Quote:
Originally Posted by Stonius
This popped up in the horsehead thread, but I thought I'd make a separate thread here.
The discussion was around how people's eyes have different sensitivities at different frequencies. Some people are more sensitive in the area where red starts heading towards the infra-red which is where a lot of emission nebula emit their light. TBH, I'm still a little foggy on what causes this variability in terms of physiology but it got me thinking - would it be possible to create an experiment to measure such frequency dependent sensitivity?
Presumably, all one would need is a standardised broadband light source and a range of filters, perhaps red, Ha, S2, maybe Hb too.
If the light source were baffled such that the only light emitted was through the filter, one could set it a standard distance away from a (standardised) white card in a dark room and by turning the light on and off one could determine detection at those frequencies.
The biggest issue I see is the procurement of a standardised broadband light source as a light source will have its own emission curve.
An alternative would be to buy some near infra-red LEDs such as these ones, available in wavelengths from 670 to 1650nm. That may be simpler to control. What are your thoughts?
Markus |
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