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Subject: Re: is there an optical sensor to measure distances? Yes

Date: 10/11/00 at 3:45 PM
Posted by: Bruce Land
E-mail: bland3@mindspring.com
Message Posted:

In Reply to: is there an optical sensor to measure distances? posted by jaktan on 09/29/00 at 3:05 PM:

There are two basic types. For distances of several feet or longer a laser diode is pulsed on and off. The time delay in receiving the reflected light is measured and distance calculated. About 15 years ago, a Korad 375MW ruby laser was used to send a pulse to the moon where it was reflected from a 10ft aluminum foil. The delay time allowed the measurement of the distance to the moon within a few feet. Now you can purchase complete laser diode distance measurement units for a couple hundred dollars and up. The effective distance is proportional to the strength of the laser and the intensity of the reflection. You can try this by inputing two fast photodiodes into two channels of an oscilloscope. Set the scope to capture a one shot pulse. Place the two diodes one behind the other by a couple of feet. Stand in front and flash a camera flash unit. You can see the time delay as the light arrives at the two photodiodes.

The second method uses low power laser diodes. A laser poiner will work. The reflected light is focused onto a linear photodiode array or a CCD chip. CCD chips are available with 1024 bits. The angle of the reflected light determines which bit is lit and the distance can be calculated from the angle. These generally work over a narrow range, say 1 to 2 inches or 10 to 11 inches. However some can resolve +/- 0.0005 inch. Both types of units have onboard microprocessors to calculate distance.

You will never be able to build either type as cheep as you can buy them.


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