To read and do close work without putting stress on your eyes, you need 100 to 1000 lux of light. Bright sunlight or specular illumination gives off 50,000 lux, while twilight or dim light only gives off 10 lux. The light from a fluorescent lamp is better for reading and close work than the light from a tungsten lamp, which is only 1600 lumens. If the eyes are exposed to dim light for a long time, they will get used to it and become very tired. Here is a circuit for measuring the amount of light. It can show a linear scale of illuminance that compares different levels of light, such as minimum, moderate, and maximum. Circuit for light intensity measurement The light sensor in the circuit is a p-n photodiode, and the display driver IC LM3915 (IC1) gives a logarithmic current scale that is proportional to the amount of light hitting it. Compared to cadmium-sulphide (CdS) photodetectors, the photodiode has a fast switching speed of one nanosecond and can respond at a high frequency. The photodiode's "n" material is made of pure silicon that has been doped with phosphor to

give off free electrons. The 'p' material is made of pure silicon with boron as the doping material. It gives away free holes. Circuit Operation When it is dark, a reverse saturation current builds up in the photodiode, and when it is light, free electrons move. In a photodiode, the amount of light is proportional to how straight the flow of current is. Usually, the light current at 5 volts is between 10 and 90 A, and a p-n photodiode's radiation sensitivity is between 0.4 and 4 A/mW/cm2. To get the light current, the photodiode is connected to the circuit in forward-biased mode through the current-limiting resistor R1. When light hits the photodiode, it sends a current to the input pin 5 of IC1 through the preset VR1. The IC LM3915 has a complete voltmeter circuit built in so that the LED scale can show how the input voltage changes. When 1.25 volts are added to the input pin 5 of IC1, each LED from pin 18 on down lights up. The IC has ten comparators with a current source output and a reference and ladder voltage divider that provides the necessary reference voltage. Pin 7 of the IC is kept at a reference voltage of 1.25 volts, and resistor R2 controls the amount of current going through the LEDs, which controls how bright they are. By changing the position of preset VR2, you can set the lowest light level at which the first LED (LED1) should light up. By adjusting preset VR1, you can set the maximum voltage at which LED7 should light up. This is done by adjusting the voltage that goes into the IC. LED1 shows the least amount of light, LED4 (yellow) shows medium light, and LED7 shows normal light. Each LED takes a little longer to turn on or off because of capacitor C1.