Re: Calculation of snr

Don Stauffer in Minnesota wrote:
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While a bar chart target is not a sine wave, which is what MTF is
designed for, a normal bar chart target is a 50-50 square wave. Hence
there is no second harmonic. The first actual overtone that comes
into effect is the third. If there is reasonable anti-aliasing
filtering, then one can use modulation measured from a bar chart to do
MTF calcs. Yeah, a sine wave chart is better, but harder to find,
especially for IR.

Indeed, yes, but you can predict the SNR on the retina of a square-wave
bar target, and then compare the results with what actual observers
achieve. IIRC, if you do a Fourier analysis of a square-wave target, the
fundamental comes out with a peak-to-peak amplitude exceeding the
peak-to-peak amplitude of the original target, so the ideal LPF might
actually increase the amplitude!

The other interesting topic for sine-wave charts is: what gamma-law should
apply to them? Linear? Log? Something more resembling the response of
the eye? For low-contrast targets (or low temperature difference
targets), perhaps it doesn't really matter.....

So in photography, is the accurate rendition of low-contrast detail more
important than a "crisp" rendition of high-contrast detail? I think the
answer is: "It depends". <G>

Cheers,
David


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