Quote:
Originally Posted by JA
Hi Markus,
I've pondered similar testing involving gauging camera frequency response, BUT in the case of the human eye its output response can really only be gauged in two ways: - an individual's perception of the brightness at a given frequency or
- some kind of reading of the signal strength in the optic nerve / brain
The 2nd option is not really that do-able for the home experimenter, so it leaves some sort of test involving a person telling you about the perceived brightness. Possibly something like .... "Oh that is twice as bright as the previous one you showed me" etc....
There is a similar problem with understanding the response of the human ear and forms the basis of such things as the Fletcher-Munson curves/ equal loudness countours in Audio and with various frequency weightings and Loudness contours can help accomodate response non-linearity. The reason I went in to the response of the ear/hearing testing is that it does give an insight in to a possible test method for the visual frequency response that you are interested in measuring.....
Something of a start would be to measure the lowest intensity at which a person, in a controlled dark environment (analagous to a person in a "silent" sound booth) could just perceive a colour out of the darkness ("nothing" so to speak). I'm almost certain that frequencies around Green will win the perception test, in the sense that they'll be perceived at lower intensities than reds and Blues. It's one of the bases of the two Green pixels in the RGB Bayer Matrix Array for one Red and One Blue: RGGB in effect.
Running that test of course requires a calibrated light source: one that you know the intensity and the frequency of for a given set of input conditions such as voltage/current etc to the calibrated light source (unless you use broadband natural daylight/sunlight and various colour and IR filters?) .... Then just adjust the source intensity until the person can no longer perceive the light source (or switch between a light/no-light channel to remove user bias/error) and note the intensity and frequency and move on to the next frequency.
But this only tests for the low level response perception limit and whether it's linear. To test the response at other than the lower limit of perception, is harder and will require more input for the subject. You could try and set the Intensity at a constant brightness from your calibrated light source and then as your scan the frequencies (colours) ask then whether the red was as bright as the orange, for instance. In doing this it would be best if the person didn't have to rely on their visual memory so 2 identical sources could be used to accomodate that difficulty. You could end up possibly making your own version of Flether-Munson curves/Equal Loudness Curves for the eye as a function of light intensity.
Or not .....
Best
JA |
That's why I love this place - so many smart people in one spot.
I agree a blinking threshold detection would be best. Measuring perceptions of brightness would be tricky. and yes, the hearing test analogy is very apt. Issues with the variability of the light source and current could be avoided by using the sun, as you say.
I would imagine something like screwing an S2 filter into a 2" adapter and creating some kind of eyecup that seals completely - essentially turning your S2 filter into a monocle.
Adapt to darkness for half an hour in a dark room with no light, then bump into the furniture until you find your way outside (aim for noon to avoid atmospheric affects?). You can then stack ND filters on top of the S2 filter to test different thresholds with known attenuation.
The thing is, what you are looking at will have different reflectivity depending on composition and sun angle. I'm thinking one of those 18%
grey balls they use for lighting callibration in visual effects shots might be good.
To test for threshold, someone else can block the sightline with a black card.
The biggest issue with this is variations in latitude and transparency, but I think a camera could be used to determine how much light is hitting a surface and compensate adequately for these variations.
In this case, the cost is the grey ball, a black card and a few ND filters.
Maybe that would work? Even if I never actually do it, it's fun figuring out how you *would :-)
Markus