometric work, FPN has to be measured for the whole array and the FPN value has to be subtracted from the measurements [11].…”