MATLAB: Light intensity or Grayvalue

grayvalueintensity

Hi.

I have a trivial question! I looked about it, but I am still not sure! Something simple confuses me! I would be grateful if someone can clear me!

In a digital image, is the "light intensity" exactly the same as "grayvalue"?

By light intensity I mean the power (energy) of the light source such as laser, and by the grayvalue, I mean the grayscales of the resulting recorded digital image (due to that light)?

I have seen in some book that these two are related, but it did not say how. I saw somewhere else that these two are the same! That is why I asked this question!

Thank you so much!

Steven

Best Answer

The intensity of the light is a physical value. The gray value of a digitized image depends on the hardware for digitizing, e.g. the sensitivity of the light sensor, the exposure time and the aperture. In addition the processing can influence the gray value, e.g. saturation (when the maximum value is reached) and interpolation, when the output size does not equal the number of sensors.

In the ideal case, a pixel with a higher intensity has a higher gray value. Depending of the hardware of the sensor, there will be a roughly linear relation between the intensity and the gray value - insider certain limits and with a limited precision. So "these two are related" is an accurate description.

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MATLAB: Relation between “Image” intensity and “Light” Intensity

Welcome to the very confusing field of optical units. An image does not have units of intensity. See the table of the bottom of this page: https://en.wikipedia.org/wiki/Candela I'm surprised you couldn't find anything because there is tons of information out there, unfortunately it will make your head spin unless you have a Ph.D. in optics (sometimes even if you do, speaking from personal experience). The units of an image are like joules, or in photometric units lux*m^2*seconds, which is lumens*seconds, which is candela*steradian*second (to get it into all base SI units). Anyway, think of it as a measure of energy (or luminous energy). Let's use regular radiometric units instead of photometric (luminous) units (which are restricted to the human visual range and a lot more complicated). So you have optical power hitting your sensor. Like 10 watts over an area of 1 cm by 1 cm. Now the CCD well integrates those photons. Each pixel might be 5 microns by 5 microns. So now we have watts per area multiplied by the area. That's how many watts are integrated by that pixel. But the pixel only integrates for a certain number of seconds, and watts is joules per second, so you have watts*seconds = (joules/second)*second = joules. That's why I say it's like joules or energy.

OK, that's more than you wanted to know, so I won't even bother to go into the "intensity" of a light source which is even more complicated. Even the "experts" don't agree. For example the American Institute of Physics says that the "intensity" of a light source is W/steradian, yet intensity is an SI Base quantity (like meter, kg, second, ampere, kelvin, and mole) and it's units are candela. My late optics professor, Jim Palmer of the College of Optics at the University of Arizona, got so worked up on the sloppy usage that he wrote a paper on it: "Getting intense about intensity", Metrologia, 1993, vol 30, pp. 371-372. I attach a partial screenshot of his paper, for educational purposes.

MATLAB: How can i observe a 3 dimensional matrix(hdr) using pcolor command

image(rgb2gray(hdr));
colormap(gray(256))

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MATLAB: Relation between “Image” intensity and “Light” Intensity

MATLAB: How can i observe a 3 dimensional matrix(hdr) using pcolor command

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