Infra-red source

Details Infra-red source Infra-red source

1993-03-18

1994-06-28

Anderson, Bruce C.

Radiant energy

Radiant energy generation and sources

250504R, G01J 100

Patent

active

053249518

DESCRIPTION:

BRIEF SUMMARY
This invention relates to an infra-red source particularly for use in a non-dispersive type of infra-red analyser.
The simplest form of such an analyser (a single beam, single waveform photometer) would normally consist of: an infra-red source, a beam modulator, a sample cell, a photo detector system, and an electronic system. In a gas analysis system, the cell is initially filled with a gas such as air which does not absorb infra-red and the system is zeroed. A calibration gas of known composition, the same as the gas to be monitored and which absorbs infra-red, is used to calibrate the instrument.
The infra-red source could be a Nernst element, a glow-bar or simply a heated wire which radiates in the infra-red region. The infra-red beam is usually modulated by mechanical means such as a chopper motor, an actuator or a tuning fork. The beam is allowed to pass or is stopped periodically by such means. An IR filter is sometimes placed on the chopper so that the beam passes through a filter selected in relation to the substance to be analysed. The modulated beam goes through the sample cell and then impinges on the photo detector which with the help of an electronic system gives a signal which is displayed.
The main reasons for modulating the infra-red beam are to be able to process the signal in an AC mode, and the need to handle transient signals such as breath-by-breath analysis of exhaled gases requiring a speed of response of the analyser in the region of 100 milliseconds.
Experience has shown mechanical choppers to be bulky and unreliable. This made some designers use small filament lamps and operate them in a switched mode, dispensing with a mechanical modulator. These however have the disadvantage of being enclosed in an ordinary glass envelope which absorbs infra-red energy. They also suffer from an uncertain life time and drift due to the movement of the filament when heated.
There are sources now available based on thin film technology and ones on semi-conductor technology which can be modulated. These however are costly.
Thus there is a need for a relatively low power low cost infra-red source which avoids the problems of mechanical beam modulators and which minimises the problems of thermal drift, short-lifetime and cost.
The present invention provides an infra-red source comprising a support base, a strip of material capable of radiating infra-red emissions, means arranged to allow connection of said strip across a source of electrical power, and two resilient mounting members provided on said support base each attached adjacent a respective end of said strip and arranged to support said strip in a tensioned state.
There is therefore provided an infra-red source which is capable of being operated in a switched mode at a frequency in the region 0.5-30 Hz. Typically the source according to this invention operates at a relatively low temperature, i.e. below 1050K.
For infra-red spectrophotometry, it is important that the infra-red source be stable both in radiation intensity and in physical position so as to reduce the drift in the apparatus to a minimum. The strip of material providing the infra-red emission in this invention is symmetrically resiliently mounted. This causes the centre of the strip to remain stationary even when the strip expands due to the increase in temperature during operation, and also maintains the tensioning of the strip.
In the source according to this invention electrical connection to the strip may be by way either of the resilient mounting members themselves or by means separate from the resilient mounting members. The source of the invention is typically low power, e.g. 1 W, and is generally of a small physical size.
In GB 917160 there is disclosed a means for supporting a tungsten filament in a high-intensity incandescent-filament source, that is to say a source of light or heat in which the filament is required to operate at temperatures exceeding 2500 K. In such a source, which generally operates continuously, a sagging filament leads to a very short life. Thus GB

REFERENCES:
patent: 4378489 (1983-03-01), Chabinsky et al.
patent: 4620104 (1986-10-01), Nordal et al.
patent: 4859858 (1989-08-01), Knodle et al.
patent: 4883971 (1989-11-01), Jensen
patent: 4922108 (1990-05-01), Modlinski et al.
patent: 5220173 (1993-06-01), Konstad

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