Design On Digital Focimeter Of Projection Type

The population of our country is more than 1.3 billion. With the improvement of civil living conditions, the requirement on spectacles is fast growing. The parameters of spectacle lens, such as spherical power, astigmatic power, prismatic power and optical center are measured by focimeter. So focimeter is one of the most important instruments to ensure the quality of spectacles and plays an important role in the consumption of spectacles. As a result, the veracity of focimeter is significant. With the deep understanding of how to detect the lens power of spectacle, a digital focimeter of projection type is designed.Firstly put the spectacle to be tested in the focimeter. Then focus to produce a sharp image on the projection screen by turning two knobs properly. The lens power will display on the LCD. Also the result can be printed by an integrated mini printer.The instrument has a single-chip as its central part. When it is under operation, a staff marking a cross will translate and rotate. The translation and rotation are transduced into respective electric signals. The two signals are sent to an AD converter via an analog switch asynchronously. Then they become digital signals and go into the single chip. After calculation, the lens powers are worked out and sent to a LCD screen. The screen also shows current measuring function and state. A keyboard contains 7 function keys and one reset key. A speaker gives out key-click sound and some other, like error alarm. The lamp will be turned off automatically if there is no operation in about 3 minutes. Once the keyboard is clicked or the staff is moved it will be turned on again immediately. The software system splits into modules including the main function, signal sampling, parameter calculating, LCD display, print control, sound control and standard storing. Each is relative to necessary functions.Experiment shows the precision is±0.017D that is better than the national standard±0.06D. The instrument offers clear projection, intuitional output, easy operation and stable performance.The paper presents the entire design in detail. At first the background knowledge is introduced. Then the measure principle is analyzed. Then the hardware and software system is explained. At last error and experiment data are given.