[Aavso-photometry] Linearity test puzzle

[Radu Corlan]

On Mon, Jan 16, 2006 at 12:57:14PM +1100, Vello Tabur wrote:
> I've been testing the linearity of my camera (SBIG ST8XE) using a LED + 
> voltage regulator, as per the circuit in AIP v1 (pg 201).
> Ten exposures (30sec each) were taken either side of a linearity test to 
> assess LED stability during the test.  The plot below shows a gradual 
> increase in LED brightness of 0.24% over a period of one hour. A curious 
> systematic feature appears on the plot: the first exposure of each group 
> shows an rapid increase in flux relative to the other exposures. It 
> occurs in each group and the same effect occurs in tests conducted on 
> other nights.  Any suggestions as to what could be causing this?

i'm not sure how that circuit looks like, but please keep in mind that 
the light output of a led is greatly affected by temperature. a red 
led's intensity can decrease by 0.5% for each degree of increase in 
temperature (at constant current). green and blue leds are a little 
better (they have a variation about 1/2 of that), but still quite 
unstable. a regular led has pretty bad cooling (mainly through the 
leads), and will self-heat with a time constant of seconds. 

a second effect appears when the source driving the led has significant 
output conductance (so it doesn't look like a "perfect" current source). 
the led voltage decreases with increasing temperature, which may 
increase the current through it and affect the light output.

when you first power a led, the combination of the above will give you 
a transient that lasts several seconds (and maybe more). 

so keep your environemnt stable, and you use the led at a 
constant, not too large, intensity (5ma or something).

radu

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-- 
Radu Corlan      

You pay now, or you pay later, but you always pay the entropy tax.