Most modern optical measurement systems require a light source to provide a uniform illumination of the light field. The uniformity of the light field has a significant impact on the accuracy of the optical measurement system. Therefore, it is necessary to detect the uniformity of the light field. The general method is to perform a point-by-point scan in the light field using a small probe to perform measurement. However, it takes a long time and the scanning mechanism of the probe is complicated. In order to overcome the above shortcomings, this paper proposes a new method for detection using linear array CCD.
2 Detection principle and method Linear CCD is a new type of semiconductor photoelectric conversion device, and its basic structure is a linear MOS capacitor array. The photo-generated charge stored in the image-sensitive cell potential well can be sequentially output under the driving of the clock pulse of the appropriate phase sequence to realize self-scanning. Using the linear distribution structure of the CCD image sensor unit and the self-scanning characteristics of the CCD, the uniformity of the light field can be detected by secondary mechanical scanning. The method scans the entire light field to be tested in two steps, and uses the pixels of the CCD to detect the light field, and uses the frame contrast method to analyze whether the light field to be measured is uniform from the CCD output signal. The first step is a circumferential scan as shown.
The schematic image of the circumferential scan shows that the effective pixel of the CCD is aligned with the optical axis of the light field to be measured, and the effective pixel of the other end is aligned with the edge of the light field to be measured. When the CCD rotates clockwise with the optical axis as the axis, each pixel of the CCD is equivalent to a small probe and is circumferentially scanned in the light field. These circles divide the light field to be measured into a plurality of concentric rings of the same width. Each CCD cell is scanned circumferentially within these concentric rings. In this way, the output signal of each pixel of the CCD can be analyzed to know whether the distribution of the light field in each ring is uniform. Through the first circular scan, it is known whether the light field distribution is uniform in each ring, but it is not possible to determine whether the light field distribution between the rings is uniform, so the second step scan must be performed.
When the CCD is translated in the radial direction, it is equivalent to a small probe scanning along the radial direction of the light field. The analysis of the output signal of the CCD collected for the same reason shows whether the distribution of the light field in the radial direction is uniform, that is, the light field between the rings. Is the distribution uniform? By combining the first and second steps, it is known whether the overall distribution of the light field is uniform. If the light field is evenly distributed along the radial direction and the circumference, the overall distribution of the light field is also uniform.
3 system composed of light field uniformity detection system is mainly composed of computer system, control drive system, CCD scanning mechanism, data acquisition system and display printing terminal. The working process of the system is as follows: firstly, the computer system scans the scanning mechanism of the CCD by the driving system to perform circumferential and radial scanning, and then collects the light field signal obtained by the CCD through the data acquisition system and sends it to the computer system for calculation and analysis, and finally the result Printed out by displaying the print terminal. Its structural block diagram is as shown.
The schematic diagram of the circuit principle uses an integrated operational amplifier feedback type constant current source circuit, which includes an adjustment tube, a sampling resistor, a reference voltage, a comparison amplifier, and the like. In practical applications requiring large output current and high precision, the use of a feedback constant current source circuit is an effective method. When working, it acts through negative feedback. The voltage drop across the current sense resistor Rs is equal to the reference voltage Vr and is applied to the inverting and non-inverting input terminals of the amplifier respectively. A constant current I=Vr/Rs can be obtained. In the state, the output current 10 of the feedback type constant current source is determined only by the reference voltage Us and the sampling resistor Rs, and has no relationship with the changes of the input voltage UI and the load resistance RL.
The auxiliary circuit has LED current display, current limit protection, soft start soft shutdown, color temperature measurement control and other circuits. LED current display circuits are common and will not be introduced here. The protection circuit adopts a current limiting type protection circuit of the Zener diode DW, and the protection circuit does not function during normal operation. When the load current exceeds the limit, the Zener diode is reverse-conducted, and the clamping action of the Zener tube makes the VT current of the adjustment tube no longer large. Current limit value Imax=(Vdw voltage can not be abrupt, need a certain charging time, the voltage can reach the rated value, adjust the starting time by adjusting the charging resistance and capacitance. The color temperature measurement control circuit is mainly designed according to the principle of two-color method, output from the probe The red and blue two-color photocurrents are amplified in the input integrated circuit, and are divided, and the output calibrated voltmeter displays the color temperature.
5 constant current source output current instability factor analysis and component selection reference voltage stability, temperature coefficient of detection resistor resistance, voltage drift of the amplifier input terminal, these three factors have a direct impact on power supply stability, proportional to Relationships are a critical factor in high-stability power supplies. The parameters of the amplifier itself, the internal resistance of the amplifier input and the drift of the current limiting resistor, the change of the load resistance with temperature, the fluctuation of the AC grid, have an indirect effect on the high stability power supply, and their changes are attenuated by KFFRS times. Then it reacts to the change of load current, which is related to the closed-loop feedback amplification factor of the system. The larger the feedback amplification factor, the better, but it is too easy to oscillate. This requires an integrated adjustment process, which can satisfy the power supply stability and enable the power supply. The entire system operates stably without oscillation.
Knowing various unstable factors that affect the output current of the constant current source, properly selecting components and wiring reasonably can minimize the influence of various unfavorable factors to improve the overall performance of the constant current source. Therefore, the key to improving the stability of the power supply is: 1 to improve the accuracy of the reference voltage. This design uses a Zener LM399 with a constant temperature bath to provide a reference voltage. The temperature drift of the LM399 is typically 3X 107/*C.2 The sampling resistance is accurate and constant. The resistor Rs is a precision wirewound resistor made of high-power silicon-manganese-copper material because of its low temperature coefficient, high resistance, good stability, oxidation resistance and heat resistance. 3 low drift feedback amplifier. Select high-performance, low drift, high input impedance operational amplifier CA3140. 6 Conclusion This circuit is simple and feasible, and the cost is low. At around 100 yuan, a precision power supply can be made to meet the power requirements of standard lamps. Used in test laboratories.
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