Design Of Light Source For Calibration Of CCD Based On LED

In CCD imaging system, because of the practical problems in the semiconductor manufacturing technology, the outputs of each CCD pixel are not the same when it is irradiated in the same light intensity. This non-uniformity of CCD pixel response affects the application performance of CCD directly, so the whole imaging system must calibrate the non-uniformity of CCD. It is the normal method that using integrating sphere as a standard light source to calibrate the CCD. This light source is usually consisted by the standard lamp, integrating sphere, optical filters and iris diaphragm. In this light source, the integrating sphere provides uniform light field, and iris diaphragm controls the illumination. However, this traditional approach is limited by the standard lamp's own light stability and the size of the integrating sphere. And the changes range of light flux and stability of the standard light source are subject to different constraints. The emergence of the high-power white LED provides a new light source for CCD calibration, especially for the use of integrating sphere as the calibration light source. Now LED is the appropriate and promising light source for CCD calibration, because of its high efficiency, low-light decline, short response time and small size.When using LED as the light source for CCD calibration, it usually combined a number of LED into an array by some form. Thus, it can reach the required illumination conditions of CCD calibration in the integrating sphere light source. However, there are also some problems in the combination of multiple LED light source. Fuzhou University, Wu Haibin pointed out: when using multiple LED to compose LED array, the different connection way, ambient temperature, p-n junction temperature and drive type of LED would affect the balance of each LED current, thereby led to the changes in electro-optical conversion efficiency and instability of the light source. This resulted that the standard light source could not be effective on the calibration of CCD device. Therefore, the stability of high-power white LED light source's luminous flux is the key issue in the calibration of CCD. Anhui Optics and Fine Mechanics of Chinese Academy of Sciences, Chen Feng, who designed spectral tunable LED light source, pointed out:the uniformity of this light source was 0.53%, and the performance of the angle was 0.77% in±10°. This new type of LED light source could be used as a transfer standard for radiometric application. This article starts research from the working principle of LED and the actual engineering design, on the basis of the latest developments in LED engineering applications at home and abroad, and research the stability of LED light array. It is the key technology to solve this problem that researching on the connection type of LED array, the drive type of LED array's power supply and the work condition of LED.This article researches LED light source surrounding the engineering applications of the LED light source as the standard light source for CCD calibration. According to the requirements of light source for CCD calibration, the type of the LED is selected. A LED luminous flux measurement platform is designed and built, including integrating sphere measurements reference and LED target plate measurement device. Through this measurement system, the stability of the luminous flux is researched when LEDs are connected in a different way. Finally, using orthogonal experimental design, LED forward current and connection way of LED array, which can affect the light stability of LED array, are optimized. Experimental results show that when the LED array is in different series and parallel mode, the stability of light source is different. Using 2×2 LED array that series first and then parallel as the basic unit of the LED array, the illumination stability of light source can reach to 99% in 8 hours, and this light source can well meet the requirements of the light source for CCD calibration.