Browse Prior Art Database

Infrared Detection System

IP.com Disclosure Number: IPCOM000077890D
Original Publication Date: 1972-Oct-01
Included in the Prior Art Database: 2005-Feb-25
Document File: 3 page(s) / 61K

Publishing Venue

IBM

Related People

Boynton, AL: AUTHOR [+2]

Abstract

The pulsed infrared (IR) detection system shown in Fig. 1, permits recovery of weak light signal 12 in a high-ambient light environment for operating a passenger barrier.

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Infrared Detection System

The pulsed infrared (IR) detection system shown in Fig. 1, permits recovery of weak light signal 12 in a high-ambient light environment for operating a passenger barrier.

Light-emitting diode (LED) 10 has a spectral peak at 9000 angstroms and is pulsed with 500 mus pulses at 200 Hz by LED driver 14, to emit weak IR light 12 through optical filter and focusing means 16. Phototransistor Q1 is also peaked at 9000 angstroms to pick up the light signal focused on it by optical filter and focusing means 20, and feeds its output signal to detector 22 where further filtering of undesired signals occurs, thence to line 24 on which appears the input signal to logic and barrier control circuits, not shown.

Filters 16 and 20 have a bandpass peaked close to 9000 angstroms and function to hide the detection system and filter the DC component of ambient, while electrical filtering within detector 22 removes the AC component of ambient light on phototransistor Q1.

Referring to Fig. 2, the collector of Q1 is connected to a high-voltage source 30, 32 and through a small emitter resistor R1 so that phototransistor Q1 takes an extremely high-ambient light level to saturate. Amplifier 34 is decoupled from phototransistor Q1 by capacitor C2 so that DC shifts at the emitter of phototransistor Q1 will not affect the bias of transistor Q2 in emitter follower 36, the input stage of amplifier 34. Emitter-follower 36 isolates the signal on the emitter of phototransistor Q1 from differentiating filter 38, which attenuates low- frequency noise developed by fluorescent lights.

In differentiator 38, the 500 mu sec light pulse effective lengt...