Olympus-OM
[Top] [All Lists]

Re: [OM] Re Infrared images/OT infra red sensors

Subject: Re: [OM] Re Infrared images/OT infra red sensors
From: HI100@xxxxxxx
Date: Sat, 14 Oct 2000 04:47:15 EDT
Jan@xxxxxxxxxxxxxx writes:

<< A detector that is at  20 degrees C simply doesn't have 
enough energy differential to detect human skin that is only 
at 30 degrees C. 
>>

 Although I tend to agree with much of Jan's post in ***connection with 
imaging film ** I disagree with the quoted section. You can go and buy an ear 
thermometer at your local drug store for $50 which does almost exactly what 
is described without any cooling. The sensor head will be at say 22C ambient 
and sensing the ear canal at 35-37C.
     They typically rely on an  IR lens and a thermopile (array of a few 
hundred micro thermocouples). The skin in the ear canal (ideally the tympanic 
membrane) approximates a black body radiator and the sensor head maintains an 
approximately isothermal radiant environment except where the IR enters 
through the lens. The thermopile generates a voltage dependent on the 
difference of  radiant heat from the enclosure on one side of the thermopile 
versus that entering from the lens going to the other side of the pile. To 
get absolute temperature the head needs two additional sensors (typically 
thermistors) to measure enclosure temperature and thermopile temperature. 
     The reason it is harder to do uncooled measurements is that the energy 
transfer depends on the fourth powers of temperature difference. So an 
uncooled sensor has very much smaller signals to work with since the 
temperature difference is small. In an idealised test environment the 
consumer ear sensors can achieve accuracies of around 0.3C. Clinically they 
are not so accurate for a variety of reasons. 
Somebody asked about heat seeking missiles and these have huge temperature 
differences to work with (1000's C) but need much fast responding sensors, 
the consumer thermopiles are pretty slow. The earliest ear thermometers used 
pyroelectric sensors and a mechanical chopping scheme. Some newer mems based 
sensors (micro bolometers etc) are just becoming available which may be used 
in these consumer IR applications in future.

        Regards,
         Tim Hughes
         >>Hi100@xxxxxxx<<

< This message was delivered via the Olympus Mailing List >
< For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
< Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >


<Prev in Thread] Current Thread [Next in Thread>
Sponsored by Tako
Impressum | Datenschutz